This glossary explains every abbreviation and technical term used in this calculator in plain language, listed in alphabetical order. If you are unsure about any value on your motor's nameplate or in the results, refer here first.
A Amperes (Amps)
The unit of electric current — the rate at which electricity flows through a wire or device. A higher ampere rating means more current is flowing. Wires, relays, and circuit breakers are all rated in amps to ensure they are not overloaded.
Arrhenius Rule Thermal Ageing of Motor Insulation
A fundamental principle of insulation chemistry that states: for every 10°C rise in operating temperature above a motor's rated insulation class temperature, the insulation life is approximately halved. Conversely, every 10°C reduction doubles expected life. For example, a Class F winding rated to 155°C operated continuously at 165°C will have roughly half the expected service life. This is why start frequency matters — frequent starts raise the average winding temperature above the steady-state running value, consuming insulation life faster. The thermal equivalent current (I_eq) in this calculator quantifies this effect.
A1 / A2 Contactor Coil Terminals
The two terminals on a contactor where the control coil is connected. A1 is conventionally the positive or switched terminal — connect it to the Kelco R1 N/O terminal. A2 is the return terminal — connect it to neutral (or the transformer secondary return, or the 24 V supply return). The coil voltage is the voltage that appears between A1 and A2 when R1 closes. In a contactor installation the KSS Super Snubber fitted at the controller goes across R1 COM and NO; a second KSS at the contactor goes across A1 and A2. Never connect A1 and A2 across two phases of a three-phase supply.
AC-3 IEC Utilisation Category AC-3
The IEC 60947-4-1 utilisation category for contactors used to start and stop squirrel-cage induction motors. AC-3 duty means the contactor closes onto locked-rotor inrush current (typically 5–8× FLA) and opens while the motor is running at full speed. The AC-3 current rating (Ie) is the maximum motor FLA the contactor can handle under these conditions repeatedly over its rated life. Always select a contactor with an AC-3 rating at or above the motor's SF-adjusted FLA. AC-3 contactors are the correct choice for all Kelco pump controller interposing contactor installations.
AC-4 IEC Utilisation Category AC-4
A more demanding IEC 60947-4-1 utilisation category covering contactors used for plugging (reversing), inching, or jogging duty — where the contactor closes onto a stationary or counter-rotating motor and/or opens against full inrush current. AC-4 is significantly more severe than AC-3 and requires a larger or higher-rated contactor for the same motor FLA. For standard Kelco pump controller installations (start, run, stop), AC-4 duty does not apply — AC-3 is always the correct category. Never substitute an AC-3 rated contactor for AC-4 duty: it will wear out rapidly.
AS/NZS Australian/New Zealand Standard
The joint Australian and New Zealand national standards body. AS/NZS 3000 (the "Wiring Rules") governs all electrical installations in Australia and New Zealand, including cable sizing, circuit protection, and earthing. All electrical work must comply with AS/NZS 3000 and be carried out by a licensed electrician.
AWG American Wire Gauge
The US/imperial system for measuring wire thickness. Confusingly, a lower AWG number means a thicker, higher-capacity wire — so 10 AWG is thicker than 14 AWG. Used on US NEMA motors and in installations following the NEC (National Electrical Code).
Back-EMF Back Electromotive Force
The voltage spike generated by an inductive load — such as a contactor coil or motor winding — when the current through it is suddenly interrupted. Inductors resist changes in current; when R1 opens and cuts off coil current, the collapsing magnetic field drives a brief high-voltage spike in the reverse direction. In contactor installations this spike appears directly across the relay R1 contacts, causing arc erosion and shortening contact life. The KSS Super Snubber absorbs this spike before it can damage the relay contacts or propagate back into the controller electronics. The HD Triac performs the equivalent function for direct motor switching.
Cap AC Voltage Margin Capacitor Supply Voltage Headroom
The percentage by which the mains supply voltage falls below the X2 capacitor's rated AC voltage. A capacitor connected permanently across the mains must be rated at or above the actual supply voltage, with adequate margin for long-term reliability. The KSS uses a 275 V AC X2 capacitor — at a nominal 240 V supply this provides approximately 13% headroom. At supply voltages approaching 275 V AC (possible at the upper end of some utility supplies) this margin narrows and is flagged as a warning. A 5% or lower margin indicates the supply voltage is very close to the capacitor's rated limit and should be investigated.
CCC Current Carrying Capacity
The maximum continuous current (in amps) that a cable can safely carry without overheating, based on its size, conductor material, installation method, and ambient temperature. Cable must be sized so that its CCC is at least 125% of the motor FLA per AS/NZS 3000 and IEC standards. Using undersized cable is a fire risk.
COM Common Terminal
The shared connection point of an SPDT relay — the terminal to which the moving contact is always connected, regardless of whether the relay is energised or not. On the Kelco controller, the motor active supply runs between R1 COM and R1 N/O. The COM terminal is always live when the mains supply is connected.
Control cable Controller-to-Contactor Wiring
The cable between the Kelco controller R1 COM/NO terminals and the contactor coil terminals A1/A2. This is a low-current single-phase circuit — the coil typically draws less than 100 mA running. All calculator calculations use 1.5 mm² stranded copper, the standard minimum for fixed control wiring per AS/NZS 3000:2018 Table 3.3 (R = 11.49 mΩ/m, IEC 60228 Class 2 stranded Cu at 20°C). The conductor size has no material effect on any result at these current levels. The length is what matters operationally: runs over 10 m require a KSS Super Snubber at both the controller (R1 COM/NO) and the coil terminals (A1/A2). The control cable is entirely separate from the motor power cable — it carries coil current only, never motor load current.
Control circuit Low-Current Coil Switching Circuit
The single-phase circuit consisting of the Kelco R1 relay contacts, the control cable, and the contactor coil. The control circuit carries only the coil current (typically <100 mA) — it does not carry motor load current at any time. The KSS Super Snubber protects the control circuit from the coil's back-EMF on every de-energisation. In three-phase installations this distinction is especially important: the KSS is always fitted to the control circuit and must never be connected to the three-phase motor power circuit.
Contactor Interposing Contactor
A heavy-duty electromechanical switch designed specifically for switching large motor loads repeatedly. When the Kelco controller cannot directly switch a motor (because the motor is too large, three-phase, or uses a DOL start above 10 A FLA), an interposing contactor is installed between the controller and the motor. The controller's Relay 1 switches the contactor's control coil, and the contactor's main contacts switch the full motor current. Coil voltage depends on the Kelco controller wiring: mains voltage (~240 V AC) when R1 COM is connected to the A terminal; ~24 V AC (mains-derived, not galvanically isolated) or 24 V AC/DC (external supply) when the LV terminal is jumpered to R1 COM — see the LV terminal entry. Always fit a KSS Super Snubber (part no. KSS) across R1 COM and NO to protect the relay contacts from the contactor coil's back-EMF. For cable runs longer than 10 m from controller to contactor, also fit a second KSS across the contactor coil terminals A1 and A2.
cosφ Cosine Phi — Power Factor
The same as Power Factor — just written using the mathematical symbol φ (phi), which represents the phase angle between voltage and current in the motor circuit. You may see this symbol on European motor nameplates. Enter the decimal value (e.g. 0.85) in the Power Factor field.
Cu / Al Copper / Aluminium
The conductor material used in the cable. Copper (Cu) conducts electricity better than aluminium (Al) for the same cable size, so an aluminium cable must be thicker than a copper cable to carry the same current. Aluminium cables require special connectors and anti-oxidant compound at all terminations and are generally only permitted in sizes of 16 mm² and above for fixed wiring.
DOL Direct On Line
The simplest starting method — the motor is connected directly to the full mains voltage the instant it starts. This produces the highest possible starting inrush current (LRC), which can be 5–8 times the running current. DOL starting is common for small motors but stresses both the motor windings and the switching contacts heavily.
dV/dt Rate of Voltage Rise
How fast the voltage rises (in volts per microsecond — V/µs) when the Triac switches off. Triacs can malfunction or be damaged if voltage rises too quickly, even if the peak voltage stays within limits. The snubber capacitor slows this rise rate to a safe level. This calculator checks dV/dt against the BTA41's specification.
% Efficiency Motor Efficiency
The percentage of electrical input power (P1) that the motor converts into useful mechanical output (P2). A motor with 85% efficiency loses 15% of the power it draws as heat. Efficiency is used by this calculator to convert P2 to P1 when needed: P1 = P2 ÷ efficiency.
EMI Electromagnetic Interference
Unwanted electrical noise generated by switching circuits (such as the Triac) that can interfere with nearby electronic equipment — radios, sensors, PLCs, and other controllers. The RC snubber recommended by this calculator also reduces EMI by slowing down the rate at which the voltage spikes rise and fall.
E_cycle / P_avg Energy per Cycle / Average Triac Dissipation
E_cycle is the total energy (in joules) dissipated in the Triac during one complete pump start-stop event — the sum of the start pulse energy and the stop pulse energy, each lasting 1.5 s. P_avg is the average power dissipated by the Triac over time, calculated as E_cycle × starts-per-hour ÷ 3600 seconds. At low cycling rates P_avg is small and the Triac runs cool between pulses. As start frequency increases, P_avg rises and the average junction temperature climbs. This is the basis for the Triac thermal check in the Start Frequency card: T_j(avg) = T_ambient + P_avg × R_th(j-a). Both values are calculated automatically from the motor FLA, LRC, and starts-per-hour entries.
½LI² Stored Magnetic Energy in an Inductor
The energy (in joules) stored in the magnetic field of an inductor — such as a contactor coil — when current I is flowing through it. L is the inductance in henries. Because inductors resist any change in current, this stored energy must go somewhere when R1 opens and the current is cut off: it discharges as a voltage spike across the opening contacts. The higher the inductance and the higher the current at the moment of opening, the more energetic — and more damaging — the spike. For a typical small contactor coil this stored energy is of the order of 10–100 mJ, but the resulting spike can reach thousands of volts because it is released in under one millisecond. The KSS Super Snubber provides a controlled path for this energy to discharge through the RC network rather than across the relay contacts.
FAIL Direct Switching Not Viable
One or more of the motor's electrical parameters exceeds the safe limits of the Kelco controller's relay or Triac. Direct wiring is not permitted. An interposing contactor must be installed. The specific reason for the failure is shown in the result details. The HD terminal link must NOT be used when a contactor is fitted.
FLA Full Load Amps
The current (in amps) that the motor draws from the supply when running at its full rated power output under normal conditions. This is the most important figure for sizing cables, circuit breakers, and relay contacts. It is printed on every motor's nameplate.
Galvanic Isolation Complete Electrical Separation Between Circuits
Two circuits are galvanically isolated when there is no direct electrical connection (no shared wire, conductor, or component) between them — energy is transferred only through magnetic coupling (a transformer core) or optically. A standard iron-core or toroidal isolation transformer provides galvanic isolation between its primary (mains input) and secondary (output) windings. This is important in pump control installations because: (1) faults on the control circuit side cannot cause dangerous voltages to appear on the motor supply side, and vice versa; (2) earth fault currents cannot flow between the circuits; (3) conducted transients (back-EMF spikes from contactors, voltage surges from the three-phase supply) are attenuated by the transformer's coupling impedance before reaching the secondary circuit. By contrast, the Kelco controller's internal LV terminal — when the controller is mains-powered — is NOT galvanically isolated from the mains, because the 24 V AC it provides is derived from the same supply through the controller's internal circuitry rather than through a separate isolation transformer. All wiring on a non-isolated LV circuit must be treated as potentially live at mains potential.
Ghost current Residual Coil Voltage from KSS Capacitor
When R1 is open (motor stopped) with a KSS Super Snubber fitted across R1 COM/NO, the KSS capacitor creates a small continuous current path through whatever is connected to the relay — in a contactor installation, through the contactor coil. This residual current keeps a small voltage across the de-energised coil (approximately 42 V with a 240 V AC coil and both KSS devices fitted). This is well below the drop-out threshold of any standard 240 V AC coil (typically 72–108 V) so the contactor releases cleanly. For 24 V AC coil installations the effect is negligible. Ghost current is a predictable and harmless side-effect of the RC snubber topology and does not affect normal operation.
HD Heavy Duty Terminal / HD Drive
Fitting the HD terminal link wire between the HD and R1 N/O terminals on the Kelco controller activates an internal Triac (STMicro BTA41) wired in parallel with the Relay 1 N/O contact. This allows the relay to switch motor loads at its full resistive contact rating by eliminating inrush current and break-arc on every start and stop cycle.
On start: The Triac fires first, absorbing the full locked-rotor inrush current (LRC) while the motor accelerates — typically within 0.2–0.5 s. The relay contacts then close onto a near-running motor (a near-resistive switching event with minimal arc). The Triac deactivates at 1.5 s; the motor runs entirely through the relay contacts thereafter.
On stop: The Triac fires again for 1.5 s, sharing the running current while the relay contacts open cleanly with no arc. The Triac absorbs any back-EMF transient before deactivating.
Thermal constraint (no heatsink): The BTA41 is PCB-mounted without an external heatsink. Locked-rotor current must not exceed approximately 60 A during the 1.5 s start window to maintain a safe junction temperature. This limits DOL direct starting to motors with FLA ≤ 10 A (LRC ≈ 60 A at 6:1). Soft-starters and VFDs reduce inrush and allow the full 16 A relay rating to be used.
⛔ Restriction: The HD terminal is at full mains potential at all times. It is strictly for direct AC motor loads only. Never use the HD link when an interposing contactor is fitted, or with any device requiring voltage-free contacts.
HP Horsepower
An imperial unit of power used mainly in the United States and on older Australian motors. 1 HP = 0.746 kW. American NEMA-standard motor nameplates rate power in HP. This calculator accepts either unit — use whatever is printed on your motor's nameplate.
Hz Hertz
The frequency of the AC mains supply — how many times per second the electrical current reverses direction. Australia uses 50 Hz. The United States and Canada use 60 Hz. Using a motor rated for the wrong frequency can cause it to run at the wrong speed.
Holdup Time SMPS Output Ride-Through Duration
The length of time a switch-mode power supply (SMPS) can maintain its rated output voltage after the input supply is interrupted or momentarily drops below its operating range. Measured in milliseconds, holdup time is determined by the energy stored in the SMPS output capacitors. In pump control applications, a short holdup time is a risk: when an interposing contactor closes, its coil draws a brief inrush current (typically 10–20× its steady-state VA for a few milliseconds) that can sag the SMPS output below the Kelco controller's minimum operating voltage. If the SMPS holdup time is too short, the controller may reset or misoperate during every contactor closure. A minimum holdup time of 20 ms at full load is recommended. Iron-core transformers are inherently immune to this issue — their output is determined by the mains supply voltage and transformer design, not by stored capacitor charge.
IEC International Electrotechnical Commission
The international body that publishes electrical standards used in Australia, Europe, and most of the world. IEC-rated motors express power in kW, use mm² cable sizes, and follow metric conventions. The Kelco controller is designed primarily for IEC/Australian electrical systems.
IEC 60947-4-1 Low-Voltage Switchgear — Electromechanical Contactors and Motor Starters
The international standard governing the design, testing, and selection of AC contactors and motor starters for low-voltage applications (up to 1000 V AC). It defines utilisation categories (AC-3, AC-4), rated operational current (Ie), rated insulation voltage (Ui), and mechanical and electrical endurance requirements. All interposing contactors recommended by this calculator should be selected and specified to IEC 60947-4-1. In Australia and New Zealand it is adopted as AS/NZS 60947-4-1. The equivalent North American standard is NEMA ICS-2.
KSS Kelco Super Snubber
The Kelco Super Snubber (part number KSS) is a pre-assembled RC snubber device made specifically for Kelco HD pump controller installations. Its impedance at 50 Hz presents a high-resistance path to mains-frequency current while absorbing high-frequency switching transients rapidly. One KSS covers all Kelco HD applications — it is a single universal part regardless of motor size, cable length, or pump type.
I_eq Thermal Equivalent Current
The single steady-state current value that would produce the same average winding heating in a motor as the actual intermittent duty cycle of starts, running periods, and stops. Calculated per IEC 60034-1 §6.1 for S4 intermittent duty: I_eq = √(FLA² + (sph × t_acc / 3600) × (LRC² − FLA²)). If I_eq exceeds the motor's nameplate FLA, the windings will run hotter than their rated temperature at that cycling rate, shortening insulation life per the Arrhenius rule. Cable CCC is based on continuous running FLA and is not affected by I_eq — the cable carries only the running current, not the starting current average.
I_pk Peak Current
The instantaneous maximum current in an AC circuit — the top of the sinusoidal waveform. For a sinusoidal supply, I_pk = I_RMS × √2. At 10 A RMS, the peak current is 14.1 A. Peak current matters for inductance calculations because the energy stored in an inductor (½LI²) is determined by the instantaneous current at the moment the circuit opens, not the RMS value. A contactor coil that draws 50 mA RMS has a peak current of approximately 71 mA — and it is this 71 mA that determines how much magnetic energy is stored and must be absorbed by the KSS when R1 opens.
kVA Kilovolt-Amperes
A measure of "apparent power" — the total electrical load placed on a supply circuit, including both the useful power (kW) and the reactive power drawn by the motor's magnetic field. kVA is always greater than or equal to kW. The difference depends on the power factor (see PF).
kW Kilowatts
A metric unit of power equal to 1,000 Watts. Used to measure how much electrical energy a motor consumes per second. Most Australian and European motor nameplates rate power in kW. 1 kW ≈ 1.341 HP.
LV Terminal Low Voltage Terminal — Kelco Controller
A terminal on Kelco pump controllers that serves two purposes depending on how the controller is powered. (1) Mains-powered controller (A & N terminals): the LV terminal outputs approximately 24 V AC, derived internally from the mains supply. This voltage is NOT galvanically isolated from the mains — it shares a common reference with the mains neutral. By fitting a jumper wire from LV to R1 COM, the relay coil circuit operates at ~24 V AC, allowing a 24 V AC contactor coil to be used instead of a mains-voltage coil. All wiring associated with this LV-derived circuit must still be treated as potentially at mains potential. (2) 24 V supply on LV & N: the controller can be powered entirely from an external 24 V AC or 24 V DC supply connected to the LV and N terminals. In this configuration, jumpering LV to R1 COM drives the contactor coil at the external 24 V supply voltage. A 24 V AC or 24 V DC coil (matched to the supply type) can then be used. This is a true low-voltage coil circuit, though correct earthing and isolation practices must still be observed.
L-L / L-N Line-to-Line / Line-to-Neutral Voltage
Terms describing how voltage is measured in a three-phase electrical system. L-L (Line-to-Line): the voltage measured between any two of the three phase conductors (lines). This is the figure printed on three-phase motor nameplates and switchboard labels — e.g. 415 V in Australia. L-N (Line-to-Neutral): the voltage measured between one phase conductor and the neutral conductor. In a balanced three-phase system, L-N = L-L ÷ √3. At 415 V L-L, L-N ≈ 240 V — the same as the standard Australian single-phase supply. This is why a Kelco controller can be powered directly from one phase and neutral (240 V L-N) when a neutral conductor is available. When no neutral is present (as on many rural or industrial three-phase-only supplies), only L-L voltages (415 V) are available between any two conductors — a transformer is then required to derive a single-phase 240 V or 24 V supply for the controller and contactor coil.
LRC Locked Rotor Current
The very high surge of current drawn by the motor the instant it starts, before it has spun up to speed. The rotor is momentarily stationary ("locked"), so the motor draws far more current than when running — typically 5 to 8 times the FLA for standard induction motors, and up to 7× for 2-wire borehole motors. This inrush lasts only a fraction of a second but is the critical figure for sizing the Triac thermal budget and the MCB instantaneous trip setting. The Kelco HD Triac handles the LRC during each start; this limits DOL starting to motors with LRC ≤ 60 A (approximately 10 A FLA at 6:1). Also written as starting current or inrush current.
MARGINAL Borderline — Verify On Site
The motor is within calculated limits but close to a component boundary. The installation may work correctly, but the actual running current should be measured with a calibrated clamp meter at full load during commissioning. If the measured current exceeds the limit, an interposing contactor must be installed.
MCB Miniature Circuit Breaker
A resettable safety switch (circuit breaker) that automatically trips and disconnects the circuit if the current exceeds a safe level. Unlike a fuse which must be replaced after tripping, an MCB can be reset by flipping the switch. The rating (e.g. C10 A, C16 A) describes the maximum current and trip curve. Always select an MCB to match the motor FLA and starting method.
MCP Motor Circuit Protector
The US/NEMA equivalent of an MCB for motor circuits — referenced in this calculator when the NEMA motor standard is selected. An MCP is sized per NEC 430.52 and provides both overload protection and short-circuit/ground-fault protection for the motor branch circuit.
mm² Square Millimetres
The metric measure of cable conductor cross-section area — how thick the copper or aluminium core is. A larger mm² means a thicker cable that can carry more current with less resistance. Common sizes include 1.5 mm², 2.5 mm², 4 mm², and 6 mm².
MOV Metal Oxide Varistor
A voltage-sensitive protection component that normally has very high resistance, but instantly clamps (limits) voltage when a spike exceeds its rated level. Recommended at the Kelco controller's A and N supply terminals to protect the controller's internal electronics from mains voltage transients and surges from other equipment on the supply network.
Motor power cable High-Current Motor Supply Cable
The cable carrying the full motor FLA from the supply source to the motor terminals. In a direct-switching installation this runs from the mains circuit breaker to the motor. In a contactor installation it runs from the contactor main contacts to the motor terminals — the section from the breaker to the contactor is the mains supply cable and is outside the scope of this calculator. The motor power cable must be sized for both current-carrying capacity (≥125% FLA per AS/NZS 3000) and voltage drop (≤3% at full load). It is entirely separate from the control cable and never carries coil current.
N/C Normally Closed
A relay contact that is closed (connected) when the relay is not energised, and opens (disconnects) when the relay activates. The R1 N/C contact on the Kelco controller is rated at 16 A — the same as N/O on the G2R-1-E relay. Note: the Kelco manual incorrectly states a lower rating of 3.5 A.
N/O Normally Open
A relay contact that is open (disconnected, no current flows) when the relay is not energised, and closes (connects) when the relay activates. The motor supply connects through the R1 N/O contact — when the controller turns the pump on, it energises the relay, the N/O contact closes, and power flows to the motor.
Option A / B / C Control Circuit Voltage Configuration (Tile 2)
The three wiring arrangements available for the Kelco controller's Relay 1 control circuit when an interposing contactor is used (Tile 2). They differ in how the controller is powered and what voltage appears across the contactor coil terminals A1–A2 when R1 closes.
Option A — Mains on A & N, R1 COM to A terminal (standard): The controller is powered from mains (A and N terminals). R1 COM is wired to the A terminal, so when R1 closes, full mains voltage (~240 V AC) appears across the coil. The contactor coil must be rated for 240 V AC. This is the simplest and most common arrangement.
Option B — Mains on A & N, LV terminal jumpered to R1 COM: The controller is powered from mains, but a jumper connects the Kelco LV terminal to R1 COM. When R1 closes, ~24 V AC (derived internally — not galvanically isolated from the mains) appears across the coil. The coil must be rated for 24 V AC. All control wiring remains at mains potential and must be insulated and labelled accordingly.
Option C — External 24 V supply on LV & N: An external 24 V AC supply powers the controller via the LV and N terminals. LV is jumpered to R1 COM. When R1 closes, 24 V AC (or DC if the supply is DC) appears across the coil. This is the only option providing a fully isolated low-voltage control circuit. The coil must be rated for 24 V AC or 24 V DC to match the external supply type.
NEC National Electrical Code
The US equivalent of AS/NZS 3000 — the code governing electrical installations in the United States. Referenced in this calculator when the NEMA (US) motor standard is selected. Cable sizing follows NEC Table 310.15 and motor circuit protection follows NEC 430.
NEMA National Electrical Manufacturers Association
The US industry body that sets electrical standards for equipment sold in North America. NEMA motors express power in HP, use AWG wire sizes, and operate at 60 Hz. If your motor has a NEMA nameplate, select "Imperial (US)" in the unit system toggle and "NEMA" as the motor standard.
mH Millihenry — Unit of Inductance
One thousandth of a henry — the unit of electrical inductance. Inductance describes how strongly a coil or winding resists changes in current and stores energy in its magnetic field. Contactor coils are in the millihenry range (typically 50–500 mH depending on frame size and coil voltage). Motor cable inductance is much smaller, in the microhenry (µH) range. The higher the inductance, the more energy is stored at a given current (½LI²), and the larger the back-EMF spike when the current is interrupted. The KSS Super Snubber's RC time constant (τ = R × C = 10.2 µs) is many orders of magnitude smaller than the coil's inductance-driven time constant, ensuring the snubber damps the transient rapidly.
nF / µH Nanofarads / Microhenries
Very small units used in the snubber (surge protection) calculations. A nanofarad (nF) is a tiny unit of electrical capacitance. A microhenry (µH) is a tiny unit of inductance. These values describe the electrical properties of your cable and are used to calculate whether a snubber circuit is needed.
Ω Ohms
The unit of electrical resistance (or impedance). A higher resistance limits how much current can flow. In this calculator, Ohms appear in cable resistance calculations — a longer or thinner cable has higher resistance, causing more voltage drop.
P1 Electrical Input Power
The actual electrical power drawn from the mains supply by the motor. P1 = P2 ÷ efficiency. P1 is always greater than P2 because motor losses (heat, friction, windage) consume additional energy. P1 is the figure that determines the current flowing through the relay, cables, and circuit breaker — this calculator always requires P1.
What to enter: Always enter P1. If your nameplate shows only P2 and states an efficiency, calculate P1 = P2 ÷ efficiency before entering. If only one power figure is shown with no efficiency stated, enter the nameplate value and also enter the motor's efficiency in the Efficiency field — the calculator will convert it to P1 automatically.
Example: A 0.75 kW nameplate on an IEC motor is typically P2 (shaft output). At 85% efficiency, P1 = 0.75 ÷ 0.85 = 0.88 kW. Using P2 directly underestimates relay loading by 15%.
P2 Mechanical Output Power (Shaft Power)
The useful mechanical power delivered by the motor shaft to the pump. This is what the pump "sees." P2 is always less than P1 because motor losses reduce efficiency. Many nameplates — especially on IEC motors — show the P2 rating only. If you enter P2 without converting to P1 first, the calculator will underestimate the current the relay must switch, potentially leading to an incorrect result. See the P1 entry above for the conversion method.
PASS Direct Switching Viable
The motor's electrical characteristics are within the safe operating limits of the Kelco controller's Relay 1 output. The motor can be wired directly through R1 N/O and COM per the Kelco wiring diagram. The HD terminal link must be fitted for all direct motor switching.
PF Power Factor
A number between 0 and 1 that describes how efficiently a motor converts electrical power into useful work. A PF of 1.0 is perfect — all current drawn does useful work. Motors have a PF below 1.0 because their magnetic coils draw extra "reactive" current that doesn't do useful work but still stresses the wiring. Also written as cosφ (cosine of the phase angle).
PIC Programmable Interface Controller
The small microcontroller chip (computer chip) inside the Kelco controller that runs the pump control logic — monitoring inputs, timing delays, and activating the relay and Triac outputs. Mains voltage transients that reach the PIC's supply rail can cause random resets, corrupted settings, or long-term damage, which is why supply-side surge protection is important.
R1 Relay 1
The main output relay inside the Kelco pump controller that switches power to the motor on and off. It is a physical electromechanical switch — a small electromagnetic coil that moves metal contacts together or apart. This calculator determines whether R1's contacts can handle the load of your specific motor.
RC Snubber Resistor-Capacitor Snubber Circuit
A small protective circuit consisting of a resistor (R) and capacitor (C) connected across the relay terminals. When the HD Triac switches off, it generates a brief voltage spike (transient). The RC snubber absorbs this spike, protecting the Triac from damage and reducing electrical interference. For Kelco HD installations, use the pre-assembled KSS Super Snubber (Kelco part no. KSS) — it is designed and rated to cover all applications.
Reflected wave Back-EMF Reflection on Control Cable
When R1 opens and the contactor coil's back-EMF spike is generated, that spike travels back along the control cable from the coil towards the controller — like a pulse on a transmission line. If the far end of the cable (at R1) presents a different impedance to the cable's characteristic impedance, the pulse partially reflects back towards the coil. This reflected wave adds to the original spike voltage at the relay end, increasing the stress on R1's contacts. The effect is most significant on control cable runs exceeding 10 m. Fitting a KSS Super Snubber at both ends of the cable — across R1 COM/NO at the controller, and across A1/A2 at the contactor coil — damps the spike at source and absorbs the reflected wave at the relay end, preventing the voltages from adding constructively. On short cable runs (<10 m) the transit time is negligible and a single KSS at R1 is sufficient, though a second at A1/A2 is always good practice.
RCD Residual Current Device
A safety device (also called an earth leakage circuit breaker — ELCB — in older Australian practice, or a GFCI in the United States) that continuously monitors the difference between the current flowing into a circuit and the current returning from it. If those two currents differ by more than the device's trip threshold — indicating current is leaking to earth, potentially through a person or a fault — the RCD disconnects power within milliseconds. The standard trip threshold for general protection in AS/NZS 3000 is 30 mA; sensitive devices for high-risk locations use 10 mA. Submersible and borehole pump installations require a 30 mA RCD as a minimum per AS/NZS 3000 cl 2.6.3. The RC snubber fitted to Kelco HD installations creates a small but continuous leakage current — typically 11–13 mA at 240 V — which is compatible with a 30 mA RCD but will cause nuisance tripping on a 10 mA device. The exact leakage figure for each installation is shown in the Snubber card. A licensed electrician must select the appropriate RCD type and sensitivity for the installation.
RCD Type AC Sinusoidal AC Residual Current Only
The basic and most common RCD type — responds only to sinusoidal AC residual fault currents at mains frequency (50/60 Hz). Adequate for simple resistive and inductive loads on DOL or star-delta started motors. Not suitable for VFD or soft-starter circuits — electronic drives produce pulsating DC and smooth DC fault current components that a Type AC RCD cannot detect, rendering it ineffective as protection.
RCD Type A AC + Pulsating DC Residual Current
A more capable RCD that responds to both sinusoidal AC and pulsating DC residual fault currents. Required for circuits supplying soft starters and VFDs per AS/NZS 3000:2018 (Amendment 2) and IEC 62423. Type A is the minimum required type for VFD and soft-starter motor circuits and is also recommended as best practice for all pump motor installations, as the pulsating DC coverage adds protection at minimal additional cost.
RCD Type F Composite AC, Pulsating DC & Low-Frequency Currents
A higher-specification RCD designed specifically for single-phase VFD circuits. In addition to the protections of Type A, Type F detects composite currents — combinations of AC, pulsating DC, and low-frequency components that single-phase VFDs can produce. Preferred for single-phase VFD motor installations where the highest level of earth fault protection is required. Consult the VFD manufacturer's documentation for their specific RCD type recommendation.
RCD Type S Selective (Time-Delayed) RCD
A Type S RCD (also called a selective or time-delayed RCD) intentionally delays its trip response by a short time (typically 10–40 ms) before disconnecting. This prevents nuisance tripping from brief transient leakage spikes — such as those generated when a large motor starts — while still providing protection against sustained earth faults. Recommended when the snubber leakage current is close to the 30 mA trip threshold, or when the installation has long cable runs with high capacitive leakage. Type S does not describe the fault current waveform sensitivity — a Type S device can be Type AC, Type A, or Type F with the addition of the time delay.
Rpm Resistance per Metre
In this calculator, Rpm refers to the electrical resistance of a cable per metre of length (in Ohms per metre), not revolutions per minute. It is used internally to calculate voltage drop and loop impedance for the selected cable size. A larger cable has lower resistance per metre.
SF Service Factor
A built-in safety margin on the motor's power rating. A Service Factor of 1.15 means the motor can tolerate up to 15% overload for short periods without damage. This calculator uses the SF to calculate the maximum safe continuous current and size the circuit protection correctly. If not stated on the nameplate, assume SF = 1.0.
Snubber Leakage Current Residual Current Through the RC Network
When the relay contact is open (motor stopped), the series RC snubber provides a small but continuous current path between line and neutral through any load connected to the relay. This steady-state current equals the supply voltage divided by the snubber's impedance at mains frequency. For the KSS at 240 V / 50 Hz it is approximately 11 mA — well below the 30 mA trip threshold of a standard RCD, but will cause nuisance tripping on a 10 mA device. In direct motor switching applications the leakage also flows through the motor windings at standstill; in contactor applications it is this current that keeps a small voltage across the deenergised coil (see "Ghost current" in the contactor wiring notes).
Soft Starter Electronic Soft Starter
An electronic device that gradually increases the voltage applied to the motor on starting, limiting the current inrush to a controlled level (typically 2–4 times FLA instead of 5–8 times). Unlike a VFD, a soft starter does not control speed during running — once the motor reaches full speed it is bypassed. This is a cost-effective way to reduce starting stress without the full cost of a VFD.
SPDT Single Pole Double Throw
A type of relay or switch with one movable contact (the "pole") that can connect to either of two fixed contacts. The Omron G2R-1-E relay inside the Kelco controller is SPDT — the moving contact can connect to either the N/O or N/C terminal, sharing one common (COM) terminal.
Starts / Hour Pump Cycling Rate
How many times the pump motor starts (and stops) per hour of operation. This is determined by the system's demand and the pressure or level switch hysteresis. A higher start rate increases thermal stress on the HD Triac, accelerates relay contact wear, and reduces motor winding insulation life due to repeated starting current heating. Industry and motor manufacturer guidelines define maximum safe starting rates by pump type: surface centrifugal pumps — typically 240 starts/hour maximum; borehole/submersible pumps — typically 30 starts/hour maximum (motor cooling is impaired between starts at depth); helical rotor/progressive cavity pumps — typically 240 starts/hour maximum (high starting torque increases winding stress); mining/heavy industrial — typically 240 starts/hour maximum. These limits apply to the motor, independent of the Kelco controller's own thermal constraints. The calculator enforces these limits and shows a warning if exceeded. The Start Frequency card quantifies Triac thermal and relay contact life at the entered cycling rate.
S4 Duty Cycle IEC 60034-1 Intermittent Periodic Duty with Starting
One of the standard duty cycle classifications defined in IEC 60034-1 §6.1 for electric motors. S4 describes a motor that repeatedly starts, runs under constant load, and stops — where the starting current (LRC) contributes significant heating to the windings during each cycle. Unlike S1 (continuous duty, which uses nameplate FLA directly), S4 duty requires calculation of the thermal equivalent current (I_eq) to determine whether the windings overheat at a given cycling rate. A motor selected for S1 continuous duty will meet S4 requirements only if I_eq does not exceed its nameplate FLA — this is the check performed by the Start Frequency card in this calculator. If I_eq exceeds FLA, a motor rated for S4 duty or with a higher service factor is required.
SMPS Switch-Mode Power Supply
A power supply that converts AC (or DC) input to a regulated DC output using high-frequency switching electronics rather than a wound iron-core transformer. SMPS units are compact, lightweight, and efficient. However, for pump control applications they have important limitations compared to galvanically isolated iron-core transformers: (1) they inject high-frequency switching noise onto their output rail which can affect sensitive controller electronics; (2) many are vulnerable to conducted transients — the back-EMF spike from a contactor coil can cause an SMPS to current-limit, latch off, or fail if not adequately protected; (3) their output holdup time (how long they ride through brief input disturbances) is limited by output capacitor size, and an undersized SMPS can sag during contactor inrush, causing the Kelco controller to reset; (4) not all SMPS units provide true galvanic isolation — the datasheet must be checked. Where an SMPS is used, a KSS Super Snubber across R1 COM and NO is even more critical than with a transformer supply, and a second KSS at the contactor coil terminals is strongly recommended. The contactor coil must be rated for DC if the SMPS output is DC — AC coils on DC supplies may not release reliably. An iron-core or toroidal isolation transformer is strongly preferred over an SMPS for all Kelco pump controller installations.
t_arc Arc Extinction Time
The time it takes for the arc across an opening relay contact to extinguish — typically 0.5 to 2 milliseconds for small relay contacts at mains voltage. During this brief period, the coil's stored magnetic energy (½LI²) is being discharged. The rate at which this energy drives the voltage — and therefore how high the spike reaches — is proportional to L × I_peak ÷ t_arc. A shorter arc extinction time (faster contact opening) produces a higher voltage spike, because the same energy is released in less time. This is why high-speed relay contacts can be more stressful to the relay circuit than slower ones, even at the same inductance. The calculator uses a conservative 1 ms value for t_arc to give a worst-case spike estimate.
τ (Tau) RC Time Constant
The characteristic time in which the RC snubber absorbs and damps a voltage transient. Calculated as R × C, it represents the time for the transient energy to decay to approximately 37% of its peak value. For the KSS the time constant is 10.2 µs — roughly 1,000 times faster than a mains half-cycle (10,000 µs at 50 Hz), ensuring the transient is fully damped long before the next switching event. A smaller time constant means faster damping but also higher peak current through the resistor during each event.
t_acc Motor Acceleration Time
The time in seconds for a motor to accelerate from standstill to full running speed after a DOL start. During this period the motor draws locked-rotor current (LRC), which is the highest current it will ever see. The acceleration time determines how long the Triac and motor windings are exposed to elevated starting current on each start. For small pump motors (typically ≤ 5 kW, low inertia load) this is usually 0.2–0.5 s. This calculator uses a conservative value of 0.4 s for the motor thermal equivalent current (I_eq) calculation. For larger motors or high-inertia loads such as large centrifugal pumps or flywheel-coupled plant, t_acc may be significantly longer and I_eq correspondingly higher — consult the motor manufacturer's data.
T_j Junction Temperature — Semiconductor Device
The temperature at the active silicon junction inside a semiconductor device such as the BTA41 Triac. T_j is always higher than the case or ambient temperature because the device generates heat internally as current passes through it. The BTA41's absolute maximum T_j is 125°C — if this is exceeded even briefly the device can be permanently damaged. This calculator checks two T_j values: (1) the peak T_j during a single start pulse (most critical — compared against the 125°C limit in the main result), and (2) the average T_j across repeated starts at the entered cycling rate (shown in the Start Frequency card). The average T_j rises as start frequency increases. The calculator warns when average T_j approaches 100°C (marginal) or exceeds it (fail).
Triac Triode for Alternating Current
A solid-state (electronic) switch with no moving parts that can switch AC current in both directions. The Kelco HD circuit uses a BTA41 Triac to absorb the starting inrush and stop-arc that would otherwise damage the relay contacts. The Triac fires for exactly 1.5 seconds on each start and each stop, then turns off, leaving the motor running through the relay contacts only.
Toroidal Transformer Doughnut-Core Isolation Transformer
A type of mains isolation transformer wound on a continuous ring-shaped (toroidal) ferrite or silicon-steel core, rather than the traditional rectangular EI-laminated core. Toroidal transformers are preferred for pump control supply applications for several reasons: (1) lower no-load core losses — they draw less current when idle; (2) lower radiated magnetic field — less interference with adjacent electronics; (3) quieter operation — the closed core geometry virtually eliminates the 50 Hz mechanical hum common in EI-laminated designs; (4) more compact for a given VA rating. Like all iron-core transformers, a toroidal unit provides true galvanic isolation between primary and secondary, is inherently robust to conducted transients from contactor back-EMF, and does not generate the high-frequency switching noise that characterises SMPS units. For pump controller supply transformers with an electrostatic screen winding between primary and secondary, toroidal construction is the preferred type in electrically noisy environments such as mining switchboards and VFD-heavy installations.
Tile 1 / 2 / 3 / 4 System Configuration Tiles
The four system configuration tiles at the top of the calculator input panel. Each describes a different electrical installation scenario and routes the calculation to the appropriate analysis path. Selecting the correct tile is the first step before entering any data.
Tile 1 — Single-phase, direct switching: The Kelco controller Relay 1 switches the pump motor directly — no interposing contactor. Requires motor nameplate data (FLA, LRC, power, voltage). The HD terminal link is required for all direct motor switching above approximately 0.37 kW. The calculator checks whether the motor is within the relay and Triac thermal limits and returns a PASS, MARGINAL, or FAIL verdict.
Tile 2 — Single-phase, interposing contactor: R1 drives the coil of an interposing contactor, which switches the motor. Used when the motor exceeds the direct switching limits, or when voltage-free contacts are required. Motor nameplate data is not entered — the analysis covers the control circuit only (coil voltage, KSS snubber requirement, control cable voltage drop). Motor and contactor sizing remain the installer's responsibility.
Tile 3 — Three-phase, neutral available: A three-phase motor installation where a neutral conductor is available at the switchboard. The Kelco controller is powered from one phase and neutral (L-N, ~240 V AC). An interposing contactor is mandatory. The calculator analyses the control circuit and KSS requirements; no motor data is entered.
Tile 4 — Three-phase, no neutral: A three-phase installation with no neutral conductor available. A galvanically isolated supply transformer is required to derive a single-phase supply for the controller and contactor coil. An interposing contactor is mandatory. The calculator provides a transformer specification in addition to the control circuit analysis.
TVS Transient Voltage Suppressor
A fast-acting semiconductor protection component that clamps voltage spikes. Similar to an MOV but faster and more precise. The Kelco controller does not include a TVS on its supply terminals — the MOV recommendation in the snubber card provides this missing protection for the controller's internal electronics.
Utilisation Category IEC 60947 Duty Classification
A classification in IEC 60947 that describes what type of load a contactor or switch is rated to handle. The most important for motor applications are: AC-3 — normal squirrel-cage motor starting and stopping (most pump applications); AC-4 — plugging, inching, and jogging duty (much more severe — use where the motor is frequently reversed or inched). An AC-3 contactor must never be used for AC-4 duty — it will wear out rapidly. For Kelco pump controllers, AC-3 duty is always the correct category.
V AC Volts Alternating Current
The voltage of the mains electricity supply. Standard Australian household and single-phase supply is 230–240 V AC. "Alternating current" means the electrical flow reverses direction many times per second (see Hz), unlike a battery which is DC (direct current).
VDRM Voltage Repetitive Maximum
The maximum voltage that the Triac can safely block between its terminals without breaking down. The BTA41 Triac in the Kelco controller has a VDRM of 600 V. When the Triac switches off, the voltage spike must stay below this limit. If the spike approaches or exceeds VDRM, an RC snubber is essential to protect the Triac.
VA_sealed / VA_inrush Contactor Coil Power — Sealed State and Pick-Up Inrush
Two different power ratings describing how much electrical power an AC contactor coil draws at different stages of operation. VA_sealed is the power consumed by the coil once the contactor armature has fully closed and the magnetic gap is at its minimum — typically 5–15 VA for small to medium frames. This is the steady-state figure that governs the running coil current and determines the coil's inductance at rest. VA_inrush is the much higher power drawn during the first few milliseconds when the coil first energises and the armature is still open — typically 5–15 times the sealed VA. The large air gap in the open armature greatly reduces the coil's inductance, so it draws high current until the armature closes. Both values are used by this calculator to estimate coil inductance (from VA_sealed and PF) and to characterise the current profile seen by R1 on each start. Actual values vary significantly by manufacturer and frame size — the figures used here are representative estimates from IEC AC-3 frame tables.
VD Voltage Drop
The loss of voltage along a cable run caused by the cable's electrical resistance. A longer run, a thinner cable, or a higher current all increase voltage drop. Excessive voltage drop (above 3% of supply voltage) reduces motor torque, causes the motor to run hotter, and increases current draw. For a 240 V supply, 3% VD = 7.2 V loss, giving only 232.8 V at the motor terminals.
VFD Variable Frequency Drive
An electronic drive that gradually ramps up the motor speed by increasing the frequency of the electrical supply from 0 Hz to the running frequency. Because the motor accelerates slowly, the starting inrush current is greatly reduced — typically to just above the running current. Also called a Variable Speed Drive (VSD) or inverter drive.
X2 Capacitor Mains Suppression Class X2 Capacitor
A special type of capacitor rated for permanent connection directly across the mains supply (line-to-line). The "X2" classification means it is designed to fail safely (open circuit) rather than short-circuit if it is overloaded. Standard film capacitors must never be used in snubbers connected to the mains — only X2-rated capacitors are safe for this purpose.