Primary Resistance Starters: Current Limiters, Not VFDs

A primary resistance starter limits motor starting current by placing resistance in series with the stator supply, then bypassing that resistance as the motor accelerates. The resistor voltage drop reduces motor-terminal voltage and starting torque while converting substantial electrical energy into heat. The number that matters is the resistor's accumulated starting energy and thermal duty, not resistance alone.

Current, Heat, and Timing Symptoms

At standstill, the motor has little back EMF and draws high current when connected directly to the supply. Series resistance creates a voltage drop equal to I × R, leaving less voltage across the motor. Lower motor voltage reduces current, but it also reduces acceleration torque.

This is heat, not logic. Instantaneous resistor loss follows P = I²R. For a balanced three-phase starter with one equal resistor per phase, total instantaneous resistor loss is Ptotal = 3I²R. Because current changes during acceleration, resistor energy must be calculated from E = ∫3I²R dt, using the measured or predicted current profile. A one-resistor arrangement dissipates I²R in that resistor and creates an intentionally unbalanced supply condition.

Voltage, Impedance, and Torque Mechanism

For a balanced per-phase equivalent circuit, starting current can be expressed as I = Vphase / |Zmotor + Rseries|. Motor-terminal voltage is then Vmotor = |I × Zmotor|. These are phasor relationships: motor impedance contains resistance and reactance, so subtracting a scalar resistor drop from the supply magnitude is not an exact sizing method.

As the motor accelerates, back EMF develops, effective motor impedance changes, current falls, and power factor improves. The resistor voltage drop therefore decreases with current, allowing motor-terminal voltage to rise progressively even when the resistor value remains fixed. This rising voltage can provide increasing torque during acceleration compared with a fixed reduced-voltage step.

At a given frequency and operating point, induction-motor torque is approximately proportional to the square of applied voltage. A large voltage reduction can therefore produce a much larger torque reduction. Resistance selection must satisfy both the supply-current limit and the load's accelerating-torque requirement.

A conventional arrangement inserts appropriate resistance in series with each motor supply phase. A contactor later shorts the resistors, applying full supply voltage. A stepped design can progressively reduce resistance before the final bypass.

The simpler KUSA arrangement, from Kurzschluss Sanft Anlauf, places one resistor in series with one winding or phase path. For a wye-connected motor, the resistor displaces the electrical star point and reduces the winding voltages unequally. A contactor shorts the resistor after the motor gains speed. The asymmetry twists the current vectors and increases negative-sequence current, so this circuit requires a different thermal and protection assessment from a balanced three-resistor starter.

Resistance Starting and Bypass Procedure

1. Identify the circuit topology. Confirm whether the starter uses equal resistance in all three phase paths, a one-resistor KUSA circuit, or staged resistance. Trace the power drawing rather than inferring the topology from the enclosure.
2. Collect the controlling data. Obtain motor starting-current and torque information, load torque versus speed, allowable acceleration time, resistor duty data, contactor ratings, and the supply or generator starting limits.
3. Define the initial current target. Use the motor's starting impedance or manufacturer starting data with the per-phase equivalent circuit. A resistance value calculated from supply voltage divided by desired current alone omits motor impedance and will give the wrong result.
4. Check accelerating torque. Calculate or obtain the motor torque at the reduced terminal voltage and compare it with load torque throughout acceleration. The difference between motor and load torque must accelerate the combined inertia.
5. Calculate thermal duty. Integrate I²R over the complete resistance-in-circuit interval. Include every resistance step and compare energy, cooling interval, and starts per operating period with the resistor's published duty.
6. Set the bypass criterion. Command bypass after the motor has accelerated far enough that transition to full voltage will not produce an unacceptable current step. Use speed, current, or a validated time setting according to the installed control scheme; derive the setting from a recorded start rather than an arbitrary delay.
7. Test under representative load. Record phase currents, motor-terminal voltages, resistor voltage, acceleration time, and bypass operation. Repeat only within the resistor and motor thermal duty.

Electrical and Thermal Verification

A successful start shows controlled initial current, rising motor speed, falling resistor voltage drop, and stable transition to full voltage. After the bypass contactor closes, voltage across each bypassed resistor should approach zero apart from measurement error and contact voltage drop. Motor-terminal voltage should then match the supply within the normal drop of the closed power circuit.

Compare the three phase-current traces. A balanced three-resistor starter should not create substantial phase-current imbalance; unequal traces point to mismatched resistance, a poor connection, an open resistor path, supply imbalance, or a motor problem. A one-resistor circuit is intentionally unbalanced, so verify negative-sequence current and motor heating against the permitted values from the motor and protection documentation.

Check resistor temperature or a validated thermal model after the start and across the required restart interval. A starter may complete one cold start successfully yet overheat during repeated starts. Confirm that the bypass contactor carries steady running current without leaving any resistor unintentionally energized.

Recurring Application Pitfalls

Resistance alone does not define performance. The same resistor can produce different current and torque profiles when motor impedance, supply voltage, load inertia, or acceleration time changes. Cold-resistance measurements also need comparison with the resistor manufacturer's hot-resistance and duty information.

High-inertia loads keep the resistors in circuit longer, increasing accumulated I²R energy. Frequent starts can begin before the resistor has cooled. Screen both conditions against the published resistor and motor thermal limits; the one-resistor method is unsuitable for higher-inertia or frequent-starting loads unless the equipment documentation explicitly rates the application.

Generator-fed starting needs particular attention because the resistors consume real power while the motor also demands reactive current. Check generator voltage dip, current capability, and engine power response using the complete starting profile. A reduced motor current does not mean the generator sees a negligible starting load.

Time-only bypass control can transition too early when load torque rises or supply voltage falls. It can also hold resistance in circuit after acceleration if the timer or bypass contactor fails. Correlate the timer with measured current or speed, and use protection that detects stalled acceleration, phase imbalance, and bypass failure.

Frequently Asked Questions