Three-Phase Motor Single-Phase Run Capacitor Calculator

Running a three-phase induction motor on a single-phase supply is a common requirement in home workshops where 400V 3-phase power is unavailable. This is achieved using the Steinmetz connection, where a permanent run capacitor creates a phase shift to simulate a third phase. While this allows the motor to rotate, it typically results in a 30% reduction in starting torque and a 20% drop in power efficiency.

Choosing the correct capacitor value is critical. A value that is too low will prevent the motor from starting or providing torque, while a value that is too high causes excessive current in the auxiliary winding, leading to rapid overheating and potential burnout. This calculator uses the empirical formula derived for 220V-240V standard mains to determine the safe capacitance range and suggests the nearest standard commercial component.

The Steinmetz formula estimates capacitance C based on motor current I and mains voltage U:

For a standard 230V supply at 50Hz, the constant k is approximately 4800. If the phase current is unknown, a power-based approximation is used:

Always choose a capacitor rated for at least 400V or 450V AC, as voltage across the capacitor can exceed mains voltage due to resonance.