Overview
This note describes methods to convert three-phase induction motors for temporary single-phase emergency operation using capacitors. These are improvisation techniques for field or rural emergency use and are not recommended for permanent installations or factory use.
Wiring for motors up to 1 kW
For motors up to 1 kW, keep the original "Y" connections inside the motor terminal box unchanged. Connect the chosen capacitor C in parallel between U1 and V1. Connect the neutral to U1 and the live to W1. If the motor rotates in the wrong direction, leave the phase on W1 unchanged and move the neutral originally on U1 to V1 to reverse rotation.
Capacitor selection for 1.1–7.5 kW motors
For conversions in the 1.1–7.5 kW range, use quality oil-immersed metalized paper capacitors for both start and run purposes. Select capacitor voltage rating Uc = 2.2·Un, where Un is the motor rated voltage. Run capacitor Cp can be estimated as:
Cp = 1600 * (In / Un)
Start capacitor Cn = (2 to 3) · Cp
In the above expressions: Uc is the voltage withstand rating of the start and run capacitors; Cp is the run capacitor; Cn is the start capacitor; In is the rated current of the three-phase induction motor; Un is the rated voltage of the three-phase induction motor.
Wiring for 1.1–7.5 kW motors
Remove the factory linking straps inside the motor terminal box and fabricate new links with 1.5–6 mm2 copper-lined flexible wire as required. Connect W2 and V2 together, and connect U2 to W1. Connect the phase directly to W1 and the neutral to U1. Place the run capacitor Cp across U1 and V1. Connect the start capacitor Cn in series with the speed-relay normally-closed contact.
Install the relay contact Sr in series and connect it between U1 and V1. To reverse the motor direction, keep the phase on W1 in place and move the neutral from U1 to V1.
General capacitor guidance for smaller motors
Motors up to 1 kW can be converted to capacitor-run single-phase motors. Motors from 1.1 to 7.5 kW can be converted to capacitor-start-and-run single-phase motors. For motors of 2.1 kW and below when making the conversion, use quality oil-immersed metalized paper capacitors as the auxiliary capacitor. The capacitor voltage rating should be 450 V or higher. Estimate capacitance by:
C = 14.6 · In
where In is the motor rated current. After calculating, round to an integer and select the nearest commercially available capacitor value.
Important note
These conversions are temporary measures for emergency use. They are not recommended for permanent or factory installations. After conversion, the motor's actual load-carrying capacity will be reduced to below 60% of the original rating.
