Engineering · Motors & Drives
Multiple Motors on One VFD
Can one drive run several motors, and what do you give up?
A variable frequency drive makes one set of output voltage and frequency, so several induction motors can be connected in parallel to it. They then all run at the same speed, start and stop together, and share the load according to their own characteristics. This saves cost and panel space, but the drive can no longer protect each motor, cannot control them separately and must be sized for all of them. This page shows how to wire and size it, how torque really shares between parallel motors, why each motor needs its own overload relay, and when separate drives are the better choice. Figures are indicative: use the nameplates and the drive manual.
The drive must carry the sum of all motor currents plus a margin of about 20 to 25%, and its motor current parameter is set to that sum. If a motor is started across a drive that is already running the others, the drive must also supply that motor’s starting current on top, which is why many makers advise starting and stopping all the motors together. The drive current list is illustrative and the short-term overload of 150% is a typical figure: check the data sheet.
All the motors turn at the same speed, so they all run at the same slip. A motor with a smaller rated slip has a steeper torque curve, so at the same speed it carries a larger fraction of its rating. Motors with different slips, ratings or cable lengths therefore share unevenly, even when they drive the same load. The model uses the straight part of the torque-speed curve, which is valid up to roughly rated load.
The drive only measures the total current, so one overloaded motor hides behind the others and the drive may never trip. Each motor needs its own overload relay (or motor protection circuit breaker) after the drive, suitable for use with drive output. If one relay trips and the others keep running, the drive output current changes, so the PLC should watch each relay and stop the drive.
Several motor cables in parallel add up their capacitance, and long leads let the drive’s fast voltage edges reflect and build up at the motor terminals. Thresholds here are an example for a 230 V class drive from one source (a reactor beyond about 30 m, a dV/dt filter beyond about 50 m); your drive manual gives the real limits and says how multi-motor cable length is counted.
| Method | How it works | Good for | Limits |
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| Criteria | One VFD, several motors | One VFD per motor |
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Rules vary by drive maker and by wiring code. Always follow the drive manual, the motor nameplates and your local wiring rules; this page is a learning aid, not a design.