Can One High-Voltage VFD Drive Multiple Motors?

A common question when planning a pump station, fan group, or compressor line is whether a single high-voltage variable frequency drive (VFD) can serve several motors. The short answer is yes, but how it is done matters. Two very different arrangements are often described by the same phrase, “one drive, multiple motors,” and they have different requirements.

Option 1: Several Motors Running in Parallel on One Drive

In this arrangement, all motors are connected to the VFD output at the same time and run at the same frequency.

What works well

• Identical or closely matched motors driving similar loads, such as a bank of parallel fans

• Applications where all motors start, stop, and change speed together

What must be considered

• Drive sizing. The VFD’s rated output current must cover the combined current of all connected motors, with a suitable margin. Adding up kilowatts alone is not enough.

• Individual motor protection. The drive protects its own output, but it cannot tell which motor is overloaded. Each motor needs its own overload, short-circuit, and earth-fault protection, plus a means of isolation.

• Control mode. Parallel operation generally means scalar (V/f) control. Vector control and motor auto-tuning are normally not applicable, because the drive cannot model several motors as one.

• Connecting motors while the drive is running. A motor should not be switched directly onto a live VFD output. The resulting inrush can trip the drive. Motors should be added or removed only when the drive is stopped, or by following the manufacturer’s procedure.

• Load sharing. Motors of different size, slip, or load will not share the work equally.

• Single point of failure. A drive fault stops every motor on it. Where this is unacceptable, a bypass arrangement or a separate drive for each motor is the better choice.

• Cable length and insulation. Long or multiple motor cables can affect voltage stress at the motor terminals. The drive topology, output filtering, and motor insulation system should be checked together with the drive supplier.

Option 2: One Drive Starting Several Motors in Turn

This is the more common use of a single high-voltage drive across several motors. The drive soft-starts the first motor, accelerates it to line frequency, and transfers it to the grid through switchgear. The drive is then released and used to start the next motor.

Benefits

• Very low starting current on the supply, compared with direct-on-line starting

• Much lower investment than one drive per motor

Limits

• Once transferred to the grid, the motor runs at fixed speed and has no speed control

• The drive no longer protects a motor that has been transferred to line

• The transfer needs correct synchronization of voltage, frequency, and phase, so it should be engineered by the drive manufacturer

A related and popular layout in pump stations uses one drive on a single pump for flow control, while the other pumps run at fixed speed and are switched on or off as demand changes.