What are some of the considerations for Multiple Motor Applications with a Single Variable Frequency Drive (VFD)?

Multiple Motor Application Considerations

 Multiple conditions must be met when applying one VFD for the control of multiple motors.
1. Each motor should have the same operating speed/RPM if coupled in some way.

2. Running different-sized motors is OK but may not operate at the same speed if nameplate data is different.

3. Running multiple motors from one Drive creates a single point of failure. If Drive fails, all motors stop.

4. Each motor must have its own overload and short circuit protection.

5. The drive must be sized properly based on the application and the motors needed to run the

6. If motors are going to be cycled in and out with the drive running, then the drive should be sized for total load current plus the difference between the FLA and the inrush current of largest motor. A load reactor is also recommended.

7. Drive motor control mode should be set to V/Hz.

8. Slip compensation should be disabled. Parameter 621 [Slip RPM at FLA] set to zero.

9. Load reactors are used based on the total cable length of all motors. Multiple load reactors may be needed due to cable charging current heating up an individual load reactor.

10. The total power cable length of all the motors should not exceed the maximum motor cable

See Chapter 1 of the Wiring and Grounding Guidelines for Pulse Width Modulated
(PWM) AC Drives

11. Do not use flying start.

Note: A Variable Frequency Drive is designed to sense its total connected load; it will output as many amps as needed up to its rated current.

 A Variable Frequency Drive that is controlling multiple motors  

The single Variable Frequency Drive cannot sense which motor is drawing high current; because of this, the VFD cannot provide the needed overload protection that each motor will require.
Note: There is no hard limitation on how many motors can be connected to one VFD. It is a variable that is determined by other related specifications like the size of motors, the size of VFD, cable lengths in the system, etc.

Overload Protection

Overload protection is designed to disconnect the individual motor from the Variable Frequency Drive when the motor draws more current than its rated full load amps (FLA). This will protect the motor and the motor conductors from excessive heating.
The most common types of motor overload protection would be:
1. Bimetal
2. Solid-state

Note: Most Solid State Overloads will not provide accurate overload protection and not function on the output of a VFD.