Many may know that a VFD can control more than one motor.  Few, however, understand how to configure a single-drive/multiple-motor system safely and effectively.  Here are three key tips and three caveats to keep in mind while undertaking what can prove to be a challenging system design.

Key Tip: FLA + 20%

The full load amps of all connected motors must not exceed the maximum amp output of the drive.  To select an appropriate drive, sum the FLA of each connected motor and then increase the total by 20%. The extra amps account for leakage inductance within the motors.

Key Tip: Use VFD Cable and a Common Mode Filter

VFD’s use pulse width modulation (PWM) to produce the AC current that drives the motor.  While PWM is at the heart of drive operation, it leads to Reflected Wave Phenomena and Common Mode Currents.  Not dealing with these effects from PWM can lead to damaged power cables, damaged equipment, and interference with adjacent electrical equipment.  VFD cable and common mode filters (Allen Bradley Bulletin 1321-M**) are some of the best weapons in your arsenal of power quality control when using drives.  Using both items is always recommended when controlling multiple motors.

Key Tip: Each Motor will Need Individual Protection

VFD’s act as overload and circuit protection for the motors they control, but this is only effective with one motor per drive.  When controlling multiples, each motor will need its own overload and overcurrent device for effective motor protection.  The Allen Bradley bulletin 140M circuit protectors provide both overcurrent and overload protection and are rated for use with VFD’s.  The 140M or a similar protection option is recommended for a multiple motor application.

Caveat: Motors Will Operate at the Same Time and Speed 

Perhaps this is obvious, but the motors controlled by a common drive will operate at the same speed and will start and stop at the same time.  This aspect of system design should be weighed carefully to ensure that the processes involved will benefit from multiple-motor control.  It should also be pointed out that the drive would be a single point of failure for the system in question.

Caveat: V/Hz Mode Only

Allen Bradley VFD’s are capable of several different control modes including sensorless vector and flux vector control.  Each of these control methods offers increased speed control and advanced torque control capabilities.  When controlling multiple motors, however, only V/Hz control will be possible as other control methods require the drive to be tuned to the specific motor it is controlling.  Something which is not possible when controlling multiple motors.

Caveat: Do Not Line Start and Stop a Motor Controlled by a Drive

This is true when not controlling multiples, but is worth reinforcing:  When a drive is running, DO NOT close an open contactor that is between the drive and motor.  The inrush current created by the contactor can damage the drive, especially if multiple motors are started at the same time.  When using motor circuit protectors (or other protection means) between the drive and motor, use a control circuit to ensure that no disconnecting means will close during drive operation.