Control Multiple Motors with Just 1 VFD

By Ashwinkumar Govindbhai Patel, product manager, Rockwell Automation

A variable-frequency drive (VFD) can be used to control multiple motors in some applications if the right design considerations are made and appropriate protection is provided for each motor. This can offer several advantages, such as lower costs and reduced panel space and control complexity.

Design Considerations and Component Selection

If the application falls within these specific conditions — and each motor can operate at the same speed and can accommodate a VFD as a single point of failure — the next step is to review design considerations and to select the right components. The VFD must be sized properly, and each motor needs protection. Recommended guidelines include:

• Total all the motors’ full load amps (FLA) and add 20% (to accommodate lower leakage inductance) to select the VFD size.
• Set the correct current ratings.
• On some drives, it’s not recommended to size below 3 Hp due to capacitive coupling (cable charging current).
• Adding a common mode core and using cross-linked polyethylene (XLPE) cable will help to minimize cable charging current.
• Drives should be set in V/Hz motor control.
• It is not recommended to “line start/stop” the motor when the VFD is running.
• Provide overload protection for the individual motor.

In multiple motor applications, NEC^® Article 430 Part III requires individual motor overload protection, which is (thermal) overload function, on the load side of a VFD. This is required because a VFD can sense only its total connected load; it can’t sense which individual motor is drawing high current. Therefore, it can’t provide appropriate overload protection.

Appropriateness of VFD Usage

Many applications use multiple motors of the same capacity operating in parallel at the same speed. Here is the opportunity to ask:

• Can I use one VFD to control these multiple motors?
• Does it help to optimize the panel for cost as well as space?
• Do I have the right components to help protect individual motors in such a configuration?

Not all types of overload protection devices are suitable for application at the output of a VFD. Because of the PWM voltage pulses and surge impedance of the motor, reflections of the voltage pulses occur at the motor’s terminals. Their amplitude is dependent upon:

• System voltage.
• The VFD’s voltage rise time.
• The motor protection circuit breaker’s (MPCB) current rating (surge impedance).
• Operational voltage.
• The MPCB’s location and cable type and length between MPCB and motor (surge impedance).

VFD Challenges

Challenges exist for overload protection devices at the output of VFDs:

• For electronic overload relays, their current sensor technology might not be able to measure the load current and harmonics correctly when operating at frequencies outside their nominal sensing range.
• For standard MPCB, their challenges are related to reflections of voltage pulse that cause high dielectric stress on the MPCB magnetic trip coils, resulting in accelerated aging.

It’s a best practice to restrict the cable length between the MPCB and the motor below the maximum permissible (“critical”) cable length at which the reflected voltage phenomena are developed fully.

Learn about Rockwell Automation motor protection circuit breakers and low voltage AC drives.

^{The Journal From Rockwell Automation and Our PartnerNetwork™ is published by Putman Media, Inc.}