How do you size a Variable Frequency Drive (VFD)?

 In Blog

Step 1: Understanding motor nameplate – the figure below shows “VOLTS 230/460” and “FLA (full load amps) 13.4/6.7” This indicates that the motor uses 13.4 FLA at 230 Volts AC while using only 6.7 FLA at 460 Volts AC.  It is critical to identify the input voltage in your application because VFDs are rated for one voltage and cannot operate at different voltages like an electric motor.

Step 2: Full Load Amps (FLA) – this is the number of amps which the motor will draw when operating at full load and full speed. It is important to consider any overload that the application might require as this may require you to move into the next size of the VFD.

Step 3: Service Factor Amps – these refer to  additional loads the motor can handle above HP (sometimes included on the nameplate, but can also be determined by multiplying running amps by service factor).  For some pump applications, the pump manufacturer will specify a motor to operate into its service factor (above FLA) to maintain flow as required.  Again, in this case, it’s important to size the VFD for the service factor amps required on a continuous basis. You will note that VFDs do have overload ratings, however these mean the VFD will be running its intermittent output current rating – which can only be sustained for a fraction of the time (best practice is to consult the manufacturer for further insight).

Step 4: Understanding Application – more difficult applications requiring constant torque, such as conveyers, cranes, blowers etc., may require 150-160% overload of motor current when in overload condition.  Again, it’s important to consider this when sizing a VFD and pay attention to terms a manufacturer uses for VFDs, such as “high overload, constant torque or automation/industrial” VFD, which signifies the ability for the VFD to be effective in high overload applications.

Other Considerations:

  • Submersible motors – are not NEMA rated B-type motors and typically draw more current per HP than conventional NEMA B-type motors.
  • Single-phase input – sometimes VFDs are used to convert single-phase power to three-phase power and operate a three-phase motor.  Some manufacturers provide specific VFD units designed for this and UL listed, while others do not.  Ensuring to use the proper, fully tested UL VFDs is critical for safety and integrity of the system.
  • Altitude – thinner air at high altitudes reduces the ability to transfer heat out of the VFD.  The less heat the VFD can exhaust the more likely it may require a derating in its ability to produce output current.  Best to check with technical support from the VFD manufacturer.
  • Ambient temperature – high ambient temperature may also diminish the ability of VFD to dissipate heat which may result in derating and/or require use of an exhaust system (as these are systems are not always fool proof, the manufacturer should be consulted). Low ambient temperature may affect capacitors, transistors and LCD displays.  It is important to warm the drive before powering because below-freezing temperature could cause capacitors to explode.

With all these factors to consider for your VFD sizing and installation, it’s important to have a partner you can trust.  Elite Controls has years of experience with VFD applications and is happy to help you with figuring out the best VFD to use for your application.  Contact us with questions – or use our online chat.