If you answer any of the following questions with YES, be extra careful in your VFD selection and setup parameters of the VFD.

  • Will the VFD operate more than one motor? The total peak currents of all motor loads under worst operating conditions must be calculated. The VFD must be sized based on this maximum current requirement. Additionally, individual motor protection must be provided here for each motor.
  • Will the load be spinning or coasting when the VFD is started? This is very often the case with fan applications. When a VFD is first started, it begins to operate at a low frequency and voltage and gradually ramps up to a preset speed. If the load is already in motion, it will be out of sync with the VFD. The VFD will attempt to pull the motor down to the lower frequency, which may require high current levels, usually causing an overcurrent trip. Because of this, VFD manufacturers offer drives with an option for synchronization with a spinning load; this VFD ramps at a different frequency.
  • Will the power supply source be switched while the VFD is running? This occurs in many buildings, such as hospitals, where loads are switched to standby generators in the event of a power outage. Some drives will ride through a brief power outage while others may not. If your application is of this type, it must be reviewed with the drive manufacturer for a final determination of drive capability.
  • Is the load considered hard to start? These are the motors that dim the lights in the building when you hit the start button. Remember, the VFD is limited in the amount of overcurrent it can produce for a given period of time. These applications may require oversizing of the VFD for higher current demands.
  • Are starting or stopping times critical? Some applications may require quick starting or emergency stopping of the load. In either case, high currents will be required of the drive. Again, oversizing of the VFD maybe required.
  • Are external motor disconnects required between the motor and the VFD? Service disconnects at motor loads are very often used for maintenance purposes. Normally, removing a load from a VFD while operating does not pose a problem for the VFD. On the other hand, introducing a load to a VFD by closing a motor disconnect while the VFD is operational can be fatal to the VFD. When a motor is Started at full voltage, as would happen in this case, high currents are generated, usually about six times the full load amps of the motor current. The VFD would see these high currents as being well beyond its capabilities and would go into a protective trip or fail altogether. A simple solution for this condition is to interlock the VFD run permissive circuit with the service disconnects via an auxiliary contact at the service disconnect. When the disconnect is closed, a permissive run signal restarts the VFD at low voltage and frequency.
  • Are there power factor correction capacitors being switched or existing on the intended motor loads? Switching of power factor capacitors usually generates power disturbances in the distribution system. Many VFDs can and will be affected by this. Isolation transformers or line reactors may be required for these applications.

Power factor correction at VFD-powered motor loads is not necessary as the VFD itself does this by using DC internally and then inverting it into an AC output to the motor. All VFD manufacturers warn against installing capacitors at the VFD output.

Solomon S. Turkel is Senior Instructor and Course Author for ATMS (Advanced Technologies Marketing and Service), Inc., Baltimore, Md.