FAQ's for AC Drives (VFD)
Frequently Asked Questions for Variable Frequency Drives (VFD)
Variable Frequency Drive (VFD) General FAQs
Q. Can I use a VFD as a 3-phase converter?
A. No. VFDs are designed to drive 3 phase motors only. A VFD cannot be used to power 3 phase loads like heaters.
Q. Can I use a VFD to control the speed of a single-phase motor?
A. No. VFDs are designed to supply power to a 3-phase motor.
Q. Can I power multiple motors from one VFD?
A. Yes. Special considerations must be applied when using multiple motors with one VFD. The full load amperes (FLA) of each motor are summed and the FLA rating of the VFD should be at least 20% above the total FLA. Short circuit and overcurrent protection must be used for each motor. Motors should not be direct line started for the VFD when it is up to speed.
Q. How do I find the circuit protection size for a VFD?
A. The VFD manufacturer provides specifications for each VFD model and size. This information is typically in the user VFD manual. Circuit breaker or high-speed fuse sizes are typically provided.
Q. Can I operate a VFD from a single-phase power source?
A. Yes. Some VFDs are designed for single or 3 phase input for a specific motor size. If the VFD is not specified for single phase power, derating the VFD based on the motor FLA (Motor FLA X 1.732) is required. The VFD FLA must be at least the calculated derating current.
We have an app note for the TD400 here.
Q. Can I use a non-inverter duty motor with a VFD?
A. A non-inverter motor will work with a VFD. It does not have the winding insulation level an inverter duty motor has. The voltage spikes generated by a VFD can arc through the weaker insulation over time and damage a non-inverter duty motor. Non-inverter duty motors can run for years from a VFD. 240-volt motors are less susceptible than 480-volt motors to insulation damage when operated from a VFD.
Q. When should I use a load reactor?
A. A 3% load reactor should be used on motor lead lengths over 100’. Disturbances on the motor leads from extended motor lead lengths can damage the motor. The load reactor will reduce those disturbances. Load reactors are recommended for applications where the motor has frequent start / stop cycles. Load reactors will also lessen the possibility of damage to non-inverter duty motors with any motor lead length. Load reactors should be sized appropriately to the VFD / motor combination.
Q. When should I use a line reactor?
A. A 5% line reactor is recommended when known incoming power quality issues exist in the facility. These issues can be from cycling of high current equipment on the same power source as the VFD. There are known power utility power quality issues. When 50% or more of the power source is powering VFDs.
Shop Line Reactors here.