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Resistors on a 3-Phase Motor Fed From a VFD

Resistors on a 3-Phase Motor Fed From a VFD

Resistors on a 3-Phase Motor Fed From a VFD

Our application involves a 3-phase AC servo-motor fed from a VFD.  The Drive vendor recommends that a switch be installed on the output side of the VFD (between VFD and motor), which would isolate the motor from the drive AND connect the motor to a 3-phase connected resistor bank.

I can see this for DC motors, however do not see the purpose or intent of this "resistor-switching" arrangement for AC induction motors fed from a VFD, unless (perhaps) the VFD does not have a resistor-transistor arrangement on the DC bus to limit transient DC overvoltages.

The VFD vendor rep was not able to assist with an answer to this question.

Are there any pearls of wisdom out there?


RE: Resistors on a 3-Phase Motor Fed From a VFD

The usual way for breaking with a VFD is to have a resistor on the DC-Link which is switched by the control of the inverter during breaking to dissipate the energy fed back.

To my mind the arrangement the vendor suggested will not be very helpful for breaking with an induction motor since the motor will not generate voltage for a long time without supplying reactive power for excitation. Are you sure your motor is an induction motor and not a permanent magnet synchronous motor?

With such an motor the arangement might have the advantage that its possible to break the motor even if the inverter is defective.

RE: Resistors on a 3-Phase Motor Fed From a VFD

Thanks for your reply.  As it turns out, this motor does have a permanent magnet rotor.  How much of a difference could this make to the stopping of the motor/load (in relative terms only)?

RE: Resistors on a 3-Phase Motor Fed From a VFD

A correctly sized resistor on permanet magnet synchronous motor will provide breaking torque forever if neccessary. On an induction motor only for some fractions of a second (Time constant of rotor field)

RE: Resistors on a 3-Phase Motor Fed From a VFD

Suggestion: There is some difference in terms of the induction motor generation of the stator voltage and energy. The induction motor will experience higher energy losses in the rotor than the permanent magnet synchronous motor. Therefore, the induction motor with shaft load will stop somewhat faster than the permanent magnet synchronous motor. This holds true under an assumption that the both motors have approximately the same amount of dynamic energy when it comes to the braking process.

RE: Resistors on a 3-Phase Motor Fed From a VFD

Is this scheme designed for an emergency stop or for a normal shutdown? You mentioned that this is a AC servo application. Is the VFD actually a servo VFD drive with regenerative capabilities? Is the application a position loop control or just speed control? It would seem that the method of braking that you decribe is fairly coarse. There are much better ways to stop a servo drive unless this is some sort of, final emergency, all bets are off, stop.

RE: Resistors on a 3-Phase Motor Fed From a VFD


This application is an e-stop scheme, thus the reason for the coarse method of braking.


RE: Resistors on a 3-Phase Motor Fed From a VFD

Electricuwe is right on with his answers. During an E-stop the PM sync. motor would be disconnected from the drive output and connected to the resistors. The inertia of the load would drive the motor as a generator with the output going to the braking resistors. The load provided by the resistors would produce torque in the motor countering the rotation, thereby slowing the motor down to a stop. The amount of time required to stop the motor would be based on the resistor sizing and the output of the motor versus the inertia of the load.

For a fail-safe E-stop scheme the braking contactor (connecting the PM sync. motor to the resistors) would be normally closed. During normal operation the contactor would be energized (opened). For an emergency stop the E-stop switch would interrupt the voltage to the braking contactor coil and allow the contactor to close. The E-stop switch must also cause the motor to be disconnected from the drive. This would be considered fail-safe because the default condition is E-stop so that failure of any component in the E-stop circuit (broken wire, bad switch, bad contactor coil, etc.) results in an E-stop. As well, in the event of a power failure you would go into E-stop. This is in contrast to the use of a normally open braking contactor where failure of a component or loss of power would render the E-stop function inoperative.

RE: Resistors on a 3-Phase Motor Fed From a VFD

Suggestion: The ac motor drives have an option of the dynamic braking resistor. There is no need for an extra electromagnetic contactor. If such contactor is added and energized, any contactor malfunction will cause an unnecessary downtime that may turn out to be expensive in the process industry. The VFD drive has a built in-capability to turn off switching devices over the E-stop pushbutton much faster than the electromechanical contactor can accomplish. Also, the value of the braking resistor can be adjusted how fast the motor is supposed to be stopped. The motor is supposed to be designed for frequent abrupt stopping via the braking resistor.

RE: Resistors on a 3-Phase Motor Fed From a VFD


the approach suggested by you will not work with a failed inverter (IGBTs or control)! So for safety reasons the contactor approach might be imperative depending on application and standards.

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