Resistors on a 3-Phase Motor Fed From a VFD
Resistors on a 3-Phase Motor Fed From a VFD
(OP)
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?
Phlipper
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?
Phlipper





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