servo amplifier or VFD as overcurrent protection
- Thread starter lquadros
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
It has been a requirement for UL listing of VFDs since 2005 that they provide not only the running over load protection, but also the motor branch short circuit protection down stream of themselves (prior to that it was OL only). VFDs that are not UL listed will often forego this, or they UL list them as a "power converter" like a solar inverter to get around it. But none of the major players participate in that scam, that's someting to be worried about with a lot of the too-cheap-to-be-true drives on the Internet.
I do not know if servo amplifiers are required to do likewise; they probably are because I have never seen an OL relay down stream of a servo amplifier, but my experiences with servos is not as deep. If not, then it will likely vary from brand to brand, and likely follow the cost, so you would have to check with each vendor.
SceneryDriver
Senior Member
- Location
- NJ
- Occupation
- Electrical and Automation Designer
VFD's will fault on overcurrent, phase fault, or ground fault if a problem occurs with the conductors between the drive and motor, or with the motor itself. The drive will limit the current to the motor and shut down if it doesn't like what it's seeing. This is why you must enter the motor nameplate data into the drive; the drive then knows what acceptable and unacceptable amperages are, and can protect the motor from overload properly.
The VFD will react MUCH faster than a fuse or breaker to protect itself. Drives powering single motors don't like being disconnected from their motors while running; doing so will often damage the output IGBT's and smoke the drive. Don't do it.
In situations where a group of motors are driven from one VFD, then each motor must have an overload between it and the VFD; a VFD large enough to drive multiple motors would not know if one motor was stalled and burning up. The VFD will still fault on phase or ground faults. Bi-metallic overloads should be used in this case, as electronic O/L's will not be happy with the waveforms coming out of a VFD. The N.O. aux contacts of the overloads should be wired in series to the RUN contact on the drive. This will shut down the drive when an O.L trips. A drive running multiple motors will be more tolerant of a single motor disconnecting due to an O/L tripping.
Servo drives, by definition, only run one motor. Do not try to disconnect a running motor from the servo drive. Do not place a contactor or O/L between the motor and servo drive. You will let the magic smoke out. The servo drive will protect itself, provided it is programmed properly (and they have to be to run the particular motor that's attached).
SceneryDriver
The VFD will react MUCH faster than a fuse or breaker to protect itself. Drives powering single motors don't like being disconnected from their motors while running; doing so will often damage the output IGBT's and smoke the drive. Don't do it.
In situations where a group of motors are driven from one VFD, then each motor must have an overload between it and the VFD; a VFD large enough to drive multiple motors would not know if one motor was stalled and burning up. The VFD will still fault on phase or ground faults. Bi-metallic overloads should be used in this case, as electronic O/L's will not be happy with the waveforms coming out of a VFD. The N.O. aux contacts of the overloads should be wired in series to the RUN contact on the drive. This will shut down the drive when an O.L trips. A drive running multiple motors will be more tolerant of a single motor disconnecting due to an O/L tripping.
Servo drives, by definition, only run one motor. Do not try to disconnect a running motor from the servo drive. Do not place a contactor or O/L between the motor and servo drive. You will let the magic smoke out. The servo drive will protect itself, provided it is programmed properly (and they have to be to run the particular motor that's attached).
SceneryDriver
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