Protecting motor on VFD
Protecting motor on VFD
(OP)
Hello,
I have encountered several designs in the past year wherein a protective relay was used to protect a motor which was on a VFD. I don't understand why someone would spend the extra money to install a relay when the VFD will protect the motor against most current/voltage related problems. Does anybody have any reasons to support the use of a relay for protecting a motor on a VFD?
Thanks
EE
I have encountered several designs in the past year wherein a protective relay was used to protect a motor which was on a VFD. I don't understand why someone would spend the extra money to install a relay when the VFD will protect the motor against most current/voltage related problems. Does anybody have any reasons to support the use of a relay for protecting a motor on a VFD?
Thanks
EE





RE: Protecting motor on VFD
RE: Protecting motor on VFD
RE: Protecting motor on VFD
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: Protecting motor on VFD
Other times, the company uses X protection relay for motor protection so that relay must be installed even though it's not needed. Sometimes in these cases only a small portion of the protection features of the relay is put into service.
RE: Protecting motor on VFD
RE: Protecting motor on VFD
If you have a bypass contactor put the OLR in the bypass path.
RE: Protecting motor on VFD
That is a great comment. I have questioned this practice ever since I first saw it a couple years ago, and that particular question had never come to mind. I will follow up with SEL and GE.
Thanks,
EE
RE: Protecting motor on VFD
RE: Protecting motor on VFD
Past methods I have seen implemented, this was before the safe off features of VFDs.
emergency stop
I think I have seen before VFDs had the safe off feature (estop) that people put contactors either before or after the VFD.
The contactor before method usually shorten the VFDs life due to powering down the VFD during estop.
The contactor after method was used when the VFD ramped to a stop the PLC opened these contactors. This was kind of like a delayed estop. I think these methods were before the wide spread use of estop relays and PLCs that had estop programming modules.
RE: Protecting motor on VFD
RE: Protecting motor on VFD
Plain heater types? I'm surprised - have you any idea on the failure mode or the reasons why it occured? The heating effect should be the same where it is putting out DC or 400Hz, and those frequencies aren't big enough for skin effect to be a problem on any normal sized OLR.
RE: Protecting motor on VFD
RE: Protecting motor on VFD
If a VFD was driving a group of small motors, than a failure of one motor may lead to current levels relative to the O/L rating.
Is it possible that the high frequency component of the VFD output in the small coil caused inductive heating of other parts?
Did only one O/L fail or were there multiple failures in a group?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Protecting motor on VFD
All of this is, as I understood the OP, somewhat slightly off topic, as I was under the impression they were referring to Motor Protection Relays added down stream, not simple OL relays. MPRs have other benefits beyond just i2t overload protection. As I said, many high-end VFDs do as good a job as some low-end MPRs, but it can be hit and miss if you don't have complete control of a bidding process, such as a public works project where a contractor will use the cheapest price to win the job.
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: Protecting motor on VFD
RE: Protecting motor on VFD
Multiple failures. They melted and burned around the heater elements. Ones that still worked were showing damage that just wasn't catastrophic yet. It didn't correspond to a motor failure. There were motor failures but the failures were to ground and the VFD was capturing them immediately.
RE: Protecting motor on VFD
It depends on your application. If this is a risk, you can equip the VFD with protection from this or you can modify the sequence and control wiring to enable the fan before operation of its isolation damper.
RE: Protecting motor on VFD
RE: Protecting motor on VFD
I agree that this is a separate conversation, but I'd like to know how you work that. I have encountered similar problems, usually resulting in the motor acting in generation mode, and the VFD consequently trips on bus overvoltage. This is especially problematic in trying to control fan speed without using a braking resistor.
EE