Protecting motor on VFD 1

[eeprom]

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

 

[micalbrch]

I always thought a protective relay between VFD and motor can cause problems for the VFD? The only separate motor protection I know for VFD driven motors are thermistors or thermocontacts.

 

[eeprom]

I do normally use separate protection for vibration and temperature. However I am unaware of any problems which might be introduced by the CTs and/or PTs.

 

[jraef]

I look at it as a "belt and suspenders" approach when I see it and I don't argue. Most soft starters and VFDs now provide very adequate motor protection, and in the case of high end VFDs, it is adaptive to speed and motor modeling, just as an expensive MPR would be. But it was not always so, in fact years ago even rudimentary motor OL protection was not required to be IN the VFD, so it ALWAYS had to be external, and even now some low-end products provide the absolute bare minimum protection to qualify them for standards approvals. By specifying an MPR on a high value asset, you remove the possibility that someone gets cheap on a budget move and buys a drive with little or even no motor protection.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[LionelHutz]

Sometimes, it's in the specification because the person did a poor job writing it. For example, they just mashed a full-voltage and VFD specification together and the full-voltage protection relay was left in the document even though it should have been removed.

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.

 

[Parchie]

Or, the owner foresees the inevitable scenario that of the VFD conking out. He will be forced to switch the bypass contactor to continue operating. That means the built-in motor protection provided by the VFD will have to be replaced by a motor protection relay external to the VFD.

 

[ScottyUK]

If the relay in question is a straight heater-based thermal relay then it will likely be a worthwhile backup within its limitations. Electronic and CT-based relays may be more hassle than they are worth because their performance at low frequencies is not often defined by the manufacturer and you won't know whether the settings you enter equate to the real world behaviour unless you actually test it with a variable frequency injection source.

If you have a bypass contactor put the OLR in the bypass path.

 

[eeprom]

ScottyUK,
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

 

[DonUK]

Apart from when using multiple motors driven from a single VFD I've never seen it done.

 

[controlsdude]

I do not see it used recently only on multiple motors on one VFD. Then you need to be careful on which type of overload can actually work with the VFD output motor lines. Since some types of overloads between a VFD and Motor will not trip.

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.

 

[LionelHutz]

I've seen heater based overload relays melt-down on the output of a VFD so they aren't always safe.

 

[ScottyUK]

Lionel,

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.

 

[LionelHutz]

I'm not positive why they failed. The heater was a small coil which could have been the reason.

 

[waross]

I have seen the heaters in overload relays glowing bright red before they tripped. They were being subject to sustained high currents. The motors were driving propeller fans that were being driven at high speed in reverse when the motor was energized.
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

 

[jraef]

We have evidence of bi-metal OL relays nuisance tripping on the output of VFDs. We've even issued a couple of advisories against using IEC style bi-metal OLs on the output of VFDs. It's not because they fail, but rather because of the observed human response to nuisance tripping. After a motor fails, Jim-Bob the 'lectrician will say "That there motor was over amping (yes, I hear them say that all the time), so I turned up the dial, now your VFD burned up my motor!" Of course you can see the real issue here being that guys will turn up the dials when they should not, so it's really a procedural issue at play. But we have tested and shown that eutectic melting alloy OL heaters are insensitive to harmonics and low frequencies, so we recommend those to customers with "less sophisticated" work forces.

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

 

[ScottyUK]

Problem being, most electronic MPRs and the CTs that feed them generally aren't qualified for operation down to a couple of Hz, at least not with any degree of accuracy. Use the drive's measurements!

 

[LionelHutz]

Did only one O/L fail or were there multiple failures in a group?

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.

 

[ChasBean1]

It’s complicated. If you have three exhaust fans on VFDs for example and they have a standard sequence of starting upon their isolation damper end switch being met, the starting fan will windmill (spin backwards) substantially due to back pressure from operating fans, so that when the motor engages, the fan trips on overcurrent. VFD or not.

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.

 

[LionelHutz]

I'm not sure how your comments about starting multiple fan motors pertain to the original question, but I could easily setup a VFD system that would start that backwards windmilling fan without overloading the motor, at all let alone cause a trip.

 

[eeprom]

LionelHutz,
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