VFD Output Overload

I have an application that will be running multiple motors on the output of one VFD. Can any recommend an Allen Bradley overload that can withstand the carrier freqencies of a VFD output and still sufficiently protect the motor? In addition to this device, I do plan to utilize motors with thermistor protection as well.


Thanks for your help!

Matt

I use many applications where I have a VFD control multiple motors. I have used both Mitsubishi and AB Powerflex with the 193-EA2DB and 193-EPM1 (DIN rail apapter) overloads and have never had any problems. Some of the multiple OL have been in service for 2 years and never had a failure (yet - I guess I just jinxed myself).

Matt,

Remember to use Bi-Metalic overloads and interlock the overloads in the hardware enable of the drive.

Dave

It depends in motor lead length.
For short runs under 20ft should be OK.
But for long runs you MUST use an output reactor between the drive and motor protective device.
Made the mistake once with motor lead lengths of 150ft and it literally melted the plastic housing of the motor overloads because their trip coils got so hot.
Keep the carrier frequency as low as possible - better a noisy motor than a melted overload.

Originally posted by BITS N BYTES:

Made the mistake once with motor lead lengths of 150ft and it literally melted the plastic housing of the motor overloads because their trip coils got so hot.

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I think that was Dave's point. If you use the older-style bimetallic strip overloads you don't have the inductance issues of the newer style electronic monitoring models. The bimetallic strip design is more like a straight wire.

Keith

I recommend the use of an individual contactor for each motor. Wire the OL so it trips that motor off line. This would allow the remaining motors to continue to operate if necessary.

kamenges said:

I think that was Dave's point. If you use the older-style bimetallic strip overloads you don't have the inductance issues of the newer style electronic monitoring models. The bimetallic strip design is more like a straight wire.

Keith

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No they were not bi-metallic strip overloads but Bulletin 140M Motor Protection Circuit Breakers. These are manufactured for AB by their Sprecher and Schuh division. Each phase has a thermal and magnetic component as well as a differential tripping function.
They work fine in multiple motor groups fed from one VFD, as long as you have a reactor on the output stage of the VFD.

I have motor runs of 150 ft (they are small motors 1-2 HP) and I do not use reactors on these. I'm not sure why you would want to wire a contactor with the overload, when the overload trips, the motor is offline and the others continue to run.

I agree, don't over complicate it.

Some overloads do not interupt the current. They only activate a switch which usually is in the coil path of a contactor.

More modern overloads, like Siemens MSP's are contactor and overload in one.

Originally posted by BITS N BYTES:

No they were not bi-metallic strip overloads but Bulletin 140M Motor Protection Circuit Breakers.

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I think we are saying the same thing but from two different perspectives. The magnetic trip component of the 140M runs the current carried by the device through a coil. This coil is what creates the trip actuator force on fault current. This coil is also what will heat up if it is subjected to the high frequency content of a VFD output.

In contrast bi-metallic overloads (like the AB Bulliten 193-T) run the current through a bi-metallic strip. Get enough current, the strip heats up and the bi-metal flexes causing a trip. Bi-metallic overloads have very low inductance because they don't have coils in them. They are not nearly as affected by the high frequency content of a VFD output. That's why one would use them in VFD output applications.

Also, as Leadfoot said, make sure you kow what you are getting with the 'overload' you select. Most devices called 'overloads' do not interrupt load current. They simply trigger a status contact and you need to do something with that contact. The AB Bulliten 140M mentioned above WILL interrupt current so that would be a single-point solution. But, as BITS N BYTES said, make sure you use an output reactor if you use those as the magnetic trip component won't like the drive noise.

Keith

Is your VFD application 230V or 460V?

Motor lead lengths, reactors and motor overload type are far less sensitive at 230V vs. 460V due to the lower VFD buss voltage and reflected wave phenomenon.

230V will not punch through the winding insulation of the motor as readily as 460V.

That being said, I would still make it a practice to install a load reactor on the output of ALL VFD's when multiple motors are being fed from one VFD. Code requires that each motor is individually protected, series wirimg the N.O. [closed when OK] auxilliary contacts of each overload to a drive input configured for overload is pretty simple and effective method.