Electronic OL for PWM?

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chris kennedy

chris kennedy

Senior Member
Location
Miami Fla.
Occupation
60 yr old tool twisting electrician
I have an application where I’m installing a new VFD that is driving 2 motors.
Obviously I have to set the VFD’s OL for the combined load.
So on the load side of the VFD I want to install an individual OL relay for each motor.

I have never tried it before but will electronic OL relays work on the load side of a VFD?
Or do I have to use thermal?

Thanks
 
Yes, that’s what I’m proposing to do. Just want to know if electronic OL’s will work on the PWM output of the VFD.
 
210709-1022 EDT

chris:

I have no idea. It will depend upon the design and manufacture of the breaker.

A VFD will likely produce a current with a high harmonic content at the breaker. The magnitude of this harmonic content at the breaker will depend upon on the filtering internal and external to the drive. Then there is the separate question of how sensitive is the breaker to harmonics.

I designed and received a patent on an electronic breaker back in the early 60s. This was for and assigned to a breaker manufacturer. From memory I would say that my design would have been relatively immune to high frequency harmonics compared to 60 Hz. Response to high frequency noise or harmonics will depend partly upon how fast is the breaker and whether there are design defects.

You may be able to run some simple experiments to evaluate the breaker performance.

Is there a particular reason to consider an electronic breaker vs an ordinary thermal magnetic or thermal fuse type?

.
 
These 2 motors drive a trummel, one on either side in opposite rotations.
If one motor overloads I would like aux relay to shut down the other.
 
chris kennedy said:
I have an application where I’m installing a new VFD that is driving 2 motors.
Obviously I have to set the VFD’s OL for the combined load.
So on the load side of the VFD I want to install an individual OL relay for each motor.

I have never tried it before but will electronic OL relays work on the load side of a VFD?
Or do I have to use thermal?

Thanks
Click to expand...
You will have to ask the manufacturer. Some are suitable for being on the output of a VFD and some are not.

AB sells a whole line of MPCB suitable for being on the load side of a VFD. 140M-D8V.
 
210709-1542 EDT

chris:

Get two "Power Cells" from Load Controls Inc. of Sturbridge, MA 01566. These are Hall Device three phase power monitors with DC output proportional to real power. You can DC filter to as low a frequency as desired. Basic response time is 15 milliseconds.

Use an appropriate difference voltage detector to trip power to both motors.

See https://www.loadcontrols.com

.
 
210709-1617 EDT

hillbilly1:

Depending on what the input power to the VFD normally looks like vs time, and compared to when one motor fails would determine whether input power monitoring would work (current is not as good a measure as power, but may be adequate). Might require some time averaging.



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