VFD protection techniques

G

Thread Starter

G. Fry

I have a customer getting ready to install some Variable Frequency Drives (single channel 4 amp max output) and he (and I) have a question about fusing and protection techniques. Please inform me of your suggestions in this matter:

1. If using 1/2 HP 480VAC motor, what size fuses would you use? (Also 1/3 hp and 1/4 hp @ 480 vac)

2. What type fuses would you use (time delay, non-time delay, etc)?

3. If using a time delay fuse, are there any considerations for the currents that can be reached while the fuse is in this "delay" period. Some currents can get rather high.

4. Are there any other considerations that need to be looked at to protect these drives?
 
T

Terence B. Creevan

My experience is that protection need be similar to "across-the-line" motor starter protection, i.e. slow blow fuses, etc. Many VDF controllers have some element of safety and/or fault protection built-in, i.e. over current protection, etc. As for fuse and wire sizing, follow the manufacturers recommendations and/or NFPA. Some manufacturers warn against MOV surge protection for relays in the same panel, opting for R-C type protection. For motors with breaks, be sure to connect the break to a separate coil possibly connected to the VDS's "Running" output signal. Avoid shutting off power to the VFD during Estop or fault, and, instead open the input signals to the VFD. Remember that when your controller signals a VFD to stop the unit will decelerate to a stop and as such some controllers have separate Estop/Fault signals.

 
R

robert trask

Dear Mr. Noname Noaddress,

The answer to your questions are not as cut and dried as first might appear, and the reasons we have those things called "PE's", who work very hard to be at the point to answer such questions. Unfortunately they have families
and lives and want to be reimbursed for their expertise and thinking. They also have the great benefit of being legally liable for their decisions so when your place burns down are you blow out the main plant breakers from your poorly
'coordinated' VFD protection and stop production in the entire plane you have an avenue to complain.

O.K. list, I'm ready for another round of PE bashing...

> I have a customer getting ready to install some Variable Frequency Drives
> (single channel 4 amp max output) and he (and I) have a question about
> fusing and protection techniques. Please inform me of your suggestions in
> this matter:

Hire a PE.

> 1. If using 1/2 HP 480VAC motor, what size fuses would you use? (Also
> 1/3 hp and 1/4 hp @ 480 vac)

Consult a PE.

> 2. What type fuses would you use (time delay, non-time delay, etc)?

Engage a PE.

> 3. If using a time delay fuse, are there any considerations for the
> currents that can be reached while the fuse is in this "delay" period.
> Some currents can get rather high.

As mentioned above.

> 4. Are there any other considerations that need to be looked at to protect
> these drives?

Yes.

Robert Trask, PE
Mind Span eSciences
Wilmington, NC
USA

 
Any fuses on the input of PWM drives should be sized to protect the input wires. Most PWM drives have internal circuitry that limits fault currents to about 200%. Do not use drives are motor starters since drives are designed to start motors at low (zero) speeds and ramp up to operating speeds. Drives can either ramp to a stop or coast to a stop. Never remove input power to stop the motor. Alway use command stop in either ramp or coast methods. Be careful of unbalanced input lines. PWM drives will take power from the highest peak voltage phases. Often detected by current imbalance on input phases. The ac line is rectified to dc by means of diodes. So highest peak will feed power through connected set of diodes. The lower peak voltage phases may not contribute if their peak is less than internal dc bus. Also with smaller HP drives, take care that output leakage current between motor leads and ground do not subtract from rating of drive since drive output sees both leakage current and motor current. Nuisance tripping may occur where high leakage exists.

 
J

Joe Jansen/ENGR/HQ/KEMET/US

<sarcasm>

Absolutely! No more of these software questions, either! These poor programmers need their food. And the electricians, and the mechanics,
and....and...

In fact, this list should just be shut down. It is taking food from the mouths of babies!

</sarcasm>

no PE bashing. There are times when a PE is a good idea. I would think these questions are answerable by any qualified electrician, however....

--Joe Jansen
 
Thank You all for your ideas and opinions. It is nice to have a Q&A quarum like this to get ideas from other professionals from different disciplines. As far as the comments from Mr. Trask, PE, I get the feeling that you are not the
type of individual that likes to share information and help others, so maybe you should not be taking the time to read the listings. However, I do appreciate your comments.
 
Don't forget to use a line reactor with your VFD, two (2) if your motor is some distance from the drive itself.
 
E

Edgardo Bello

hello
i am an engineering student, in Chile (please, excuse my english)
i read a post about vdf, and i want to know if you please could send me any kind of information or a web page where i can found vdf information, i need to know about this to present a report at the university
thank you very much

Edgardo Bello
Estudiante de ingenieria de ejecucion en electronica
Universidad de Antofagasta, Chile
 
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