Help with 3 phase motor running at max. rated current 1

[superdumpy]

Hello All,

I have a three phase VFD driven delta motor running at 375volts at roughly 1,350 rpm and 45 hertz. The problem I have though is the current is running at 130amps while its full load is 131amps. Could I have a problem with motor burn-out with this high current. The motor and fan are located in a 10 to 14 degrees celcius HVAC chamber.

We have been experiencing belts being melted and/or being thrown from the pulleys and have changed out one motor already.

Any advice would be appreciated.
Kind regards,
Thomas Byrne

 

[MetalworkerMike]

In that cold environment I can't imagine it having a problem running at an amperage within its FLA providing that it is VFD rated at that current, and the vendor doesn't stipulate de-rating for VFD use or something like that.
Did you check all three legs for amperage?

This is a fairly smooth power demand application? Fan perhaps?

The belt problem makes me think that this motor is working quite hard. Providing the sheaves are in alignment and the belts are properly tensioned, then adding another belt or two (or switching to a posi-drive belt) might be worth doing. If the belts are slipping on the motor sheave then that adds to the heat load on the drive end through friction.

Mike

 

[superdumpy]

Thanks Mike,

I have placed a Dranetz meter on the phases to test all legs. I'm starting to lean towards changing to a toothed belt and pulley combination on this motor and fan.

We have maybe 50 of these HVAC units and every month we have maybe one or two unit failures due to these belts jumping off pulleys.

Again thanks for the help.

Thomas

 

[itsmoked]

Go to the GATES site as they have a belt selection program. Run their program,(It's quite nice), and see if your actual belts are even close to the correct match. I suspect they aren't.

The program also helps you select alternatives by spitting out part numbers.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[MetalworkerMike]

The posi-drive belts that Gates makes (I think they call them something like 'chain-belt'... I haven't installed one in years) are really, really nice. They are fiendishly expensive, but properly installed they last a good, long time, they have tons of capacity available (I've put single belts on 300HP motors) and they are just nice to work with. You _do_ need to align the pulleys properly for optimal usage life.

Mike

 

[waross]

I assume from the FLA of 131 amps that this is a multiple belt drive.
This sounds like a chronic mis-alignment problem.
When the drives are mis-aligned one or two belts take all the load and the others run progressively slacker. The few belts driving the load tend to stretch and wear. This leads to burning.
The slacker belts may flap. The flapping belts will sometimes contact and interfere with adjacent belts. This often throws belts.
If the belt pulleys are not parallel then burning and flapping are common. Sometimes you will have only burning, sometimes only flapping, sometimes both.
The multiple belts under one backing are nice but in your case you may be spending a lot of money to mask a problem and still suffer very short life on expensive belts.
I suspect that your service people are not aware of proper alignment procedures.
I have installed a lot of multiple belt drives on large motors. There are probably millions of applications of multiple belts running without problem. There is the very rare instance where a combination of center distances, pulsating loads, and tension produce chronic flapping that may be mitigated by the increased mass of a uni-belt, and there are some plants who have decided on the better quality of uni-drive belts.
However I strongly suggest that you have a competent serviceman check your alignment before changing belt types.
I believe that the motor burnout is not related to the belt problem and may come under the heading of "Stuff happens".
You may consider changing sheave sizes and letting the motors run at or a little above synchronous speed instead of a little below synchronous speed. That will reduce the current and the motors should run cooler.
respectfully

 

[superdumpy]

Thanks to all your replies.

I suspect that waross is very close to the root of the problem and it may be the route I take in finding an answer.

Cheers again.

Ill keep you updated.

Thomas

 

[superdumpy]

Hi Waross,

Would you have any literature on the alligning of fan and motor pulleys for multiple belt installation.

Many thanks,
Thomas

 

[waross]

We had an alignment question in thread thread 237-189495.
Some answers dealt with belt alignment and some dealt with coupling alignment. Several of the members contributed good advice.
respectfully

 

[superdumpy]

Hi Waross,

Again thanks. I feel like I'm back in school here but can you tell me how to search for this thread you mentioned. I'm very green on this forum site.

Cheers,
Thomas

 

[ScottyUK]

Thomas, here's a direct link. Note there is no space between 'thread' and the number. Eng-Tips does the automatically from there.

thread237-189495

I'm waiting for the search tips too because 'search' often seems more like 'hide' to me. I can't find threads I know exist!


----------------------------------

Sometimes I wake up Grumpy.
Other times I just let her sleep!

 

[itsmoked]

The search here is soooo bad that I often resort to using google to search thru eng-tips with much better results!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[superdumpy]

Cheers Scotty. Some good info on that thread.
Rgds
tommy