VSD Question 5

[JRLAKE]

I recently supplied a 250 HP pump to a quarry and included the pump manufacturer's 250HP VSD drive to control it.

At startup I learned that the electrical supply (lines, transformer, 0.5 s time delay fuses) were rated for 200 HP. I can control the max amp draw using the PLC on the VSD to prevent an overload. However, I am concerned that if a condition occurs suddenly in the pump to cause it to lock up, a sudden rush of current could cause problems. Basically, the motor is capable of drawing more amps than the transformer can handle and all that we have preventing this from happening is a setting on a PLC. I don't want the transformer to blow up, if this is possible.

I am a mechanical engineer and I am weak in the electrical area. The quarry's electrician says that my concerns are not possible because of the PLC setting that limits amp draw. However, this is the same guy that wired for 200 HP when 250 HP was stamped on the pump motor and VSD.

Do I have a legitimate concern?

 

[Bestboy]

Sorry for this. I rushed to posted a suggestion without reading CJCPE's post, he had mentioned the same point earlier.

 

[macmil]

Apart from all the valuable comment above, what are the design parameters for the pump ? What will the required horse power be at the design point ? Conceivably it could be anywhere between 200hp and 250hp, and the problem may not be as significant in practice, in normal operation. And yes, I agree that the protection must be in place for the abnormal.

 

[waross]

In regards to the transformer.
Heat is what kills overloaded transformers.
A prolonged overload will overheat a transformer and so shorten the life of the transformer.
A transformer with a projected life of 20 years may have its life shortened to 15 years, 5 years, 1 year, 6 weeks, hours or minutes.
It depends on the magnitude and duration of the overload.
Transformers withstand short term motor starting overloads very well because the short duration of a starting overload does not allow the temperature to rise much.
In your instance,
1> If the VFD is improperly configured and allows more line current than the transformer is rated for, you may expect to shorten the life of the transformer.
2> If the VFD is properly configured to limit the line current to the transformer rating or less you may expect to enjoy the full life of the transformer. Any momentary overloads due to jam-ups will not have a noticable effect on the transformer if the VFD limits the current before the transformer temperature has a chance to rise.
If care is taken setting the parameters of the VFD you should have no problems with the transformer. The question then will be
"Can the pump perform its task under these limits?"
If someone in the future increases the current limit setting and increases the pump workload the transformer may be destroyed rapidly.
The 250 amp time delay or dual element fuses should give excelent protection to the transformer.
They will withstand the possible short time overloads that are not dangerous to the transformer, but will open on the sustained overloads that are dangerous to the transformer.
respectfully

 

[JRLAKE]

Thank you waross. That is exactly the information I am looking for.

I'm not implying that the other posts are not helpful because they are. Thank you all.

 

[jraef]

What a way with words...