Connecting 208v Supply to a 240V Machine?

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augie47 said:
If memory serves, Buck & Boost would only be a viable option if there is no
120 v requirement (no L-N load)
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Since the equipment (motor) was described as 240V three-phase, it seems unlikely that it includes any L-N 120V loads in its design.
Now that could be 240 delta or 240Y that it needs. But far more likely 240 delta. So boosting each 120V wye leg will provide 240 delta from a wye source.
And the 120V would still be available from the original supply tap if needed, via separate wires.
 
GoldDigger said:
Since the equipment (motor) was described as 240V three-phase, it seems unlikely that it includes any L-N 120V loads in its design.
Now that could be 240 delta or 240Y that it needs. But far more likely 240 delta. So boosting each 120V wye leg will provide 240 delta from a wye source.
And the 120V would still be available from the original supply tap if needed, via separate wires.
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OK. Boost just the motor supply from 208V to 240V. Small step, small transformer, small price.
 
GeorgeB said:
But ... 208 would be the distribution voltage also, sqrt3*120. Analogous utilization voltage would be sqrt3*115, or 199V. Doing just a little rounding, SHOULDN'T the nameplate be "200-230V"?

I'm not being picky, just trying to understand. This isn't the first time I've noted 208 to follow "different" rules.
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Yes, 208V would be the distribution voltage, and officially* the utilization voltage for a 3 phase motor on a 208V system would be 200V. So you will see that many motor mfrs offer true 200V motors for that reason. The problem is, you cannot connect a 200V motor to a 240V supply, it's just too high, so those motors are unpopular.

The "208-230V" labeled motor is the market compromise position; in reality it is neither a 200V motor for use on a 208V system nor a 230V motor for use on a 240V system, it is a design compromise that can USUALLY be used on either one.

* "Officially" is a loosely defined concept here. There is an ANSI specification for all of this (C84.1), but nobody is charged with enforcing ANSI specifications, it's really market driven.
 
Jraef said:
Yes, 208V would be the distribution voltage, and officially* the utilization voltage for a 3 phase motor on a 208V system would be 200V. So you will see that many motor mfrs offer true 200V motors for that reason. The problem is, you cannot connect a 200V motor to a 240V supply, it's just too high, so those motors are unpopular.

The "208-230V" labeled motor is the market compromise position; in reality it is neither a 200V motor for use on a 208V system nor a 230V motor for use on a 240V system, it is a design compromise that can USUALLY be used on either one.

* "Officially" is a loosely defined concept here. There is an ANSI specification for all of this (C84.1), but nobody is charged with enforcing ANSI specifications, it's really market driven.
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Is it safe to assume 208-230 volt motors are actually 220 volt designed but cover both spectrums?
 
AUC-Distribution said:
Thanks everyone. The Machine is made in China for Use In North America.It's been delivered. with 240V/60Hz/3Phase power and 7.5KW .How can I make this work without killing the motor.
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You would need to use a step up transformer. 120/208 to a 120/240 3 phase. that's it. the cost is about $1800.00 or get a new piece of equipment for the voltage you have.
 
mbrooke said:
Is it safe to assume 208-230 volt motors are actually 220 volt designed but cover both spectrums?
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My favorites are the ones in HVAC systems (air handlers) - 1/2hp to 2hp, 208 - 230 volt nameplate. i suspect the suppliers get the thinnest motors available.

Just my personal opinion: They are 230V motors that if loaded to near horsepower max, and connected to 208 source (that is at 200v at motor), they will over amp, run hot, and fail quickly.

I tell my customers to buy a 200V, 1.15sf. They usually squwak, "Too long a lead time. Too much money."

Well, okay, Then buy a few spares. Inexplicably - they still squwak.

ice
 
mbrooke said:
Is it safe to assume 208-230 volt motors are actually 220 volt designed but cover both spectrums?
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Safe to assume? That's a loaded element of that question, I don't like either the "safe" or "assume". Let's just call it a "reasonable expectation". I find few motor mfr people who will openly admit it, but one of Baldor's engineers, Edward Cowern P.E., published something similar in a booklet called the Cowern Papers while he worked there (he is retired now).

http://www.baldor.com/pdf/manuals/pr2525.pdf

He discusses this on page 53, although he dampened his wording from what he said in a class I attended years ago where he talked about motor design history and the "great compromise" of starting with a 220V design. I guess that's kind of an insider dirty little secret. Opps... :slaphead:
 
GeorgeB said:
But ... 208 would be the distribution voltage also, sqrt3*120. Analogous utilization voltage would be sqrt3*115, or 199V. Doing just a little rounding, SHOULDN'T the nameplate be "200-230V"?

I'm not being picky, just trying to understand. This isn't the first time I've noted 208 to follow "different" rules.
Click to expand...

In ratings we use nominal voltages.
 
not compliant with NEC to connect utilization equipment rated @ 240v to a 208v system without increasing voltage. Buck boost transformer is the only way to make it compliant. You may have to increase conductor/branch citcuit size, all depends on the particulars. agree with Besoeker
 
AUC-Distribution said:
Thanks. Now what size transformer do I need?
Machine is 3phase 240V/60Hz and motor power is 7.5KVA and the rated current is 25A. The building supplies 208V
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I recall that you posted that the motor is 7.5 kW. Not 7.5 kVA.
The 25A is reasonable if you take a combination of 0.8 pf and 92% efficiency.
But that makes it 10.2 kVA if I've done my arithmetic correctly.

However, the input kAV is usually lower than the motor kVA - different power factors you see.
I normally rate input transformers at drive rating in kW plus 20%. So 9.0 kVA in your case.

But, if we are looking at a boost autotransformer, then just the 208V to 240V bit of the winding, 32V, will be subjected to load current which is why they can be small.
If it was my project I'd go to a transformer manufacturer and explain what you need.
Unless you can get something like that off the shelf. And maybe you can there. Here, it would be a rare requirement and it would have to be made to order.
 
Besoeker said:
Perhaps T181052 would comfortably do the job?
208/230V,
12.45kVA
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Not arguing just wondering why not the 81059?

It is rated at 9.5 Kva (2Kva higher than the motor name plate load) and will boost 208V-240V.
 
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