Step Up Transformer 4

[Ripcord13]

Our plant manager just informed me that he bought a 125 HP 460v 3 phase air compressor. The area it is to be installed has only 208v 3 phase. Don't worry he says, the salesman is going to give (sell) us a transformer that will step up the voltage to 460. Is this a good idea?

 

[mpparent]

As long as it's installed properly and to your local code, it shouldn't be a problem. Can you ask the salesman if he can get it in 208V?

Mike

 

[waross]

I usually reconnect the motor for 230 volts. This may require a larger contactor in the control panel including a larger overload relay, or a 208/240 volt rated control panel.
Many 230 volt motors will work happily on 208 volts. If the full load current is within the nameplate rating you are safe at 208 volts. There are millions of 230:460 volt motors running happily on 208 volts. If the motor wants a full 230 volts, use two autotransformers in open delta. The cost of the two auto-transformers and a larger contactor is usually much less than the cost of a 208:480 volt transformer. (460 volt motors are commonly used on 480 volt systems. 208:480 is a more common transformer voltage than 208:460. Don't be alarmed if the salesman supplies a 208:480 volt transformer).
respectfully

 

[Ripcord13]

The NEC says that a 125 HP motor at 460(480) volts has a FLA of 156 amps (3/0 conductors) What amperage am I going to need to feed the 208 volt step up transformer to get 156 (200)amps at 460 volts?

 

[stevenal]

I usually don't recommend using 230 motors on 208 systems, although I know it is done. By NEMA, motors are able to handle + or - 10%. Also by NEMA, 208 systems are allowed to go as low as 197, and infrequently go as low as 191. Both of these lower limits are less than than 230*0.9. The result can be high current and over heating.

Regarding the transformer: Why have the extra expense and space requirement and possible failure point? They ought to be able to supply the compressor with a 200V motor for your system.

Of course rules 1 and 2 always apply to a situation like this.

 

[stevenal]

Sorry, I was looking at the ANSI/NEMA service voltages above. Utilization minimums are 191 and 184 on that 208 system.

 

[Ripcord13]

The manufacturer of the compressor has told our manager that 460 volts is the "industry standard" and they do not make a 208 volt version, which at 359 FLA would require a 400 amp service. They even sent an air dryer with a 5 HP motor at 460 volts, had they sent a 5 HP at 208 volts I could just replace our old one which is 208.

 

[stevenal]

Still need that 400 amp service on the 208 side. The amperage is inversely proportional to the voltage. Of course if you are planning on redoing your service anyway, you might want to go to a 480 delivery.

 

[sreid]

Using the stepup transformer, the full load amps on the primary is still going to be 360 amps and 400 amp service will be required. There is no free lunch.

 

[itsmoked]

waross; As much great advice you pass here I wish you wouldn't pass this "240V motors will run at 208V stuff". I disagree vehemently. There may be millions 230V motors running on 208V but I guarantee you they are [red]not[/red] happy!! They are running 2 years in stead of 6 or 3 years instead of 8.

Ripcord13; Do not try to run a 230/240 volt compressor on 208... I have seen over a HUNDRED motors that have failed because they were 240V motors run at 208V. Some panty-weight application that needs only 80% of the name plate horsepower or needs name plate HP for 1 minute out of an hour will probably work fine but compressors of any type present "hard labor".

I saw at one site three 240V compressor motors running a device fail in three years in 208V service . The fourth one was installed with three boost transformers. It has now run for 11 years! How can I make it any clearer.


Your motor will not last if subjected to this serious abuse. If it is a 240V motor make sure the transformer/s will get you to 240V!!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Ripcord13]

Thanks guys now I have the "confirmed info" to present to the "Boss". We have a 600 amp 460 volt service that is "maxed out" and a 2000 amp 208 volt service that is about 70% taken hence the step up transformer. Looks like it's time to upgrade the 460 to about 2000 amps also. Thanks Again.

 

[mpparent]

Ripcord,

You'll still need a 400A, 208V feed to your step up transformer, regardless. Like sreid said, "there is no free lunch", and there isn't in your case. I don't know that a 480V, 125HP is the industry standard, I'm just saying that adding a transformer into your system adds just a little more complexity to your existing 208V system. It would be nice, in my opinion, to have a 208V motor.

Mike

 

[DickDV]

And, in the day of expensive energy, the transformer adds about 3% loss to your system. That will be measureable extra $ at the end of each year.

 

[waross]

Hi itsmoked;
Your point is well taken. The majority of those 208 volt motors that are running happily were built when the NEMA allowable voltage drop was 15%. This has been changed to 10%, and I keep forgetting. Now 200 volt motors are used for 208 volt applications.
Ripcord;
460 volts is not the industruy standard.
460 volts is one of several industry standards.
230 volts is one of several industry standards.
200 volts is one of several industry standards.
Changing or installing motors isn't rocket science. Try to buy the compressor without a motor and instal your own 200 volt motor. The salesman is probably trying to sell what he has in stock. If he thinks he may lose a sale, it's amasing what may suddenly become available.
Re the transformers. I have done a lot of voltage correction with transformers and have never yet had to use three.
An open delta autotransformer connection works well and just requires two transformers. Less losses. Given the available sizes of transformers, and the small sizes needed for small voltage adjustments with autotransformers, I have always been lucky enough to find that the available sizes were adequate for the application with only two transformers.
Beware of 240 volt motors. I won't say that they don't exist, but this is not a NEMA standard voltage. A motor rated 240 volts is unlikely to be built to NEMA standards.
Respectfully

 

[itsmoked]

I agree! Get a 460V motor before getting a 150HP transformer!!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com