Step Up Transformer
Step Up Transformer
(OP)
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?





RE: Step Up Transformer
Mike
RE: Step Up Transformer
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
RE: Step Up Transformer
RE: Step Up Transformer
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.
RE: Step Up Transformer
RE: Step Up Transformer
RE: Step Up Transformer
RE: Step Up Transformer
RE: Step Up Transformer
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
RE: Step Up Transformer
RE: Step Up Transformer
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
RE: Step Up Transformer
RE: Step Up Transformer
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
RE: Step Up Transformer
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com