240V Motor Rating Explanation

[nonsinusoidal]

If a single phase motor is rated for 240V, what are the consequences if it is feed from a 208 single phase voltage source. Is there a NEMA or other standard that motors are manufactured according to? Can anyone explain how motors are rated and what type of motors are available for 208v Single phase voltage source. Thank you

 

[edison123]

Assuming the frequency is constant for both the voltages, the torque of the motor will come down by (208/240)^2 and the HP will come down by (208/240).

 

[waross]

If the motor is made in North America it will probably be rated at the standard rating of 230 Volts. If the motor is less than 100% loaded it will probably work well on 208 Volts as many motors do. Your options are to look for a dual rated, 230/208 Volt motor, look for a 200 volt motor, or reconnect the 230 Volt motor for 120 Volts. If the motor is actually rated for 240 Volts, suspect offshore substandard construction.

Bill
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"Why not the best?"
Jimmy Carter

 

[jraef]

By the way, there is a standard, it's called NEMA MG-10 and although you must buy the full version from NEMA, you can download many parts of it in reviews and compilations on-line.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[ScottyUK]

Bill! If it is old and says 240V it is probably one of ours and might well be a decent motor. If it is new and says 230V it might be a European one and will probably be junk because we make far more junk than we do decent motors. Some big-name brands are included in this, it's not just anonymous junk over here.


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If we learn from our mistakes I'm getting a great education!

 

[waross]

Hi Scotty
I forget the details but I saw a small gen-set (about 1 KVA) earlier this year from an emerging nation. I remember thinking that if the nameplate data were to be posted as part of a question the OP may be red flagged for lack of basic professional knowledge. JUNK with capital letters.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[nonsinusoidal]

Thank you very much for all of your prompt responses. I still have a few other questions.

What are the allowable minimum and maximum voltages that a 230 volt rated motor should be fed from (as per standard)?

If the 230 volt rated motor is fed from a 208 single phase voltage source, will it start (since it will be already feed from a lower voltage level 208 in this case) if the voltage drops five percent from the 208 nominal voltage. I acknowledge that it would be better to buy a single phase motor rated for 230/208 Voltage source. However, I am looking into feeding existing motors from a 120/208 single phase source.

Your assistance to this question is highly appreciated. Thank you for your time in responding.

 

[waross]

It was common for decades to run 230 Volt rated refrigeration compressors from 208 Volts. Most supermarkets and malls have 120/208 Volts. But for some years the trend has been to use 200 Volt rated motors for 208 Volt service. If here is an indication that there may be performance issues with 230 Volt rated motors in your installation, such as chronically low incoming Voltage or frequent Voltage dips,, you may consider boosting the Voltage to the motors with open delta auto-transformers. You may use one set for each motor or one set for a group of motors. Most desirable for a large group of motors is three sets of auto transformers with the common leg on each of the phases.
Called away, more later if you consider voltage boosting. How many what size motors?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

waross; It was common to try to run 230V refrigeration motors on 208V. This destroyed literally millions of them. Most compressor burnouts were directly related to this ignorant (cheap) behavior. Refrigeration is the one field you should NEVER attempt to run a 230V motor on 208V as it will cost you a compressor, and often the entire system. A Burn Out in a semi or completely hermetic system results in acid distributed throughout the entire system. You should always boost 208V to something higher if you are running a 230V refer system in 208V land. This goes for any other motor that runs fully loaded for long periods. For instance an air compressor, or possibly a pump, or fan application.

nonsinusoidal; You have never said what your motor is going to run. If it's a drill press then it will likely have less power but not burn out. This would be because drill presses rarely run continuously nor are they usually fully loaded. It depends on what the motor's load is.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[nonsinusoidal]

During my research I found the following document, please review and advice as appropriate. Thank you

http://www.brithinee.com/240vs208volts.htm

 

[ozmosis]

I would say the article from Richard L. Nailen is about right and what the above posters are saying.
Basically, if your motor says 240V and you supply it with 208V then it might not work, but then it might work. Should you do this? No, because you have a motor working outside of the limits assuming it is running at full load continuously. If it isn't running at full load continuously then we don't know.

 

[itsmoked]

It's correct, pointing out the biggest problem with running 230V motors on 208. 208 is commonly 200 or 198, etc.

In refrigeration the compressors find themselves running continuously during the hottest part of the day, moving the maximum energy, while the utility voltage is the lowest, due to maximum network loading and possibly dial-back. A fatal mix for the poor motors.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ozmosis]

That is why those (shush..) fixed optimising 'energy controllers' were the death of most refrig.compressors. They 'fixed' themselves during startup on a point at low load, phase back, reduce voltage and then uhoh, the load is there and "what's that funny smell coming from the bottom of the fridge?"
Then they come in and say "hey, we've saved 30% of your energy bills..!" sure, a third of the fridges have stopped working tosser..

 

[waross]

Have you seen those dual rated 230/208 Volt motors? They are fairly common in Canada. I assume that they are rated for full output at 208V but have enough iron to accept 240 Volts without saturating.
You may have issues running a 460:230V motor on 208 but when the nameplate says something like 460:230/208V 14:7/7.5A it should be safe on 208 Volts.

Bill
--------------------
"Why not the best?"
Jimmy Carter