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#130240 - 02/24/06 05:30 AM 3 Phase Motor Voltages: 208 vs. 240
rhpope Offline
Member

Registered: 02/23/06
Posts: 18
Loc: North Carolina
Any opinions on running 3 phase motors rated at 220/230/240 volts on 208 volts? I have seen this done before without any problems, but I am looking for opinions on the extra current draw and possibility of overheating by running at the lower voltage. My main reason for asking is that I am trying to spec the proper utility service voltage for a machine shop with the power company. 120/208Y seems the most flexible, but 120/240 Delta is a little better voltage wise. There will be a mix of 3 phase and single phase. 10HP and less on 3 phase machines. 5HP and less on single phase machines. Then there are some single phase and 3 phase welders.

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#130241 - 02/24/06 07:24 AM Re: 3 Phase Motor Voltages: 208 vs. 240
Radar Offline
Member

Registered: 04/30/04
Posts: 344
Loc: Los Angeles, CA
From my experience, running a 230V 3Ø motor on a 208V system will (as you noted) result in a higher current flow than would result from running on a 230V system. The motor will do whatever work is required of it, and starving it of voltage will demand increased current (P = E x I).

This is OK to do IFF the motor is not at or near full load. in other words, the HP rating of the motor MUST be degraded by the factor of the increase in current caused by the diminished voltage. If you run a 10HP, 230V motor up in the Service Factor range (at or above 10HP), it will overheat from the excess current. Keep the actual motor load down to 80% - 90% of rated output and you should be OK (I'm figuring 208V is about a 10% reduction from 230V).

Radar
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There are 10 types of people. Those who know binary, and those who don't.

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#130242 - 02/24/06 09:34 AM Re: 3 Phase Motor Voltages: 208 vs. 240
Dave T Offline
Member

Registered: 01/22/05
Posts: 157
Loc: Waukesha, WI, USA
A simple thing to remember is that a motor that is designed to run at a given voltage and 5 HP, for example, but applied at a lower voltage will draw more current because because you can't get something for nothing, that to get 5hp when the voltage decreased, the current must increase to provide that 5hp let alone considering torque. That motor isn't designed with that extra current in mind resulting in increased heating.

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#130243 - 02/24/06 07:47 PM Re: 3 Phase Motor Voltages: 208 vs. 240
Joe of NJ Offline
Member

Registered: 12/05/05
Posts: 24
Loc: West Orange, NJ, USA
Question:

If a 5 HP motor designed for 230 volts is used in 208 volts, shouldn’t it deliver less power (HPs) instead of draining more current?

Thanks,

Joe.-

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#130244 - 02/24/06 08:27 PM Re: 3 Phase Motor Voltages: 208 vs. 240
Radar Offline
Member

Registered: 04/30/04
Posts: 344
Loc: Los Angeles, CA
Joe - No it won't. Look at it this way, a motor does not decide how much power it will put out, that's a function of the load applied. If you have a 5HP load, any motor you attach to it will attempt to carry the load. If the motor is rated 5HP or above, it will drive the required load with relative ease. If the motor is too small, it may still do the work but draw more than rated current in the process and will overheat (#1 cause of motor failure). If the motor is small enough, it of course will stall out and overheat (more quickly).

Sometimes we tend to think about motors in a way similar to how non-electricians think of circuit breakers. By that I mean many people presume that if a CB has a number 20 stamped on it, there must automatically be 20 amps flowing thru it. We know the actual current flow is a function of the applied load. Same thing is sort of true for a motor, the HP rating is a maximum capacity rating, and that must be within the other nameplate ratings.

So, asking a 5HP motor to carry a 5HP load with less than rated voltage will not stop the motor from doing the work, but it will cause it to overheat from higher than rated current (sort of making up for the reduction in voltage). You can get away with a 230V motor on a 208V system as long as the motor is loaded to less than rated HP.

Radar
_________________________
There are 10 types of people. Those who know binary, and those who don't.

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#130245 - 02/25/06 07:19 AM Re: 3 Phase Motor Voltages: 208 vs. 240
Larry Fine Offline
Member

Registered: 02/18/05
Posts: 684
Loc: Richmond, VA
To add:

Horsepower is the physical analogy to power (wattage) in electricity. Since power (in watts) is equal to potential (in volts) times current (in amps), if the voltage changes, the current will (attempt to) change inversely, in order to keep the power from chenging.

Note: this is where motor loads are unique as compared to regular resistive loads, where current changes proportionally (but not inversely) to voltage. Normally, current is said to be "drawn by" a load, whereas it's more accurate to say "allowed to pass through" the load.
_________________________
Larry Fine
Fine Electric Co.
fineelectricco.com

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#130246 - 02/26/06 02:26 PM Re: 3 Phase Motor Voltages: 208 vs. 240
jraef Offline
Member

Registered: 07/21/04
Posts: 88
Loc: San francisco, CA, USA
Here is the reality.

Only the oldest motors still carry 240V nameplate ratings, and there are often good reasons to replace those anyway if they are running continuously, mainly efficiency. If they are only running for short periods, low voltage (208V) will have little detrimental effect anyway.

New NEMA motor designs are supposed to be able to tolerate up to 10% low or high voltage without adversely affecting rated torque and temperature rise. So if a motor is rated at 230V, 230 x .9 = 207V, so a 208V service is just fine. However, whenever that 208V drops down to 200V, as it may likely do in some areas, the 230V motors will be out of spec and run hotter than allowed. Theoretically. If you really look at most new energy efficient motors, you will see that they are rated to be used at 208V now, the motor mfgs know that 120/208V 3 phase is the most common commercial voltage. What they do is rate them for 220V, because 220 x .9 = 198 so they work on 200V, and 220/.9 = 244 so they work on 240V as well.

If you have a large percentage (more than 10%) of your loads being 120V, do not use the 120/240 3 phase delta service, it will be severely unbalanced and that will have much more of a detrimental effect on the 3 phase loads than 208V would.



[This message has been edited by jraef (edited 02-26-2006).]
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