# 240 volt motors run on 208 volts



## Chris1971

What would be the long term effect if you ran 240 volt motors on 208 volts? Reduced torque, higher current draw and reduced motor life? Would it be better to get a boost transformer to operate the machines at 240 volts?


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## wildleg

these are good questions, but you left out the next question:

is there a one-size fits all answer to the above questions ? 

I don't know the answer to your questions, but I would assume they are yes. But I would also assume that it depends on the service factor of the motor, and also the usage.


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## FastFokker

And type of motor.


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## Wirenuting

Here I have seen them last from 10 minutes to 10 years.. 
These were mostly air handlers and circ pumps for heating & domestic water. 
They never last as long as a 200 volt motor. 

A 208 system uses a 200 volt motor best. 
Them triple tap motors suc & 240/480 motors hold out ok.


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## John Valdes

Many new motors are rated for 120/208/230.
Or the nameplate will say 208 is acceptable.

This is how I would approach using 208 to run 230 volt motors.


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## Chris1971

What issues would their be on a 240 volt single phase motor running on 208 volts?


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## bkmichael65

It may have a shorter life from the increased current draw


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## ksmith

*3phase generators*

can someone help me what would happen to the generator if you hook a 3phase generator up to a single phase set up ?


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## micromind

In reality, a pretty fair number of 230/460 volt motors will draw less current when operated on 208. 

I've connected tons of 230/460 motors to 208 systems, and I can't remember having trouble with any of them. 

The vast majority of 230/460 motors are rated for operation on 208 systems at full rated HP, but often with reduced service factor and a different (not always higher) amp rating. 

If a motor is started frequently, lower voltage will make it last longer because of reduced magnetizing and locked-rotor current. Especially magnetizing current.


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## xlink

Chris1971 said:


> What issues would their be on a 240 volt single phase motor running on 208 volts?


A 240 volt motor on 208 volts will run a little slower. Running slower decreases the counter EMF (back emf?) and the current will go up. However, a motor running slower does less work so the current doesn't go up as much as you might expect. For example, a fan running slower moves less air and draws less current. Is it less than what it would draw at 240 volts or more because the counter EMF is lower? I don't know.

In the end, if the current is below the rated amperage and the motor runs fast enough for the fan to remove the heat, you won't have a problem. But if the motor runs an air compressor that was loaded to 135% at 240 volts, you should expect problems.


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## hardworkingstiff

xlink said:


> A 240 volt motor on 208 volts will run a little slower.


That goes against what I was taught (in the field, not formal training). I was told that the motor will attempt to operate at a certain level and it will just draw more amps with the lower voltage. This only works for so long before the motor will have damage from too low of a voltage.

I'm not saying I'm exactly correct, just that's the way it was explained to me.


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## chicken steve

micromind said:


> If a motor is started frequently, lower voltage will make it last longer because of reduced magnetizing and locked-rotor current. Especially magnetizing current.



i don't understand this mm....

~CS~


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## backstay

xlink said:


> A 240 volt motor on 208 volts will run a little slower. Running slower decreases the counter EMF (back emf?) and the current will go up. However, a motor running slower does less work so the current doesn't go up as much as you might expect. For example, a fan running slower moves less air and draws less current. Is it less than what it would draw at 240 volts or more because the counter EMF is lower? I don't know.
> 
> In the end, if the current is below the rated amperage and the motor runs fast enough for the fan to remove the heat, you won't have a problem. But if the motor runs an air compressor that was loaded to 135% at 240 volts, you should expect problems.


Speed is dependent on source frequency, number of poles and load.


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## xlink

backstay said:


> Speed is dependent on source frequency, number of poles and load.


and slippage.


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## backstay

xlink said:


> and slippage.


That's what load does.


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## xlink

I'm thinking of the little speed controls that you can put on a shaded pole motor, like a bathroom exhaust fan or a ceiling fan. It doesn't change the frequency, number of poles or the load.


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## backstay

xlink said:


> I'm thinking of the little speed controls that you can put on a shaded pole motor, like a bathroom exhaust fan or a ceiling fan. It doesn't change the frequency, number of poles or the load.


I didn't think about that. My point comes from thinking in the terms of a 3 ph motor or large 1ph motor.


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## xlink

backstay said:


> I didn't think about that. My point comes from thinking in the terms of a 3 ph motor or large 1ph motor.


I ran a compressor on a cord that was too long. It started okay and seemed okay until it started to load up. When the voltage drop happened, the motor slowed down.

What happens really depends on the type of load. I think that is why we all have different answers.


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## xlink

A local motor supply shop had some interesting information on this. They tested a Baldor 3 phase, 208 volt motor at 208 and 240 volts, under no load.
208 volts = 2.9 amps
240 volts = 3.6 amps

On their shelf, they had another 208 volt motor that was brought back for warranty. It was running on 255 (approx) volts and the vibration damaged the motor. It is thought the problem came after the poles were fully magnetized.

They were more concerned with the imbalance between phases. The motors are wound with the same length conductors but have a different number of wraps. When the windings are ohmed, they are all the same but when the motor is running, the current on the phases differs significantly.


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## micromind

chicken steve said:


> i don't understand this mm....
> 
> ~CS~


There are basically 3 currents that a motor sees;

1) Magnetizing current. This is the initial inrush, it lasts somewhere around 1/60th of a second. The current here is a function of the resistance of the wire in the windings and the fault current available at the motor terminals. There is no counter EMF, it's basically a bolted fault. 

This amount of current is a lot like smacking the stator with a hammer. The larger the wire feeding the motor, the more current delivered and the harder it hits. Smaller wire = less current, and less abuse. 


2) Locked rotor current. This is what we usually refer to as the inrush. Counter EMF is alive and well here, and it limits the current to about 6X the full-load current. 

If the voltage is reduced during starting, this current will be lower; thus less heat. If however, the voltage is reduced too much, the motor will have a hard time starting, thus more heat.


3) Operating current. This will be full-load current or less. Small long wire will reduce voltage at the motor, but this is not always bad. 

Choosing wire size and breakers for motors is more of an art than a science. The code is a minimum for wire size, and often this is best. The code is a maximum for breaker size, but often a smaller (and less expensive) breaker will do just fine.


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## bkmichael65

hardworkingstiff said:


> That goes against what I was taught (in the field, not formal training). I was told that the motor will attempt to operate at a certain level and it will just draw more amps with the lower voltage. This only works for so long before the motor will have damage from too low of a voltage.
> 
> I'm not saying I'm exactly correct, just that's the way it was explained to me.


You're correct. Speed is determined by frequency and number of poles. You lower the voltage and it will increase amp draw until it eventually breaks down the winding insulation


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## xlink

micromind said:


> In reality, a pretty fair number of 230/460 volt motors will draw less current when operated on 208.


It seems that is correct but I think the fair number depends on the type of load. It is certainly true at no-load.


micromind said:


> I've connected tons of 230/460 motors to 208 systems, and I can't remember having trouble with any of them.


That is my experience, too. I connected a 240 volt elevator motor that was about 1940 vintage to 208 volts and it has been okay for a decade.


micromind said:


> If a motor is started frequently, lower voltage will make it last longer because of reduced magnetizing and locked-rotor current. Especially magnetizing current.


Again, I think that depends on the type of load. An air compressor at full load and reduced voltage sucks.


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## JRaef

In an AC induction motor, lowering the voltage lowers the TORQUE produced by the motor. If the torque is lower and the load remains the same, the load creates more slip, which makes the motor draw more current. There is no direct relationship to voltage and speed, only the indirect relationship to slightly more slip, IF the load remains high with regard to the motor torque capability. So if you are only using 75% of the motor's torque, and you lose 15% off the top end because of low voltage, you may never see a difference in speed. So bottom line, as was said way earlier, there is no blanket statement that can be made about the effects of running a 230V rated motor at 208V, because "it depends...".

Shaded pole motors are a little different in that their design allows for the reduced torque to cause a very high slip, but the motor design is such that it naturally current limits itself, so it runs slower without overloading. The trade off is, not much torque to begin with, that's why you only see them on small fans and such. But still, the relationship to voltage and speed is still indirect, you are really reducing TORQUE, which allows the LOAD to determine speed.

Measuring no load amps on uncoupled motors at different voltages is a completely meaningless test, I'm surprised a motor shop would have even bothered with that experiment (it was more likely done by a rookie that had no clue). The totally predictable higher amp reading at the higher voltage would have been the result of the higher magnetic losses, which when there is no load, become a greater part of the power consumed. But under load, those losses become a fraction of a fraction of the load power and relatively insignificant.


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## hardworkingstiff

JRaef said:


> In an AC induction motor, lowering the voltage lowers the TORQUE produced by the motor. If the torque is lower and the load remains the same, the load creates more slip, which makes the motor draw more current. There is no direct relationship to voltage and speed, only the indirect relationship to slightly more slip, IF the load remains high with regard to the motor torque capability. So if you are only using 75% of the motor's torque, and you lose 15% off the top end because of low voltage, you may never see a difference in speed. So bottom line, as was said way earlier, there is no blanket statement that can be made about the effects of running a 230V rated motor at 208V, because "it depends...".
> 
> Shaded pole motors are a little different in that their design allows for the reduced torque to cause a very high slip, but the motor design is such that it naturally current limits itself, so it runs slower without overloading. The trade off is, not much torque to begin with, that's why you only see them on small fans and such. But still, the relationship to voltage and speed is still indirect, you are really reducing TORQUE, which allows the LOAD to determine speed.
> 
> Measuring no load amps on uncoupled motors at different voltages is a completely meaningless test, I'm surprised a motor shop would have even bothered with that experiment (it was more likely done by a rookie that had no clue). The totally predictable higher amp reading at the higher voltage would have been the result of the higher magnetic losses, which when there is no load, become a greater part of the power consumed. But under load, those losses become a fraction of a fraction of the load power and relatively insignificant.


I was hoping you would show up. Thanks. :thumbsup:


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## xlink

JRaef said:


> Measuring no load amps on uncoupled motors at different voltages is a completely meaningless test, I'm surprised a motor shop would have even bothered with that experiment (it was more likely done by a rookie that had no clue).


Nope. It was done by a very experienced person measuring the imbalance between phases on a new motor. He happened to have done the test at two voltages. I only remembered one phase and reported it here for general information.

More interesting, though, is the effect of over-voltage on the motor.


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## quanchai

hardworkingstiff said:


> That goes against what I was taught (in the field, not formal training). I was told that the motor will attempt to operate at a certain level and it will just draw more amps with the lower voltage. This only works for so long before the motor will have damage from too low of a voltage.
> 
> I'm not saying I'm exactly correct, just that's the way it was explained to me.


That is correct, when you have a load on the motor the load will slow down the motor, slowing down the motor reduces the CEMF generated by the motor that limits the current draw, so as the load gets heavier the motor speed is slowed down and the generated CEMF force is reduce which causes the current draw to rise, while reducing voltage to the motor reduces the motor torque 

Voltage is related to torque, the more closer to the rated voltage the more torque the motor produce, the more torque the heavier load the motor can run without slowing down which reduces the CEMF


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## Switched

bkmichael65 said:


> It may have a shorter life from the increased current draw


Every time I see your avatar, I read what you wrote in the dog's voice....in my head of course!:laughing::laughing:


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