240 feeding 208 volt motors

[mbrooke]

I cannot really say "most". But although a dual voltage motor with no wiring change is certainly common (either rated 230 with a tolerance band down to the high end of 208 or rated 208/230 with different nameplate currents, etc.) I can see specialized equipment which is designed and rated for 208 (with 230 OK at the upper end of its tolerance band.
I will let others with wider experience say whether "most" applies. Maybe for NEMA motors?

IMO a 230 volt motor is technically ok for a 208 volt supply provided it doesn't fall below 200 volts. 208 is the low end bandwidth of 230 volts.

 

[mbrooke]

I wouldn't say most. If they say 230/208V on the nameplate that's what has been done. There are plenty 230V and 200V motors though.

Ive seen straight 230 volt motors placed on 208 and worked fine. IMO it can be done if 208 volts can be guaranteed at the motor terminals.

 

[fifty60]

My tagline to this point has been "A motor that has a nameplate of 230V/208V means that the motor is actually rated for 230V with lesser performance implied at 208V". Is this "lesser performance" because there is more slip at the lower voltage, which causes more current? So more current for RPMs further below synchronous speeds at 208V as compared to 230V?

What has always confused me is that the NEMA rating implies a + or - 10% voltage range around the nameplate. 208V is approximately 10% below 230V, and if a 230/208V rated motor implies less performance at 208V already, does the NEMA rating gurantee operation at as low as 188Volts, or is the + or - 10% only based off of the 230V number?

The last I had it explained to me was the + or - 10% means 230V + 10% and 208V -10%, so the true range would be 253V to 188V...

 

[Strathead]

I'm not arguing, I'm actually asking for some educating.........

We hook up motors wired for high voltage and run them on low voltage when we test them, 240V and even 480V tested with 120V. With the windings in the high voltage configuration they pull less amps at the lower voltage than they do at the high voltage. There is no load so I know that is part of the deal, but the point is more voltage does in fact mean more amps. What's up with that?

Out in the field our power runs a little high in my fair city so 480V can run 510-520 or so. Motors will pull a little more juice then too.

I am surprised smart$ hasn't responded with the firm science. kwired gave a response that is accurate in my opinion. I will give you my layman's understanding of AC motors. As kwired stated, there is a synchronous speed of a motor. It is fixed value derived by the frequency of the power and the number of poles on the motor. That is why it is always and interval of 60, 1800 RPM being a VERY common synchronous speed. If an induction motor reached synchronous speed (I think) the magnetic field would no longer have the push pull effect and would have no torque. When you look at the motor RPM is is always lower than the synchronous speed, because that is where the motor reaches equilibrium between drag (load) on the motor and max speed (synch). So now on to the voltage.

A motor does work rated in horsepower. At rated RPM, it produces rated horsepower. Our standard physics says that if all other factors stay the same, (temperature, physical characteristics and work which is measured in wattage for electricity) and you decrease voltage, the amperage has to go up. If you observe any other phenomena, then less or more work accordingly is being done by the motor. Given one speed, less voltage produces less induction, less induction means less impedance, less impedance means higher amperage, but the same amount of work. This all assumes that both voltages are adequate to run the motor at is rated RPM, which it will always try to attain. When the load is too great and causes the motor to run slower, then induction goes down, amperage goes up and you have an overload condition.

So, respectfully, if your city voltage runs at 510, then your motors should draw less amperage, period.

Again, the above isn't intended to be a physics lesson, more of a perspective.

 

[Strathead]

My tagline to this point has been "A motor that has a nameplate of 230V/208V means that the motor is actually rated for 230V with lesser performance implied at 208V". Is this "lesser performance" because there is more slip at the lower voltage, which causes more current? So more current for RPMs further below synchronous speeds at 208V as compared to 230V?

What has always confused me is that the NEMA rating implies a + or - 10% voltage range around the nameplate. 208V is approximately 10% below 230V, and if a 230/208V rated motor implies less performance at 208V already, does the NEMA rating gurantee operation at as low as 188Volts, or is the + or - 10% only based off of the 230V number?

The last I had it explained to me was the + or - 10% means 230V + 10% and 208V -10%, so the true range would be 253V to 188V...

Given my understanding, you are close. You don't get more "slip" as much as you get more resistance (as in opposition not electrical resistance) to the designed slip of the motor. This resistance actually causes less induction or opposition to the flow of electricity due to current. So more current must flow to maintain the same speed. Same speed = same amount of slip.

 

[kwired]

My tagline to this point has been "A motor that has a nameplate of 230V/208V means that the motor is actually rated for 230V with lesser performance implied at 208V". Is this "lesser performance" because there is more slip at the lower voltage, which causes more current? So more current for RPMs further below synchronous speeds at 208V as compared to 230V?

What has always confused me is that the NEMA rating implies a + or - 10% voltage range around the nameplate. 208V is approximately 10% below 230V, and if a 230/208V rated motor implies less performance at 208V already, does the NEMA rating gurantee operation at as low as 188Volts, or is the + or - 10% only based off of the 230V number?

The last I had it explained to me was the + or - 10% means 230V + 10% and 208V -10%, so the true range would be 253V to 188V...

I wouldn't say there is lesser performance - at least to any significance, the motor still will put out same power at 208 instead of 240 volts it will just pull more current in doing so and may see relatively minor decrease in overall efficiency.

I am surprised smart$ hasn't responded with the firm science. kwired gave a response that is accurate in my opinion. I will give you my layman's understanding of AC motors. As kwired stated, there is a synchronous speed of a motor. It is fixed value derived by the frequency of the power and the number of poles on the motor. That is why it is always and interval of 60, 1800 RPM being a VERY common synchronous speed. If an induction motor reached synchronous speed (I think) the magnetic field would no longer have the push pull effect and would have no torque. When you look at the motor RPM is is always lower than the synchronous speed, because that is where the motor reaches equilibrium between drag (load) on the motor and max speed (synch). So now on to the voltage.

A motor does work rated in horsepower. At rated RPM, it produces rated horsepower. Our standard physics says that if all other factors stay the same, (temperature, physical characteristics and work which is measured in wattage for electricity) and you decrease voltage, the amperage has to go up. If you observe any other phenomena, then less or more work accordingly is being done by the motor. Given one speed, less voltage produces less induction, less induction means less impedance, less impedance means higher amperage, but the same amount of work. This all assumes that both voltages are adequate to run the motor at is rated RPM, which it will always try to attain. When the load is too great and causes the motor to run slower, then induction goes down, amperage goes up and you have an overload condition.

So, respectfully, if your city voltage runs at 510, then your motors should draw less amperage, period.

Again, the above isn't intended to be a physics lesson, more of a perspective.

That is basically my understanding of what happens in an induction motor. If you input 60 Hz, it is going to pull whatever current is necessary (or die trying to do so) to reach a point that is near synchronous speed. The more load there is on the output shaft the more current it will draw.

There may be other factors that limit the current - like when the core becomes saturated, or overall impedance of the source or supply circuit could be current limiting as well. Impedance of a winding itself may also be current limiting if you apply a very low voltage compared to rated voltage.

 

[ActionDave]

.....So, respectfully, if your city voltage runs at 510, then your motors should draw less amperage, period.

To a point yes, but there is a tipping point. Once you have enough voltage to make the motor work efficiently, not too much slip, adding more voltage will mean more amperage just like a resistive load.

K8MHZ was right, Ohm's Law is still in effect.

....There may be other factors that limit the current - like when the core becomes saturated, or overall impedance of the source or supply circuit could be current limiting as well. Impedance of a winding itself may also be current limiting if you apply a very low voltage compared to rated voltage.

Had a chance to chat with my motor guru. What I took away from it was once you saturate the iron (still working on what that means exactly) you have the right amount of slip to make the motor do the work it is supposed to do. Too little voltage and you will have too much slip and draw more current, the right amount of voltage and you draw the right amount of current, add more voltage above that and there will be more current.

Works for me, and is consistent with what I have seen.

 

[ActionDave]

Ive seen straight 230 volt motors placed on 208 and worked fine. IMO it can be done if 208 volts can be guaranteed at the motor terminals.

Yes, I agree it can be done. I've done it more than once.

 

[Besoeker]

Yes, I agree it can be done. I've done it more than once.

I suppose it depends on the loading. I'd worry about it stalling if it is required to operate at full power on the lower voltage especially if that lower voltage, 208V, was at the bottom limit of permitted range.

 

[GoldDigger]

Isnt it where the at a certain point the voltage will cause the iron to saturate and any increase in voltage there on increases the current? My understanding is a motor has a complex voltage/torque/current curve.

You absolutely do not want the magnetic circuits of a motor to saturate. That causes current to increase limited only by winding resistance instead of inductance.
Yes, the slip rate of the motor, representing the dragging of magnetic field lines through a conductive medium, leads to very complex shaped voltage/current/torque/speed curves. But not because of hysteresis or saturation.

And neither of those really relate to CEMF either.

 

[GoldDigger]

I suppose it depends on the loading. I'd worry about it stalling if it is required to operate at full power on the lower voltage especially if that lower voltage, 208V, was at the bottom limit of permitted range.

Keep in mind that the OP was talking about applying 240 to a motor with a nameplate voltage of 208.
Saturation would be one possibility if the motor used a non-conservative design.
Similar effect to applying 50 Hz instead of 60Hz without dropping the voltage. Only it would be only be a 1.15 overload instead of a 1.2 overload.

 

[ActionDave]

I suppose it depends on the loading. I'd worry about it stalling if it is required to operate at full power on the lower voltage especially if that lower voltage, 208V, was at the bottom limit of permitted range.

If the voltage is at the bottom limit and the motor really needs full power you could have a problem. Good thing engineers rarely design things right on the cutting edge. The full power of the motor is a bit more than what it says, the next guy picks a motor that is a bit more than what is needed,......

 

[Besoeker]

If the voltage is at the bottom limit and the motor really needs full power you could have a problem. Good thing engineers rarely design things right on the cutting edge. The full power of the motor is a bit more than what it says, the next guy picks a motor that is a bit more than what is needed,......

Might depend on how commercially competitive the bid needs to be.

 

[iwire]

Might depend on how commercially competitive the bid needs to be.

Don't start going all Haji on us.

 

[Strathead]

Had a chance to chat with my motor guru. What I took away from it was once you saturate the iron (still working on what that means exactly) you have the right amount of slip to make the motor do the work it is supposed to do. Too little voltage and you will have too much slip and draw more current, the right amount of voltage and you draw the right amount of current, add more voltage above that and there will be more current.

Works for me, and is consistent with what I have seen.

Don't know enough about motor theory to dispute or confirm what you say, but please explain how your statement seems to contradict the information stamped on motor name plates. Specifically that the amperage of a motor at 208 volts, is higher than the amperage at 240 volts, but the motor RPM is, as far as I know the same?

 

[iwire]

Specifically that the amperage of a motor at 208 volts, is higher than the amperage at 240 volts, but the motor RPM is, as far as I know the same?

It does not contradict anything at full HP output.

 

[K8MHZ]

Update:

The duct guys were there the other day and I told one of them about the 240 in the main and the 208 label on the unit. He opened up the unit and the motor was labeled 208-230 VAC. So, that's more like it, but the unit also had a control board on it. I mentioned that, too.

I did manage to make a new friend. One of the apprentices on the crew. They sure are getting younger and always seem to be under foot. A friendly guy with lots of energy but didn't really seem to get much done other than running around hanging out with the other trades.




View attachment 13432

 

[gar]

150925-0955 EDT

Various comments, I have not read all the posts, but I consider some as being incorrect.

Probably not mentioned in this thread, but Ohm's law does not include the power equations, the power equations are Joule's law.

Ohm's law and its extension to include imedance applys anywhere that resistance or impedance is essentially constant over the range that voltage and or current varies. Thus, consider a forward biased diode. This has a non-linear V-I curve, but over some small range I can consider it as a constant resistance equal to the slope of the curve at that point.

An AC synchronous motor has an output speed that is constant, except for a phase shift, as motor load is changed up to the point where the load is sufficient to break sync.

An AC induction motor has an output speed that is synchronous speed (the speed of a synchronous motor) minus a slip speed that increases with load. The slip speed is very dependent on the rotor resistance. Higher rotor resistance and the slip is greater for a given load. Most induction motors are made with low resistance rotors. Fan motors are probably made with high resistance rotors.

Motor core saturation, when reached, this is gradual for most motors, may increase input current rapidly and thus motor heating.

Experiment on a rectangular box fan about 20" square on the fan face.
A variac (Powerstat) and a Kill-A-Watt EZ were used in the test.

V In    Amp    Watts   VA      PF

100    0.78      56.8     79    0.71
120    0.95      81.3    113    0.71
145    1.08     113      157    0.73

Motor speed varied a great amount, low at low voltage, and high at high voltage. Sensed by noise level. Note: speed will never exceed synchronous speed.

Since 240/120 V systems usually run high on voltage these days, mine is about 125 V, I woulf not put a 208 motor on a 240 V system. Life will be shorter, and in the case of a fan the noise will be greater if the fan has a high resistance rotor.

Many questions on this forum could be answered more quickly if the questioner did some simple experiements before asking the question.

Important criteria for most electrical products are:
Maximum ambient temperature.
Temperature rise vs input conditions.
Maximum internal temperature vs life.
Speed in the case of motors, intensity for lights, etc.
And others.

.

 

[ActionDave]

Don't know enough about motor theory to dispute or confirm what you say, but please explain how your statement seems to contradict the information stamped on motor name plates. Specifically that the amperage of a motor at 208 volts, is higher than the amperage at 240 volts, but the motor RPM is, as far as I know the same?

Because the motor is doing work and it needs a certain amount of juice to do that work. Once you go past that level you are jamming more juice than the winding needs and it acts like a resistor. More volts through a resistor equals more amps. Gars experiment shows what I have seen.

 

[GoldDigger]

Because the motor is doing work and it needs a certain amount of juice to do that work. Once you go past that level you are jamming more juice than the winding needs and it acts like a resistor. More volts through a resistor equals more amps. Gars experiment shows what I have seen.

Or in this case, as long as you do not reach saturation, an impedance which is mostly inductive but with a resistive component that leads to heating.