what happens when you run a 50Hz 3 phase motor at 60Hz

[ignator]

G7PSK:I don't see that anyone said this directly.
Think of the motor as an inductor. What does impedance do in an inductor when you change frequency. The 2 pi f L impedance equation. So the input current given the same voltage will decrease.
As Johansen very well describe, the torque capability is reduced, which makes sense if the input power is less.

[Simon]

that is why I assumed, faster speed less power, supper fail

[IanB]

that is why I assumed, faster speed less power, supper fail

But the faster speed part is not directly important.

The power requirement of a fan is determined by the characteristics of the fan and the operating conditions (and not the motor). So if a fan requires a certain power to run at 750 rpm, then it requires the same power to run at that speed whether the motor is supplied at 50 Hz or 60 Hz. The motor power demand at 750 rpm is exactly the same whatever the line frequency.

Now the power is also given as the product of shaft torque and rotation speed. Therefore to deliver the required power at 750 rpm the motor must be able to provide sufficient torque on the output shaft when turning at that speed.

What happens at the higher line frequency is that the motor has no trouble turning at 750 rpm with no load, but when the fan is attached the motor can no longer generate sufficient torque to develop the required output power at this speed. Therefore the motor runs at a slower speed until the power developed by the motor and the power consumed by the fan become equal.

One way to compensate for this is to replace the fan impeller with a smaller diameter one that has a lower input loading (but then it will move less air). Another possibility is to boost the input voltage to compensate for the higher frequency, but this may not be cheap or easy.

[Simon]

well the motor is speced at 875rpm @ 60 Hz so we need the extra power to run the same fan. although it sounds like it will just slow the motor down if the motor can't provide the power. Will the motor inherently take more or less power at 60 Hz ?

[johansen]

well the motor is speced at 875rpm @ 60 Hz so we need the extra power to run the same fan. although it sounds like it will just slow the motor down if the motor can't provide the power. Will the motor inherently take more or less power at 60 Hz ?

no load idle draw will be slightly more, but this is voltage dependent.

what you fail to understand is the cubic power curve. this is why, as you said earlier "the motor isn't getting there"
its probably running at 300 rpm or something.
google:
torque-rpm curve induction motor
pull out torque
pull in torque.

depending on that motor's specific torque curve, you could be running the motor anywhere from 300 to 700 rpm on the 60hz line.

unless you change the pulley diameter(if it has one?), or cut the blades down you need to find a 7KW motor, which is about 10 hp.

[G7PSK]

Increase the load or decrease the power into an induction motor and you will increase the percentage of slip, that means that the bars of the squirrel cage cut fewer lines of force so the back emf is reduced therefore the current rises and you get increased heating.

Percentage of slip = Difference between actual speed and  synchronous speed / synchronous  speed X 100.

Synchronous speed = HZ X 60/ number of pairs of poles.

Normally the slip % is around 2 to 5  much more than this and the motor is in trouble.

[IanB]

well the motor is speced at 875rpm @ 60 Hz so we need the extra power to run the same fan. although it sounds like it will just slow the motor down if the motor can't provide the power. Will the motor inherently take more or less power at 60 Hz ?

The motor and fan are designed together as a combination. The characteristics of the motor are matched to the characteristics of the fan. It's a bit like impedance matching in electrical systems.

Just because the motor has 60 Hz on its nameplate, it doesn't mean that the motor and fan in combination are specced to run at 60 Hz.

In electrical systems impedance matching can be achieved by a transformer. In mechanical systems impedance matching can be achieved by gearing. So in this case, if there is a pulley system connecting the motor to the fan, then it may be possible to change the gear ratio with different pulleys so the faster speed of the motor can be matched to the slower speed of the fan.

You haven't told us where this motor/fan combination came from, and why it is being used on the wrong specification of electrical supply? Is it a surplus component that is being re-used, or did incorrect information get given to the original supplier? Someone would have designed it to do a certain job, and now it seems it is being used outside its design. If the opportunity exists, you could go back to the designer to have the problem remedied.

[ConKbot]

If you cant pass the problem onto someone else, then it sounds like a VFD may be an easy way out.  Since your load is a fan, a simple V/Hz one should suffice.  Plus that adds 'features' like a logic-level on/off, variable speed, and a ramped start.  Obviously the motor is rated at 60 Hz, so you probably shouldnt run it past 60 Hz on the VFD, especially if its not rated for a VFD. I'm no expert in VFDs, but if its non rated to run on a VFD, then reactor on the motor may be required to keep harmonics and parasitic capacitance in the motor from causing issues, but I'd get a second opinion on that.

[johansen]

Obviously the motor is rated at 60 Hz, so you probably shouldnt run it past 60 Hz on the VFD, especially if its not rated for a VFD. I'm no expert in VFDs, but if its non rated to run on a VFD, then reactor on the motor may be required to keep harmonics and parasitic capacitance in the motor from causing issues, but I'd get a second opinion on that.

you can run most small induction motors at 400hz if you think the bearings can handle it. the problem is, he needs a 7 KW motor to run a 4KW fan at 20% faster than design rpm. 
as it is, the fan "isn't getting there" which most likely means the motor is not coming up to synchronous speed.

[IanB]

Let me ask this question, for which I am not clear about the answer:

Suppose we take a 750 rpm motor that can generate its designed maximum power output when supplied with 50 Hz, and we change the supply frequency to 60 Hz so that it wants to run at 900 rpm. If we keep the supply voltage the same, will its maximum power output at the new faster speed be the same, be less, or be greater than it was at 50 Hz?

My impression is that the total power will be less, because the decrease in torque will outweigh the increase in speed. Is this correct?

[johansen]

Suppose we take a 750 rpm motor that can generate its designed maximum power output when supplied with 50 Hz, and we change the supply frequency to 60 Hz so that it wants to run at 900 rpm. If we keep the supply voltage the same, will its maximum power output at the new faster speed be the same, be less, or be greater than it was at 50 Hz?

http://www.usmotors.com/TechDocs/ProFacts/Voltage-Frequency-Variation.aspx

[IanB]

Suppose we take a 750 rpm motor that can generate its designed maximum power output when supplied with 50 Hz, and we change the supply frequency to 60 Hz so that it wants to run at 900 rpm. If we keep the supply voltage the same, will its maximum power output at the new faster speed be the same, be less, or be greater than it was at 50 Hz?

http://www.usmotors.com/TechDocs/ProFacts/Voltage-Frequency-Variation.aspx

Unfortunately neither that document nor others I have found give a direct and unambiguous answer to my question.

However, from considering the information provided it would appear that the power output would remain approximately the same. Speed is proportional to frequency and torque is proportional to voltage/frequency, so if the frequency is changed without changing the voltage the two effects of frequency will cancel out.

[pickle9000]

Does this help?

http://myelectrical.com/tools/induction-motor-calculator

[johansen]

Suppose we take a 750 rpm motor that can generate its designed maximum power output when supplied with 50 Hz, and we change the supply frequency to 60 Hz so that it wants to run at 900 rpm. If we keep the supply voltage the same, will its maximum power output at the new faster speed be the same, be less, or be greater than it was at 50 Hz?

http://www.usmotors.com/TechDocs/ProFacts/Voltage-Frequency-Variation.aspx

Unfortunately neither that document nor others I have found give a direct and unambiguous answer to my question.

However, from considering the information provided it would appear that the power output would remain approximately the same. Speed is proportional to frequency and torque is proportional to voltage/frequency, so if the frequency is changed without changing the voltage the two effects of frequency will cancel out.

i don't understand your question. maximum power output does not equal maximum efficiency.

A 1 hp motor's maximum power output is approximately 2 hp, but could be as much as 3 hp or as little as 1.5, depending on what it was designed for.
Its reasonable temperature rise will limit the hp to 1, its "rated" load.

running the motor at 20% more hz but keeping volts the same will reduce the "maximum power output" by a lot. however, it will still operate at its "rated" load, provided its "rated load" is measured in watts, not torque, at similar conditions. the slip will be higher however, due to the reduction in voltage.
if the voltage is increased as well, you get 20% more power for a slight increase in windage and iron losses.