400V / 50Hz motor run at 60Hz

Status
Not open for further replies.

PhaseShift

Senior Member
We have a brand new motor rated at 400V and 50Hz. Our supply here is a 480V 60Hz supply. I am trying to determine weather or not I can run this motor on our supply?

First thing I realized is that operating a motor designed for 50Hz on a 60Hz system will cause the motor to run 10% faster. I am o.k. with this.

Second thing I look at is the V/Hz ratio. I see that 400V/50Hz = 8 and see that 480V/60Hz = 8. Therefore both V/Hz ratios are the same and will have the same flux densities.

The thing I question, is even know the V/Hz ratios are the same do I have to worry about operating the motor within the +/- 10% value of the nameplate rating? Will opeating this motor at 480V (+20%) cause damage to the motor?

Will I be delivering more torque, by operating at a higher voltage, or is torque striclty dependent on the V/Hz ratio?
 

broadgage

Senior Member
Location
London, England
The motor should in practice be fine, It will run about 20% faster, not about 10%, and will develope more HP as a result.

Overheating is unlikely since the internal cooling fan will also run faster.

Remember that the nameplate current should not be exceeded or overheating and even fire risk is probable.
Most mechanical loads, if driven faster than intended, will absorb substantial extra HP and could lead to the motor drawing more current than intended.

Also check that the driven load can accept the 20% higher speed without danger or harm.
 

Besoeker

Senior Member
Location
UK
I've ran 60hz motors on machines at 90 hz for years and not hurt them. Much past 90hz you start losing torque
Any frequency above nameplate rating without a proportional increase in voltage will reduce the available torque.
So yes, you can run them at above nameplate frequency subject to the good advice given by broadgage*.
The rotor on cage motors are normally pretty robust and can tolerate higher speeds. As a matter of fact we used to routinely run steel works 50Hz roller-table motors at 100Hz.

*wonder if he is a railway enthusiast....
 

PhaseShift

Senior Member
So when NEMA says that motor should be operated in a +/- 10% of voltage which voltage is this referring to? Most of the time it is the nameplate voltage since thats what the motor was operated for. But does this really refer to +/- 10% of whatever voltage keeps the V/Hz ratio at the desinged V/Hz ratio?
 

Besoeker

Senior Member
Location
UK
So when NEMA says that motor should be operated in a +/- 10% of voltage which voltage is this referring to? Most of the time it is the nameplate voltage since thats what the motor was operated for. But does this really refer to +/- 10% of whatever voltage keeps the V/Hz ratio at the desinged V/Hz ratio?
I imagine the former rather than the latter but I don't know for sure. I'm sure there are others here who are far more conversant NEMA standards and application guides.
But, given that the motor is 400V, 50Hz, it probably wasn't manufactured in North America so NEMA might not be applicable anyway.

That said, I would expect a motor of reasonable quality to be able to tolerate a +/-10% variation in V/f ratio.
And here is a quote from an article on EU voltages:
To cope with both sets of limits an equipment will therefore need to cover 230V +/-10% i.e. 207-253V. This will actually become the official limit for the whole of the EU in 2003
The nominal 230V is, of course, the line to neutral value of 400V 3-phase.
 
Last edited:

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Most mechanical loads, if driven faster than intended, will absorb substantial extra HP and could lead to the motor drawing more current than intended.
This is a very important point and should not be take lightly.

If, for example, the load on the motor is a centrifugal pump or fan, in all likelihood the higher speed will result in higher flow, which translates to needing more HP. Although technically you will get higher HP out of the motor, it may or may not be enough because the motor HP will increase at a rate directly proportional to the speed, but the load HP requirements will increase at the cube of the speed.

So for example a 10HP motor run at 20% higher speed will become a 12HP motor, but if the centrifugal load takes 10HP at 50Hz, the 20% increase in speed will increase the load to 1.2 x 1.2 x 1.2 or 17.28HP. If your motor was over sized to start with, that may still be OK, i.e. it was a 10HP motor but the pump only needed 7HP, and with the increase needs 12.1HP. But if it was sized close to the bone, it may not work without overloading.
 

PhaseShift

Senior Member
This is a very important point and should not be take lightly.

If, for example, the load on the motor is a centrifugal pump or fan, in all likelihood the higher speed will result in higher flow, which translates to needing more HP. Although technically you will get higher HP out of the motor, it may or may not be enough because the motor HP will increase at a rate directly proportional to the speed, but the load HP requirements will increase at the cube of the speed.

So for example a 10HP motor run at 20% higher speed will become a 12HP motor, but if the centrifugal load takes 10HP at 50Hz, the 20% increase in speed will increase the load to 1.2 x 1.2 x 1.2 or 17.28HP. If your motor was over sized to start with, that may still be OK, i.e. it was a 10HP motor but the pump only needed 7HP, and with the increase needs 12.1HP. But if it was sized close to the bone, it may not work without overloading.


Jraef

Good points above.

I always thought that motor current was related to motor torque. In other words the motor current was related to the torque and the overall hp was that torqe at a given speed. In other words on a given motor speed vs torque curve as the load increases and slip increases we sift higher on the motor torque curve with equates to a larger current on the motor speed vs current curve.

So I guess I 'm trying to figure out if current is more related to torque or hp? For instatnce if a motor was driving a load at a given toruqe and then you increased the motor speed but the toruqe stayed the same, technically the hp would increase but the toruqe would stay the same, so would the current increase?

Or am I wrong and due to increasing the motor speed we would slide up on the load toruqe curve and thus require more torque at an increased speed?
 

Besoeker

Senior Member
Location
UK
I always thought that motor current was related to motor torque. In other words the motor current was related to the torque and the overall hp was that torqe at a given speed. In other words on a given motor speed vs torque curve as the load increases and slip increases we sift higher on the motor torque curve with equates to a larger current
Yes, torque and current are related. More torque requires more current but it isn't a simple relationship. A typical three-phase cage motor might take 30% of rated full load stator current with no mechanical load applied to the shaft. Load the motor up, and several things change. As far as the input supply is concerned, it is mainly increased current, increased power factor, and increased input power.

So I guess I 'm trying to figure out if current is more related to torque or hp? For instatnce if a motor was driving a load at a given toruqe and then you increased the motor speed but the toruqe stayed the same, technically the hp would increase but the toruqe would stay the same, so would the current increase?
If you have constant V/f, then a constant torque load would, to a first approximation, take the same current and output power would increase in proportion to speed.
 

PhaseShift

Senior Member
If you have constant V/f, then a constant torque load would, to a first approximation, take the same current and output power would increase in proportion to speed.

O.k. so for a constant torque load the current would not increase with an increase in speed although the hp would increase.

For a varialbe torque load the torque would increase with an increase in speed, and therefore the current would increase and the hp would increase as a function of both the increased torque and increased speed. Is this correct.

So confirming my op above this 400V motor as long as it has the same V/Hz ratio at 480V and 60hz will work fine even though voltage is 20% higher. There would be no effect on torque either since torque is dependent on V/Hz ratio?
 

Besoeker

Senior Member
Location
UK
O.k. so for a constant torque load the current would not increase with an increase in speed although the hp would increase.

For a varialbe torque load the torque would increase with an increase in speed, and therefore the current would increase and the hp would increase as a function of both the increased torque and increased speed. Is this correct.
Yes, if the load torque and motor speed increase, the output power would increase. Fundamentally, power is torque times speed.



So confirming my op above this 400V motor as long as it has the same V/Hz ratio at 480V and 60hz will work fine even though voltage is 20% higher. There would be no effect on torque either since torque is dependent on V/Hz ratio?

If you keep the V/f constant, the available torque remains the same. The required load torque might not. Truly constant torque loads are not very common. For most loads, the torque required increases with speed. If it does for your particular case, the increased torque required might very well exceed the motor rated torque and thus, exceed its rated current.
 

PhaseShift

Senior Member
Yes, if the load torque and motor speed increase, the output power would increase. Fundamentally, power is torque times speed.





If you keep the V/f constant, the available torque remains the same. The required load torque might not. Truly constant torque loads are not very common. For most loads, the torque required increases with speed. If it does for your particular case, the increased torque required might very well exceed the motor rated torque and thus, exceed its rated current.

So as long as the v/hz is kept constant the motor will always supply the max torque? Or will the torque supplied by the motor be dependant on the torque required by the load with thw v/hz determining how much torque is avaliable?

For instance if I have a conveyor that is lightly loaded at a fixed speed and fixed v/hz then is the motor supplying the max torque? Or only when the conveyor becomes loaded and the load torque requiremet causes the motor load to increasem

For this same conveyor I'm assuming that a conveyor is a constant torque load. So if we keep the conveyor feed rate the same and increase the speed o the conveyor then the required torque will stay the same but the hp will increase due to the speed increase. Because torque stays the same current will also stay the same?
 

Besoeker

Senior Member
Location
UK
So as long as the v/hz is kept constant the motor will always supply the max torque? Or will the torque supplied by the motor be dependant on the torque required by the load with thw v/hz determining how much torque is avaliable?
Within its limitations, the motor will supply the torque required by the load.
 

PhaseShift

Senior Member
Within its limitations, the motor will supply the torque required by the load.

Yes this I understand. As a motor becomes loaded the slip increases and thus the motor moves to the left on the speed toruqe curve increasing the torque and also increasing the current. I guess this can be seen by moving the load torque verically upward for increased load and then finding where it intersects with the motor curve. This part I understand when we are talking about a fixed speed.

It is when we look at changing the speed and thus effecting hp that I am not seeing the full picture. Is the real current (exluding magnetizing current) drawn by a motor always strictly a function of torque only? Torque increases current increases, torque decreases current decreases. Therfore current is not necessarily a direct function of hp but rater the torque element that makes up the hp.

Because the torque provided by a motor will always be what the load requires this tells me that the V/Hz ratio just ensure maximum torque is avaliable for use if the motor requires it depending on load.
 

Besoeker

Senior Member
Location
UK
Is the real current (exluding magnetizing current) drawn by a motor always strictly a function of torque only? Torque increases current increases, torque decreases current decreases. Therfore current is not necessarily a direct function of hp but rater the torque element that makes up the hp.
Yes, that's pretty much it.
 

Besoeker

Senior Member
Location
UK
Perhaps I ought to add that, over the normal operating range, torque is proportional to slip to a fairly close approximation.
 
Status
Not open for further replies.
Top