50/ 60 Hz motor question 1

[ARenko]

I know there are a lot of questions related to operating on different frequencies, but I haven't seen anything that answers my questions. I understand how operating on different frequencies than rated affects a motor's speed, power, etc..., but I'm wondering about motors that are dual rated for 50/60 Hz, yet have only one power rating on the nameplate. Are these motors configured to output the same power regardless of frequency? If so, is it something internal or does it depend how the motor is wired up?

I have an application for a motor in a mobile application that will run on either 460V/ 60Hz or 400V/ 50Hz depending on where it is. If the motor wiring needs to change each time frequency changes then I think a better solution would be choose a single frequency motor and size the motor/ pump combo to handle the differences.

 

[jraef]

A motor winding system, the magnetics if you will, is designed to produce torque, the frequency determines the speed (assuming the same number of poles). Torque production is however dependent upon the relationship to voltage and frequency together, called the "V/Hz ratio" and you will get rated torque so long as the V/Hz ratio is within tolerance, usually +-10%. 460V 60Hz has a V/Hz ratio of 8:1 and 400V 50Hz has a V/Hz ratio of... 8:1! Hence, you will get the same torque of of that motor regardless of the frequency, so long as the voltage is matching to maintain that ratio. This by the way is exactly what a VFD does for a motor; maintains the motor V/Hz ratio so that you can get rated torque as the speed (frequency) changes.

If the ratio were off by more than the +-10% tolerance, it changes the dynamics. Too low of a ratio, meaning less V per Hz, results in a loss of torque and more importantly, the loss of PEAK torque, that which takes place at the Break Down Torque level and is responsible for the motor responding to step changes in load. Peak torque drops at the square of the difference, so if the motor wants an 8:1 ratio and you give it 6.7:1, such as giving it 400V 60Hz, the difference of 83.3% results in 16.7% loss of running torque and a 31% loss of peak torque capability from the motor. If the ratio is too high, i.e. 460V 50Hz, a 9.2:1 ratio, the difference results in over saturation of the windings and increased heat in the motor for the same amount of work performed, so the motor life is cut short.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[ARenko]

Thanks jraef. Let me say this motor will not be on a VFD, just a starter, or maybe a soft starter. This is a hydraulic pump application, which I usually size the motor by HP to meet my pressure/ flow demands. If the torque stays the same and speed changes, then the HP will change and my flow/ pressure capabilities will change. This is why I'm wondering for a motor that has a given nameplate HP, but is good for 50/60 Hz if it needs to be wired different at the motor. I'd want to avoid that. Forgive my ignorance as I'm not used to a motor that may be one month on 50Hz and the next on 60Hz. And maybe I didn't make it clear that this motor will move around to different locations with different power sources.

 

[waross]

The safe course is to take the HP at 50 Hz and verify that the torque is adequate for 60 Hz operation.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Bill is right (as per usual), size it for the worst case scenario (50Hz operation), then when you have 60Hz, the motor capacity (from a HP standpoint) is more than you need. But torque will be the same either way.

And no, on a motor like that you would not need to change the wiring on it.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[ARenko]

Thanks waross and jraef. I plan to base speed on 50Hz because it affects my flow rate.

jraef, when you say the HP is more than you need, this is what confuses me. I mean, that's the way I've understood it when moving equipment between US and Europe for example, but when I see motor nameplates that give the same HP rating for both 50 and 60 Hz I wonder how that works if the torque remains the same as you mentioned in your first post.

 

[waross]

That may be true of some fractional HP, very high slip motors.
On the integral HP motors that I see the most, there is only one frequency shown.
I have done a number of conversions, both 50 Hz to 60 Hz and 60 Hz to 50 Hz.
The torque stays the same. Both speed and HP change in the ratio of either 6:5 or 5:6
If you are using centrifugal pumps, the increase in HP at 60 Hz may not match the increase in torque demanded by the faster spinning pump.
With a positive displacement pump, the flow will be greater on 60 Hz.
I suggest going up a size on the motor and sleeping easier.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ARenko]

Thanks Bill. Here's an example of a nameplate that suggests (to me at least) it's 20 HP at both 50/60 Hz. That just threw me for a loop as I've always thought of it the way you guys explained it. It's a hydraulic motor - I've sized it and the motor to accomodate the lower speed and HP at 50 Hz. Thanks everyone for all the input.

 

[waross]

Is this a no-name motor out of China?
Some of those nameplates defy reality.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ARenko]

Looks like I forgot to put the link, so not sure what you're looking at. Here's the link...

http://www.mgmelectricalsurplus.com/Details/Motors/AC Induction - Squirrel Cage/219.php

What I've seen on other motors is it might say 20 HP under the 60Hz section, but under the 50Hz section HP is left blank.

 

[jraef]

It’s just math.
HP = Torque(ft-lbs) x RPM / 5252
So even if torque remains constant, when RPM changes so does HP. Since induction motor speed is based on frequency, a given motor going from 50Hz to 60Hz increases the speed by 6/5 (120%), ergo HP increases by 120%.

Someone has “taken liberties” with that nameplate data, the math doesn’t work. The motor was designed and built as a NEMA 460V 60Hz motor, so that’s why it says 20HP. I know that because the 50Hz voltage says “380/190” instead of 380/220, the Wye relationship. The 380V 50Hz “HP” (most 50Hz counties use kW), should have been 16HP if the torque remains the same, yet they show it as 20, which can only be true if they magically get 20% more torque from the motor at a lower voltage and frequency. I can’t say how they got the math to work out, you would have to ask US Motors (now Nidec). Most likely: laziness. There is no such standard as “16HP”, so someone just rounded up. That might be a problem for you, or might not. Most loads need torque, speed is secondary.

Most likely, the motor design was


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[ARenko]

Yes, I get the math. It was the nameplate (like in the example I linked) making no sense to me that made me ask my original question. To me that nameplate is wrong. I just wondered if there was something else I was unaware of. Thanks again for your help.

 

[waross]

They seem to be fudging the numbers.
At 60 Hz the SF is 1.15, or 23 HP.
That equates to 19.17 HP at 50 Hz.
They are running at 30 RPM slip at 60 Hz and at 45 RPM slip at 50 Hz.
That pushes the motor a little further up the torque current curve.
I note that the full load current rating is higher on 50 HZ.
The motor will run a little hotter on 50 Hz current.
Even at a full 20 HP load 24/7 it will probably take years before the slightly shortened life is experienced.
Well off the warranty period.
I suggest basing the overload settings on the 50 Hz rated current.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[zlatkodo]

Some notices:
- HP is the same according to the nameplate data. Pay attention that the PF and efficiency are not the same at 50 and 60 Hz.
- Also, each 50 Hz motor with SF = 1 has a SF = 1,15 at 60 Hz.
- It should be checked what effect the increase in rpm (at 60 Hz) has on the load, as Waross mentioned.

Winding Design and Motor Repair