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Joined: Mar 2004
Posts: 947
twh Offline OP
If I take a motor that is rated for 50 Hz and connect it to 60 HZ, what happens? The motor is rated for 230 Volts at 50 Hz so the supplied voltage can remain the same.

I think the speed changes from 1425 rpm to 1725 rpm.

How much HP could the motor produce at the same FLA?

Running a motor connected to something like a fan at a different speed changes the load, so I wonder if it's possible to predict what the change in current would be if the frequency changed and the type of load remained the same (like being connected to the same fan).

This question was passed on to me, so I don't have access to the motor for testing. I just want to return a reasonable guess.

Joined: Dec 2002
Posts: 205
First thought is that power output will reduce as less current will flow in the windings.
(Assuming a fractional horsepower squirrel cage type, though I haven't thought the maths through)

Joined: May 2002
Posts: 381
I have indeed done this when my South African origin bench grinder and pillar drill lived in my American workshop for five years. Suitable 230V sourced and supplied.

Both ran 20% faster. No problem other than I couldn't use the highest speed belt setting on the drill - it stalled the motor. It was on this board that I said at the time (2003?), the grinder ran sweeter on 60Hz than 50Hz - yes the wheels were rated for the increased speed.

Both now happily consuming 250V (last time I checked), 50Hz here in greenest Oxfordshire, UK.

Joined: Mar 2004
Posts: 947
twh Offline OP
Thank you for this information. I think I'll say that the motor will run faster and have a noticeable reduction in HP at the higher RPM. Lower HP at the same voltage will probably translate into lower current.

Joined: Jul 2002
Posts: 8,412
Likes: 3
How on earth are you?
Haven't "seen" you around these parts in ages!
Hope all is well with you.


Joined: May 2011
Posts: 10
I agree that a 50Hz motor supplied with 60Hz will run faster.

I don't agree with your HP reasoning. The HP needed is defined by the load, the motor has to provide it. With something like a fan the mechanical power varies with the square of the speed, so doubling the speed requires 4 times the mechanical power & will shift a lot more air.

So, if the motor is running faster then the load is likely to demand more mechanical power & this may well overload the motor.

The current in the windings has two components - the first is a reactive component that creates the magnetic field. I agree that this will be less at 60Hz. The other component is that current which is responsible for the 'work'. This will be directly proportional to the mechanical load - since the mechanical load is likely to be greater (its being spun faster) then this component of the current will be bigger.

Joined: Mar 2004
Posts: 947
twh Offline OP
I agree that a fully loaded motor at 50 Hz will probably be overloaded when it drives the load at 60 Hz. I can't think of a load that isn't greater when it's driven faster.

I get hung up on the same voltage at different frequencies. A vfd will vary the voltage with the speed - lower Hz needs a lower voltage. Then, would a higher Hz need a higher voltage to overcome the counter emf?

My guess, from the example given by Hutch, is that dragging down the speed of the motor with the load might not return the motor to it's original HP. That is, a 50 HZ motor has more power at 50 Hz that it does at 60 Hz because it is designed for 50 Hz.

I have seen motors rated for both frequencies but they have different voltage ratings or HP ratings, or both. I haven't seen one with the same voltage and HP.

Joined: May 2002
Posts: 381
Ah Trumpy, good to "hear" from you again. Yes, I appear to have taken a little (and not deliberate!) sabbatical but at 50Hz, there appears to be only 9,261,837,000 cycles between the posts. Excel is a wonderful beast. smirk

Last edited by Hutch; 05/28/15 06:02 PM.
Joined: Jun 2014
Posts: 129
dsk Offline
Usually a 3 phase 50 Hz motor will work well at 20% higher voltage and frequency.

An European 400V motor will work well on 480V at 60 Hz and of course be 20% stronger.

Not always so the other way, because lower frequency may need more Iron in the cores.


Last edited by dsk; 08/07/15 06:29 PM.

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