running a 120 volt 60Hz single phase induction motor on 120 volt 50Hz

[robsims]

Hi,
What will happen if i run a 120 volt 60Hz single phase induction motor on 120 volt 50Hz

and what will happen if i run a 120 volt 50Hz single phase induction motor on 120 volt 60Hz

[Xena E]

I'm left making some assumptions about this question, ie., what particular type of single phase induction motor.
Shaded pole; split phase; Switch start; capacitor start and run; capacitor start, induction run; capacitor start/capacitor run;

however the short answer is in the first part, proportionately lower starting torque, lower mechanical power, lower sync, and full load slip speed. Full load current will also be different

Second part would be a reversal of the answer above.

Usually induction motors are rated for both 50 and 60 Hz supplies so just look at the ratings for your particular motor.

Sorry if the answer is seems vauge but...

[alpher]

Split phase and PSC motors will run fine on 50Hz, just 20% slower, motors that use centrifugal switch have to be rated 50/60Hz to work properly, otherwise the rpm on 50Hz will be too low to reliably disconnect starting winding especially under load. Going the other way (50Hz motor on 60Hz power) should pose no problems other than running 20 % fast.

[james_s]

It depends on the type of motor. Shaded pole motors do not seem to be affected by the frequency within reasonable bounds. Other types of induction motors will run slower on 50Hz. Some cheap motors designed for 60Hz only may saturate and overheat on 50Hz but most are fine. If you look closely at the label you may see the RPM rating is something like 2880/3450, and that will be the speed it runs on 50/60Hz.

[robsims]

I'm talking about single phase induction motors

[robsims]

I'm left making some assumptions about this question, ie., what particular type of single phase induction motor.
Shaded pole; split phase; Switch start; capacitor start and run; capacitor start, induction run; capacitor start/capacitor run;

however the short answer is in the first part, proportionately lower starting torque, lower mechanical power, lower sync, and full load slip speed. Full load current will also be different

Second part would be a reversal of the answer above.

Usually induction motors are rated for both 50 and 60 Hz supplies so just look at the ratings for your particular motor.

Sorry if the answer is seems vauge but...

The motor is a capacitor start, induction run  motor

[james_s]

I'm talking about single phase induction motors

There are many types of single phase induction motors. Shaded pole, split phase, PSC, capacitor start, capacitor start-capacitor run, and repulsion motors which are rare today are technically another type of single phase induction motor.

[robsims]

I'm talking about single phase induction motors

There are many types of single phase induction motors. Shaded pole, split phase, PSC, capacitor start, capacitor start-capacitor run, and repulsion motors which are rare today are technically another type of single phase induction motor.

I'm talking about a capacitor-start induction motor

[alpher]

Capacitor start motor most likely uses a centrifugal switch to disconnect start winding. Therefore if the motor is rated 60Hz only running it on 50Hz is a bad idea. It will probably start fine without any load, but if loaded centrifugal switch may engage periodically or even steadily, motor will not last long under these conditions, hours maybe if overcurent protection doesn't shut it off first .

[robsims]

Capacitor start motor most likely uses a centrifugal switch to disconnect start winding. Therefore if the motor is rated 60Hz only running it on 50Hz is a bad idea. It will probably start fine without any load, but if loaded centrifugal switch may engage periodically or even steadily, motor will not last long under these conditions, hours maybe if overcurent protection doesn't shut it off first .

No the motor has no centrifugal switch. If i leave it on running on 50Hz i think it will overheat because it turns slower. Is that a good assumption?

[james_s]

If it has no centrifugal switch then it isn't a capacitor start motor.

I would not expect a centrifugal switch to be an issue anyway though, they typically engage at just a few hundred RPM and drop out as the motor coasts to a stop.

[alpher]

It will be an issue, trust me. But you're right it may not be a capacitor start motor, only way to know is to know the capacitor value. If it's in the 100uF plus range then it's a cap. start motor, may not have a centrifugal switch, it is rare but there are motors with electronic switch or PTC.

[james_s]

Now that you mention it, I have seen PTC start switches, but only on hermetic compressors. There's no technical reason why one couldn't be used on a more conventional motor but so far I've never seen it done.

[amyk]

Now that you mention it, I have seen PTC start switches, but only on hermetic compressors. There's no technical reason why one couldn't be used on a more conventional motor but so far I've never seen it done.

Single-phase induction motors without centrifugal start switches are becoming more common now that the cost of a centrifugal switch exceeds that of a start relay. They're mostly found in fractional horsepower sizes, above that at which shaded pole motors start being too inefficient.

https://woodgears.ca/motors/starter.html

[WattsThat]

The motor is only one half of the equation. The other half is what is attached to the output shaft of the motor.

A centrifugal fan? No problem. A pump designed to run at 90% of motors rated power? That’s a problem.

[Siwastaja]

You should scale voltage linearly, too. Going from 60Hz to 50Hz is 83% of original frequency. 120V should be lowered to 83% * 120V = 100V; then you can say it just runs slower.

Of course, this is such small difference that it's likely not catastrophic. The situation is equivalent to overvolting the motor. It creates more torque, current increases, the slip frequency decreases, reactive power increases more than useful mechanical power.

If the motor is crappy and marginal, and used in demanding conditions, such slight abuse might bring it over the edge, and it burns out in lesser time. But very likely it's OK.

[WattsThat]

the slip frequency decreases

Why would slip change with applied frequency and or voltage? Isn’t slip is a constant and proportional to rotor resistance?

Perhaps I misunderstood. I’m not familiar with the term “slip frequency”. I define slip as drop in speed from no load (which is not exactly synchronous but darn close) to full load speed.

[Siwastaja]

the slip frequency decreases

Why would slip change with applied frequency and or voltage? Isn’t slip is a constant and proportional to rotor resistance?

Perhaps I misunderstood. I’m not familiar with the term “slip frequency”. I define slip as drop in speed from no load (which is not exactly synchronous but darn close) to full load speed.

Slip is the difference between the equivalent rotational speed of the mains (magnetic field of the stator) and the actual rotation of the rotor. Slip frequency is just the same as expressed in electrical frequency (Hz) instead of RPM. More slip means the motor is running slower, and vice versa.

If one increases voltage and thus winding current, the motor is capable of generating more torque and "catches" the stator field in speed. But if there is no use for this torque, it just means worse power factor and high reactive current.

Under no load, induction motor runs closer to synchronous speed. Indeed, the first improvement from linear V/f relationship towards optimum efficiency, constant torque control is to implement constant slip control, which adjusts voltage so that less voltage is used with small load (making slip actively higher when it naturally tries to go too low), and more voltage is used under heavy load (making slip actively lower).

Optimum slip is indeed kinda constant (and proportional to rotor resistance, so rotor windings heating up makes it a non-constant again), so I think this is where you got mixed up. But for a fixed-voltage, fixed-frequency (i.e., direct mains) use, optimum slip is only achieved at the optimum mechanical load designed for that motor. It is well known that an induction motor which runs at either very light or very heavy load significantly improves its efficiency when driven with a VFD in FOC mode. (Constant slip control is worth experimenting with if you ever try to build this stuff. I remember mentally struggling with the whole FOC concept but slip control made a lot of sense to me and I was able to make it work experimentally very well, and difference in dissipated power in power stage transistors thanks to getting rid of all that reactive current was just huge, compared to V/f control.)