What are the drawbacks of Using a 50hz motor on a 60hz system

[mpiblue]

Question for you guys, I work in a industrial plant that is owned by italians but we are located in north america. The spare parts we recieve come from our parent plant in italy. We have been dealing with alot of premature motor failure in the plant and i was curious if it could be related to the name plate rating's on the motors. I.E. we have a machine that uses a 18.5kw 1440rpm 50 hz motor that cycles on/off around 20 times a minute. I was wondering if running this motor on a 460v 60hz supply could be leading to the motor failures. We are getting an average life of around 4 months per motor. These motors are not locking up bearings, most all are burning up the windings.

Thanks

 

[Skogsgurra]

Cycling on/off 20 times/minute is tough for any motor.

Doing this with 20 % higher speed than rated causes 44 % more rotor losses and correspondingly higher stator losses.

That, coupled with the fact that the cooling isn't active all the time, will definitely shorten motor life. And it is, in my mind, no wonder that your windings are burning.

I can guess that the motors live somewhat longer in Europe. But probably not the 10 - 20 years you expect from an induction motor.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[dpc]

cycles on/off around 20 times a minute

I agree with Gunnar - this is probably not going to work well for any induction motor of this size. You really need to rethink your process and find a way to reduce the number of motor starts/stops.

David Castor

http://www.cvoes.com

 

[jraef]

Might be time to consider a VFD, but that depends on a lot of other factors that you left out.

Speaking of leaving things out, can we assume the motor was rated somewhere between 380 and 415V at 50Hz?

What kind of load?

Why is is cycling so fast?

Was the motor over sized for the load in order to accommodate the high duty cycle? If so, did anyone consider the effects Gunnar mentioned when operating in 60hz power?

In other words it might be as simple as using a larger motor to allow for the added heating effect of running it here. But if the load is a centrifugal pump for instance, the load on the motor will actually increase to 172% with a 20% increase in speed! (However I can't imagine a centrifugal pump cycling every 3 seconds.)

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[mpiblue]

Sorry about the information i left out. Yes in this case the motor im reffering to is rated at 380v. The reason it cycles so fast is it is running a gear box that runs a extrusion mill that extrudes a clay block. The cycle time is is figured by the extruded length meeting a preset number read by an incrimental encoder. I have 5 of these units on site and all experience quick motor failure. The cycle times vary per extrusion mill. some put out 50mm blocks some put out 200mm blocks. I have external fans blowing on the motor at all times.i was just curious if i got an american made 460/60hz motor if i would see an increase in motor life. The motor company we use now is directly across the street from out parent company in Italy so thats why we recieve these motors at 50hz.its such a normal occurence here that everyone just accepts it and puts a new motor on. I believe there may be a solution to this problem.
thanks for your imput.

 

[waross]

A clutch will probably save the motors and may improve accuracy.
I agree with the others. This would probably be borderline at 50 Hz. and is tougher on the motor at 60Hz.

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

 

[Skogsgurra]

The voltage ratio is 1.21 and the frequency ratio is 1.20. So voltage shouldn't be a problem.

It is all about accelerating to a higher speed which means speed ratio squared. Good that you have external blowers.

Extrusion means that torque also increases with speed squared (usually) because it is viscious load. That doesn't exactly help. Jraef mentioned that. An increase from rated to 1.72 times rated is nothing to take lightly. A magnetic or hydraulic clutch as proposed by waross, is probably the way to go. Remember to cool the clutch, as well.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Skogsgurra]

viscious? Doesn't sound right. What should it be?

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[carnage3]

Do you gear the output to a 6:5 ratio to achive the desired input speed of the machine you are using? by gearing the speed down you would reduce the motors power load and the required heat dissapation.

 

[sonic02]

I encountered a similar event about 2 months ago. We ordered a 60Hz IEC electromechanical relay that was to trip a circuit breaker for one of our clients. But when we went on-site, we found out that a 50Hz relay with the same no.of contacts and physical dimensions were installed instead. And suprisingly, it went ok during commissioning.

So i'm guessing that the particular relay could work for both 50Hz and 60Hz system.

 

[waross]

I have also seen some effective air operated clutch-brake combinations.

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

 

[edison123]

A higher rated motor may be needed here ? Did you mention the duty cycle to the OEM when you ordered the motors ? Frequent starts and stops is the worst kind of duty for a motor, given the I^2t heat. You probably need S8 (?) duty rated motor.

Muthu

http://www.edison.co.in

 

[DickDV]

skogs, that would be "viscuous", I believe.

Isn't language an adventure! I still remember struggling to get along in Swedish and having my rep keep telling me "It can't be that hard. I see little children speaking it every day!"

It was always said kindly and in jest so it is a pleasant memory.

 

[zlatkodo]

Hi, mpiblue,
You have not told us the most important thing: what is the full-load and no-load currents of motor?
Without these details it is difficult to say anything.
Zlatkodo

 

[tygerdawg]

"viscous"

I suggest you do a sizing analysis on this application, or get someone who can. Twenty starts per hour is not crazy, but is tough for the wrong motor. I know that SEW Eurodrive has an Engineering Manual that goes through a very rigorous methodology that sizes their motors including a given starts per hour quantity. It usually results in applying a larger motor for a given application just to absorb & dissipate the heat generated from frequent starts.

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering

http://www.bluetechnik.com