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Home > Forums > Electrical / Electronic Engineers > Activities > Electric motors,

generators & controls engineering > FAQsElectric motors, generators & controlsengineering FAQ

Motor Power Supplies

Motors: Changing between a 50 and60Hz supply.faq237-1224Posted: 20 Mar 06 (Edited 9 Jul 06)

Due to the frequent questions about running motorsat "the other frequency" this FAQ was created.

This FAQ is directed at other engineers and so willcontain details the average user may not careabout. You are invited to pay attention only to what isuseful to you.

This FAQ applies to induction motors, both single, andpolyphase. Single phase motors have an added aspectthat will be discussed at the end of this FAQ.

AC motors are quite complex for all their functionalsimplicity - turn them on they spin.

There are a huge number of interacting relationshipsin a motor's design. There are first order, second orderand probably even third order aspects that are allbalanced to produce a dependable motor with thedesired characteristics.

This FAQ will deal only with the First Order aspects.

These complexities include:1) Rotational speed is a direct function of the powerfrequency.2) Cooling is a direct function of rotational speed.3) The magnetic capacity of the motor's magnetic(iron)circuit is designed to the relationship: voltage/frequency(V/f).4) Back-emf decreases as a motor slows down.5) Current increases with reduced back-emf.

Lets look at these complexities each in turn. If you drop the frequency the motor will slow down. If you raise the frequency the motor will speed up.

If the motor slows down it's cooling will drop (and at a

faster rate then the s low down). If the motor speeds up its cooling will increase rapidly.

If the frequency drops the V/Hz goes up. This meansthat the motor needs a larger magnetic circuit. Withoutit the magnetic circuit can saturate. This leads to arapid increase in current draw and a correspondinglarge increase in temperature.(A motor's chief enemy) If the frequency increases the V/Hz drops. This is nota first order consideration. [The motor may have aworse power factor.]

A motor is a device that 'wants' to turn at its designedspeed, set by its designed operating frequency. Itdelivers the required horsepower(hp) the load needswhen the load is spun at the motor's designedspeed. Different loads change their hp demands invery different manners, depending on the type of load

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they are.If a motor's load increases for some reason the motor

will be slowed, this means the back-emf drops, thiscauses the motor to draw more current. More currentis where the motor gets more power to turn the heavierload. Here the prudent motor user makes sure thatthe motor is pulling less current then the motor's fullload amperage(FLA) rating.

If the load remains the same and the voltage islowered the motor will draw more current to continuemeeting the load's hp requirement. Remember themotor will still be running at the same speed since the

frequency wasn't changed.

So looking at the case shifting a 50Hz motor to 60Hzduty.a) It will turn 20% faster.b) The cooling will increase dramatically.c) The load's horsepower requirement will increase,possibly dramatically.d) The V/f will drop which will not cause a current drawincrease.

You should discuss what the load will do when sped up20% with someone knowledgeable with that type ofload. (Forum members are very knowledgeable in thisaspect!)

If the load can take the speed increase then run theload and promptly check the motor's current draw. If it

is under the FLA you should have no other problems.

Using a 60Hz motor in 50Hz duty.w) It will turn 20% slower.x) Cooling will drop dramatically.y) The load's horsepower requirements will drop,possibly dramatically.z) V/f will increase possibly causing a large increase incurrent draw.

Make sure the load will still do what is required of itwhen running slower. For instance, a fan load wouldnow move less air. Still enough?

The cooling will reduce dramatically. Is it enough tomatter? This depends on how the load hasdecreased. If the hp requirement dropped because theload has decreased the current will drop and less heat

will be released internally. You would want to taketemperature measurements until the motor reaches asteady state temperature running in its new frequencyapplication.

The motor's hp will drop because hp is a function ofspeed x torque. The motor's torque doesn't change butits speed has dropped so it is now a lower hp motor. Ifyou change pulley sizes to return the load to its originalspeed your motor will likely be undersized, possiblyseriously. Example: A 10 hp motor is now an 8 hp.

The most serious issue is the V/f issue. The V/f willincrease. Likely enough to cause a large increase inthe motor's current. This coupled with the reducedcooling may cause rapid overheating. However the V/fproblem can be fully mitigated! You reduce the voltageto the motor by the amount required to return the V/fback to its original value. This removes the hazard ofexcess current from an increased V/f.

Example: A 60Hz 240Vac motor is going into 50Hzservice.V/f = Y Hence: 240/60 = 4.0

So ifY x f = V then by plugging in the new frequency of50 we see:4.0 x 50 = VV = 200Running the motor at 200V at 50Hz will remove the V/f

problem.

Another way to think of this is:New voltage = Old voltage x 50/60New speed = Old speed x 50/60New Horsepower = Old horsepower x 50/60

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Note: This would instead be 60/50 for a 50 to 60 Hzconversion.

Single phase complications.Single phase motors must be assisted because single

phase power does not have an inherent rotationalaspect to it. This means an additional start winding isrequired to provide rotational starting torque. Acommon method for controlling power to thistemporary winding during starting is a centrifugal switchmounted on the motor's rotating shaft. This switch isnormally closed during start up. Once the motorachieves a certain speed during start up the switch

opens de-powering the start winding. If a single phasemotor is moved to a new frequency domain theoperation of any rotational switch must be checked. A60Hz motor in a 50Hz application turning 20% slowermay not achieve a speed sufficient to open thecentrifugal switch. This would likely result in animmediate burn out. Likewise a 50Hz motor placed into60Hz service may switch off the start winding at too lowa speed for the motor to make the transition torunning.

Some single phase motors have start or run capacitorsand some both. If the motor is being shifted from 50 to60Hz then their effect will increase. This will usuallypose no problem.

Alternatively, taking a motor from 60 to 50Hz willreduce the capacitor effects. This will result in possibly

lower running and starting torques. This may mean themotor cannot successfully start its load or maintainrunning a load.

A failure to start may result in a rapidburnout(seconds) so this should be carefully monitoredinitially. Larger capacitors or additional ones may beneeded.

Other points to note:With respect to the previously mentioned V/f ratio,460/60 motors match 380/50 motors.

If you need any other clarifications please post a clearand specific question in the Electric Motors forum andmembers will be happy to assist you..

If you see a need for a correction or an addition in this

FAQ please let me know in a thread or directly.

Thanks for assitance by:warossjraefdavidbeach

Keith CressFlamin Systems, Inc.-http://www.flaminsystems.com

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