Overcurrent on a 60Hz motor feed under 50Hz...

captain_fracasse · Jan 3, 2011

Hello everybody and happy New Year!

Simple question. I actually use a 3~ motor (230V , 60Hz , 375W). In my country our voltage frequency is 50Hz.
I experiment different time that when I'm feeding a 60Hz motor under 50Hz its current is double.
In my example nominal current should be around 2A but when I measure it's around 4A (and of course heat a lot but without mechanical issue of course).

Does somebody can explain me why it's not possible to use this motor in european frequency (50Hz)?

Kidblue · Jan 3, 2011

This may help explain...

http://www.eng-tips.com/faqs.cfm?fid=1224

Regards,

Rob

DickDV · Jan 3, 2011

Captain, the current in a three phase motor under the same load conditions is determined by the ratio of voltage to frequency. The North American motor is designed for 230V 60hz which is a ratio of 3.83. You have it connected to 230V 50hz which is a ratio of 4.6. Thus the high current.

You would need to reduce the voltage to 230x50/60=192V to keep the ratio and the current the same.

You could take another route with these motors. Since they are almost certainly labelled for dual voltage 230/460V, you could operate this motor on 400V 50hz with the motor connected for 460V and it should be happy. That is because 460/60 is the same ratio as 400/50 or close enough.

Karsten · Jan 4, 2011

captain_fracasse said:
Hello everybody and happy New Year!

Simple question. I actually use a 3~ motor (230V , 60Hz , 375W).

Are you really running at 3*230V and not 3*400V?

Gil47 · Jan 4, 2011

Karsten said:
Are you really running at 3*230V and not 3*400V?

Those are my thoughts exactly

My question is how many terminals are there inside the motor terminal box and how are you connected to them.

List the details from the motor label for us to see

DickDV · Jan 4, 2011

A North American dual voltage 230/460VAC 60hz motor will have connection instructions for only those voltages.

There is no provision for 400V 50hz.

captain_fracasse · Jan 5, 2011

Thanks for all your answers. The motor is rightly connected, voltage is 3~ 230V.
I made different machine for our US local supplier who supply its own US motors.
What I understand is that the current is directly proportional to the magnetic flux (that is proportional to the frequency...).
If somebody has more information this could be interesting.

Nic

DickDV · Jan 5, 2011

Captain Nic, please re-read my earlier post. The current (and flux) is NOT a function of the frequency but of the voltage to frequency ratio.

So, with the motor connected according to the 230V 60hz directions, you will need to drop the voltage to 192VAC at 50Hz to keep the current/load relationship the same. If you were to reconnect the motor to the 460V 60hz arrangement, then you would need 400V at 50hz to keep the current/load relationship the same.

Note also that the available hp (kw) from the motor will be reduced by 5/6 as well. The torque will remain the same but the speed drops by that ratio.

There's really nothing more to it than that!

shooter · Jan 6, 2011

use a frequency converter it will solve all your problems.
and they are cheap for 2 amps.
oke i know it is a good lesson to try to run it in 50 Hz.

Jeff23spl · Jan 6, 2011

shooter said:
use a frequency converter it will solve all your problems.
and they are cheap for 2 amps.
oke i know it is a good lesson to try to run it in 50 Hz.

Do you know other converter than a VFD drive ?

I will say the same said earlier but you can understand it by the ohm laws as the impedance of the motor change accordig to frequency for the same load. (Impedance change a lot with the load but if we remove that part from the calcul we can understand it this way:
Impedance (OHMS) = Resistive ohm(Very low) + inductive resistance that increase with frequency (6.2832 x Frequency x L(mH))
Than because of (Amps=Volts/Resistance) when the resistance decrese with the lower frequency, the amperage increase at the same voltage

Because motor speed is lower with 50hz, (3600/3000 or 1800/1500) i assume that you change gear ratio to get the same machine work ? Than with the same HP at the end, you ask for more torque from the motor itself ?

Pego · Jan 6, 2011

frequenty converter or VFD...

Jeff23spl said:
Do you know other converter than a VFD drive ?

I know a lot of other converters, but this is just a "language problem".
In other languages a "variable frequenty drive" can not be translated properly.

So... we call it a "frequenty converter", the input to the "converter" is ??? Hz and the output frequenty is ??? Hz. So what does it do? It converts the input frequenty to another (output) frequenty.
I leave the other statement(s) to the original poster

leitmotif · Jan 6, 2011

Yes there are several types of frequency converter

DC or AC motor driving a 400 Hz alternator is one.
Hi freq source in a TIG welding power supply another

Dan Bentler

Jeff23spl · Jan 6, 2011

leitmotif said:
Yes there are several types of frequency converter

DC or AC motor driving a 400 Hz alternator is one.
Hi freq source in a TIG welding power supply another

Dan Bentler

ok but in that kind of case: is there cheap converter that do only 50 to 60 hz or 60 to 50 hz cheaper than a motor VFD or a (motor-alternator with physical speed change)
If yes i would like to know because i don't know any and it would help me too in some case.

leitmotif · Jan 6, 2011

I would say for most cases VFD would be least expensive and most appropriate. Motor generator units would be most appropriate in multiples of 10 kW range - still fairly expensive.

Most common in this day and age is units to supply ground power (400 Hz) to aircraft - maybe(??)

The good things about VFD -- variable speed, multiple motor control, accel and decel ramps would be difficult to do with MG sets.

Dan Bentler

DickDV · Jan 6, 2011

(sigh!) You don't need a cheap frequency converter. You need a cheap transformer. Just a transformer to get your voltage to a level that gives you the right V/Hz ratio.

Anybody out there got a frequency converter that's cheaper than a transformer?

Overcurrent on a 60Hz motor feed under 50Hz...

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