50 Hz motors

Hi all,

I need some advice from motor experts. We have a machine that was transferred from a plant in Europe to the U.S. This machine has 8 motors, 2 of them VF drives, and they run at 380 VAC 50 Hz. When it was shipped to the U.S. they installed an alternator to keep power to the motors at 380 VAC 50 Hz. I would like to eliminate the alternator and convert everything to 460 VAC 60 Hz. The 2 VF drives can be easily adjusted and most of the others I can change the output sprockets to keep all the speeds the same. It sounds easy but what effect will this have on the motors (going from 380 to 460 VAC and 50 Hz to 60 Hz) aside from the speed change?

Look at the motor nameplate and see what its rated for, if its wound and rated at 380vac then at 480 you will have a problem at some point. If the VF drives can be used at 480 then have the motors rewound to match and you should be ok.

Howsa `bout buying a 480 to 380 volt xformer.
120 times frequency / # of poles = synchronous speed.

Look at the motor nameplate and see what its rated for, if its wound and rated at 380vac then at 480 you will have a problem at some point. If the VF drives can be used at 480 then have the motors rewound to match and you should be ok.

Click to expand...

I do not agree.

The motors should be ok running at 480 Volts, 60 Hz as you are keeping approximately the same volts/hz ratio as at 380 Volts, 50 Hz. The motors will run 20% faster and may be overloaded at the higher speed. If you change the sprockets, as you said, so the machine runs at the same speed as before, everything should be fine. I have seen several European machines running this way.

Provided the VFDs are rated for 480 Volt input (most are rated 380-480 Volts), you should not have to do anything to them. Leave the drives programmed for a 380 Volt, 50 Hz motor and everything will be fine.

Be sure to check that the controls can be modified to 480 volts. You may have to change the control transformer to one with a 480 volt primary.

Good Luck

tomneth said:

Hi all,

I need some advice from motor experts. We have a machine that was transferred from a plant in Europe to the U.S. This machine has 8 motors, 2 of them VF drives, and they run at 380 VAC 50 Hz. When it was shipped to the U.S. they installed an alternator to keep power to the motors at 380 VAC 50 Hz. I would like to eliminate the alternator and convert everything to 460 VAC 60 Hz. The 2 VF drives can be easily adjusted and most of the others I can change the output sprockets to keep all the speeds the same. It sounds easy but what effect will this have on the motors (going from 380 to 460 VAC and 50 Hz to 60 Hz) aside from the speed change?

Click to expand...

What HPs are your 6 motors using line power (and presumably "across the line" starters)? VF drives have become SO versatile and cheap youd probably be smart to make 'em all VF, and you can link them with "master-slave" functions to maintain speed ratios. Truth is, you'll save power, and add protection and quite likely life to your motors too.

I'm just an electrician, with lotsa machinery experience, NOT a drive salesman.

In my view, Vic has it right. Running 380V 50Hz motors on 460V 60Hz is done all the time in the US. If you are going to have problems with doing this, it will be because the application demands the same or more load torque at the higher speed of 60Hz as at 50Hz. Doing the math reveals that you have just asked the motor to develop 20% more HP and that won't go on for long. The comment about increasing the power train ratio to reduce speed back down to 50Hz speed will also limit torque and is exactly what needs to be done to protect the motors running like this. Generally, the insulation in IEC 380V motors will tolerate 460V well.

The suggestion to use a transformer to reduce 460V 60Hz down to 380V 60Hz will lead to disappointment and motor trouble. The problem is that, at 380V 60Hz, the current is reduced by the higher frequency and therefore, the torque is reduced as well. The result will be a motor not able to develop torque needed to run the machine. The motor will slip excessively and will overheat much the same as if you were taking a NEMA motor and running it on low voltage. Of course, if the motor is way oversized for the load, you might get away with a transformer but, my luck isn't usually that good! I don't know about your luck!

As someone told earlyer:
Use transformer to 380V and VFDrives if economycally sense.
Then voltage is correct, speeds are adjustable and VFDs give good protection and more start-torque !

Hold on, You will burn the windings, The windings must work at the voltage they are designed for by the manufacturer. Have them rewound for 460 V or if mechanically possible change them with 460 V motors.

We are about to export a machine to Mexico and we bought an inverter to change the frequancy but I was unaware of the voltage change. I assume Mexico and US use the same power supply? I knew the domestic power was 110v is the commercial supply not related to that? 60Hz is not a problem to most motors designed for 50hz but using a motor designed for 60hz on 50hz without reducing voltage will overload the motor. I have questioned to idea of exporting the inverter from UK as although the inverter is not bothered on frequancy as all power is changed to DC the contactors controling it are and since bought in UK are 50Hz also before machine starts the load will only be the power to PLC and I am not sure if the inverter will run correctly on such a low load. I would have left all heaters on 60Hz and used inverters one for each motor. But my boss was in favor of all on same inverter so all speeds are the same. The plant makes bubble packs to sell the product in. Seems transport costs to US are too high as now to export in bulk and then pack in Mexico. Hope I get a visit?

Thanks Eric

Commonly available voltages and frequencies in Mexico match those of the US.

Thank you everyone for the replys. I like Vic's reply the best and will probably take that route after I ensure the other components (starters, power supply, etc.) can handle 460 volts. The largest motor on the machine is 2 hp and that one is on a VF drive running at about 40 Hz. The rest are .5 to 1 hp and replacing them if necessary is also an option. Thanks again.

Motors

A lot will depend on the quality of the motors but I can give you the general effect of voltage variation on induction motors.

At 120% voltage;
The starting and running max torque will increase 44%. Synchronous speed will not change, Percent Slip will decrease 30%, Full load speed will increase by 1.5%, Efficiency will increase slightly, Power factor at full load will decrease 5-15 points, Full load current will decrease by 11%, Starting current will increase 25%, Temperature rise at full load will decrease 5-6degC, Maximum overload capacity will increase 44%, and a noticable change in magnetic noise will be present.

Source: Fink & Carroll engineers hand book.

Hope this helps
Roger

Roger

This is correct if you are only changing the voltage. In this case, both the voltage and frequency are being changed so none of these effects apply.

Vic

Frequency

Vic

The changes I posted are still good with Frequency changes, I just didn't include them but they are as follows.

At 95% frequency; Starting and maximum torque is increased 11%, Synchronous speed decreases 5%, Percent slip is practically no change, Full load speed decreases by 5%, Power factor at full load slight decrease, Full load current slight increase, Temperature rise at full load slight increase, Maximum overload capacity increases slightly, and the magnetic noise also increases slightly.

That about covers it, these general effects of voltage and frequency variation are shown on the same chart.

The following formula can be used for determining the induction-motor torque that can be developed when operating at off-standard voltages.

Tr= rated, standard, torque.
Er= rated, standard, voltage.
To= Off-standard torque.
Eo= Off-standard voltage.

To= Tr(Eo/Er)squared

Roger