Supply voltage undersized

[johnnysparks24]

We have a new machine that requires 240v ac @30 amps. Our Electrical contractor hooked it up to 208v ac @30amps, as we don't have 240 in that area of the plant.

Will this have an effect on the motor RPM? The machine runs ok however it seems slow.

Thanks John

 

[DickDV]

The motor will try to run at the same speed since speed is determined by frequency but it will not be able to develop quite as much torque. If your machine requires all of the torque the motor produces at 230V, the motor may well draw excess current and overheat when run on 208V

You should check motor amps at the heaviest load condition on your machine. If they are within motor nameplate specs, you'll be ok.

 

[Sheldn]

DickDV is correct regarding frequency and amp draw. If you have a system that has motor nameplates with an RPM of 1750 at 60 Hz it will run at 1458 RPM at 50 Hz. Also the motors will draw more amps at a lower voltage.

For the motors and power devices make sure each nameplate allows for the lower voltage and associated amperage. This is usually the case for motors which will have this on the nameplate along with Hp, Volts (208-240/480). This may result in premature failures. In many cases you will not see the failure for months or years and will not realize that the two are related.

You should adjust the fuses for each motor and the heater elements or overload setting and or the motor protective device (European style) to match the nameplate ratings to assure proper motor protection.

 

[shaan]

hi johny... voltage has nothing to do with the rpm of the motor.
but the torque produced by the motor will reduce because T is proportional to the square of the voltage.

 

[johnnysparks24]

Given this instence how much torque are we looseing by only running 208, instead of 240v?

John

 

[TConnolly]

Its not something that I would worry about. 208 is every bit as common as 240, and there are thousands and thousands of motors running on it all over North America. The motor draws slightly more current on 208 that on 240. However 208 is so common that all most all motors are manufactured with that in mind. The extra current that the motor draws at the lower voltage will assure that the motor achieves the nameplate rated horsepower.

 

[Jim Dungar]

I would ask for a buck-boost transformer to be installed to raise the voltage from 208 to 230V minimum.

Every situation is a little different but in general a lightly loaded motor will not care if it is at 208 but a heavily loaded one will. A motor controlled by a speed drive won't care either but an old servo might.

Electrical equipment is usually designed to operate at -10% voltage. 208V is -13.77% of 240V and -9.56% of 230V (normal rating for motors), which is definitely marginal. Now what if you have any voltage drop in your wiring or if the 208V is really 198V (the lowest most utilities will supply)?

 

[elevmike]

I would ask for a buck-boost transformer to be installed to raise the voltage from 208 to 230V minimum.

Every situation is a little different but in general a lightly loaded motor will not care if it is at 208 but a heavily loaded one will. A motor controlled by a speed drive won't care either but an old servo might.

Electrical equipment is usually designed to operate at -10% voltage. 208V is -13.77% of 240V and -9.56% of 230V (normal rating for motors), which is definitely marginal. Now what if you have any voltage drop in your wiring or if the 208V is really 198V (the lowest most utilities will supply)?

All this is true.

Either get the transformer or get a new motor with the proper nameplate voltage to suit the power supply. The motor is running outside of what would be considered the acceptable range.

If you have some type of control panel associated with this project, you also want to check that. We often use control transformers with mutible primary taps. (208, 220, 230, & 240).

 

[Vic]

Most motors will run fine on 208 Volts

As Alaric said

However 208 is so common that all most all motors are manufactured with that in mind.

Most motors are designed to run satisfactorily on either 208 or 230 volts. If this is the case, the motor nameplate will show 208/230 volts and xx/xx amps for the current. The higher value will be the 208 volt current. The motor will produce its rated torque but draw a little more current than if it was connected to 230 volts. Be sure the overload is set for the higher current and everything will be ok.

 

[Lancie1]

You could do as I usually do in this situation: Go to the transformer supplying the 208 volts and kill the power, remove the cover, loosen the wires and move them up to the next (or second) higher voltage tap. Adjust so that you get 218 volts instead of 208. Then quit worrying about the motor.

 

[elevmike]

You could do as I usually do in this situation: Go to the transformer supplying the 208 volts and kill the power, remove the cover, loosen the wires and move them up to the next (or second) higher voltage tap. Adjust so that you get 218 volts instead of 208. Then quit worrying about the motor.

But then there would be nothing to talk about...

 

[Jim Dungar]

Transformer taps are designed to compensate for low incoming voltage NOT to raise the output voltage above normal. The incorrect use of taps may lead to early failure of the trandformer due to overheating caused by the improper flux density in the core.

Most reconnectable 230/460V motors are not rated for 208V. Although there is plenty of anecdotal evidence that they may work, why risk it if your livelyhood depends on this machine?

 

[keithkyll]

A transformer can be used to raise or lower the voltage. The KVA rating is the important part.

Major motor manufacturers don't want to build several different models for each country. It's more cost effective to build them universal. 50/60 Hz, low/high voltage. 200V for Japan, 208/240 for US, 220 for Europe, etc. The only unique voltage is 380, but some are built for this too.

A closer look at a rating of 220V most likely will reveal a +/- 10% spec. This makes it 200-240V. When it's rated for 230V, the spec is +5/-10%.The only motors I've seen that were critical about voltage is fractional horsepower types, because low cost is a factor in these.

 

[Tinker]

Well every time I get into this topic it seems to be one that is left unresolved. Everything is made to work at a preset value. The fact that there are +/- tolerances only states that the device will work at the fudge factor but at reduced life expectancy. As well the tolerance is there for line drupe and spikes not long time runs. The same manufacture that tells you it will work at the outer edges of the tolerances will be the same one that tosses the warranty out the window because you used it that way.

 

[Lancie1]

Jim,
You said:

Transformer taps are designed to compensate for low incoming voltage NOT to raise the output voltage above normal.

Not very logical! Why do you think the windings in the transformer CARE where they are tapped? It is all the same: For a certain turns ratio of Primary to Secondary windings, you get a certain voltage output. It doesn't matter if you are changing the taps to compensate for a low input voltage, OR the input voltage is normal, and you merely want to raise the output voltage to a higher level. The results are the same. If the transformer will work (has proper flux) for the first, undervoltage condition, then it has to work for the second, overvoltage condition.

(PS: You wouldn't be in the business of selling transformers, by any chance?)