240v supply and 208v motor

[isu19151@verizo]

I have a question about connecting service to a bridgeport cnc mill.I have 120/240 single phase service from power company running through a 5HP phaseamatic rotary converter and want to connect to a 208 volt machine.I see that the transformers can be wired for 230 volts .What else needs attention? Motor? Motor contactors?Motor overload heaters?I thank You for any info that could be provided.I just registered onto this site recently and have been reading and learning from all the threads and responses.

[mxtras]

I am not clear on what you are asking. What's this about a transformer?

Running a 208V motor on 230-244V is not a problem but it sounds like you might also be looking to run a complete machine - including a processor??

Scott

[Al_The_Man]

I don't think you would need to worry going from 208 to 240, It is not quite as drastic as going from 440 to 220, where wiring gauge and O/L heaters would maybe need to be changed.
A 208 device will draw slightly more on 240, but wire & O/L's are usually rated quite a % higher.
Al.

[isu19151@verizo]

Scott and Al the Man thank You for your reply.I am looking to connect the mill to a 230v 3 phase line after it goes through a rotary phase converter.The transformers are in power distribution enclosure and supply power (110-120vac)to all systems except spindle motor.Jim

[awright]

I'm no motor expert, but I'd be very cautious about applying 240 volts to a 208 volt motor. That's a 15% overvoltage and you could run into magnetic saturation of the motor stator laminations. If saturation does occur, you can get very high peak currents.

If the motor manufacturer can be contacted, consider calling them to ask about overvoltage tolerance. Or just ask any motor manufacturer.

At the very least, when you power the motor up on 240 volts, listen carefully for excessive hum and watch for any signs of excessive current. If things sound normal, measure current in each phase, turning the motor off between measurements. If measured currents under load are not more than 15% above rated current, I guess it would be reasonable to proceed cautiously.

It's a little cumbersome, but one available and easy method of reducing the voltage applied to the motor is to use a separate 240 volt to 36 volt transformer to reduce the voltage applied to each winding. The low-voltage winding of the transformer only needs to have a current rating equal to the rated current of the motor under load. The primary is connected across the 240 volt supply to the winding and the secondary is connected in series BUCKING with the motor winding. This reduces the voltage applied to the motor by an amount equal to the secondary voltage of the transformer.

36 volt transformers are pretty rare, but 24 volt transformers are very common on surplus. If it was my machine, I'd feel better reducing the voltage applied to the motor, even by 24 volts.

awright

[gus]

most motors are rated 208/230

it is rare that 240 is over 240, but sometimes it can be, that is the only time I would be concerned

[Al_The_Man]

Originally Posted by awright I'm no motor expert, but I'd be very cautious about applying 240 volts to a 208 volt motor. That's a 15% overvoltage and you could run into magnetic saturation of the motor stator laminations. If saturation does occur, you can get very high peak currents.

I am not sure magnetic saturation would occur and be retained, this is an AC current being dealt not DC, also the common inrush current in an AC motor exceeds 10 times the normal running current every time it is started, decreasing with rpm. This is increased by the shorted turn effect of Armature conductor bars.
Al.

[isu19151@verizo]

Thank You for your response.I have reconfigured the transformers for 230-240v input and now am thinking about adding a 3 phase line monitor as added protection for the 208v motor.According to the description it will balance the phases at 2-4-6-8 or 10 % per a selector switch.I was also thinking that it would keep the DC voltage in the drive and controller circuit within specs if the AC voltage was as it should be then DC voltage would be also.???? Any Thoughts.Jim

[awright]

I'm trying to understand what you are describing regading the 3 phase line monitor. It sounds like you believe that the monitor will influence and control the phase balance. If it is what I have seen as line monitors, I believe that what it will do is give an alarm if the phase balance exceeds the setting you have selected. It will not do anything to modify or control the phase balance.

Still a good idea, but since they are primarily intended to monitor solid three phase lines coming in from the utility, the balance of which may be influenced by the loads placed on the various phases throughout the plant, I fear that the softer and erratic phases supplied by a phase converter may give you alarms fairly regularly to the point that you ignore it or disconnect it. Have you heard from other users of phase converters that phase monitors are useful?

Don't forget that two phases are being supplied directly from the line, but the third is being artificially generated by the idler motor ond the run capacitors. That third phase is pretty squirrilly and subject to fluctuation with loading, depending upon the size and design of the phase converter.

awright

I also do not believe that a phase monitor will do anything to maintain your DC voltage at a desired level.