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Has anyone actually run a 208V motor on 230v?

bill1608

Plastic
Joined
Apr 17, 2008
Location
Central Illinois
I know that you shouldn't theoretically run a 208V motor on 230V, but I would like to hear from someone who has actually tried it. My 2 applications are not full hp continuous duty and I can put a temperature sensor on the motor to monitor the temperature. My motors are 3 phase and will be running off a rotary converter. One of my 208V motors is on a wood shaper and the other is on a metal cutting vertical band saw. If I burn up one of these motors, I just end up replacing it anyway, so not much to lose by trying.
 
Right, wrong or indifferent, its a common thing. As a commercial/industrial electrician I often run into 240 volt grounded delta, secondary in older commercial mills and other buildings.
 
I have run 200V motors off of 240 V . They started to run very hot. I shut them down before I burned up a good motor. Bob
 
It will run, but how hot it may get are depend on many factors, specific to each situation.

Design of the 208V motor should be rated for +10% in voltage which puts you at 229V, but if your RPC is running at 245V out, your a little further than the range +10% Overheat could be possible, coupled with extra heat from not perfectly balanced voltages from the RPC. Lightly loaded would reduce the risk.

Then there's the issue of the overload relay setting. For a 208V setting it could be too large for 245V line current to properly protect the motor.

If it were critical or expensive to replace / repair, you could use a buck boost transformer to reduce the voltage below 230V to get within the +10% rating.

A 16V buck transformer would reduce 240V to 224V. Right in the sweet spot between the 208V equipment and the 240V rated equipment. Then there shouldn't be excessive heating with either type equipment being used within the +/- 10% envelope.

SAF Ω
 
Two more jokers in that deck....

1) The motor "service factor". If larger than 1.0, that's how much more you can apply in percent voltage.... i.e. SF 1.1 means an added 10%. Lots of motors have SF of 1.0. You can;t do much with that.

2) Yes, the 10% gets you some added range of voltage. BUT, there is also 10% on the supplied mains voltage. So, a 240V nominal mains circuit would be within spec at just below 264V. That's a long way from 208V, and considerably less happy-making for the motor.

Obviously if you have already "used up" your motor's + 10% tolerance just getting to the nominal mains voltage (with or without buck transformers), you have nothing left for a "high line" condition. So if the line does go high and stay there, you may have some overheating.
 
Most modern small motors (5HP and under) are now built as "208-230/460V" meaning they are designed around a compromise in terms of the lower voltage, making them essentially equivalent to a 220V +15, -10% giving them a range of 198 to 257V. If your motor is SPECIFICALLY labeled as 208V only, then it is not like this. But typically a motor DESIGNED to only run on 208V would be MARKED as "200V", what's called the "Utilization Voltage". So if it actually says 208V on the nameplate, that would be unusual.

When you give a motor more voltage than it is designed for, you over saturate it and it runs hotter, but produces only a little more work for the effort. That tends to make them short lived. Here is a chart of the effects of voltage variation on AC induction motors.

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Take note that at +10% of rated voltage, not only do the amps go up and the deficiency goes down, but the STARTING torque jumps a LOT. Torque varies by the square of the voltage difference, so at 110% voltage, your starting torque becomes 121% of normal. Normal starting torque is already 160% of running torque, so now it will be almost 200% torque on start-up. That can cause mechanical issues.
 
Two more jokers in that deck....

1) The motor "service factor". If larger than 1.0, that's how much more you can apply in percent voltage.... i.e. SF 1.1 means an added 10%. Lots of motors have SF of 1.0. You can;t do much with that.

2) Yes, the 10% gets you some added range of voltage. BUT, there is also 10% on the supplied mains voltage. So, a 240V nominal mains circuit would be within spec at just below 264V. That's a long way from 208V, and considerably less happy-making for the motor.

Obviously if you have already "used up" your motor's + 10% tolerance just getting to the nominal mains voltage (with or without buck transformers), you have nothing left for a "high line" condition. So if the line does go high and stay there, you may have some overheating.

That's not quite so.. Here's the actual definition:
Service factor — Service factor (SF) is an indication of how much overload a motor can withstand when operating
normally within the correct voltage tolerances. For example, the standard SF for open drip-proof (ODP) motors is 1.15. This
means that a 10-hp motor with a 1.15 SF could provide 11.5 hp when required for short-term use. Some fractional horsepower
motors have higher service factors, such as 1.25, 1.35, and even 1.50. In general, it's not a good practice to size motors to
operate continuously above rated load in the service factor area. Motors may not provide adequate starting and pull-out
torques, and incorrect starter/overload sizing is possible.
This is from the Electrical Construction and Maintenance website.
 
Thread: Running motor with 208 V on 240 V feed

Looks like I should just try it according to this thread. I can put a thermocouple with digital readout on the motor housing or in the motor cooling air airstream to monitor temp.
 
That's not quite so.. Here's the actual definition:
Service factor — Service factor (SF) is an indication of how much overload a motor can withstand when operating

Yes, however, overvoltges are typically also short term, so there is a reasonable match. Depends if the overvoltages are really short term. And short term can be quite a while with a device that has a thermal time constant of 30 to 45 minutes.

No, you do not really want to operate them long term "nominal" at a condition requiring the SF. Then you don't have any more range for an overvoltage, to mention the least of the problems.
 








 
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