50/60Hz Motor Nameplate Question

[fifty60]

Been a big fan of Dave Jones for years now.  Anyway, new to the forum.  Here is my question:  I know that a nameplate that reads 208-230V and 60Hz means that the motor is optimal at 230V, with degraded performance at 208V.  Just because the motor has 208-230V on the nameplate, does not mean that it is good for 10% below 208V.  The 10% above and below rule would apply to the 230V portion of the nameplate, and not the 208V portion.  If the nameplate read 208/230 instead of 208-230, then it is an entirely different ballgame.  That being said, what if the name plate read 208V-230V and 50/60Hz?  Is the optimal motor voltage 230V for 50 and 60Hz?  If it were just a 60Hz motor, and I were derating it for 60Hz, then I would think the 208V would be the optimal voltage and it should maintain good performance in a 10% band around 208V.  To sum up my question, does the voltage range on a nameplate that reads 208V-230V for a 50/60Hz motor mean that the optimal voltage for 50 and 60Hz operation is 208V (degraded performance) 230V (optimal performance.)  Thanks for taking the time to answer this question.

[tom66]

I don't know much about motors, but I've had 230V motors spin on less than 5V. Not very fast, and sometimes requiring a kick start, but they do work. I would doubt the motor will fall off suddenly below 208V, it will just run slower and/or may have lower output power.

[Pentium100]

I don't know much about motors, but I've had 230V motors spin on less than 5V. Not very fast, and sometimes requiring a kick start, but they do work. I would doubt the motor will fall off suddenly below 208V, it will just run slower and/or may have lower output power.

When I asked a similar question on stack exchange (can I use lower voltage to make an AC fan spin slower) I got an answer that while the motor would spin slower because of a lower voltage, the current may be higher and could burn the motor, so the only proper way to control the speed is to use variable frequency controller.

[G7PSK]

My understanding of motor voltage ratings as stated on the plate is that the upper and lower ratings should not be exceeded. So a 208-230 is the operating voltage range and the motor should not be run at 200 volts or 250 volts.

[sacherjj]

I don't know much about motors, but I've had 230V motors spin on less than 5V. Not very fast, and sometimes requiring a kick start, but they do work. I would doubt the motor will fall off suddenly below 208V, it will just run slower and/or may have lower output power.

It would not run slower, as almost all AC motors are synchronous with a multiple of the frequency of input AC.  Varying voltage doesn't change the frequency.  It would have lower output power.  This could only cause it to run slower, only as much as the motor lags to produce the torque required with the lower voltage.  If you are getting much slower, you are running the motor in a very bad torque area and wearing the motor out.

[Gall]

The range between 208V and 230V is the range of voltages that cannot harm the motor. Higher voltage will cause overheating. Lower voltage may cause overheating or may be harmless, but the motor is not guaranteed to work. The overheating is caused by prolonged use at incorrect voltage, so it is virtually impossible to damage the motor if it has proper thermal protection. (It may help even at huge overloads - I once connected a 127V motor to 220V mains by mistake with no damage.)

It is usually safe to run the motor at low voltage with no load or very little load, i.e. for short-time testing only. Better do not try this with a loaded motor.

[Kremmen]

I don't know much about motors, but I've had 230V motors spin on less than 5V. Not very fast, and sometimes requiring a kick start, but they do work. I would doubt the motor will fall off suddenly below 208V, it will just run slower and/or may have lower output power.

It would not run slower, as almost all AC motors are synchronous with a multiple of the frequency of input AC.  Varying voltage doesn't change the frequency.  It would have lower output power.  This could only cause it to run slower, only as much as the motor lags to produce the torque required with the lower voltage.  If you are getting much slower, you are running the motor in a very bad torque area and wearing the motor out.

If we are talking about induction motors, as we almost certainly are, then a couple of minor points: 1) Induction motors are not in fact synchronous and in fact you say so too ("lags to produce the torque..."). The magnetic field in the motor stator has a synchronous rotation speed that the rotor never quite achieves but comes close under no load. In fact induction motors are called _a_synchronous motors in many contexts. 2) The way you express voltage vs torque is a bit confusing. I'm sure you mean to say that lower voltage decreases motor torque thus decreasing speed and that is right. Whether that will harm the motor is entirely dependent on the actual operating point, i.e. the stator current vs. rotation speed (and consequently, cooling). If you don't exceed the nominal current and the cooling is sufficient to dissipate the generated heat, you are OK.

[AlfBaz]

I agree with Kremmen.
Synchronous motors usually have a rotating magnetic field in the stater to which the rotor locks onto due to dc applied to rotor fields. This way the rotor speed is synchronised with the rotating magnetic field.

As for the torque its proportional to slip, which is great, as you load the motor up mechanically the rotor slip increases and therefore it's torque increases. This happens up to a point known as the breakdown torque. At this point torque is now inversely proportional to slip.

Torque is also proportional to voltage, so if you drop the voltage its torque will drop also but for the voltages mentioned you would have to have the motor loaded up substantially to see any perceivable speed change.

As for the op's question I wouldn't be surprised that if you did some quick calcs you would find the currents at 203V 50Hz would be very nearly the same at 230V 60Hz which would give you similar power ratings in both these scenarios.

To double check your assumptions based on the 203-230V 203/230V idea how many wires are coming out of the motor, is it 3phase on single, are there any connection diagrams showing to ways to hook it up (in case it is a dual voltage job) and what does the power rating say on the name plate. Does it have the duty cycle written on there and if it is, is there one value or two?