208 Volt Supply Running a 240 Volt Inverter

J

Thread Starter

jeremy

We have a single phase 240v unit with an inverter powering an electric motor. However this is going to the U S where the voltage is 208v. Will this still run or will the voltage be to low?
 
Without knowing the inverter spec's and characteristics, hard to say from here. The inverter will likely tolerate a lower voltage. How much?
 
B

Bob Peterson

By inverter I am going to presume that you mean variable frequency drive. A variable frequency drive that expects 240 volts will probably still work at 208 volts however the maximum output voltage will be 208 volts instead of 240.

If you are running in volts per Hertz mode and that is the most common way to run a vfd you probably won't be able to go at whatever the maximum speed is for the motor you have.

Your best bet might be to just replace the motor with a us-style 240 volt 3-phase motor which usually is able to operate at full load at 208 volts 3 phase. It's also hard to know whether you're intending to run the thing it 50 or 60 hertz as its maximum speed.
 
>Your best bet might be to just replace the motor with a
>us-style 240 volt 3-phase motor which usually is able to
>operate at full load at 208 volts 3 phase. It's also hard to
>know whether you're intending to run the thing it 50 or 60
>hertz as its maximum speed.

In case you did not understand this reference, here in North America, most small motors (5HP and below) built for 230V are also designed to operate at 208V input, because that is the predominant 3 phase distribution system at that voltage class. The motors have a little more iron in the frame to absorb the added heat from the higher current draw at 208V. But IEC motors designed for 230V 50Hz are not designed with this in mind, hence Bob's suggestion of changing the motor out with a NEMA designed 230V motor. Motors designed like this will have it shown on the nameplate, saying "208/230V" or showing two different FLC ratings, one at 230V, one at 208V. You would need to make sure.

If however your machine is designed around 230V at <b>50</b>Hz, and the VFD is only going to allow 50Hz operation, any NEMA motor will be fine even if not labeled as 208/230V. That's because your drive will only be able to put out 208V at 50Hz, but the NEMA motor is designed for 230V <b>60</b>Hz, so turning the motor speed down to 50Hz means the voltage needs to be at 191V, so you would have plenty.

Your only issue will be if you are using an IEC motor designed for 230V 50Hz and you can only give it 208V 50Hz. The motor will have about 90% of rated torque and therefore pull more current than it is designed for if the load requires full torque. If your load does NOT require more than 90% of full motor torque anyway, you may never know the difference when running at a steady speed. But maybe more importantly, that motor will only be able to provide about 80% of PEAK torque, which is used to re-accelerate a motor after a step change in load. That means it will be significantly easier to stall the motor.
 
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