Why must a motor be inverter duty for vari-speed applications? 5

[Lovison]

Is there a rule of thumb for just how low a motor
speed can be reduced before the motor itself stalls?

If you have a motor speed of 1775 RPM and you want to
slow it down - let's 50% what comes into play within
the motor? Does it pull more or less amps? Just what
takes place when the voltage is reduced? I suspect
overheating would take place because the fan is not
cooling the outer shell adequately? Can you still
maintain phase balance at lower speed ranges or does
it drift? Wayne E. Lovison
service-parts@naglepumps.com

 

[kassad]

In my experience you can use standard motors with inverters. But you have to be carefull to select a higher rating than your load. eg with a load of 10 HP you can use a 15HP motor. But this only works if the speed variation is not greater than 50%.
Using a standard motor with inverters will cause overheating due to low cooling but that is compensated if you use a motor of higher rating which will have a larger surface area for cooling. But another problem arises is of the heating caused due to the 5th harmonic produced by the inverters. But new IGBT based inverters produce less 5th harmonic than their non IGBT based inverters. So its a question if whether you are trying to buy a new motor. If the answer is yes than go for the inverter duty motor. If you have the motor already installed try to decrease the load if you can.

kassad

 

[electricuwe]

To kassad:

5th harmonic on the output side has alrady been elimiated with sine-modulated PWM-inverters using bipolar power transistors, Mosfets, force commutated thyristors or GTOs before the IGBT appeared on the market. I do not the exact date, but I guess the first inverters using this modulation technique was built nearly 25 years ago.

But, thats true, the IGBT was the power semiconductor which made VFDs to a product widely used in industry.

 

[kassad]

To electricuwe
Thanx for the tip. I was measuring it in terms of industrial acceptance.

kassad