OT - Motor at 100Hz question

Hi
Before I start, I did not specify or supply this....

I have a inverter rated motor (50Hz = 100% speed). The motor drives an agitator, and the motor is fitted with a thermistor.Due to the 'hygenic' requirments there is no cooling fan on the motor.

Apparently the user has to run the drive at 100Hz (200% speed) to reach the desired agitator RPM and the 'drive is tripping out'

I am going to visit this site next week to take a look at it.

My immediate thinking is that the motor is incorrectly specified, or needs a gearbox (or different gearbox). The motor must be running hot and the thermistor is tripping.

I woould never exceed about 150% on inverter applications as a rule of thumb so am after general opinions really

In the area above nominal speed, the vfd would theoretically have to increase the voltage to keep the torque constant (u/f should be a constant), but since that is normally not possible, the torque will drop exponentially above 50 Hz. At 100 Hz the available torque will be 50%.

In other words, both motor and VFD must be overdimensioned to run above nominal speed. Everything should be OK, if the person who did the design was aware of the above.
But if someone just took a standard motor, and a VFD suitable to run it at nominal speed and load, then it just wont happen.

Hi Jesper
I agree with everything you said. Also it gets worse... it is a 240V supply inverter, with I believe a 0.55kw 415V 3 phase motor on it.

Looks like a specification issue

Another thing to watch here is the motor rated speed.
If the motor speed is 1450 RPM then 200% speed is 2900 which would be ok.
If the motor speed is 2800 RPM then 200% would be 5600 RPM which is dangerous.

cjd1965 said:

... it is a 240V supply inverter, with I believe a 0.55kw 415V 3 phase motor on it.

Click to expand...

In not sure if the 240V supply is a problem or not. Modern single-phase-supply VFDs has some wizardry inside to boost the voltage above the input voltage. I think they stage two DC rectifiers on top of each other.
Anyway, the VFD specs will say clealy what output voltage it can generate.

http://catalogo.weg.com.br/files/we...nical-guide-028-technical-article-english.pdf

I had read this PDF about 10 years ago
it is well worth reading
specifically the insulation resistance to higher voltages.

It is possible that the motor is nameplated for 100hz operation with a base speed of 50hz. Some inverter duty motors are capable of this. In that case, the torque at 100hz would be half the torque at 50hz and the kw would be the same over the range from 50 to 100hz.

Smaller motors not nameplated for 100hz overspeed are still capable of 100hz operation usually but the torque will drop off faster above 150% speed so, at double speed, there will be less than one-half the base-speed torque.

Above base speed, assuming full rated voltage, the amps will hold to FLA as the overspeed hp holds at rated hp. So, at FLA, the torque goes down as the inverse of the overspeed.

In the application described, I would expect that the user is expecting constant torque all the way up to 100hz and, as a result, the drive is being taken to overcurrent or, at least, to motor overload. Those are the faults I would expect.

Note that all of this assumes full voltage on the motor. If the drive cannot supply motor FLA at full voltage up to 100hz, then the problems are worse.