Wrong pole number parameter in inverter setup

[panou]

Hello to all.
I want to ask what would be the result of setting the motor nominal rpm to 1000 instead of 1500 rpm in an inverter. This happened to my site and i noticed a great amount of armonics from this inverter. Can this be the reason ?

 

[DickDV]

Generally, when you enter motor base speed into an inverter, that becomes the frequency point where the output voltage reaches the maximum level. That, in turn, alters the V/Hz curve and the output torque capability of the motor.

Some motors are designed a little "voltage starved" which permits a little adjustment of base speed to improve torque output but this seems extreme. I would think that the motor is saturating before it reaches full speed and that reduces available torque, of course.

Before just changing the parameter, I would investigate the motor characteristics and try to discover why such an unconventional software configuration was chosen.

 

[ScottyUK]

For 50Hz supply, 1000rpm is base speed for a 6-pole machine and 1500rpm is base speed for a 4-pole machine. In both instances the base speed is reached when the output frequency is 50Hz. If the drive has no means of measuring speed - i.e. not a closed loop system - it calculates motor speed based on the motor parameters that you enter and those it 'learns'.

I'm not at all convinced that the drive will overflux the motor unless you manage to enter a conflicting pair of parameters - e.g. base speed 1000rpm while also entering that the motor is a 4-pole machine. This would give the situation DickDV indicates. Otherwise if the drive is in an open loop mode I don't think there will be any negative effect other than the indicated speed which the drive implies from its internal motor model will be incorrect.

Where did you measure harmonics? On the output or on the input? The input of a 'standard' 6-pulse rectifier is laden with harmonics. If on the output, vector drives do not produce pure sinewaves - the output waveform is modified to get the best performance from the motor.



----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[DickDV]

ScottyUK, if as you mention, the drive has separate parameters for motor poles and base speed, then likely no harm has been done except, as you mention, the speed readout would be erroneous.

However, few drives in my experience have these separate parameters. Instead, you have only nameplate speed and the drive calculates the poles from that. In that case, the motor in question would reach full line voltage at 1000rpm instead of 1500rpm and saturation, torque loss, and overheating are likely.

 

[ScottyUK]

Hi Dick,

Thanks for the reply. I agree, it would be unusual for a drive to have separate parameters for pole number and speed. But since a 6-pole machine rotates at 1000rpm when fed by a 50Hz source, and a 4-pole machine rotates at 1500rpm with a 50Hz source, how does the drive distinguish? As far as I can see it will reach full voltage at 50Hz regardless of which motor is connected, unless a different parameter has been set to produce full voltage at a lower frequency, 33.33Hz for example to achieve 1000rpm with a 4-pole machine. This parameter is rather different to what the OP was describing. If the drive produces full voltage at 50Hz then both the 4 and 6-pole machines will be ok. I'm not sure how the drive knows anything about actual speed without an encoder- it only sees frequency and implies the speed based on the information entered. I know you know all this: I'm recording my thought process because I'm not sure what you are getting at.

I'm just wondering: are you considering an application where the demand input is scaled directly in RPM? So a 1000 rpm demand would equate to a 50Hz output at full voltage if the drive thought it was connected to a 6-pole machine, but only 33.33Hz output at 66.67% voltage if it thought it was connected to a 4-pole machine? I think I understand what you are saying now - we were probably looking at it from very different perspectives!


----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[DickDV]

No, Scotty, I was just plain making a mistake. You are correct.

I spend so much time around precision (sensorless and flux vector) drives these days, I've gotten out of the habit of thinking in Hz.

Again, thanks for fixing me up on this.

 

[jraef]

Before the widespread use of space vector algorithms in VFD setups, it was the case that the only thing it would affect would be the display. But now some drives use that information to "tweak" the motor model used in the vector control calculations, so it may have an effect on the output. It will depend on the drive design.

"Venditori de oleum-vipera non vigere excordis populi"

 

[aolalde]

Hi panou

As I understand all VFD’s will develop “a great amount of harmonic and non-harmonic frequencies“ even though the set up is correct.
What is the percentage and order of the frequencies you found?