VFD Increase Motor Rated Current results in Lower RPM?

[BK21]

Hoping someone could help me understand this situation I'm having.

We have a MOVITRAC LT VFD connected to a SEW Eurodrive DFT71D2 induction motor.

Originally, the Motor Rated Current was incorrectly set on the VFD as 1.8A. At this time, we were getting a measured RPM of 400RPM @ 25Hz.
It was then noticed that the Motor Rated Current should be set on the VFD to 3.05A. Once we changed the setting we got a measured RPM of 276RPM @ 25Hz.

Can anyone please help me understand why increasing the Motor Rated Current results in the RPM decreasing for the same frequency setting. I thought that the Motor Rated Current is only for ensuring safe operation of the motor. I.e. Motor will trip at a higher rated current if set to 3.05A.

(Note: Measured RPM is from after the gearbox).

Thank you!

 

[itsmoked]

The motor rated current is used for many things, heck, almost everything! In your case I believe you were snared by the excitation current. The drive calculates the amount of current needed to magnetize the rotor. By grossly understating the the motor rating the drive grossly under-excited the motor. This actually can cause a motor that's not fully loaded to speed up considerably. I believe this is what's happened.

BTW: That's called "field weakening".

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Field weakening is a DC motor effect, Keith.
BK21; What is your synchronous RPM at 25 Hz?
An induction motor will approach synchronous speed.
Is your load overhauling and driving the motor?
That may cause greater speed with less current.


Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[itsmoked]

Yes, it is a DC motor effect, as I described it is also an AC motor effect.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

I think it's backwards.
A weak excitation field will allow an induction motor to slow down more under load, then of course, when the load is removed there will be a greater speed increase.
But.
The motor speed will not exceed the synchronous speed.
A DC motor has a speed that roughly corresponds to synchronous speed;
That is the speed where the motor is slightly over-driven so that the motor is neither motoring nor generating, similar to an induction motor at synchronous speed.
Weakening the field in an induction motor does not speed it up, nor raise the synchronous speed but allows more slip RPM at a given load.
Weakening the field in a DC motor raises the base speed.


Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[itsmoked]

Ah ok. Noted. Thanks.
I see VFD overspeeding say setting to speed of a 230V60Hz motor/drive to an output speed of 70Hz. Frequently called "field weakening" but by virtue of violating the motor's V/Hz rating.



In the op's case they've told the VFD the motor is smaller than it is. So the VFD is going to reduce the its output to supply lower current since it expects a lower excitation requirement. The only way to send a lower current is to send a lower voltage. It would seem to me that the motor would see that lower voltage as a lower V/Hz because the same Hz is still showing up.


Of course there are also torque-boost, compensation, and PID functions that might be involved with the current limit. And what mode the VFD is in... Perhaps FOC or vector could be a mind-twister about what mis-setting the current limit would result in.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Thank you for the information Keith.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[LionelHutz]

BK21 - If you want an answer it would help if you could post the rated rpm and frequency of the motor and the gearbox ratio. For example, the motor might be 1450rpm @ 50Hz.

You need to ratio the motor speed for the applied frequency and apply the gearbox ratio to see which output speed makes sense.

The motor rated current is used for many things, heck, almost everything! In your case I believe you were snared by the excitation current. The drive calculates the amount of current needed to magnetize the rotor. By grossly understating the the motor rating the drive grossly under-excited the motor. This actually can cause a motor that's not fully loaded to speed up considerably. I believe this is what's happened.

No, an AC induction motor will not speed up when using a VFD and maintaining the same output frequency simply by lowering the V/Hz ratio. The only time it could happen is if the motor is being over-driven and no longer creates enough torque to hold back the load. You can sometimes run to a higher frequency and speed by not maintaining the V/Hz ratio, but that requires increasing the frequency as well, not just applying a lower V/Hz ratio to the motor.