Motor stalling. 3

[itsmoked]

I'm going to check out a VFD that is turning a grinding motor today. The customer is telling me, "Yeah, it comes to a complete stop sometimes".

I asked what alarms are being thrown.
None.

I told him that can happen with a VFD in V/Hz mode since the VFD has no idea what the motor is doing. I told him to switch it to 'vector mode' where it should've been all along.

He called me back to tell me, "It is in vector mode!".

Turns out he's never 'tuned it' but I'm still scratching my head about this. I can not imagine how this can happen. Can anyone give me rational for a vector driven motor to stop without alarms. I presume the drive would have to be reading the off winding in realtime and would choke instantly on uncommanded stalled rotation.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[itsmoked]

The motor name plates on ~20 "identical" motors are the most confusing poorly labeled bunch I've ever seen.

They thought they'd bought all the same units yet most are 4A FLA and one is 2A FLA and one is 1A FLA.

Here's an example of the plates:

Why two lines identical??
Some have the first line the same and the last line states:
230V 120Hz 7200 RPM

Others state the same first line and the last line states:
1.5HP 460V 60Hz 2A

Head scratching.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

Isn't that a synchronous motor? 3600 RPM @ 60 Hz points to that. Then the VFD may be special and not the standard plain vanilla. Lots can happen then. If feed has stiction and doesn't move for a few seconds - then you get a lot of sudden feed and that usually means problems like uncogging and such. Strange that you didn't get an alarm...

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[itsmoked]

Geez. Good point Gunnar!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

Spindle motors like this are now often IPM (Interior Permanent Magnet) or SPM (Surface Permanent Magnet) motors, collectively referred to as PMAC which because they don't need induction to the rotor, are in fact synchronous. There is evidence on Setco's website indicating that they use Fanuc BiS-B Series motors which are offered in both of these types, on their motorized spindles for CNC machines. If not the Fanuc, they likely used something similar.

They (PMAC motors) cannot be used without VFDs, but at the same time, not all VFDs can run them, it requires a highly complex control algorithm and often times a specific motor must have a specific drive. If the drives you are looking at were the original drives used by the OEM, chances are they knew what they were doing and used a VFD capable of powering their PMAC motors. If not, and someone swapped out that drive not knowing this, you could be in trouble. Given some of the things you meantioned about mounting etc., I'd suspect these are not the original drives. If you post the make and model of the VFD I can probably tell you, I've had to investigate who is and is not capable of running PMAC motors for some of my customers.

The reason for the different nameplates is because they will often do what I referred to above as the "double voltage trick". When you need to run at speeds above the motor base speed, what you do is to use a motor rated for 230/460V, strap it for 230V, but use a 480V VFD fed from 480V and PROGRAM the VFD to put out 60Hz at 230V. That way when you increase the frequency above the base motor speed, in this case 60Hz, you can keep increasing the voltage with it, maintaining the V/Hz ratio that the motor was designed for so that are not losing torque. So you can get full torque at 2x the base speed. In YOUR case because it sounds as though you don't really NEED all of the available torque anyway, they are likely not doing that trick, they are knowingly weakening the motor at 80Hz, but it's irrelevant because you don't even need that much torque anyway. So they can use any motor off their shelf for this, hence some are 230 only, some are 230/460, some are marked as 230V 120Hz, it wouldn't matter so long as the VFD is programmed correctly for each motor. That may be an issue as well, for instance if they pulled one VFD off of one machine and put it on another one that had a different motor.

And no matter what, EACH motor and VFD must be specifically tuned to each other. There is no fudging on PMAC motors.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[waross]

I have heard of the double voltage and frequency trick being used for skid mounted air compressors.
A skid would be designed for 100 HP at 3500 RPM.
Then they would use a 50 HP, 1750 RPM motor rated at 230 Volts with a 480 supply and a VFD to run up to 120 Hz and 3500 RPM.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[edison123]

Thanks Jeff.

Muthu

http://www.edison.co.in

 

[LionelHutz]

Your second example is likely a motor run into field weakening, which could cause stalling. It could also be a dual voltage motor 115V/230V that is connected for 115V and then run on a VFD that produces a V/Hz pattern that reaches 230V @ 120Hz. What was the HP on the second line?

Your third example is likely just a dual voltage motor.

After seeing that, you should investigate further to figure out exactly what is there for both the motor and VFD in the applications that are giving issues.


Typically, the VFD is programmed for 460V and 120Hz to maintain the V/Hz ratio up to 120Hz when running a 230V motor. This will double the motor HP at 120Hz. These settings usually set the knee point where the voltage quits rising with frequency, so setting 230V at 60 Hz would mean the voltage stops rising above 60Hz.

I've also seen a VFD programmed to 575V @ 75Hz used to extend the full torque speed range of a 460V motor.