Continue to Site

Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

  • Congratulations The Obturator on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

Monitoring VFD on ??HP motor

Status
Not open for further replies.

mpparent

Electrical
Sep 26, 2001
399
All,

I have a motor (of which I do not have a nameplate to verify HP rating), which belongs to a process. The process owner wants to purchase a new motor, but with no nameplate, we are unsure of what to purchase (a new motor was purchased at one point and it turned out to be wrong). The motor is driven by an old VFD (not supported), and I have a PQ meter on both the line and load sides of the VFD. I'm not really a VFD expert, but I keep reading that circuit monitors can become confused by taking readings on the output of the drive (because of switching). A colleague said we should monitor the load side, however, since the input into the drive is the real work done by the motor, I would think monitoring the line would be enough to figure out the actual motor HP requirements. Would love to get some feedback.

Thanks,
Mike

Mike
 
Replies continue below

Recommended for you

Typically, you match the motor voltage with the line voltage when using a VFD. So, that should give you the voltage.

As for power, if you measure the power going into the VFD you'll get a pretty good idea of the power required. You will have included the VFD losses which would be add like 5% more compared to the motor only so it's not a big difference. You have to run the motor at rated speed with the highest load to get the worst case power required.

 
Take a few physical measurements and determine the frame size.
You may end up with a motor one size to large but that's better than another unusable motor.
respectfully
 
waross,

The process owner really isn't sure if the existing motor is adequate. Else, we would have certainly done that.



Mike
 
Incidentally,

We were reading unbalanced currents on both the line and load sides of the VFD. That can't be good...

(I'm not sure of the % of unbalance. I'll know more in a couple of weeks...the monitoring period)

Mike
 
Please realize that without some very expensive specialized equipment, readings on the output side of a VFD are not necessarily reliable or accurate.

Doesn't your VFD provide an Output Current display? Those are usually plenty accurate enough for this purpose. Other than that, LionelHutz's suggestion is probably your best option. By the way, if your VFD is so old as to not be capable of that it is probably an analog drive, you may want to consider replacing it as well. VFDs have improved greatly since the days of analog.
 
If you have a make, type and maybe serial number of the VFD then I'm sure between us we should be able to find out something about it.
Regarding your motor, try and find out the shaft diameter and motor speed at rated frequency. Knowing this will give you a rough idea of the motor rating.
 
Status
Not open for further replies.

Part and Inventory Search

Sponsor