OT multi motor/VFD startup inrush protection

mordred · May 9, 2008

We have an application where we have several AB powerflex 40 VFD's
which are running 8 fans each all have the same FLA of 1.2, 1hp 575 v. They had installed seperate solid state overloads one per motor we have a line reactor AB cat number1321-3r12-b 2.5 mh, 12 a on the input to the drive
on the output side of the drive we have a TCI drive output filter KLCU12A1 which is 3ph 600 Vmx 12Amx. The drives are configure Volts/hertz linear ramp. We were informed froma Rockwell field technician that senserless vector should not be used on multidrive applications. I'm still detecting large inrush currents on motor startup and am looking for recommendations on how we can offer better protection as we have been experiencing output burnouts on the VFD's. Also the average distance from the drive to motors is 200 feet farthest is 327 ft

rdrast · May 9, 2008

More info required...

Is this a multi-DRIVE application? one motor per drive? If so, Sensorless Vector would work, even though it seems unnecessary on fans.

Since Rockwell is saying no to SV, and suggesting only Volts/Hz, I'm thinking this is a multi-MOTOR on one drive application. If that is the case, you aren't starting/stopping (connecting/disconnecting) motors on an already running drive, are you?

Also, if these are fans, is there any chance that they are being driven by the airstream? That could be bad.

mordred · May 9, 2008

Sorry guess I wasn't clear enough its one VFD per 8 motors and air stream isn't a factor its for a dehumidification system

MartB · May 9, 2008

Are you stop or starting fans individually whilst the drive is in operation?

mordred · May 9, 2008

Unfortunately the designers did not place any provisions in place for the Delta PLC to handle a slective startup so they all start at the same time and they did not supply contact just overloads nor is there any extra outputs available on the PLC

allscott · May 9, 2008

What's the rated output of the drive?

mordred · May 9, 2008

output on the drive is rated for 9.9 amps cat number 22b-E9p9n104 5.5kw/7.5 hp 3ph input 48-63 hz 13.7 amp. I'm wondering if extending the linear ramp will help or going to an s curve ramp style.

rdrast · May 9, 2008

If the drive is properly sized, to handle all 8 motors there shouldn't be a problem, unless it is indeed from the distance to the motors. 200 to 300 feet is a long way.

What is the accel time on the drive set to? (may be too low) Do you have boost set? (may be too high).

And, should be obvious, but all of these fan motors are Three Phase, yes?

Also, that drive is just barely sized for the load you are running. 8HP is 5.9KW. 8*1.2A = 9.6A.

I would have gone up at least one size in an application like this.

milldrone · May 9, 2008

mordred,

Just a shot in the dark. Could the minimum output voltage or the minimum hz be set too high? Or maybee a conflicting skip frequency setting?

mordred · May 9, 2008

accel time is 2 seconds were currently not using boost and yes the motors are 3 phase I will probably extend the duration is ther any way to test for reflective wave? We've already confirmed the min freq and and min output voltage> haven't looked into conflicting skip frequency. And I agree that a lager drive size would be a benefict. We have looked at the charts for reflective wave and added the output filters to compensate with Rockwells assistance in sizing them. And yes we are using XLPE cables

mordred · May 9, 2008

I think what I'm going todo next is get an amp logger hooked up or a power quality meter and take some more recordings perhaps that will reveal more. The scope is showing a clean output signal comparable to other powerflex 40's on single motor apps we have in plant

allscott · May 9, 2008

Your drive is REALLY marginally sized for the load. I agree with RDrast about going up a drive size for an app like this.

Stretching out the Accel time will probably help a little, I wouldn't bother with an S curve though. If you want to log the current I would hook up a meter to an analog out of the drive, you won't get good data if you use an amp clamp on the output.

milldrone · May 9, 2008

mordred,

I just rememberd this thread that DickDV responded to. Obviously this was a much harsher application than yours but still simular.

http://www.plctalk.net/qanda/showthread.php?t=3501

Check his responses concerning the capacity of the drive.

mordred · May 9, 2008

Absoluteley agree with both your accessments and am currently talking with the various powers that be here concerning the drive size thanks for the tip on hooking up the amp meter I had intended that method but its nice to get confirmation. and I'll try extending ramp time see how that works out
rest of it is in the try it till it fails stage. Thanks for the link I am currently perusing it

DickDV · May 9, 2008

The drive is way too small for the motors. The rule for multiple motors on a single drive is 10% extra for every motor over one. That would be the motor FLA times 8 times 1.7 for the drive output amps.

Don't bother to put a current monitor on the drive output leads. Use the drive current output (usually 4-20ma) instead. The signals are so complex and fast on the motor leads that no current monitor will read correctly. I've tried it and am sure that statement is true.

OT multi motor/VFD startup inrush protection

mordred

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