View Full Version : Difference between inverters

March 11th, 2005, 07:26 AM
Dear experts,

Sorry for disturbing you all in the first place. Hope you all don't mind to answer me a question... :)

Well, may I know is it bigger capacity drive Omron inverter cannot be functioning or control by plc when turning on a motor with smaller load? I got read a statement in that manual mentioning that, need to use a smaller size of capacity to drive the motor with 'about the same' load. I change to a smaller capacity of inverter and it works. Is it correct by the way or I missed out something? If is it correct, then any expert don't mind to provide some valuable explanation for me? Is it some other brand of inverter also giving the same problem like this?

Thank you to those who willing to spend time to read this thread... (",")


March 11th, 2005, 07:32 AM
Firstly, (/sigh) a PLC has nothing to do with an Inverter (VFD, AC DRIVE, SERVO DRIVE, VECTOR DRIVE, et al) other than telling it to "GO", "STOP", "REVERSE", "RUN AT x SPEED/TORQUE".

Now, second question, in general, an Inverter (VFD, AC DRIVE, SERVO DRIVE, VECTOR DRIVE, et al) should be close in size to the total rated load of the motor(s) it is connected to.

Yes, you can oversize a drive, but there are limits. You can probably run a 10KW motor on a 25 or 30KW drive, but trying to run a 10KW motor on a 100KW drive will (almost always) cause problems. The simplest reason? Drives need to control and monitor the current delivered to a motor. The feedback devices for current in drives are optimized to a certain range of loads, generally 30% to 150% of the rated drives capacity.

Running too small a motor can cause problems with stability in the current/torque loops, as well and potentially damage the connected motor.

Think of it like a load-cell. Controlling 50 to 100KG tension on a loadcell rated for 150KG isn't much of a problem. Controlling 10g to 20g on the same loadcell is.

March 11th, 2005, 04:12 PM
On an open loop or V/Hz or scalar drive, mismatching isn't much of a problem within a 5 or 6 to 1 ratio.

The problem comes when trying to use Flux Vector or Sensorless Vector Drives. They probably should stay within about a 3 to 1 ratio to function properly. And, as stated above, the reason is poor detection and measurement of unexpectedly small motor lead currents.