I have two 1305's that turn seperate plate feeder turntables. These turntables meter material out of overhead bins onto a conveyor for mixing.

Due to increased efficiency in the use of materials, I need to slow these things down. I have 1175 rpm 1 hp motors turning a small gearbox and all works well as long as I command (via a 4-20 ma signal) the drive and motor combination above about 10 hz. Below that, the motor stops turning although current doesn't rise and the drives do not trip on overcurrent.

AB tells me that "slip" may be causing this scenario, and I used their formula for calculating it. When I entered the newer parameters, I actually had to use a higher freq. command to get the motors started.

I also wondered about high torque starting causing the problem... so I started the drive at 15 hz. and it turned the motor fine. As I incrementally dropped the command to about 7 hz. the motor stalled again. I tried eliminating the torque problem by issuing a high freq command, let the drive come to speed and slowing it down. That didn't work either.

Any suggestions as to what's wrong here would be greatly appreciated. One of these 1305's is a rev B and the other is rev C. I mention that only because I know there were some parameters added in rev C that B doesn't have.

Thanks for the help...

John Kelley

 

This may sound stupid, but have you looked at how much torque the motor has at 1/6th the rated RPM? A standard AC motor rated at 1hp at 1175 rpm will NOT have 1hp at 200rpm. I would expect an error on the drive letting you know that the motor has stalled, but I don't know for certain. Find out the rated torque at the given rpm and make sure that's enough to drive your load. This all has nothing to do with the drive.

The only thing I know to actually fix this is double your gear box reduction so you can run at 20Hz instead of 10. Of course, that means at 60Hz you'll be running half the speed as you were before.

-James Ingraham
Sage Automation, Inc.

 

You can try creating a custom Volts to Hertz curve. You do this by setting up a break point so you will have more voltage at lower speeds. With
480VAC use start boost of 40 volts,set your break frequency at 15 hertz with a break voltage of 115 volts. You could also wire the motor delta instead of Y, a delta motor has more torque at lower speeds. But if the gear ratio is to small you will loose torque between poles which could stall the motor or create cogging. You may have to replace the gearbox

 

John,

The solution to this problem is that you should carefully disconnect the drives from the control circuitry and power / motor leads. Remove them from the backpan and then gently set them on a shelf somewhere where you will never have to look at them again. You need vector control to get the
performance that you really want. Give the Control Techniques UniDrive a shot at it without an encoder (set it up for open loop vector control) if this doesn't give you the satisfaction you are looking at, purchase an
encoder and connect to the drive for closed loop vector control. . . You will not be dissatisfied with the performance or the price.

Ken Brown

 

If this is a plain V/Hz type inverter, it can be that the voltage boost adjustment needs to be increased. Typical V/Hz drives have a voltage boost feature to allow compensation of the proportionally higher stator resistance voltage drop at low Hz. If boost too low, magnetizing current will be too low and torque drops off rapidly. This combined with low Hz command (slip close to zero) could result in stall.

 

Your application is constant torque your motor and drive will produce the same amount of torque at 10hz as well as 60hz. We have to remember that this drive is only a v/hz drive it realy has no idea how fast the motor is actualy turning. My suggestion is to lower your gear ratio or if posible any sproket or pully ratio to allow the motor to run faster and still obtain you output speed. Then run your upper end over frequency i.e. 70,90,120 hz whatever it takes to obtain your max speed. V/hz drives dont like to control speed in the mud if you can't change your ratios go with a sensorless vector drive or a DTC drive.
Good luck.

 

Anonymous,
Your assumption that the motor will produce the required load torque is flawed. Obviously, it's not, as it stalls. A deeper understanding of how a drive really controls at low Hz, e.g. stator IR drop/voltage boost, magnetizing current, slip speed etc., most likely explains the problem and solution. In short, motor could have very poor torque producing conditions at low Hz with simple V/Hz drive, especially if Vboost not set properly.