encoder feedback on PowerFlex70 drive?

[cfordyce]

I have a standard 5hp 3ph induction motor running a pump through a gear reducer. Inverter controlled (Allen-Bradley PF70 drive). I have very poor speed accuracy (4% error) using open loop mode. I want to install an encoder on the motor and run in closed loop control.
I am wondering what ppr encoder is recommended. I noticed that lower ppr encoders are lower cost. I am thinking that 1024ppr is overkill. Can I get away with 60ppr encoder and still get good speed accuracy? Is there any benefit to higher ppr in my application? Motor typically runs ~1500rpm and is controlled by a pressure controller.
Thanks.
CAF

 

[Skogsgurra]

It depends a lot on how the drive measures speed.

In older days, a simple pulse count over a fixed gate time was used. That is seldom the case any more. Instead, a modified reciprocal technique is used in most drives. That makes PPR less important.

What is important is that you get updated speed information fast enough to avoid serious ripple in the speed controller output. At 1500 RPM, that usually means that you need new speed data every 5 - 10 millisecond. Since your shaft is turning at 25 RPS, you could actually get away with 4 PPR.

That opens a lot of possibilities. The SKF Sensor Bearing is one. It has a rather low number of PPR, 32 is typical and using quadrature decoding (which drives do these days) you get 128 pulses per revolution. A lot more than your application needs.

I would definitely have a look at the Sensor Bearing. The beauty with it is that you don't need any adapters or couplings. Just mount it like any other bearing. Google Sensor Bearing for more information.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[blacksea]

Inverter with Vector control instead used by you V/F control may help you to increase speed accuracy. Such type of inverter is more expensive 2..3 times.

 

[DickDV]

I believe that the PowerFlex70 drive is capable of sensorless vector control. If it is of at least average capability, you should be able to squeeze the speed error down to about one-quarter of motor nameplate slip.

That means that, with a motor with 3% slip (fairly typical of 5hp motors these days), you can get to .75% speed error from no-load to full-load.

Since this is a pump application and the load generally does not swing all the way from no-load to full-load, you should be able to get better than .75% speed error in your operation.

If you are getting more error than that, I would hold off on the encoder and figure out how to use the drive capabilities that are already there.

Are you familiar with the sensorless vector setup procedure for the PF70?