PID motor control vs ramping up/down to a set-point - is one better for motor?

[USAeng]

Just wondering - for health/lifespan of motor - is controlling motor through a PID that only makes slight changes better than controlling by ramping up and down between 2 set points?

Currently the motor changes between 500 and 1300 RPM via ramp up/down with VFD about 6 times per hour to maintain a system set point. Its a 2HP 3500 RPM AC motor

 

[LionelHutz]

I doubt there is any difference for the motor. The process might be happier.

 

[GroovyGuy]

Actually I'd have more concern wrt the 2-pole motor only operating from 500rpm to 1300prm. I take it that the load profile must be variable-torque, and not constant-torque, else the motor would have been fried by now.
What is the application?
GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[cswilson]

As long as your VFD can ramp the speed/frequency at a rate your motor can follow reasonably, I see no reason to go to a closed-loop solution. (And I say this as a servo jock.)

If the VFD were to ramp too fast (or step) and create very large slip values, you would have surges in current similar to DOL starting, which could be problematic over time.

Curt Wilson
Omron Delta Tau

 

[GroovyGuy]

Howdy Wilson,
Not to worry wrt surge currents with a VFD, as the inherent current limiting feature of the VFD will always override any setpoint changes (ie to protect the drive from overcurrent).
GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[cswilson]

Thanks, GG. As a general rule, though, I hate to intentionally command an action that requires the safety limits to kick in.

 

[jraef]

I'm still curious why a 2 pole motor is being used here when the max speed is below that of a 4 pole motor. Why not go with a 1HP 4 pole motor?


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[waross]

1300 RPM is quite a low speed for a 3500 RPM motor.
500 RPM on a 3500 RPM motor needs supplementary cooling or severe de-rating.
What is the nature of the process?
What is the load profile?
What is the controlled variable? Flow? Pressure? Level? Temperature? Something else?
If you want a better answer, you have to provide better information.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

Further to my previous comments, VFD may be the hands-on way to go, but we need more information.
If the implementation of PID control results in the motor speed varying slowly between about 900 RPM and 1100 RPM, then PID control may be a better choice.
Consider changing to an 1200 RPM base motor, or changing the drive ratio so as to run the 3600 Base motor much faster.

for health/lifespan of motor

DON'T RUN A 3500 RPM MOTOR AT 500 RPM. IT WILL NOT BE COOLED PROPERLY.
TEFC MOTOR ARE PARTICULARLY AT RISK.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

The motor is currently going between 500rpm and 1300rpm and apparently surviving fine so it doesn't seem like there are any concerns right now.

Also, if there is no issue now then running at a constant speed somewhere between those 2 speeds instead should not be an issue either.

So, do the PID loop if it makes the process better.