vfd response time

[ptonsparky]

I was in a large Dairy a couple weeks ago pretending I knew something. Local POCO is doing a PQ check .

One of the things I did not like was the VFD controlling a vacum pump motor. Noticable variation in rpm ever second or two. In the hundreds.

This may or may not have influence on PQ, but how tough is this on the motor?

 

[Besoeker]

I was in a large Dairy a couple weeks ago pretending I knew something. Local POCO is doing a PQ check .

One of the things I did not like was the VFD controlling a vacum pump motor. Noticable variation in rpm ever second or two. In the hundreds.

This may or may not have influence on PQ, but how tough is this on the motor?

If the VFD is set up correctly the motor should be fine. Unless the speed is too low to provide effective cooling for any length of time.
Be interesting to know what causes the speed variation though. What is the controlled function?

 

[Cow]

The VFD is trying to maintain a certain vacuum in the milkline for milking the cows. They are set up on PID loop over here. Are you sure it isn't a loop that isn't tuned very well rather than a PQ issue?

 

[kwired]

The VFD is trying to maintain a certain vacuum in the milkline for milking the cows. They are set up on PID loop over here. Are you sure it isn't a loop that isn't tuned very well rather than a PQ issue?

That was my guess right away - something in the controls malfunctioning or not set up right, including tuning of a PID loop - which would make sudden changes as it is over compensating the next necessary change.

 

[Jraef]

Probably someone used just a PI loop control, no D. That's what the "D" is supposed to prevent. In a VFD, it's often said that "you don't need the D", but that's because the VFD has a ramp time function anyway, which can mitigate this in a different way. Sounds like someone took that statement literally, but then set the ramp time too short as well.

As Besoeker said, no harm to the motor if all things are set right. But other one thing worth checking (beside his suggestion of minimum speed) is if the VFD is set up for Sensorless Vector Control, but nobody did an Auto-tune, or someone swapped out the motor since it was done and was unaware that it had to be re-tuned afterward. That can cause some erratic and unpredictable behavior, some of which might end up being harmful to the motor.

 

[ptonsparky]

The VFD is trying to maintain a certain vacuum in the milkline for milking the cows. They are set up on PID loop over here. Are you sure it isn't a loop that isn't tuned very well rather than a PQ issue?

I believe it is a loop problem.

Almost unbelievable, but they do not complain about "stray voltage".

 

[ptonsparky]

I talked to the owner this morning about this and was informed that the VFD is doing exactly as intended. It is maintaining a constant vacuum at the teat during milking. Seems strange with the wide variation in speed but I do not know all the other variables involved.

 

[rcwilson]

The 1960 vintage Surge milkers on my uncle's farm pulsed or surged every couple of seconds. They did not continuously suck. Maybe the milkers are randomnly in synch and the VFD is responding to the repetitive variation in load.

(During power outages, we parked the old Ford tractor outside the milk house window and connected a hose to the manifold to use engine vacuum to run milking machines. I don't know if the little petcock valve on the manifold was an after market addition or a standard feature on Ford tractors).

 

[ptonsparky]

The 1960 vintage Surge milkers on my uncle's farm pulsed or surged every couple of seconds. They did not continuously suck. Maybe the milkers are randomnly in synch and the VFD is responding to the repetitive variation in load.

(During power outages, we parked the old Ford tractor outside the milk house window and connected a hose to the manifold to use engine vacuum to run milking machines. I don't know if the little petcock valve on the manifold was an after market addition or a standard feature on Ford tractors).

Ahhh, that's right, now the pulsing of it makes sense.

 

[Cow]

The 1960 vintage Surge milkers on my uncle's farm pulsed or surged every couple of seconds. They did not continuously suck. Maybe the milkers are randomnly in synch and the VFD is responding to the repetitive variation in load.

(During power outages, we parked the old Ford tractor outside the milk house window and connected a hose to the manifold to use engine vacuum to run milking machines. I don't know if the little petcock valve on the manifold was an after market addition or a standard feature on Ford tractors).

My folks dairy was the same way up until about 4-5 years ago when they remodeled. They also used the same old Surge vacuum operated pulsators and they did the same thing during an outage as well. They would park an old truck or tractor with a valve on the intake manifold and run a hose inside. When it got dark, light would come from the pickup outside shining it's headlights through the door.

Like RCwilson said, if it's maintaining proper vacuum, I wouldn't worry about it. When we installed some Yaskawa drives for the vacuum pumps over here, they would simultaneously drop six machines at once(start the suction) to see how much it would affect vacuum. The Yaskawa salesman who set it up had the setpoint at 13.8 on the display and it wouldn't hardly waver, even with a setpoint using tenths, maybe 13.5-13.9 would be all it'd fluctuate.

 

[kwired]

I talked to the owner this morning about this and was informed that the VFD is doing exactly as intended. It is maintaining a constant vacuum at the teat during milking. Seems strange with the wide variation in speed but I do not know all the other variables involved.

I haven't really worked on the milking equipment in a dairy farm - usually done by equipment specialty people, but would think you would want a main relatively constant vacuum system and individual regulation of what happens on individual branches of that system. The larger the dairy operation the more I would think you want it that way. Possibly even to the point where they may be parallel operating pumps - try to synch those if they need to vary in that kind of fashion.