Odd VFD behavior 5

[itsmoked]

I setup a control system for a customer with some heating and a 3HP VFD to run a 2HP motor.
This is that accursed chocolate curtain thing probably mentioned elsewhere here. It's a prototype that's having severe teething problems. The customer had to change pumps from a 2" to 3" positive displacement type. During attempts to get the result right the poor two horse pump was pushed very (too) hard.

Here's what I can't explain.

He set the speed (frequency) to 60Hz then turned the motor on. He stated, "The pump sped up to about 45 Hertz then slowed its acceleration taking about 30 seconds to creep up to about 55Hz before the drive tripped off."

I asked him how he knew about this acceleration behavior, "That's what it sounded like?"

"No, I watched the pumping results and display on the VFD where it showed the frequency slowly creeping up."

I'd set this drive up as 'dumb' V/Hz type. I would expect it to take my pre-set 5 seconds to get to 60Hz motor-be-damned and for the display to show exactly that even if the motor did indeed struggle and could never get to 60Hz.

Is that not what I should expect?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LittleInch]

I would say the motor is slipping a lot as the load is increasing beyond its torque?

Not sure what happens in this instance but if it struggled to get 55 htz and then tripped maybe the VFD just changes itself to be the same as the actual speed of the motor?

If the power the pump needed to run at >45 htz is more than the motor can supply then it simply won't speed up.

The electrical side of this I don't know that much about tbh.

The only other way to check it would be to run the pump in full recycle and turn it on to 60 htz and then slowly close the recycle and see what happens.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[LionelHutz]

Many VFDs have a current limit setting and a VFD limits current by reducing frequency.

 

[Gr8blu]

The "new and improved" pump has a specific load torque curve during acceleration - which may be dependent on how the flow is run through the rest of the system (valves open / closed, leftover material in the casing, condensation, etc.). The motor also has a defined torque profile as it accelerates to running speed. The drive is (probably) running up against a current limit (set constant volt/hertz to obtain a relatively constant current).

In the bottom half of the acceleration window, the motor can drive the pump and thus accelerates from zero to some speed. Somewhere along the line, the pump starts to require more power (torque) than the motor can deliver and the acceleration rate (difference between load and motor torque) gets smaller, and the time to increase speed extends. If it gets bad enough, either the motor quits altogether, or the drive times out on the time-to-accelerate (i.e. it sees the problem as some sort of "stall" condition, even though the shaft is turning).

Converting energy to motion for more than half a century

 

[edison123]

Motor got torqued out? What was the load current?

Muthu

http://www.edison.co.in

 

[btrueblood]

Um...you have a 3hp VFD, originally spec'd for a 2HP 2" positive-displacement pump.

You/customer changed out the 2" pump to a 3" pump.

A 3" pump is going to draw a LOT more hp than a 2" pump; power should roughly scale by the pipe cross section area, 3^2/2^2 or 9/4, roughly double plus a bit.

It sounds like the drive is current limiting/running out of juice. You need to look at the specs for the new pump and probably upgrade the VFD.

You can probably run the PD pump at a lower frequency and have it run stably and get good results. Or, convert it to run on a v-belt and put a smaller drive pulley on the motor.

 

[LittleInch]

For a PD pump the flow is directly proportional to frequency / speed, but pressure is probably proportional to flow squared. Chocolate though is strange stuff, so its frictional pressure drop may be shear dependant and reduce as flow increases but then power still stays the same or more as flow increases.

So it gets to 45 htz and then slowly increases flow as the chocolate changes its rheaology until it runs out of power.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[itsmoked]

Ah.. you're ALL making sense. I'm now recalling the same sort of frequency modification due to too short an acceleration time.

Lionel; Ah okay.. good point.

Gr8blu; I'm sure that's exactly what was happening.

edison; I wasn't present until after he ran the tests and said the motor was toast. I actually don't think it's toast as the drive was set correctly for the 2hp motor. I think he just kept hammering it and the drive tripped faster and faster keeping track of the time/thermal aspects.

btrueblood; Excellent point about pipe size verse loading. This is running thru a 5:1 right-angle gear drive.
Get this.. It's supplied with single phase.... So while that wasn't a problem with 2hp and a bunch of heating I can't say "no problem" with a 5hp switch up! Not to mention that's going to require a 10hp VFD while the a 3HP comes with a 1/3 phase VFD. Urk!

Little; Chocolate is definitely a non-Newtonian fluid. I'm concerned about the 3" necking down to 2" at the manifold. I think that could be a much bigger restriction with chocolate as compared to water. I believe the plan is to split that into two 2" inlets. Not sure that's even an equivalent trade. Checking.. a 3" pipe is equivalent to (2.8) 2" pipes..

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

Many VFDs have a current limit setting and a VFD limits current by reducing frequency.

First thing that popped into my head too. It might be referred to as “stall prevention” as well, but the net effect is the same; the drive attempts to reduce load on the motor as you approach the overload condition by reducing speed, but that doesn’t work on a PD pump very well because it’s more of a torque issue than a flow issue. It’s not doing this to protect the drive itself, it’s doing it to protect the motor. But since it isn’t working, eventually it either gets to your minim speed or trips on motor overload. You can probably turn this feature off in programming, but that will likely just result in it tripping on OL sooner.

Bottom line, not enough motor for the task.


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