VFD Accel and Decel Time

[StarCat]

I am looking for simple rules of thumb for basic types of motor loads. As one example, I used 7 SEC. ramp up time for a High inertia Fan load which seems to be ok. I have another load which is a centrifugal pump which is not currently on a VFD, but may be soon. This would be an application that would benefit in this case from a longer ramp up time and would utilize 1-3 PRESET running speeds. Because this pump is started dead headed in several operational modes. Increase on the ramp up time could possibly be balanced with the time it takes the operator to get to the downstream valve and open it. Up to 15 SEC might work, but what would be considered too long? This also would likely be a speed selection which would limit top speed to 40 HZ. The decel times seemingly would be left at default or close for pump duty. Pretty quick I think.

 

[Jraef]

For a centrifugal pump, longer accel times can often be problematic because of hydraulic issues, i.e. at slow speeds you are not creating enough head pressure to actually move the water yet, so it can end up boiling in the volute. But I don't think 15 seconds is something that would fall into that category. Another potential issue though is if the pump uses the fluid as the bearing and you need to get the pressure high enough to facilitate that. In some pumps they may dictate that it must be at a minimum speed within a few seconds because of this. It's really something that the pump supplier needs to be consulted on (if possible). For example some pump mfrs will void their warranty if they find out the accel time was longer than 5 seconds.

I generally play with Decel times based on trying to eliminate or reduce water hammer, which is variable based on the head pressures involved and what type of valve they are using (if any).

 

[GoldDigger]

Do you also have to worry about the fluid bearing layer collapsing during decel? Or is there a hysteresis effect that mitigates that problem?

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[Jraef]

Do you also have to worry about the fluid bearing layer collapsing during decel? Or is there a hysteresis effect that mitigates that problem?

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Hysteresis effect. Once it is established it takes less to keep it.

 

[StarCat]

This application is Brewery and the specifics are dealing with wort that has just been boiled, and is very hot. Even after the whirlpool stage there is too much cavitation on the original configuration. The goal is to moderate the top speed of the pump to reduce breakout, and also to make it where the pump is not " dead headed " at full speed , in theory on start up. This impeller style does not take dead heading very well. Waukesha c-216. The seal member is carbon to ss with wet wash over. The cavitation ia violent enough to destroy seals pretty quick, as in 90 days and also start destroying the stub shaft and impeller hub due to the forces encountered. I have only briefly spoken to some pump experts on the matter, so my intel is limited.
Thanks, and let me know if you have anything further. Most all of the Modern Brewhouse systems have speed control being sold these days. The idea would be 1-2 presets for different duty.
Star

 

[Jraef]

I think this is going to take several on-site visits to fine tune the settings...

 

[Eddie702]

Most pump MFGs for water at least recommend 20hz minimum speed for typical HVAC applications. There is usually less problems with vfds and motors if the ramp up and down time is extended.

We had several 100 HP centrifugal fans belt drive that were breaking belts 5 groove sheaves and 200+" belts that have to be a matched set get to be a lot of $$$$. We finally found the problem to be the vfd programming. Before the drives were programmed we ran these fans across the line for a month with no issues. The fans were large and had a lot of inertia, on shut down the ramp down time was too short. The drive told the motor to stop when it was still coasting and it would snap or throw the belts

I usually lengthen the ramp up and ramp down time unless there is some reason to shorten it.

 

[ptonsparky]

Most pump MFGs for water at least recommend 20hz minimum speed for typical HVAC applications. There is usually less problems with vfds and motors if the ramp up and down time is extended.

We had several 100 HP centrifugal fans belt drive that were breaking belts 5 groove sheaves and 200+" belts that have to be a matched set get to be a lot of $$$$. We finally found the problem to be the vfd programming. Before the drives were programmed we ran these fans across the line for a month with no issues. The fans were large and had a lot of inertia, on shut down the ramp down time was too short. The drive told the motor to stop when it was still coasting and it would snap or throw the belts

I usually lengthen the ramp up and ramp down time unless there is some reason to shorten it.

If it coasted to stop with no problems while ATL, coast to stop is still a very good option with a VFD.

 

[StarCat]

Most pump MFGs for water at least recommend 20hz minimum speed for typical HVAC applications. There is usually less problems with vfds and motors if the ramp up and down time is extended.

We had several 100 HP centrifugal fans belt drive that were breaking belts 5 groove sheaves and 200+" belts that have to be a matched set get to be a lot of $$$$. We finally found the problem to be the vfd programming. Before the drives were programmed we ran these fans across the line for a month with no issues. The fans were large and had a lot of inertia, on shut down the ramp down time was too short. The drive told the motor to stop when it was still coasting and it would snap or throw the belts

I usually lengthen the ramp up and ramp down time unless there is some reason to shorten it.

 

[StarCat]

Sorry on the double click. Coast to stop agree. Eddie, interesting. I can see this. Years ago I had a VFD failure seem to mimick bearing problems. Without really knowing exactly what was going on. The way things appeared is that the drive was throwing off the frequency on 1 leg only. Once I realized what was happening, I put that fan drive on across the line taking the drive out of the circuit and all the weird vibrations stopped cold. It was a lower end brand of drive that I do not really care for. Lots of issues on that type. The thing on this pump application is it deasls with fluid close to boiling temperature, so the dynamics are different that pumping cooler water.

 

[Eddie702]

@StarCat

I don't claim to be a VFD expert at all. We had another one on a water tower fan where the fan would vibrate at a certain HZ. Ramping up on start up at 35 hz it would vibrate like crazy and at 44hz it would stop vibrating and run smooth to 60hz. We had the vfd rep in and he changed something in the drive. I think it had something to do with torque setting like constant torque-variable torque. After he changed it it was fine.

@ptonsparky That fan was coasting down but the vfd was programmed for a faster ramp down that the large inertia would allow so the vfd was acting as a brake trying to stop the motor while the fan was still spinning and destroyed the belts

 

[ptonsparky]

Eddie, I understand. Did it operate successfully before without the braking? Does it require braking now?

 

[StarCat]

@StarCat

I don't claim to be a VFD expert at all. We had another one on a water tower fan where the fan would vibrate at a certain HZ. Ramping up on start up at 35 hz it would vibrate like crazy and at 44hz it would stop vibrating and run smooth to 60hz. We had the vfd rep in and he changed something in the drive. I think it had something to do with torque setting like constant torque-variable torque. After he changed it it was fine.

@ptonsparky That fan was coasting down but the vfd was programmed for a faster ramp down that the large inertia would allow so the vfd was acting as a brake trying to stop the motor while the fan was still spinning and destroyed the belts

Very interesting Eddie. Back at A&M I had 2 250 Ton Marley Cooling towers on older generation VFDs. I noted the same kind of weird seemingly harmonic vibration as that drive was hunting all around in the lower speed range. It was annoying, but the boss insisted it was not a problem.
I only recall minor gearbox MTC issues on those machines, but never liked the way they ran at low speed. One time on a when working to get a European 50 hz machine to run over here, I found " Flux Opimization " in the ABB menu to be a total godsend, and fully resolved an overcurrent situation on a bastard 4 HP motor after I had made the decision that a VFD was the only way. We had some bigger AHUs on Reliance drives with multiple groove sheaves, but no problems on the config. They always worked well from startup. The Alitvar drives were the worst. My VFD experience is limited, but somehow I have always gotten them to work. Last one was a trick.

 

[Eddie702]

Eddie, I understand. Did it operate successfully before without the braking? Does it require braking now?

No it's doesn't need braking, just coasts down

 

[Eddie702]

@StarCat
I think ABB are the best drives. Altivar, those are made by Square D right? We had issues with those. Most of the work I did was HVAC related. Most of the manufacturers have HVAC drives which are less complicated and don't have as many features but easier to set up and usually are more or less preprogramed you usually just put the motor information in and they will run.

For a good laugh we did a pump install (2 direct drive pumps 20HP) that we changed out and ran them across the line. The customer which was an engineering firm (this was in their building) wanted to "save money" and went out and bought the pumps and motors and VFDs.

Trouble is the job was 208 volt. They ordered everything for 480 volt so the drives were wrong. The motors were 240/480 dual voltage but lucky for them the motors were tagged "suitable for use on 208 volt"

Unfortunately, one of the two drives was mounted and wired befoer the problem was noticed but not energized