Trouble switching motor from VFD to line power

[eeprom]

Hello,
I am trying to drive a 50 HP induction-motor pump alternately with a Yaskawa VFD, and line power. What I am trying to do is to use the VFD to control the pump while the synchronous speed is less than 60 Hz. When the synchronous speed reached 60 Hz, I use a set of reversing contactors which disconnect the pump from the VFD, and then connect it directly to the grid. Sometimes this works fine. The problem is that occasionally the connection to the line is very rough, and it results in popping a breaker. It seems that the transition should be smooth. I can start the motor directly from the line with no problem. But when the VFD drops out and I try to connect from the line, I get the bump. Any ideas why this would be happening? And any ideas on how to fix it?

thanks,
Andrew

 

[Skogsgurra]

Learning from this.

Gunnar Englund

http://www.gke.org

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

 

[itsmoked]

Jeff; What kind of application is that(effort) for?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

Keith,
The most common is a pump station, such as a lift station for a wastewater system or a pressure booster for a water delivery system. Because of the nature of the beast, the flow or pressure requirements can be extremely variable.

 

[ozmosis]

Going back to the original poster, assuming the 3 x pumps are feeding into the same system, I can't quite work out why you would want to switch the VFD from one pump to the next.
Within our VFD, and it is similar to a number of drives on the market, there are functions called "motor staging" or "Pump Staging".
This method would operate with the VFD using it's internal PID loop and the pressure transducer feeding back to the VFD the PID FB. Scenario is this:
Pump 1 is connected to VFD directly, no switching required between VFD and motor. The VFD starts Pump 1 and runs in a PID closed loop controlling pressure. As pressure increases then the VFD pump slows down, as the pressure drops then the VFD increases pump speed to try and maintain pressure SP. If the SP cannot be reached and Pump1 is running flat out, then an internal timer will kick in within the VFD. After a minute or so of running flat out and still not reaching required pressure, an internal relay will change state and bring in Pump 2.
Pump 2 will be operated either DOL (Direct on Line) or via a Y-d starter or a softstarter. At the moment the relay on-board the VFD was to bring in the Pump 2 then it would momentarily drop in speed, cut out the PID loop to try and eliminate a large in-rush as the fixed speed pump was brought in. This is the difficult part and one that generally needs the most 'fine tuning'.
The position now is that Pump 2 is running at full speed and Pump 1 would typically be running quite slow (the PID is now back in operating). The feedback from the pressure transducer is providing the VFD with the pressure demand and again, if the pressure is still lower than the SP then the VFD increases the speed of Pump 1. If the same scenario happens with pressure dropping and Pump 1 running at full speed, then the 2nd relay on the VFD will bring in Pump 3 (again, at fixed speed). Pump 1 would have dropped off to try and compensate for the surge.
Now with P1 varying and P2 & P3 at fixed speed, you will have a system that should be designed to full capacity and cope with the worst case scenario in pressure drops.
If the pressure starts to rise and the P1 is running at minimum speed, then the opposite process will operate with P3 being taken out of service with the Rly onboard the VFD and so on.
This is a typical Pump Stage process that a large number of VFD's on the market can function with all the logic on-board the VFD.

This system would not necessitate the need to bring in and out the output stages of the VFD and potentially lead to the problems originally explained.
Hope this makes sense.

 

[ScottyUK]

sed2,

This is more-or-less what I was alluding to in my earlier post. Seems these pump guys like the facility to allow any of the pumps to be assigned to the function of P1, P2, P3 in your example in order to balance running hours. That makes things a little trickier.


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Sometimes I only open my mouth to swap feet...

 

[jraef]

sed2developer,
I agree that is the most commonly applied application as far as VFD manufacturer's built-in drive control macros, but there are a lot of engineers who will not use it. The reason (besides the "difficult part" you mentioned) is that Pump #1 ends up with a lot more running time than the others because it cannot be included in the alternating scheme since it is the only one on the VFD. Granted, being on the VFD means the pump is not being worked as hard, but things like seals and bearings wear out based on run time as well. I did a quadraplex system several years ago and the VFD driven pump now has around 7 times the running hours as the other 3. The risk is that it wears out faster and when it fails or is taken out of service for maintenance, they have now lost almost all of their system control.

 

[DickDV]

ABB supplies a "Pump/Fan Macro" in their 550 and 800 series drives. The operation is as described above with one pump being variable speed and the others on contactors.

You get the choice of operating the system that way or with pump rotation. Pump rotation periodically moves the lead and variable speed control to the next pump. After a fixed number of operating hours, the rotation is to the next and so on until all have about the same hours.

You need one drive sized for the pump motor hp and two contactors for each motor---one to connect the motor to the line and one to connect to the VFD. The drive logic takes care of the sequencing and rotation leaving only the contactors to be added.

I've done one of these in a hospital water supply system and it works really nicely. You can even take one pump off line and the sequence and rotation will jump over that position and just keep on working.

Very nice!

 

[eeprom]

Two main concerns of this design are that the pump run times remain balanced, and that we don't cause water hammers by bringing auxiliary pumps online too abruptly. Turning on the auxiliary pumps has been a bit of a trick. A soft-start for each motor would be nice, but the system is already built and paid for. And three soft starts would be a significant cost. One idea proposed has been to bring auxiliary pumps online by first dropping the VFD to a fixed setting corresponding to about half flow. With the lead pump running at half flow, we can start one of the auxiliary motors straight off the line. As soon as the contactors for the second motor are closed, we put the VFD back into control mode. The result should be a minimal water hammer.

 

[ScottyUK]

That's interesting Dick,

Is there an application note on this scheme anywhere? A ready built macro would save a lot of engineering time - I'd like to have a look at this one.


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Sometimes I only open my mouth to swap feet...