One drive, two motors.

[marke]

I have become involved in an application where the end user wants to use one drive with two x 850KW 400V pumps.

 

The plan is to start one pump and get it to full speed and then switch it onto the supply and connect the drive to the second pump and use that to control the flow.

 

The pumps will probably be coupled by a long shaft.

The plan is to use open transition switching from the output of the drive to the supply. I am concerned that this will result in a high curent and torque transient and this could cause problems with the long drive shaft, plus potentially compromise the drive.

 

One option is to use the drive on one pump and a reduced voltage starter on the other, or to use a drive capable of shunt switching.

 

Has anyone had any experience with this type of application?

 

Best regards,

[JTC]

Marke

 

I would assume that open transition 'bypassing' would cause these current and torque transients you mentioned but would also apply voltage to the output of the drive. I would recommend using a contactor to break the connection between drive output and the bypass connection.

 

I have heard that some drive manufacturers are looking at synchronising features in order to perform this type of application and also cascading applications, particularly those who are involved with solar power generation.

 

Alternatively the system could be set up to start with the first motor controlled via the VSD, then bring in the second motor via a soft starter and decelerate the first motor once the second is at 75% speed. Then you can use the first motor controlled via the VSD to control the system.

[chaterpilar]

Dear Marke,

 

We have used VSD's for similar requirements but in a different way.

 

There are 5 pumps in parrallel with 1 pump having VSD and other 4 being controlled by VSD outputs...

 

The VSd gets input from pressure transmitter and regulates the speed of master pump to desired pressures and if pressure falls below setpoint and the master pump is already at full speed then it gives signals to other 4 pumps to maintain pressure.

 

regards

 

chaterpilar

[niallnz]

Hello Marke,

 

Your plan will work, open transition with a contactor on the output of the drive (K1) to disconnect it and a contactor to reconnect (K2) the motor. Obviously, the contactors have to be elecrtically interlocked so that K1 opens before K2 closes. The really trick here is a synch transducer with an analogue output you can feed into the drive to alter the speed. The second trick is that when the synchroniser is happy, it stops the drive from modulating, opens K1 which then closes K2.

 

I do know of one drive manufacture that has this synchroniser as a standard kit, and I can confirm that the current surge when K2 closes would be no more than 20% of the load current at the time of synchronising.

 

Happy synchronising.

Cheers Niallnz

[GGOSS]

Hello Marke,

 

This sounds like one of those jobs that may best addressed by allowing others (your opposition) to win.

 

There has been much discussionion about drives with synchronisation capabilities but I do not know of one supplier who can do it reliably, and more to the point who can do it with a drive rated at 850kW 400VAC. If there are some suppliers out there please let me know.

 

The soft start + drive solution is the one that I would be pushing but care must also be taken here. First and foremost the supply needs to be capable of delivering sufficient power to start one motor via the soft starter. That alone would result in a starting current of around 5600 amps assuming 1400 amp FLC and a start current ratio of 4 x FLC to accelerate the pump. I have suggested 4 x FLC because the starting characteristics of motors in the range are pretty poor. Needless to say, the drive will more than likely be running at full speed when the starter is called to start, and therefore the total amps drawn will be closer to 7000.

 

The THD at the point of common coupling could also be beyond acceptable levels if the transformer is not sized correctly, and even if it is I would envisage some sort of harmonic mitigation device would need to be installed.

 

As you may already be aware soft starters measure frequency in order to self calibrate. I would expect distortion on the voltage waveform caused by the presence of such a large drive may in fact upset the zero crossing to the point where the soft starter cannot reference, causing the start to be aborted. One way to help minimise that is to install a choke in front of the drive but even then, I would not like to be the one providing a guarantee of operartion.

 

To the best of my knowledge there are no block style contactors available for with a 1400 amp AC3 rating, so you would need to conisder bar contactors which as I am sure you know tend to be very large, very expensive and on occassions difficult to work with.

 

My 20 cents worth.

 

Regards,

GGOSS

[jraef]

Marke,

I think maybe GGOSS interpreted that yu want to start motor #2 with the soft start, in which case he makes a good point. If what you were referring to, however, was to use the soft starter to connect the 1st motor to the line power after shutting off the VFD and thus avoid the switching transient from the motor reconnecting at a lower speed, we have done so successfully. What we did was use a very low current limit setting on the soft starter and no ramp time, so the starter goes into CL immediately when energized and prevents the transition spike. We have done this for 2000HP 4160V turbines. It works great. I'll send you a U2U on this as well.

[Charlie]

I have become involved in an application where the end user wants to use one drive with two x 850KW 400V pumps.

The plan is to start one pump and get it to full speed and then switch it onto the supply and connect the drive to the second pump and use that to control the flow.

The pumps will probably be coupled by a long shaft.

The plan is to use open transition switching from the output of the drive to the supply. I am concerned that this will result in a high curent and torque transient and this could cause problems with the long drive shaft, plus potentially compromise the drive.

One option is to use the drive on one pump and a reduced voltage starter on the other, or to use a drive capable of shunt switching.

Has anyone had any experience with this type of application?

Best regards,

 

Hi!

 

I've got an experience with inverters of Moeller, which allow the user to switch load on the output. Solving the problem the way you describe seems from my point of view quite good, I shout to do it the same way. But, unfortunately, Moeller inverters gives not such a big output power...

 

With regards,

Charles

[kens]

Hi Mark, I dont understand the reason for the coupling of the pumps with a shaft. If the pumps are coupled how will you vary the speed of one and not the other?

 

Ken

[marke]

Hi kens

 

There are two pumps and two motors. The pumps are separated from the motors and are coupled to the motors by a long shaft. No interconnection between the two pumps.

 

Best regards,

[kens]

Thanks Mark that makes more sense. I think that jraef may have a good solution there as i would be concerned about the transients at the time of change over also. It would be very similar to those at the star - delta change i would imagine. As GGOSS mentioned you would need a very substantial supply to cope with the starting requirements on a SS also i have had problems in the past with SS faults caused by drives on the same supply point (especially in rural areas) so care must be taken there also. I am assuming that the cost factor is preventing the use of 2 drives? If this installation is in your area there maybe some significant advantages as far as network charges go in having the ability to limit connection size and also load shed at peak demand times. If you want to send a personal message with a few more details i can give you some figures to work with.

 

Ken

[marke]

Hi kens

 

Thank you for your input.

This particular application is now under way with 2 x 900KW drives on order.

The question was from the client on ways to cut costs and only use one drive only.

 

Best regards,

[GGOSS]

Yes, thanks for that.

 

Regards,

GGOSS

[mariomaggi]

Dear Mark,

 

This particular application is now under way with 2 x 900KW drives on order.

 

I hape that thay selected an output voltage higher than 400 V, at least 690 V or higher, less than 1 kV.

Otherwise efficiency will be too low.

 

Regards

Mario

 

 

[jOmega]

Dear Mark,

I hape that thay selected an output voltage higher than 400 V, at least 690 V or higher, less than 1 kV.

Otherwise efficiency will be too low.

 

Regards

Mario

 

 

Mario ...

 

Perhaps you would be kind enough to enlighten us all as to how a 400V application is LESS efficient than the same application at 690V (neglecting cable loss) and by how much the efficiency would degrade at 400V as opposed to 690V or higher.

 

Thank you.

 

jΩ

[mariomaggi]

Dear jOmega,

(neglecting cable loss)

 

why? Losses in the cables, rectifiers and IGBTs will reduce efficiency if voltage is low.

 

Regards

Mario

[jOmega]

 

Losses in the cables, rectifiers and IGBTs will reduce efficiency if voltage is low.

 

Regards

Mario

 

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

 

Hello Mario,

 

So, if I understand you correctly, you are saying that a 400 kW 400v vfd is less efficient than a

 

400 kW 690v vfd.

 

 

Somehow, the vfd manufacturer data doesn't support your argument, as shown below..

 

 

Perhaps that is because of the way in which Efficiency is defined ~~ ( Power out ÷ Power in )

 

And since Power is defined as P = E I

 

It follows that for the same value of power i.e., 400 kW ....... as the voltage goes up, the current goes down.

 

The same is true for the cables........ for same power i.e., 400 kW,........ larger size cables are used for the 400v application than for the 690v application. But the cable used in the 400v application has lower resistance per meter than does the smaller size 690v cable ......or even the 2.3kV cable....... so the losses stay relatively constant.

 

Also consider that a 400 kW 400v motor has the same efficiency as the 690v and even 2.3 kV motor of the same power rating.

 

If you have the time, and are interested, I would refer you to

 

ABB industrial drives

ACS800, single drives, 0.55 to 2800 kW

Technical catalogue

 

which you can download as a PDF file or view online at the following link:

 

ACS800 Single Drive Technical Catalogue

 

where on pg. 10 you will find the efficiency for the ACS-800 series drives stated as follows:

 

Efficiency (at nominal power)

ACS800-0x series 98%

ACS800-1x/-3x series 97%

 

you will note that VOLTAGE is not a factor in stating efficiency. That is because, for a given power rating, in a given hardware/feature configuration. the efficiency is the same at all voltage 208v thru 690v

 

Ok, that should state the case of the low voltage drives up to 690vac ... now for the Medium Voltage Drives.

 

Once again we turn to ABB and their ACS 1000 product for 2.3/3/3/4.0 kV

 

I refer you to their literature..... ACS 1000 Datasheet ... which is available as a PDF file download or you can view it online at the following link

 

ACS1000 Datasheet

 

There you will find that the efficiency is stated as Nominal 98% which is the same as the ACS800-0x series above.

 

I leave it to you to use a wire table to look up the resistance/meter of wire cables that would be used in a 400 kW 400v, 400 kW 690v, and 400 kW 2.3 kV application and determine the IR drop loss in say 20 meters of cable run for each size ....... I am confident that you will not find sufficient difference in losses to validate your argument.

 

Kind regards,

jΩ