PeroFlex 700 VC with another motor

Hi:

In the plant that I am working for now there are two PowerFlex 700 VC 50 HP to control two pumps. The problem is that the 50 HP pumps were not enough to empty the tanks so they changed to a 75 HP motors, but they did not change the VDF, so they are running 75 HP bombs with a 50 HP VDF. They insist that is not a poblem, but I have my doubts about this. Moreover, the NP current of the motor is bigger than the rated current of the VDF (I don't remeber the exact value, but is like the VDF of 65 A and the motor NP is about 84 A).

Any suggestion to talk with this guys?Any experience about this?

Thanks!

Do not know what your pf700 vc model is, but if it was rated for a 50 hp motor , I guess is not suitable for a 75 HP one, as rule of thumb phase current is aprox HP times 1.2 , so in this case your current is 75 times 1.2 = 90 Amps.
One thing you can try to do, in case that you drive has enough current, It's to try to close pump discharge side up to a level that current motor is less that maximum of the drive. If that can fulfill your needs, other wise you need to change drives.

rleon said:

... the NP current of the motor is bigger than the rated current of the VDF ...

Click to expand...

That information is enough to tell you that you need a bigger drive to fully utilize the motor. You might get away with the set up you have right now, if the load being driven by the motor requires less than the drive current rating.

Monitor the drives' displayed current levels during all phases of its operation and you should be able to tell what the loads actually call for.

Also note that those drives have a normal duty and a heavy duty rating, so a heavy duty rating of 50HP may be actually good for a 60HP load.

Paul

rleon,

In this thread the original poster was able to "get by" with a smaller VFD by using a current limiting parameter.

http://www.plctalk.net/qanda/showthread.php?t=44301&highlight=danfoss+pump

It's always best to forget hp ratings and deal with current to size drives and motors.

The motor and pump together require a certain amount of amps to get the maximum pump output needed. Whatever that current is, if the drive can supply it on a continuous basis, then you are all set.

Just to give this case some numbers, say you have a 75hp motor that has an FLA of 90amps. At maximum pump output, the motor draws 71 amps. Let's say the drive has a maximum continuous rating of 72amps. You have a working system. If the drive instead has a maximum continuous rating of 68amps, you do not have a working system.

Determining the drive maximum continuous rating is generally not a published value. You will see continuous amps with 150% overload, a different continuous amps with 10% overload, but no ampacity for continuous amps with no overload. For that, you need to ask the drive distributor, rep, or manufacturer.

Thanks for all answers, I am more clear now. Actually, the rated value for the VDF is 65 amps, and the pump working with load and max rpm draws 63 and 64 amps. So I think that there is no problem. I will continue "monitoring" the drive with a trending to see is there are some peaks or overcurrent values.

thanks!

Thanks for all answers, I am more clear now. Actually, the rated value for the VDF is 65 amps, and the pump working with load and max rpm draws 63 and 64 amps. So I think that there is no problem. I will continue "monitoring" the drive with a trending to see is there are some peaks or overcurrent values.

thanks!