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Star delta or vfd

Star delta or vfd

Star delta or vfd

(OP)
Hi guy I've one doubt in the pump system we are using 90kw pump for recirculation of sugar juice pump specifications are 700 cu m/hr. We used two pumps (one is running and another one is stand by) we are using star delta starter for both motors then my question is we required 700 cu m/hr discharge lifted by single pump but it cannot lift more than 600 cu m/hr so we using vfd starter any chances is their lifting 700 cu m/hr discharge.

RE: Star delta or vfd

The discharge at a given speed is decided by the pump, not by the motor, which only provides the requisite power. Are you asking to increase the motor speed via VFD (which by the way is more than a starter) to increase the pump discharge? If yes, you need to talk to the pump supplier first.

Muthu
www.edison.co.in

RE: Star delta or vfd

You cannot change physics. If it is a centrifugal (quadratic) pump, increasing the speed with a VFD will increase the kW required by the pump at the cube (3rd power) of the speed increase; one of the “Affinity Laws”. So for example if the pump speed must be increased to just 120% to get the higher flow, the kW required by that same pump increases to 1.23 times or 173%, so the pump requires 155kW, but your motor is still only capable of 90kW. So it will not work.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

RE: Star delta or vfd

Motors in industrial plants often do not match the load HP requirements exactly.
If your motor is using less than rated HP to pump 600 cu m/hr then you may safely increase the speed.
By guided by jraef's post regarding load increase with speed increase with a centrifugal pump when evaluating the amount of spare capacity of the motor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Star delta or vfd

One mitigating factor to jraef's caveats is that at higher rpm the motor is capable of delivering more power (though the torque stays the same). So the actual overload increase goes up by the square of the speed increase rather than the cube.

RE: Star delta or vfd

Quote:

that at higher rpm the motor is capable of delivering more power (though the torque stays the same)

Are you sure about that?

The motor stays constant HP once it reaches its nameplate and nameplate voltage. As you speed the motor up further the torque drops keeping the motor power the same.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Star delta or vfd

Keith, I believe what you state is true if you keep the voltage constant. If you keep the volts/Hz constant , then the power capacity goes up with frequency. The limitations will be supply voltage and voltage rating of the wire insulation.

RE: Star delta or vfd

Yes! I agree and thought that might be what you were thinking it just wasn't obvious and I could see the OP scratching his head.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Star delta or vfd

One option that may appease both of you:
At 90 kW the motor is most likely wye connected.
Also the mains voltage is probably so close to the motor rated voltage that there is no additional voltage available.
How about reconnecting the motor in delta. Then we can go up to 173% of the delta rated frequency, voltage, speed and HP.
If the load presently demands the full 90 kW we will be out of luck with a direct drive.
If by any chance this is a belt drive we have a chance.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Star delta or vfd

Increasing the voltage above the supply voltage is not a simple thing to accomplish...

And yes, the motor power increases, linearly, while the REQUIREMENT increases at the cube. So using my 120% value (we have no idea if that would work, I was just using it as an example) and assuming he can increase the voltage to 120% of his source, then the motor kW increases to 120%, but the REQUIRED power is still 173%. So instead of being 73% short, he is "only" 53% short. Then assuming that MAYBE the motor was over sized by 20%, he is only 33% short.

The motor still overloads...

PS to Bill's post:
Yes, that's an old trick of the trade, ASSUMING that the supply source voltage is the higher (Wye) value of the motor nameplate. But you also DO have to make sure the motor can operate at the higher speed, i.e. cooling fans, balancing and bearing speed rating.




" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

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