6.6kV Motor VSD Upgrade Advice 2

[scott88]

Hi,

We are in the process of beginning a feasibility study into upgrading our main cooling water pump motors to VFD and ideally removing the gearboxes as these are prone to failure.

Motor Details;
6.6kV, 4 Pole, 1500rpm, 2200kw

Pump Speed;
380rpm (Speed range would be 200-380rpm)

My question is; would it be possible to use the existing motor directly couple to the pump and a VFD to control the speed or would we need to replace the motor with say a 12pole to bring the designed operating speed closer to the required speed? If so what are the reasons for this?

Many Thanks,
Scott

 

[ScottyUK]

Your gearbox acts as a speed reducer and torque multiplier (or vice-versa), so neglecting losses it is a constant power device. An electrical analogy would be a transformer.

When you slow the motor down it capable of delivering the same torque as when it is at full speed, so when you reduce speed your motor output power reduces proportionally. It is 'possible' that your motor is so ludicrously oversized that it might have enough torque available to run the pump with a direct drive, but I would be very surprised. I expect that you will need a new motor.


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If we learn from our mistakes I'm getting a great education!

 

[jraef]

What he said.

By the way Scotty, I never thought of the transformer analogy, nice one. Something EEs can relate to. I plan to plagiarize it.

I just ran into a similar issue, a local water utility wants to replace an ancient vertical 900HP 2300V 600RPM synchronous motor with an off-the-shelf 4 pole induction motor. I talked myself blue in the face trying to get the EE to understand why it wasn't going to work by just putting in another 900HP 4P motor and turning the speed down with a VFD, he was either going to have to use a 12 pole 900HP motor which would NOT be off-the-shelf, or a 4 pole 2400HP motor to get the same shaft torque. Finally the ME came in and immediately understood the relationship to torque and pumping capacity.

I'll bet if I had used the transformer analogy for the EE he would have understood sooner... Live and learn.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[scott88]

Hi,

Firstly thanks very much for the informative replies. Please could I ask you to expand on your answer to help me understand further, so is torque proportional to speed and power output. That's where I am slightly confused. How can I calculate if the motor is oversized or what motor would be compatible and what curves are required?

Again thanks, your responses are greatly appreciated........in the mean time I will begin to use google as a self learn tool

Thanks,
Scott

 

[waross]

For simplicity let's assume a motor with an auxilliary cooling system so motor cooling is not speed related.
As the motor speed is reduced the safe allowable torque load remains constant. As the speed is reduced, the HP is reduced proportionately.
As an exercise calculate the torque of your 2200 KW motor at 1500 RPM.
Now calculate the HP of a motor of that torque turning at 300 RPM.
You will find that the HP is now 300rpm/1500rpm x 2200 KW = 440 HP.
I doubt that 440 HP will be enough to drive your pumps. The fact that the gearboxes are prone to failure suggests that the motors are fairly well loaded.
ScottyUK and jraef; Great answers, but sometimes another way of explaining is helpful.

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

 

[ScottyUK]

jraef,

I've used that one for so long that I can't even remember who told me originally. Does that mean I can claim originality?


Bill,

Your North American horses must be 1000W horses instead of our puny European 746W ones.


Scott,

You're going to need some assistance from your mechanical or process engineering brethren to find out what the power requirement of your pump is at 380 rpm. There should be a set of pump curves from which you can determine the power input to the pump. I really doubt that you will be able to justify over-sizing the motor by such a margin based on energy saving over a typical RoI period. That leaves you with either using a custom motor, or replace the pump with a higher speed design.

Have you considered why the gearboxes are failing? If the gearbox is failing as a result of the savage acceleration during a DOL start then the ability of a VFD to ramp the speed may solve a lot of your reliability problems and also give you the variable flow control you desire. I did something similar with some large cooling tower fans a few years ago: the fans with DOL starters used to break gearboxes all too frequently and would occasionally throw a blade, while failures on those we modified to VFD became very infrequent. The VFD also allowed us to replace an expensive two-speed motor with a standard 4-pole machine without any loss of functionality.


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If we learn from our mistakes I'm getting a great education!

 

[waross]

I've been trying to limit my coffee intake with the results you see. If you read 440 kW for 440 HP does it come out properly. (grin?)

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

 

[ScottyUK]

Reads perfectly Bill.


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If we learn from our mistakes I'm getting a great education!

 

[jraef]

Limit coffee intake!!???

Blasphemer!

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376