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A VFD system with a MG set

A VFD system with a MG set

A VFD system with a MG set

(OP)
I am planning on a VFD system for testing large MV motors (3.3 & 6.6 KV) motors up to 5000 HP on no-load along the following line:

A 150 HP DC Motor driving a 150 KVA AC 415 V generator. The generator output is stepped up by 150 KVA, 415 V/3.3-6.6 KV trafo. The motor under test has a power factor correcting capacitor to offset the motor no-load current, leaving the whole system needing to feed the motor no-load losses only, which I figure will be 2-3% of motor HP.

This way, by maintaining a constant V/F (by controlling the DC motor speed), the motor inrush current will be the same as the motor no-load current right up to the full speed, without the use of an expensive VFD.

Do you see any flaws in the above scheme ?

Muthu
www.edison.co.in

RE: A VFD system with a MG set

(OP)
The PFC capacitors will be rated for 3.3 KV and 6.6 KV ranging from 50 to 200 KVAR with a total of about 1500 KVAR in each voltage.

Muthu
www.edison.co.in

RE: A VFD system with a MG set

Of course you'll have to make sure the system is undercorrected at the highest frequency. Otherwise you can get resonance and high voltages.  

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: A VFD system with a MG set

(OP)
Of course pete. I try to mtach about 90% of no-load current.

Muthu
www.edison.co.in

RE: A VFD system with a MG set

150 HP driving 150 KVA apparent power is a PF of about 75%.
How about selecting a larger generator and letting the generator supply more of the reactive current.
There will be very little increased load on the drive motor.
The reason that I suggest a larger alternator is the fall off of VARs when both frequency and voltage are reduced.
You may have very little VAR contribution from the capacitors at low speeds. You may have to supply most of the VARs from your generator until the speed, frequency and voltage come up near rated values.

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

RE: A VFD system with a MG set

(OP)
A good idea Bill. Oversizing the generator could give me a bit more breathing space.

I don't understand your point that the caps will not give sufficient vars at low frequencies. Could you expand on that ? I was thinking with a constant V/F, the source impedance of motor inductance and the capacitor would remain constant from zero speed to full speed. Am I missing something ?

Muthu
www.edison.co.in

RE: A VFD system with a MG set

It works the other way round with capacitors. While reactor impedance goes down with frequency and therefore works 'your way' it is opposite with the capacitors. Their impedance increases when frequency goes down. At half frequency, they have twice the impedance. And, since voltage is also halved, you will have 25 % of the full frequency vars. At one tenth speed, you will only have 1 % vars.

Bill's cath is a very valid one and I think you will be sorry if you try to get any vars out of the capacitors at low frequency.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: A VFD system with a MG set

(OP)
Bill & Gunnar - Got it. While the inductive current remains the same for constant V/F and the capacitive current varies with the product of V & F.

Muthu
www.edison.co.in

RE: A VFD system with a MG set

The good news is that you don't need a larger prime mover, just a larger generator. Supplying motor VARs with a generator rather than capacitors will mean a little more I2R losses in the generator but not a great percentage increase in real power demand.

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

RE: A VFD system with a MG set

(OP)
Yes Bill. I need to tweak my system a bit more, like reduce the V/F to the extent it will start that damned motor while simultaneously increasing the cap Vars so that I land up in an economical generator.

I also want to scope out the one minute overcurrent capability of the generator. What do you think ? A 5 to 1 overcurrent for one minute (which is what I expect the time to get to full speed with reduced V/F will be) will kill the generator or not ?

Muthu
www.edison.co.in

RE: A VFD system with a MG set

(OP)
pete

Could you please e-mail me at my sig-line mail id ? Thanks.

Muthu
www.edison.co.in

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