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 ?
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
Muthu
www.edison.co.in
RE: A VFD system with a MG set
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RE: A VFD system with a MG set
Muthu
www.edison.co.in
RE: A VFD system with a MG set
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
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"Why not the best?"
Jimmy Carter
RE: A VFD system with a MG set
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
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
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: A VFD system with a MG set
Muthu
www.edison.co.in
RE: A VFD system with a MG set
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: A VFD system with a MG set
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
Could you please e-mail me at my sig-line mail id ? Thanks.
Muthu
www.edison.co.in