DC Motor Replacement with AC 1

[sewerfix]

We have a centrifugal pump that is powered by a 675kW DC motor (two speed) which we want to replace with AC motor and AC-VSD control.
Is it as simple as requiring a 675kW AC motor ? Are there other factors , derating factors to consider in relation to the starting characteristics of AC versus DC motors?
Thanks for any feedback

 

[SS7]

Dear Sewerfix,
Correct engineering practice will demand invertor duty motor chosen for starting characteristics of your pump, review of cable(type,insulation,armour etc), length of cable is a restriction for AC-VSD.

Good luck.

 

[jbartos]

Suggestion: Typically, a motor-load marriage is based on the coordination of the load required torque-speed characteristic with respect to the motor output torque-speed characteristic. DC motors as well as AC motors do have output torque-speed characteristics available. Therefore, the load and motor characteristics must be properly aligned for the AC motor the same way as they were aligned for the DC motor.
Additional requirements for the AC motor with its appropriate drive should be implemented via the instruction manuals coming with the equipment and referenced tech support.

 

[CB2]

As long as you are matching the base speed of the motors, this should be a simple change out. Purchase an "inverter duty" motor, and I would not worry about output filters unless the motor wire runs are >500'.

I see your biggest risk in selecting a VFD vendor. The size you are dealing with is at the top of many VFD manufacturers capability. Suggest you seek out VFD manufacturers that are in the big Hp market, ie. ABB, Toshiba.

CB2

 

[jbartos]

Suggestion: A VFD with Active Front End (AFE) tends to produce a low VFD harmonic content.

 

[jOmega]

JB.... you said :

"A VFD with Active Front End (AFE) tends to produce a low VFD harmonic content."

Only if it is DESIGNED from the git-go... to do so. This requires a special design considerations for harmonic mitigation. That is not the same as an active front end for the purpose of allowing the motor load to regenerate back to the ac line. In fact, JB, REGENERATIVE front ends on VFD's make "dirty" on the ac line.... Yea, verily.!

 

[jbartos]

Suggestion to the previous posting: Please, would you provide some references or proofs where these VFDs are designed to make "dirty AC line" and by which manufacturers are they offered?

 

[jOmega]

Suggestion to JB:

Do a harmonic analysis of a drive with a regeneative front end drive in the regenerative state. You could even use a full wave regenerative DC drive as your model.

And, BTW ... They are NOT designed to make dirty on the ac line... No one intentionally sets out to design something that distorts the ac line, JB. Nor was that inferred in my previous post. The distortion that occurs is a consequence of the topology and the process of regeneration into the ac line.

And JB, an AFE, in, of, and by itself doesn't do a thing for harmonic mitigation. Active Front Ends that afford harmonic mitigation are specifically designed for that purpose. They include a controller for the gating of the devices in the AFE. The gating is determined by complex algorithms that determine when current is permitted to be drawn from the phases of the ac line so as to minimize the distortion currents. You could NOT USE such an AFE for regeneration of the overhauling load energy back to the ac line. That also requires a specific gating schema which is not the same as the harmonic mitigation gating schema.

An AFE that does both: harmonic mitigation in both motoring and regenerating quadrants is a very special design and I do not know of any manufacturer that is offering such a product as standard at this time.

Apples and Bananas, JB = Fruit Salad.

 

[CB2]

jOmega, suggest you check out the ABB model ACS800-17 Regenerative VFD, 75 to 1,250 Hp, active front end. The front end of this VFD has 2% current distortion, 2% voltage distortion, power factor can be controlled leading or laging, all in both motoring and regen modes. Standard product but pricey for harmonic mitigation application.

 

[jOmega]

CB2...

Thanks for pointer.

the ACS 800 is a reworked ACS 600 ... (Cost reduced and smaller foot print)... And it looks like they've added some features to it that the 600 didn't have.

My curiosity is piqued about their claims for this series. By chance do they mention anything about line imedance requirements necessary to meet their claims?

I've ordered up some literature on it look forward to being enlightened.

jO