DC motor vs. AC motor Efficiency.

[KeepCool]

I am working on a project where they replaced a 3000hp DC motor with a 3000hp AC motor. They are claiming significant energy savings by going with an AC motor with VFD control. The old DC motor also had speed control. The 2 motors have very similar efficiences (95.4%-DC, 96%-AC).

Is there significant energy savings inherent in an AC motor with VFD control as compared to a DC motor with speed control?

 

[RadarRay]

I've never done a direct compare of power unit efficiencies but I would guess they are pretty close. The AC drive has to convert from AC to DC and then AC so it might be a little less efficient than DC.

Where the big gain occurs is in Power Factor. The DC motor roughly goes from 20% at low speed to 80 to 85% at high speed, with load having some affect. More load slightly better.

The AC power unit is probably somewhere between 90% to 95% power factor. Why? Well how do you correct power factor? You use capacitors. What is an inverter full of Capacitors.

So my guess is they are mainly talking about the power factor benefit when they are saying it is more efficient.
And that means less power is used from the line to create reactive power on the AC drive than on the DC drive.

 

[GusD]

The associated costs of maintaining a 3000hp DC motor versus a 3000 hp AC drive, will in short time offset all
other comparative costs.
GusD

 

[EdDanzer]

If you need adjustable speed I hope your luck with AC adjustable speed drives is better than mine.
So far the maintance cost of the AC drive exceeds the costs of the DC drive.

 

[SolDirJoe]

radarray is right. Power Factor is the key unless you are always operating at 80% or above base speed.

IGBT transistors and the rectifier volt drops AC Invertor drives], are lower than the voltage drop across hi-power thyristors [dc drive convertors] so heat is not quite as high.

However, the switching losses [very high frequency current pulses] in the cables connecting the invertor to the motor can be horrendous and lead to cable and EMC problems cuased by harmonic curents. This is not a fault of invertors, usually lack of knowledge of the installer. Screened cables should be fitted to all AC induction motors fed by AC Invertors. This can be expensive if the installation is badly managed. Speak to a reputable supplier. They have a vested interest in foreseeing any problem before you get there. !!

SolDirJoe

 

[jbartos]

Suggestion to the original posting: Please, would you elaborate on the motor load, any potential for regeneration, motor duty, etc. what may be used for more precise motor comparisons?
Also, the motor ac drive could be AC-AC rather than AC-DC-AC.

 

[KeepCool]

In Response to jbartos:
The motor drives a fan for a cement kiln. It's loading varies from 30% to 100%. The AC motor controller is a standard PWM VFD. No potential for regeneration.