What is the best variable speed drive and why?

Salem Straub

Salem Straub

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Oct 20, 2008
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Anyone with experience running machines with VFD/AC drives vs. say, a Minarik Motor Master on a DC motor care to comment on the pros and cons of each? What is smoother, what is cheaper, and what is more energy efficient? Say for a 2 h.p. motor...
 
Compared to DC in the 2hp size, you'll find that a VFD-AC package is much less expensive. In fact, a new 2 hp dc motor alone may cost in the $1200-2000 range...compared to a about $250 for a 2hp AC motor.

Performance wise, they are very similiar in terms of HP and torque....depending on the specific settings for the drives.

The VFD's offer motor protection in terms of thermal overload which is not available on DC drives...VFD also offer dynamic braking, instant reversing, and a wider speed range with over speed settings several times the motors base speed. VFDs are most costly than a DC drive in the same size....but the trade-off in motor cost make them more affordable as a package...in the 2hp size.

Go with a VFD because the cost, motor protection, and the addition features.


-Rob
 
I have six setups with variable speed drives...four with DC and two with AC. I can set up the DC cheaper by buying on ebay as most people have no use for 2-3 hrspwr DC motors. As far as life of the setups...all run great and provide no problems whatsoever...just put controllers in dust free boxes.
 
Thanks fellas, I am just retrofitting right now but my dream is to have a couple KMGs one day with variable on both, I hope you don't stop making those anytime soon Mr. Frink...
 
What Rob said plus:

With AC induction motors, you have 100% torque from zero rpm. Also, until your load exceeds the hp rating of your motor and drive, you have absolute control of the motor speed.

At work, we are seeing many (large) applications where customers are changing from DC drives to AC drives. I don't know of anyone developing new DC drives. The protection features as well as other features such as newer control algorithms trickle down from development work aimed at the larger drives (50hp and up) to the smaller drives used here so you get them "for free".

As the price for semiconductors continue to fall, the difference in the cost between a DC drive and an AC drive is now just a small fraction of the final selling price.
 
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