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var. speed, no more DC motor systems?

S

Steve Tiedman

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Oct 25, 2020
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Hey motor/drive experts- why have the DC motor and drives fallen out of favor, with 3-phase var. speed drive being the go-to system now? Just curious...

20 years ago and more, 3-phase was a rare bird used mostly on the $$$ machines like Oneway, et al. Weekly there were web discussions about salvaging treadmill motors and running them with Minarik drives (similar to how Craft Supplies shipped Woodfast and Vicmarc lathes).

My early 2000s CS-outfitted Vicmarc VL200 shipped with a new Leeson 1.5hp DC and Minarik drive, still working to this day, and if I need it to I can creep down to lower RPMs than the 3-phase system on my new Oneway 1224 can do.

Thanks for the insight and experience.
 
Cheap and powerful semiconductors.

Ultimately we're just talking about different ways of commutating a motor. Classic DC motors use commutator bars and brushes to energize different motor coils and generate torque. There are several disadvantages with this approach, namely brush wear, noise, and low efficiency.

ECMs (electrically commutated motors) use semiconductors to electronically commutate the motor windings. Windings are energized in sequence - switched by e.g. IGBTs, a type of high-power transistor - controlled by a small computer chip. The cost of these chips, and most importantly the power transistors, has fallen precipitously. In light of this, mechanical commutation is all but obsolete in a wide variety of applications. When packaged in a sophisticated control module, fine control of the phase current provides several new features, such as variable power to maintain constant RPM with varying load, overcurrent protection, different acceleration profiles, etc. This is what a VFD is. Combine this with even finer positional control, a permenant magnet rotor, and high-frequency polling, and you have a servo motor.

As for low-RPM performance, this is something that VFDs can achieve but overall there are design tradeoffs where most manufacturers might not prioritize extremely low RPM performance. If this is a priority for you, a servo motor would be ideal - high power, compact, efficient, and extremely tight positional control. AFAIK only Harvey has begun to introduce servo motors to the market, but the motors are available from suppliers for retrofit.
 
It's all about technology. Thanks Aaron.
 
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