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Old follow up: AC motors replacing series DC

Old follow up: AC motors replacing series DC

Old follow up: AC motors replacing series DC

(OP)
Greetings.

Quite some time ago, I started a discussion about a customer situation which called for us to replace a big, nasty DC series field motor with gearcase, jackshaft, couplings, gearing with modern gearmotors, to move a machinery carriage.

Some readers opined that it would work fine, others were dubious. A few asked me to report how it worked out. I am probably cursing myself by doing this, but I'd like to report favorably after about 6 months.

So far it has worked very well and the users LOVE it. We used two 15hp German gearmotors driven by one VFD with dynamic braking. Accel/decel is now beautifully smooth (S curve) and predictable. Every motion of the machine now has "two step" control. First notch gets minimum speed; second notch acts as an accelerate point. Drop the button completely and pick it back up before stopping, and it holds the reduced speed. We even added radio control. I believe the machine is much safer and will last much longer. Not to mention we removed a LOT of dead weight from the machine and increased energy efficiency by eliminating the old electromechanical controls and resistors. Eventually they may need new drives, but that seems pretty cheap compared to new electromechanical controls at $65000.
 
I've worked on similar projects recently, and the biggest single realization I've had is: if you can't quite identify or (affordably) match the starting torque of the series DC motors, can you simply slow the acceleration a bit or use S-curve? Unless the cycle time is absolutely critical, slightly slower operation might be perfectly acceptable when the user realizes they can place a machine right where they want it after a few minutes practice, and save wear and tear on components. One example would have replaced 6 large open spur gears, 4 large bevel gears, a driveshaft, a bunch of friction bearings, one DC motor with two AC gearmotors running in synthetic lube. We haven't gotten that contract, but I'm confident it would work well.

Regards...

 

RE: Old follow up: AC motors replacing series DC

Hey eeinpa, thanks for the feedback. It's much appreciated.

Keith Cress
kcress - http://www.flaminsystems.com

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