DC drive and motor questions; regen and speed ranges

[ctolbert]

OK, I thought I understood this more than I do. Can someone enilghten me a little.

1.) The primary reason for using a regen dc drive versus a non-regen is brakeing as I understand. The real question is it because a non-regen dc drive cannot control the deceleration effectively and a regen drive can?

2.) Someone had mentioned to me that a dc motor/drive can hold full torque at 0 speed. I always understood that a dc motor was more like 20:1 speed range so similar to an ac motor of the same range it would oscilate at the lower speed ranges.

Don't get me wrong....I have had 1000:1 speed range ac motors with encoders and ABB ACS800's never move at 0 speed like on cranes or position apps (with the correct firmware). I'm not as familiar with the dc side of this equation.

Thanks,
Carl

 

[waross]

Regen is used to recover energy. There are dynamic breaking systems that give good control of deceleration but they consume more energy than a regen system.

Some examples of full torque at zero speed or close to zero speed are diesel electric locomotives starting and carriage drives in a sawmill (Which use the same type motor).
Motors to be used over a speed range that includes very slow speeds are often cooled by an external, seperately driven, fan.
A carriage drive will go from full speed in one direction through zero speed to full speed in the opposite direction smoothly.
Although full torque at zero speed is possible it is usually avoided if possible. That's what brakes are for.
Applying torque to balance the load before releasing the brake is often used, but the brushes and comutator may be damaged if high current is maintained indefinitely without rotation.
respectfully

 

[jraef]

Regen is used to recover energy.

Yes but...

An equally valid reason to use it is when the duty cycle is so high that braking resistors become impractical, case in point your example of a sawmill carriage drive. While it's true they recover energy, the primary reason they use regen is because it is a lot more practical to put the braking energy to use elsewhere than to burn it off in resistors who's life is shortened as a result (not to mention the fire hazard of red hot resistors in a sawmill).

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http://www.jraef.com

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[Skogsgurra]

DC motors are, in fact, very well suited to zero speed operation. As Waross says, some are not always happy to produce full torque at zero speed, but that is more a question of cooling the collector (or making it sturdy enough).

Examples of DC motors working happily at zero speed are elevators (yes, there is a brake, too), machine-tool servos (no brake to keep the X still while treading in the Z direction), cable drum tensioners (yes, AC can also be used) and lots of other applications.

The AC motor zero speed problem is inherent in the machine equations. There is no "transformer action" between stator and rotor at zero Hz. So there has to be a low frequency to maintain torque at zero speed. The opposite is true in a DC motor. There is always a main field (from main pole current) and there is always an armature current, even at zero speed. So the transition from CW rotation and torque to CCW is smooth and fully controlled.

As we used to say only fifteen years ago: "If you want DC performance, then try DC!"



Gunnar Englund

http://www.gke.org