MOTION SYSTEMS HANDBOOK
Brakes, clutches,
and torque limiters
There are three types of electrical braking for motors — regenerative braking, dynamic braking, and plugging. Of the three methods, plugging provides the fastest stop, but it can be harsh on both the electrical and mechanical components. Because of this, it’s the least commonly used method of braking … though appropriate for some applications. Plugging (called reverse current braking in some contexts) is possible on both dc motors and ac induction motors. For dc motors, plugging is via reversal of armature-voltage polarity. When this happens, back EMF voltage no longer opposes supply voltage. Instead, the back EMF and the supply voltage work in the same direction, opposing the motor’s rotation and causing it to come to a near-instant stop. The reverse current produced by the combined
supply voltage and back EMF is extremely high, so resistance is placed in the circuit to limit the current. One potential problem with plugging for braking (especially when braking time is short) is that it can be difficult to brake the motor at exactly zero speed. Another drawback to plugging is that it can induce high mechanical shock loads on the motor and connected equipment, due to the abrupt stop that it causes. Plugging is also very inefficient for stopping and generates significant heat. So for most modern motion designs requiring high throughput — external brakes and clutches are required to get sufficiently quick stops or disengagement. This applies to conveyors, airport-baggage handlers, escalators, and elevators … as well as other axes that make frequent stops and starts — even as few as 10 cycles a minute in some cases. With stops and starts at much higher cycle rates,
Torque limiters versus overrunning clutches ROLLER CLUTCHES •
SOMETIMES CALLED ROLLER-RAMP CLUTCHES
Related components include those classified as mechanical overrunning clutches.
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DESIGN WORLD — MOTION
Brakes & Clutches & Torque Limiters — Motion Control HB 08-19 V4.indd 38
8 • 2019
CAM CLUTCHES
EXHIBIT HIGH TORQUE DENSITY
RATCHE TING CLUTCHES ARE SIMPLE TO INTEGRATE
motioncontroltips.com | designworldonline.com
8/21/19 2:02 PM
