DC-injection lets motors do the braking

Much is made of increasingly dynamic performance available from electric motors (and drives), and rightly so. The other side of the equation is ability to stop all that speed and acceleration efficiently, when needed. External braking is the more familiar stopping method, but three-phase ac induction motors provide internal electronic means to slow and brake their motion—among them, dc-in...

04/01/2006


Motors, Devices & Motion Control

Set by a time delay or closed-loop monitoring of ac voltage, start of dc-injection braking often is delayed to allow motor voltage to drop to a “safe level.”


Much is made of increasingly dynamic performance available from electric motors (and drives), and rightly so. The other side of the equation is ability to stop all that speed and acceleration efficiently, when needed. External braking is the more familiar stopping method, but three-phase ac induction motors provide internal electronic means to slow and brake their motion—among them, dc-injection braking .

As the term implies, dc-injection braking (DIB) generates electromagnetic forces in the motor when the controller, in stop mode, injects direct current (dc) into the stator windings—after it has cut off alternating current (ac) supply to two of the stator phases—thus turning off the normal rotating magnetic field. A linear braking torque (ramp) results, which does not decrease with motor speed. DIB current is adjustable over time as well as over a range of full-load ampere (FLA) values. DC-injection can range as high as 300% of motor FLA.

DIB is used when stopping action quicker than coast down is desired. It operates on the principle that imposing direct current on the stator windings produces stationary flux in the motor's air gap (in contrast to the normal rotating magnetic field of induction motors) to which the rotor attempts to align, thereby resulting in strong braking action. Appropriate electronic circuits provide for smooth deceleration to zero speed.

Safety in mind

Time duration for dc-injection must be limited to prevent overheating of the rotor. Experts say that a DIB cycle puts about the same amount of heat energy into the rotor as a direct on-line start with acceleration to full speed. Motor sizing (and the rotor's capacity to dissipate heat) must account for this heating effect if DIB is used for other than occasional emergency stops. Sensors in the control circuit typically determine when the motor has stopped, at which point DIB is turned off, stopping further heat input, and the motor is ready for restart.

While dc-injection braking will effectively stop and hold the motor load, it's not intended to serve as a holding brake. For safety-critical applications an external brake is necessary. DIB also requires a power source at all times; and it's essential to separate, in time, the on-state of ac and dc voltages to avoid short circuits.

Other ac motor electromagnetic braking methods are dynamic braking , where power is dissipated through a resistive load to the dc bus, and regenerative braking , which returns power to the ac supply line. Regenerative braking and DIB also are used effectively in combination. DC-injection braking is a standard feature on a number of variable-frequency drives and motor starters.

Frank J. Bartos, P.E.

fbartos@reedbusiness.co





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