DC injection braking

Lore & Background

DC injection braking operates by applying direct current to the motor stator windings after the AC supply is disconnected, creating a stationary magnetic field that applies static torque to the rotor. This slows and eventually halts the rotor completely, and as long as DC voltage is applied, the rotor is held in position and resistant to spinning. The braking force and holding power increase with higher voltage, but current must be limited to a few seconds to prevent motor overheating.

In a thyristor-controlled injection braking unit, the injected voltage is obtained by rectifying the supply voltage using two thyristors connected as a phase controlled rectifier. Braking torque depends on current magnitude, which is varied by phase control of the thyristors. When the motor is switched off, a motor contactor relay disconnects AC supply and causes a braking relay contactor to close, initiating a sequence: a time delay of about 300ms allows residual current to diminish, then thyristors fire to produce braking current for a few seconds, after which the current decays and the braking relay contactor opens after about 1.5 seconds, allowing the motor to be restarted. Two potentiometers typically adjust braking torque and timer duration, balanced to avoid exceeding rated motor torque and to limit braking time to prevent overheating.

Reader's Guide

DC injection braking is significant as an alternative or addition to friction brake systems. It requires only a small module located with motor switchgear or drivers, mounted remotely, whereas friction brakes must be mounted on the rotating system and have consumable parts that wear out and require replacement. DC brake modules have no consumable parts and require no maintenance, while friction brakes need actuation by a human operator or system-controlled actuator, adding complexity. DC brakes integrate easily into motor control circuitry. However, for motors controlling hanging loads such as elevators or cranes, friction brakes remain necessary as a primary stopping method to securely hold the load. The legacy of DC injection braking lies in its ability to reduce downtime and provide emergency stopping without mechanical wear, though it is limited by overheating risk if applied too long and by the need for friction brakes in load-holding applications.

Did You Know?

• DC injection braking uses direct current injected into the AC motor winding after AC voltage is disconnected.
• The braking current should only be applied for a few seconds or the motor will overheat.
• A thyristor-controlled unit includes a time delay of about 300ms before braking current begins.