Braking a Motor
Two common basic methods used for braking a motor are DC injection braking and dynamic braking. We will look at both in detail, starting with electric braking.1. DC Injection Braking
DC injection braking is a method of braking in which direct current (DC) is applied to the stationary windings of an AC motor after the AC voltage is removed. This is an efficient and effective method of braking most AC motors.DC injection braking provides a quick and smooth braking action on all types of loads, including high- speed and high-inertia loads.
Recall that opposite magnetic poles attract and like magnetic poles repel. This principle, when applied to both AC and DC motors, is the reason why the motor shaft rotates.
In an AC induction motor, when the AC voltage is removed, the motor will coast to a standstill over a period of time, since there is no induced field to keep it rotating. Since the coasting time may be unacceptable, particularly in an emergency situation, electric braking can be used to provide a more immediate stop.
By applying a DC voltage to the stationary windings once the AC is removed, a magnetic field is created in the stator that will not change polarity.
In turn, this constant magnetic field in the stator creates a magnetic field in the rotor.
Since the magnetic field of the stator is not changing in polarity, it
will attempt to stop the rotor when the magnetic fields are aligned (N
to S and S to N).DC injection motor braking demonstration (VIDEO)
The only thing that can keep the rotor from stopping with the first alignment is the rotational inertia of the load connected to the motor shaft. However, since the braking action of the stator is present at all times, the motor is braked quickly and smoothly to a standstill.Since there are no parts that come in physical contact during braking, maintenance is kept to a minimum.
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