DC Injection braking is one of the braking methods used for AC Induction Motors Braking. In this article Electrical Engineering XYZ shares the fundamentals of DC Injection braking.
If a spinning AC induction motor’s stator coils are energized with DC rather than AC, the rotor will find itself spinning inside a stationary magnetic field. This causes currents to be induced in the rotor bars, which in turn causes a braking force to develop in the rotor in accordance with Lenz’s Law. The effect is exactly opposite of what happens when a motor is energized from a stand-still: there, currents are induced in the rotor bars because the rotor is stationary and the stator field is rotating. This method of braking is quite effective, with only small amounts of direct current through the stator winding being necessary to cause a large braking torque.
The braking torque produced by DC injection varies directly with the magnitude of the DC injection current, and also directly with the speed of the rotor. This means the braking force created by DC injection tends to diminish as the motor slows down to a stop.
When any motor acts as a brake, the kinetic energy of the motor and the mechanism it attaches
to must go somewhere. This is a basic tenet of physics, codified as the Law of Energy Conservation: energy cannot be created or destroyed, only altered in form. When DC injection is used to brake a motor, the braking energy is dissipated in the form of heat by means of the induced currents circulating through the rotor bars and shorting rings. This is something one must be careful to consider when choosing DC injection as a braking method: can the rotor safely dissipate the heat when needed? Repeated braking cycles, especially with little time between cycles, may overheat the rotor and cause damage to the motor.
Modern solid-state AC motor drives easily provide DC injection for braking. All they need to do is energize their output transistors in such a way that one or more of the stator windings sees a constant voltage polarity instead of an alternating polarity as is the case when the motor is running. The following diagram shows the power flow into the motor during DC injection:
The figure below shows a VFD injecting DC to the motor
The intensity of the DC injection current may be varied by altering the pulse-width duty cycle of the transistors used to switch the braking current.
Summary:
DC Injection Braking is one of the braking methods used for braking AC motors. List of all braking methods:
- DC injection braking
- Dynamic braking of AC Induction Motors
- Regenerative braking of AC Induction Motors
- Braking using Plugging
References/Further reading:
- AC Motor Braking methods PDF Handbook extracted from Instrument Handbook authored by Tony R Kuphaldt under CC4.0L