DC Injection Braking System

 DC Injection braking is a method used to rapidly decelerate and stop the rotor of AC induction motors without using friction. 




Application And principle Of Operation.


A typical application of this is in the tripper car conveyor system above, the tripper car is driven by direct-drive motors coupled to the tripper wheels. The car can move freely(forward and backward) on the rail and the car is stopped at precise locations to discharge product. The image below would give you a clearer understanding.


We don't want a situation where we stop the electric motors but the car continues to roll. This is undesirable because you would struggle to stop the tripper car at the exact point where you want to discharge the products. It could also create a dangerous situation where you stop the car but, the moment of inertia could roll the car until it hits the mechanical stoppers at the end of the rail, potentially derailing the car.

Electronic brakes are desired due to the fact that they have no moving part and there is little risk of applying too much braking force and locking the wheels. If the wheels lock up on the rail, there is a possibility it could skid and derail or skid off its desired position stopping position. Finally, constant skidding could erode the wheels of the tripper car and railway. 

The principle of operation of an electronic brake is pretty straightforward. When AC current has been switched off,  a DC current from a Brake Module or VFD is applied to two of the windings of the motor, this creates a stationary magnetic field and a stationary torque which causes the motor to quickly brake.

In the normal operation of an induction motor, when 3 - phase AC current is supplied to the induction motor stator circuit and this produces rotating magnetic field. The rotor is magnetized and it  attempts to catch the magnetic field that keeps moving creating motion on the motor shaft. When braking is needed, DC current is injected into two windings of the 3 phase induction motor This DC current produces a static magnetic field. The spinning rotor is also magnetized and it attempts to align itself with the static magnetic field, thereby resulting in a strong braking action. The braking force depends on the magnitude of the current applied to the stator windings. 

Finally, there are some points to add.
1) There is a slight time delay between the time AC current supply is disconnected from the windings and when the DC braking current is injected. This is to allow current still flowing in the motor windings to decay to a safe level.

2) Keep in mind that DC current applied across a winding, acts like a short circuit, so the braking current must be applied for a limited time and the magnitude of the braking current needs to be controlled to avoid damaging the windings.

3) In the tripper car example used above, DC braking is used to bring the car to a standstill but not used to hold the car in place. A thruster brake which is a friction type brake is used to hold the wheels of the tripper car. 

 

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