Methods of Electrical Braking of AC motors | Electrical Machine II | AC Machines

There are generally three types of electrical braking for motors: regenerative braking, dynamic braking, and plugging. Of the three methods, plugging provides the fastest stop, but it can be harsh on both the electrical and mechanical components.

1. Methods of Electrical Braking of AC Motors ELECTRICAL MACHINE II (EE-353) ANEEQUE AHMAD ********- 012 Group No.04 AHMAD FARAZ ********- 019 ALI HAMZA ********- 020 ARBAB HASSAN ********- 035

2. What is Braking? The term braking comes from the term brake. A  brake  is a  mechanical  device that inhibits motion by absorbing energy from a moving system, like vehicles, locomotives. Two types of Braking Mechanical Braking Electrical Braking

3. Continue… Mechanical braking In this process of braking, the kinetic energy of the rotating parts is dissipated in the form of heat by the brake shoes of the brake lining that rubs on a wheel of vehicle or brake drum. Electric braking In this process of braking, the kinetic energy of the rotating parts of the motor is converted into electrical energy which in turn is dissipated as heat energy in resistance or in sometimes, electrical energy is returned to the supply. Here, no energy is dissipated in brake shoes

4. ADVANTAGES Small maintenance cos t  Energy saving   Reliability Smooth operation   System Capacity

5. METHODS OF ELECTRICAL BRAKING Electrical braking of AC motors can be done by different methods, which are- Regenerative Braking Dynamic Braking Plugging Braking DC injection Braking

6. REGENERATIVE BRAKING In the Regenerative braking method the motor is not disconnected from the supply. It remains connected to it and feeds back the braking energy or it kinetic energy to the system. As no energy is wasted in this method and it is supplied back to the system, thus overall energy is saved.

7. REGENERATIVE BRAKING Thus the regenerative braking is better than Plugging and Dynamic braking. This type of braking is used in traction whenever the train runs on down. The regenerative braking is applied to DC shunt motor, series motor and 3-phase induction motors

8. Explanation We know the power (input) of an induction motor is given as. P in = 3VI s cos φ s Here, φ s the phase angle between stator phase voltage V and the stator phase current I s . Now, for motoring operation φ s < 90 o and for braking operation φ s > 90 o .

9. Explanation When the speed of the motor is more than the synchronous speed, the relative speed between the motor conductors and air gap rotating field reverses, as a result the phase angle because greater than 90  , and the power flow reverse and thus regenerative braking takes place.

10. Explanation If the source frequency is fixed then the regenerative braking of induction motor can only take place if the speed of the motor is greater than synchronous speed, But with a variable frequency source regenerative braking of induction motor can occur for speeds lower than synchronous speed

11. Advantage & Disadvantage The main advantage of this kind of braking can be said that the generated power is use fully employed and T he main disadvantage of this type of braking is that for fixed frequency sources, braking cannot happen below synchronous speeds.

12. DYNAMIC BRAKING ANEEQUE AHMED 18143122-012

13. TYPES AC dynamic braking Self-dynamic braking DC dynamic braking

14. AC DYNAMIC BRAKING This type of induction motor braking is obtained when the motor is made to run on a single phase supply by disconnecting any one of the three phase from the source . It is either left open or it is connected with another phase. When the disconnected phase is left open, it is called two lead connection and when the disconnected phase is connected to another machine phase it is known as three load connection .

15. AC DYNAMIC BRAKING

16. Explanation When the motor is running on 1-phase supply, the motor is fed by positive and negative sequence, net torque produced by the machine at that point of time is sum of torques due to positive and negative sequence voltage. At high resistance the net torque is found to be negative and braking occurs. From the previous figure the two and three load connections can be understood.

17. SELF DYNAMIC BRAKING In this method there capacitors are kept permanently connected across the source terminals of the motor. The value of the capacitors are chosen depending upon their capability to deliver enough reactive current to excite the motor and make it work as a generator. The braking connection and self-excitation process is shown in the figure.

18. SELF DYNAMIC BRAKING

19. DC DYNAMIC BRAKING To obtain this type of braking the stator of a running induction motor is connected to a DC supply. Two and three load connections are the two common type of connections for star and delta connected stators.

20. DC DYNAMIC BRAKING

21. Explanation Now coming to the method of operation, the moment when AC supply is disconnected and DC supply is introduced across the terminals of the induction motor, there is a stationery magnetic field generated due to the DC current flow and as the rotor of the motor rotates in that field, there is a field induces in the rotor winding. As a result the machine works as a generator and the generated energy dissipates in the rotor circuit resistance and dynamic braking of induction motor occur

22. PLUGGING BRAKING AHMAD FARAZ KASHIF 18143122-019

23. What is Plugging Plugging is the method of inducing negative torque in the rotor of an induction motor to rapidly bring its speed of rotation to zero. This is done by reversing the supply connection at the stator terminals.

24. PLUGGING Plugging is the most powerful method of braking an electric motor, consisting of actively applying power to the motor in the opposite direction of its rotation. This is analogous to reversing the engine thrust of a power boat or an airplane in order to quickly bring it to a halt. For a VFD, this means a reversal of phase rotation while carefully applying power to the AC induction motor.

25. What is DC injection DC Injection Braking is a braking system used with 3-phase motors. The VFD outputs a DC current that gets applied to two of the windings of the motor, which causes the motor to quickly brake .

26. Continue… Like DC injection braking, plugging requires power be applied to the motor in order to make it stop, and it also results in all the kinetic energy being dissipated in the rotor. The advantage held by plugging over DC injection braking is that the braking torque may be maintained and precisely controlled all the way to zero speed

27. PLUGGING BRAKING OF INDUCTION MOTOR Plugging induction motor braking is done by reversing the phase sequence of the motor. Plugging braking of induction motor is done by interchanging connections of any two phases of stator with respect of supply terminals. And with that the operation of motoring shifts to plugging braking. During plugging the slip is (2 – s), if the original slip of the running motor is s, then it can be shown in the following way.

28. PLUGGING BRAKING OF INDUCTION MOTOR

29. DC INJECTION BRAKING ARBAB HASSAN 18143122-035

30. 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.

31. DC INJECTION BRAKING DC injection braking works just as its name implies by injecting DC voltage into the motor windings. Before DC injection braking can begin, the AC power to the motor stator must be disconnected typically by the opening of a relay. In turn, a relay controlling motor braking closes, DC bus allowing DC voltage sourced by the  to be applied to the windings .

32. DC INJECTION BRAKING To prevent the braking current from exceeding the drive and motor ratings, circuitry in the VFD(variable frequency drive) or in an external braking module controls the amount of voltage applied .

33. Explanation When DC current is applied to the motor windings, it creates a fixed (rather than rotating) magnetic field. Braking action is produced by the rotor working to align to this stationary field. The higher the DC current, the stronger the braking force. 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 .

34. APPLICATION Industrial AC motors run at high speeds so they can power heavy duty equipment and machinery. Using high power motors is crucial to any operation that requires a reliable motor for speed and quality. AC motors are used in a variety of applications. They can power fans, planers, conveyors, rolling mills, cut-off saws, wood hogs, rubber mills, presses, rock crushers, shredders, chippers, centrifuges, grinders, and other machines or equipment.

35. Limitations The energy generated during braking is dissipated as heat by the motor (particularly the rotor) and the controller, so thermal limitations of these components dictate how much braking current can be applied and the amount of time for which it can be applied before overheating occurs . If DC injection braking is used frequently, the additional heating created during braking must be taken into account when sizing the motor .

36. Limitations Although DC injection braking can be used to hold a load stationary, it’s not generally recommended due to heat generated in the motor. And because DC injection brakes require a constant power supply, they’re not considered fail-safe devices.

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