Electrical Machines - Questions and Answers
Information courtesy of ALSTOM.

BRAKING OF A.C. MOTORS

1. When is braking of an electric motor required?

When there is a need to bring a drive quickly to rest, to hold a drive at standstill after some operation has been completed, or to check the speed rise of a motor with an overhauling load.

2. What are the principal ways of braking electric motors?

By mechanical braking, by arranging the motor itself to exert a braking torque, or a combination of the two methods.

With mechanical braking, a drum or disc friction-type brake is usual, the brake shoes being held off against spring loading by a solenoid or electrically-operated thruster gear normally connected across the motor terminals. Apart from mechanical braking, there are three principal methods:

1. Regenerative braking, applied to overhauling loads; it can only be used to reduce the speed to the no-load or synchronous value.
2. Counter-current braking or plugging, achieved by reconnecting the machine so that its output torque reverses.
3. Dynamic braking, obtained by disconnecting the machine from the mains and establishing a fixed magnetic field from a DC supply which causes e.m.f.s. to be induced in the rotor windings. Powerful dynamic braking may be obtained by introducing capacitors into an induction-motor circuit, with or without DC injection.

3. How is plugging applied to AC motors?

The phase sequence of the supply (in the case of 3-phase) is reversed by interchanging two leads. The usual arrangement for squirrel-cage motors is by a reverse contactor which closes when the stop button is operated. The supply to the motor must be disconnected as the speed nears zero. This is effected by having a reverse-rotation relay coupled to the motor shaft and connected to open the braking-contactor coil before the motor reverses. The motor takes heavy current from the supply unless resistance is added to the stator circuit. Rotor resistance may be employed to ensure that a high braking torque is obtained.

Plugging is the commonest braking method used with Scharge type AC commutator motors. With this type, provision must be made in the control gear to insert a resistance in each secondary phase during plugging to limit current to a safe value.

4. How is DC-injection braking applied to induction motors?

Direct current is injected into the stator winding after this winding has been disconnected from the supply. This sets up a stationary field, inducing e.m.f.s. and hence currents in the rotor circuits. This method provides a high-braking torque with low losses, but the braking effect is small at high speeds for machines of normal slip and it is usually necessary to use heavy DC exciting currents to effect a quick-stop. Some improvement of the braking characteristic can be made by increasing the rotor-circuit resistance at high speeds and reducing it as the speed falls.

Fig. 37- Basic connection diagram for capacitor and magnetic dynamic braking.
Operation of stop button opens supply lines and inserts capacitors 'C' across motor terminals. Voltage relay 'VR' provides the necessary delay before operating to short-circuit the motor terminals. Instead of 'VR' a timer or limit switches maybe used R1 are resistors for controlling braking torque. R2 discharge resistors and R3 control resistor for relay drop-out voltage.

The control gear must include a transformer and rectifier to give a low-voltage heavy-current DC which is applied to the motor windings when the stop button is operated. If secondary resistance is included to increase the available braking torque, provision for this is required. The direct current is usually disconnected after a time delay. Direct-current-injection braking has the advantage over plugging that reversal of the motor cannot take place.

5. Can DC-injection braking be applied to other types of AC motor?

It is commonly applied to stator-fed shunt AC commutator motors and may also be used with Scharge motors. Dynamic braking can be applied lathe synchronous motor. The supply is disconnected while the DC field of the motor is maintained and resistance connected across the stator winding. The machine then runs as an alternator feeding into a load of fixed resistance.

6. How is capacitor braking applied to induction motors?

Capacitor dynamic braking uses the ability of the induction motor to self-excite if sufficient capacitance is connected across its terminals when the supply is removed. The motor then runs as an induction generator and dissipates power with subsequent braking effect.

If the motor terminals are then short-circuited, magnetic braking follows. A typical basic connection diagram is shown in Figure 37. Capacitor braking followed by simultaneous magnetic and DC-injection braking may be applied when load inertia is very high.