Controlled slowing or stopping of a motor and its driven load is as important as starting in many applications like cranes and electric traction. Sometimes electric braking is better than other breaking systems due to its greater economy and absence of brake wear.
Therefore, electric braking of dc motors has its own importance, keeping it in mind, in this article I am discussing electric braking of dc motors.
Electric Braking of DC Motor
The DC motor is still being widely used due to its excellent braking characteristics and ability of smooth transition from the motor to generator mode and vice versa. During the breaking period, the motor is operated as a generator and the kinetic or gravitational potential energy (cranes or hoists) is dissipated in resistors or returned to the supply. There are three methods of electric braking of dc motors:
- Dynamic or Rheostatic braking and,
- Regenerative braking
Plugging Braking of DC Motor
This method involves the sudden reversal of the connections of either the field or the armature winding of the motor. A strong breaking torque is achieved by maintaining the supply voltage to the armature reversed. To limit the heavy inrush of the current to the armature, the breaking resistor (Rex) must be brought into the circuit. The kinetic energy of the moving system is dissipated in the armature and breaking resistance.
Electric breaking of any type becomes less effective as speed decreases with a consequent decrease in braking torque. The supply must be switched off close to zero speed and applying back-up mechanical or hydraulic breaks to bring the motor to a halt. As the large initial current and the resultant high mechanical stress occurs in the motor during the plugging, the application of plugging is restricted to small motors only.
Dynamic or Rheostatic Braking
In this method, the armature is disconnected from the supply and then a breaking resistor Rex is immediately connected across it. The field circuit is left connected to the supply. The motor acts as a generator, driven by stored kinetic energy.
The whole kinetic energy of the moving system is dissipated in breaking resistor Rex and motor comes to a halt. One very important point to be kept in the mind is that if supply fails, breaking also fails.
In this method, most of the breaking energy is returned to the supply and is used especially where the duty cycle requires the breaking or slowing of the machine very frequently and is most useful in holding a descending load of high potential energy at a constant speed. The condition for regeneration is that the rotational e.m.f is more than the applied voltage so that the current is reversed and mode of operation changes from motoring to generating.
Regenerative breaking of DC Shunt Motor
High inertia loads like electric trains, hoists and cranes are used to control the downward speed while lowering the loads.
We know that a DC shunt motor or a separately excited DC motor will behave like a generator if it’s back EMF, Eb becomes more than applied voltage, V. This situation comes in the scenario when the speed of the motor becomes more than its ideal no-load speed while lowering the load of high inertia. When this happens, the machine starts working as a generator.
Now generator mode is helping us in two ways – controlling the downward moving load (Lenz law) and at the same time, it is converting the potential energy of the load into electricity.
Regenerative breaking of DC Series Motor
We know that when the speed of a DC series motor increases, the current and hence, the field flux decreases. Therefore, we cannot get more back EMF, Eb than applied voltage, V as we were getting in the case of shunt motor while the downward movement of the load. Therefore, regenerative braking is not possible with ordinary DC series motor.
However, in electric traction, where the regenerative braking of the DC series motor is used, the motors are reconnected as separately excited machines while breaking.
Thanks for reading about “electric braking of dc motor”.
DC Motor Braking | MCQ
1. The most economical method of electric braking is
(b) dynamic braking with separate excitation.
(c) dynamic braking with self excitation.
(d) regenerative braking.
2. The electric braking system employed with dc shunt motors, in which the armature connections are reversed so that the motor tends to run in opposite direction is called the …….. braking.
3. Plugging of dc machine is normally executed by
(a) reversing the field polarity.
(b) reversing the armature polarity.
(c) reversing both field and armature polarity.
(d) connecting the resistance across armature terminals.
4. A dc shunt motor, running lightly at 1,000 rpm, is operated under plugging. With plugging connections left as it is the final speed of the motor will be
(b) 1,000 rpm.
(c) —1,000 rpm.
(d) — 2,000 rpm.
5. In plugging of a dc shunt motor, the resistance is inserted in the armature circuit to
(a) increase the braking torque.
(b) stop the motor quickly.
(c) limit the armature current.
(d) have a shorting effect on the shunt field.
6. The plugging provides …………. braking torque in comparison to rheostatic and regenerative braking systems.
7. The electric braking system commonly employed in rolling mills, elevators and printing presses is
8. The dynamic braking is generally used with ……… dc motors.
9. Dynamic braking is very effective if the dc motor
(a) is series excited.
(b) is shunt excited.
(c) is separately excited.
(d) has cumulative compound excitation.
10. For non-reversing dc drives it is preferable to use
(b) regenerative braking.
(c) dynamic braking with separate excitation.
(d) dynamic braking with self excitation.
11. During rheostatic braking of dc series motors,
(a) the speed is reduced.
(b) the direction of rotation is reversed.
(c) the machine operates as a generator.
(d) none of the above.
12. During rheostatic braking the braking torque is proportional to
13. Regenerative braking is based on that the back emf is
(b) equal to applied voltage.
(c) less than applied voltage.
(d) more than applied voltage.
14. In regenerative braking, the motor energy is
(a) dissipated in armature heating.
(b) dissipated in windage losses.
(c) returned to the supply mains.
(d) none of the above.
15. In case of dc shunt motors, the regenerative braking is employed when the load
(a) has an overhauling characteristic.
(b) is variable.
(c) is constant.
(d) also acts as a braking force.
(e) is constantly reducing.
16. Regenerative braking
(a) can be easily applied to dc shunt motors.
(b) can be easily applied to dc series motors.
(c) can be used for stopping the motor.
(d) cannot be used when the load on motor has overhauling characteristics.
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