In this article we will discuss about:- 1. Meaning of Electric Braking 2. Types of Electric Braking 3. Advantages 4. Disadvantages and Limitations.
Meaning of Electric Braking:
If the load is removed from an electric motor and supply to it be disconnected, it will continue to run for some time due to inertia. The time elapsing before it stops will be especially long if the motor is massive and has run at high speed.
It is essential, however, in many cases that the motor and its driven machine be stopped quickly (in machine tools, cranes, hoists etc). In fact, quick stopping of a motor is more essential than quick starting. Delay in starting up a motor only causes the machinery to stand idle; a delay in stopping a motor may result in heavy damage to equipment or to the manufactured products and even the loss of human life.
Based on the purpose for which braking is employed, it is of the two types, viz, braking while bringing the drive to rest and braking while lowering loads. In the first type, the device employed for braking absorbs the kinetic energy of the moving parts while in the second one, it absorbs, in addition to the kinetic energy, potential energy, usually gravitational which can drive the system at an excessively high speed.
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Braking, while stopping, may be used for any one of the following objectives:
(i) Reducing the time taken to stop.
(ii) Stopping exactly at specified points, for example in lifts; sometimes such precise stops are necessary for reasons of safety.
(iii) Feeding back, atleast a portion of the power, to the supply system.
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Braking, while lowering loads, may be employed for any of the following purposes:
(i) Controlling the speed at which the load comes down and limiting it to a safe value.
(ii) Feeding power back to the supply system.
The main features of a satisfactory brake are that:
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(i) The braking should be quick and reliable in action,
(ii) The braking torque must be controllable,
(iii) Some suitable means must be provided for dissipation of kinetic energy of the moving parts of the motor and its driven machines and
(iv) Failure of any part of the braking system must result in application of brakes.
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Braking torque can be applied either by mechanical (or friction) brakes or electrodynamically. In mechanical braking, the stored energy of rotating parts is dissipated in the form of heat by a brake shoe or band rubbing on a wheel or a brake drum. In electrical (or electrodynamic) braking the stored energy of rotating parts is converted into electrical energy and dissipated in the resistance in the form of heat or returned to the supply. Here we will deal with electric (or electrodynamic) braking only.
Types of Electric Braking:
There are three types of electric braking, all of which are applicable to the usual types of electric motors, viz. plugging (or counter-current braking), dynamic (or rheostatic) braking and regenerative braking.
1. Plugging or Counter-Current Braking:
This is the simplest type of braking. Plugging or counter-current braking occurs when the motor windings are connected for reverse direction of rotation at a time when the armature is still rotating in the forward direction either under the action of an external torque or due to inertia. Plugging involves reconnection of power supply to the motor so that it tends to drive in the opposite direction. It is obvious that, left to itself, the system will come to rest and then accelerate in the reverse direction. In case, it is required to bring the drive system to rest, it is necessary to include a special device to cut off the supply exactly at the instant when the motor stops.
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During the braking period, the energy is drawn from the supply. The energy drawn from the supply and stored or kinetic energy of the rotating parts of motor and its driven machines are dissipated in the series current limiting resistor. Thus this method is wasteful of energy. Other drawbacks of this method of electric braking are the shock caused to the motor and equipment and heavy inrush of current at the time of braking.
This method provides greater braking torque than that provided by rheostatic (or dynamic) braking. This method is used to get either a quick reversal or to get a rapid stop. This method is commonly used in controlling rolling mills, elevators, printing presses, machine tools etc.
This method can be applied to direct current, alternating current induction and synchronous motors.
2. Dynamic (or Rheostatic) Braking:
In this method of braking the motor is disconnected from the supply and operated as a generator driven by the kinetic energy of the rotating parts of the motor and its driven machines. Thus the kinetic energy of rotation is converted into electrical energy, which is dissipated in the external resistance connected across the motor at the braking instant. By this method, energy required from the supply to brake the motor, has been eliminated as compared to the previous method (plugging) and this is marked advantage. This method of braking can be applied to brake the direct current motors, synchronous motors, and induction motors (with a separate source of dc excitation during braking).
3. Regenerative Braking:
In the previous two methods of electric braking namely plugging and rheostatic braking stored energy of the rotating parts of the motor and its driven machine is wasted whilst in plugging extra energy is drawn during the braking period and is wasted. In regenerative braking, mechanical energy is converted into electrical energy, part of which is returned to the supply and the rest of the energy is lost as heat in the windings and the bearings of the electrical machines. Regeneration does not, in general, involve any switching operation, unless it is required to change the speed at which it becomes effective. Most of the electrical machines pass smoothly from motoring to generating operation, when overdriven by load.
For regenerative braking it is necessary for:
(i) Supply voltage to drop,
(ii) The motor to be overexcited or
(iii) The motor to be running at a speed higher than no-load speed.
In all of these cases the armature current is reversed and a retarding torque is produced, slowing down the motor until the back emf of the armature becomes equal to the supply voltage. Regenerative braking will not stop the motor. It is effective only for braking hauling loads. They may occur in a crane motor due to action of the lowering load or in the motor of an electric locomotive moving down grade.
Advantages of Electric Braking:
(i) Since mechanical braking, due to excessive wear on the brake blocks or brake lining, needs frequent and costly replacement but no such replacement is required in electric braking, hence there is saving on this account.
(ii) Frequent adjustments are required to compensate for the wear in mechanical braking thereby making maintenance costly. In electric braking there is no cost by way of maintenance of the brake shoes or lining.
(iii) By employing electric braking the capacity of the system can be increased by way of higher speeds and haulage of heavy loads.
(iv) In some cases of electric braking, a part of energy is returned to the supply thereby affecting a considerable saving in the operating cost. This is not possible with mechanical braking.
(v) In electric braking, heat product, in no way, is harmful to the braking system where as in mechanical braking heat is produced at brake blocks or brake lining which may result in failure of brake.
(vi) Mechanical braking produces metal dust which, if finds ingress to bearings, will cause over-wearing of the bearings. No such dust is produced in electric braking.
(vii) Incorrect adjustment of mechanical brakes may result in very harsh and sudden braking being applied which may produce shock loading of ropes and mechanical parts or in case of passenger lifts and trains it may result in the discomfort to the occupants. Electric braking on the other hand is smooth and without shock.
Disadvantages and Limitations of Electric Braking:
(i) In electric braking, the driving motor operates as a generator during the period of braking, and motor ceases to operate as a generator at standstill so that although an electric brake can almost stop a machine or load, but it cannot hold it stationary, therefore, a friction brake is required in addition.
(ii) Since the motor has to function as a generator during the braking period, therefore, it must have suitable braking characteristics i.e., choice of motor is limited.
(iii) Additional complications, high initial cost, special motors capable of generating electrical energy make electric braking costly.
Economic considerations have a considerable bearing on the use of electric braking.