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Motors are generally permitted to slow to a stop when disconnected from the power line, but there may be in stances when that isn't an option or not convenient.
There are several methods that can be employed to pro vide braking for a motor. Some of these are:
• Mechanical brakes
• Dynamic braking
Mechanical brakes are available in two basic types, drum and disk. Drum brakes use brake shoes to apply pressure against a drum (Fgr. 1). A metal cylinder, called the drum, is attached to the motor shaft.
Brake shoes are placed around the drum. A spring is used to adjust the amount of pressure the brake shoes exert against the drum to control the amount of braking that takes place when stopping the motor. When the motor is operating, a solenoid is energized to release the pressure of the brake shoes. When the motor is to be stopped, the brakes engage immediately. A circuit of this type is shown in Fgr. 2. Mechanical brakes work by converting the kinetic (moving) energy of the load into thermal (heat) energy when the motor is stopped. Mechanical type brakes have an advantage in that they can hold a suspended load. For this reason, mechanical brakes are often used on cranes.
Disc brakes work in a very similar manner to drum brakes. The only real difference is that brake pads are used to exert force against a spinning disc instead of a cylindrical drum. A combination disc brake and magnetic clutch is shown in Fgr. 3.
Dynamic braking can be used to slow both direct and alternating current motors. Dynamic braking is sometimes referred to as magnetic braking because in both instances it employs the use of magnetic fields to slow the rotation of a motor. The advantage of dynamic braking is that there are no mechanical brake shoes to wear out. The disadvantage is that dynamic brakes cannot hold a suspended load. Although dynamic braking can be used for both direct and alternating current motors, the principles and methods used for each are very different.
Dynamic Braking for Direct Current Motors
A direct current machine can be used as either a motor or generator. When used as a motor, electrical energy is converted into mechanical energy. When used as a generator, mechanical energy is converted into electrical energy. The principle of dynamic braking for a direct current motor is to change the motor into a generator.
When a generator produces electrical power, it produces counter torque, making the armature hard to turn. The amount of counter torque produced by the generator is proportional to the armature current.
Dynamic braking for a DC motor is accomplished by permitting power to remain connected to the shunt field when the motor is stopped, and reconnecting the armature to a high wattage resistor (Fgr. 4).
The resistor may actually be more than one resistor depending on motor size, length of braking time, and armature current. High wattage resistors are shown in Fgr. 5. The braking time can be controlled by adjusting the resistance value. If current remains connected to the shunt field, the pole pieces retain their magnetism. Connecting a resistance across the armature terminals causes the motor to become a generator.
Dynamic braking for a DC motor is very effective, but the braking effect becomes weaker as the armature slows down. Counter torque in a generator is proportional to the magnetic field strength of the pole pieces and armature. Although the flux density of the pole pieces will remain constant as long as shunt field cur rent is constant, the armature magnetic field is proportional to armature current. Armature current is proportional to the amount of induced voltage and the resistance of the connected load. There are three factors that determine induced voltage:
• Strength of magnetic field. (In this instance, the flux density of the pole pieces.)
• Length of conductor. (Also stated as number of turns of wire. In this instance, it's the number of turns of wire in the armature winding.)
• Speed of the cutting action. (Armature speed) As the armature slows, less voltage is induced in the armature windings, causing a decrease of armature current.
Dynamic Braking for Alternating Current Motors
Dynamic braking for alternating current motors is accomplished in a different way than that described for direct current motors. Dynamic braking for an AC motor can be accomplished by connecting direct current to the stator winding. This causes the stator magnetic field to maintain a constant polarity instead of reversing polarity each time the current changes direction.
As the rotor of a squirrel cage motor spins through the stationary magnetic field, a current is induced into the rotor bars. The current flow in the rotor causes a magnetic field to form around the rotor bars. The rotor magnetic field is attracted to the stator field, causing the rotor to slow down. The amount of braking force is proportional to the magnetic field strength of the stator field and the rotor field. The braking force can be con trolled by the amount of direct current supplied to the stator.
When direct current is applied to the stator winding, there is no inductive reactance to limit stator cur rent. The only current-limiting effect is the wire resistance of the stator winding. Dynamic braking circuits for alternating current motors generally include a step down transformer to lower the voltage to the rectifier and often include a series resistor to control the current applied to the stator winding (Fgr. 6).
In the circuit shown, an off-delay timer is used to determine the length of braking time for the circuit.
When the START button is pushed, motor starter M energizes and closes all M load contacts to connect the motor to the line. The M auxiliary contacts change position at the same time. The normally closed M contact opens to prevent power being applied to the dynamic brake relay (DBR). The two normally open M auxiliary contacts close, sealing the circuit and supplying power to the coil of off-delay timer TR. Since TR is an off delay timer, the TR timed contacts close immediately.
The circuit will remain in this position until the STOP button is pressed. At that time, motor starter M de energizes and disconnects the motor from the line. The normally open M auxiliary contact connected in series with timer coil TR opens, starting the timing sequence.
The normally closed M auxiliary contact closes and provides a current path through the now closed TR timed contact to the coil of DBR. This causes the DBR contacts to close and connect the step-down transformer and rectifier to the power line. Direct current is now supplied to the stator winding. Direct current will be supplied to the stator winding until the timed TR contact opens and de-energizes coil DBR.
Plugging Plugging is defined by NEMA as a system of braking in which the motor connections are reversed so that the motor develops a counter torque that acts as a retarding force. Plugging can be used with direct current motors but is more often used with three-phase squirrel cage motors. Plugging is accomplished with three phase motors by disconnecting the motor from the power line and momentarily reversing the direction of rotation. As a generally rule, the reversing contactor is of a larger size than the forward contactor because of the increased plugging current. There are several methods that can be employed when a plugging control is desired.
One type of plugging control depends on an opera tor to manually perform the operation. A manual plugging control is shown in Fgr. 7. The circuit's basically a forward-reverse control circuit, with the exception that there is no holding contact for the reverse contactor. Also, the PLUGGING push button is a double acting push button with the normally closed section connected in series with the forward contactor. This permits the PLUGGING push button to be used with out having to press the STOP button first.
One method of providing plugging control is with the use of an automatic timed circuit (Fgr. 8).
This is the same basic control circuit used for time con trolling a dynamic braking circuit in Fgr. 6. The dynamic brake relay has been replaced with a reversing contactor. A modification of this circuit's shown in Fgr. 9. This circuit permits an operator to select if a plugging stop is to be used or not. Once the opera tor has pressed the PLUGGING push button, the timer controls the amount of plugging time.
Although time is used to control plugging, problems can occur due to the length of plugging time. If the timer isn't set for a long enough time, the reversing circuit will open before the motor completely stops. If the timer is set too long, the motor will reverse direction before the reversing contactor opens. The most accurate method of plug stopping a motor is with a plugging switch or zero speed switch (Fgr. 10). The plugging switch is connected to the motor shaft or the shaft of the drive machine. The motion of the rotating shaft is transmitted to the plugging switch either by a centrifugal mechanism or by an eddy current induction disc inside the switch.
The plugging switch contact is connected to the coil of the reversing starter (Fgr. 11).When the motor is started, the forward motion of the motor causes the normally open plugging switch contact to close. When the STOP button is pressed, the normally closed F contact connected in series with the reversing contactor will re-close and reverse the direction of rotation of the motor. When the shaft of the motor stops rotating, the plugging switch contact will reopen and disconnect the reversing contactor.
Plugging switches with two normally open contacts can be obtained for use with forward-reverse controls. These switches permit a plugging stop in either direction when the STOP button is pressed (Fgr. 12). The direction of motor rotation will deter mine which switch closes. The switch symbol indicates the direction of rotation necessary to cause the switch contacts to close.
1. Name three methods of braking a motor.
2. How is the braking force of drum type brakes controlled?
3. Why are mechanical brakes often used on cranes?
4. What is the advantage of dynamic brakes over mechanical brakes?
5. What is the disadvantage of dynamic brakes when compared to mechanical brakes?
6. The amount of counter torque developed by a direct current generator is proportional to what?
7. When using dynamic braking for a direct current motor, how is the braking time controlled?
8. Name three factors that determine the amount of induced voltage.
9. How is dynamic braking for direct current motors accomplished?
10. How is the dynamic braking force of an alternating current motor controlled?
11. How is a plugging stop accomplished?
12. What device is generally used to accurately stop a motor when a plugging stop is used?
13. Refer to the circuit shown in ill. 11. When the START button is pushed and the motor starts in the forward direction, the plugging switch will close. What prevents the reversing contactor from energizing when the plugging switch contact closes?
Interesting use of dynamic braking: High-speed train/railroad locomotives
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