ELECTRIC MOTOR BRAKING METHODS TUTORIALS
What are the methods in stoppng/braking an electric motor?
MECHANICAL BRAKING
Mechanical braking with magnetic lifting is the technique most widely used for the braking of electric motors. At standstill brakes of this type provide a holding torque, and are therefore used where loss of braking in the event of power failure could be dangerous. However, in certain cases it may be necessary to lift the brake without starting the motor. This can be done by supplying the brake coil from a separate power source, or with a manual release device.
The mechanical brakes used for electric motors are shoe, multiple-plate or disc brakes. ABB Motors brakes are disc brakes with asbestos-free brake pads or linings. During braking, the braking torque is constant with mechanical braking. At standstill the brake has a holding torque. On some brakes the braking torque can be reduced for softer deceleration. When the motor is started again, the holding torque ceases automatically.
ELECTRICAL BRAKING
Countercurrent braking gives a very high braking torque. The current during braking is about the same as during starting, so that there is a considerable temperature rise in the motor. Consequently the permitted frequency of braking with the countercurrent technique is only about one-quarter of the number of permitted brakings for a brake motor. Since the permitted frequency of braking can easily be exceeded with countercurrent braking, temperature sensors should always be used to protect the motor windings from overheating.
For slip-ring motors the starting and braking times are both determined by the dimensioning of the rheostatic starter. With countercurrent braking there is no braking action in the event of power failure. The technique is therefore unsuitable for use in plant where loss of braking could cause danger.
Direct-current braking gives a far longer braking time than countercurrent braking, however high the excitation current is, but thermal losses are lower, so more frequent braking is permissible.
What are the methods in stoppng/braking an electric motor?
MECHANICAL BRAKING
Mechanical braking with magnetic lifting is the technique most widely used for the braking of electric motors. At standstill brakes of this type provide a holding torque, and are therefore used where loss of braking in the event of power failure could be dangerous. However, in certain cases it may be necessary to lift the brake without starting the motor. This can be done by supplying the brake coil from a separate power source, or with a manual release device.
The mechanical brakes used for electric motors are shoe, multiple-plate or disc brakes. ABB Motors brakes are disc brakes with asbestos-free brake pads or linings. During braking, the braking torque is constant with mechanical braking. At standstill the brake has a holding torque. On some brakes the braking torque can be reduced for softer deceleration. When the motor is started again, the holding torque ceases automatically.
ELECTRICAL BRAKING
Countercurrent braking
With countercurrent braking, an ordinary standard motor is switched at full speed for the opposite direction of rotation. This can be done with a reversing switch. After braking to a standstill, the motor starts in the opposite direction of rotation, unless the current is switched off at the right moment. A low speed detector is therefore used to cut off the supply to the motor when the speed approaches zero.
With countercurrent braking, an ordinary standard motor is switched at full speed for the opposite direction of rotation. This can be done with a reversing switch. After braking to a standstill, the motor starts in the opposite direction of rotation, unless the current is switched off at the right moment. A low speed detector is therefore used to cut off the supply to the motor when the speed approaches zero.
Countercurrent braking gives a very high braking torque. The current during braking is about the same as during starting, so that there is a considerable temperature rise in the motor. Consequently the permitted frequency of braking with the countercurrent technique is only about one-quarter of the number of permitted brakings for a brake motor. Since the permitted frequency of braking can easily be exceeded with countercurrent braking, temperature sensors should always be used to protect the motor windings from overheating.
For slip-ring motors the starting and braking times are both determined by the dimensioning of the rheostatic starter. With countercurrent braking there is no braking action in the event of power failure. The technique is therefore unsuitable for use in plant where loss of braking could cause danger.
Direct-current braking
When braking with this technique, the A.C. supply to the motor is disconnected and the stator is excited with direct current instead; this causes the motor to produce a braking torque. An ordinary standard motor and suitable equipment for D.C. excitation may be used. The A.C. voltage follows a decay curve, and the D.C. voltage must not be connected until the A.C. voltage has fallen to a value at which it will not harm the D.C. equipment.
When braking with this technique, the A.C. supply to the motor is disconnected and the stator is excited with direct current instead; this causes the motor to produce a braking torque. An ordinary standard motor and suitable equipment for D.C. excitation may be used. The A.C. voltage follows a decay curve, and the D.C. voltage must not be connected until the A.C. voltage has fallen to a value at which it will not harm the D.C. equipment.
Direct-current braking gives a far longer braking time than countercurrent braking, however high the excitation current is, but thermal losses are lower, so more frequent braking is permissible.
source: 2010 IIEE Technical Manual
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