Braking of Electrical Motors

Whenever an electric motor is disconnected from the supply, the speed of the driving motor gradually decreases and become zero after a few minutes. Braking includes rapid stopping of the electric motor, holding the motor shaft to a specific position, maintaining the speed to a desired value of preventing the motor from over speeding.

How braking works

During the braking process, the energy can change its flow between the source and load. In this period, the motor drive acts as a generator, which pumps the energy back to the electric supply. The complete operational cycle of an electric motor in highly dependent on braking method.

Types of Braking

• Mechanical braking
• Electrical braking

Mechanical braking

In mechanical braking method, the friction force between the rotating parts and brake drums provide the instant braking. Mechanical equipments such as brake linings, brake shoe and brake drums are required. Mechanical braking require frequent maintenanace.

Electrical braking

In electrical braking, the motor is made to work as a generator. So that it produces a negative slip and negative torque i.e) braking torque. The motor can easily made to work as a generator by suitably changing the electrical connections of the motor. Whether mechanical or electrical braking, of the drive should be such as to stop the motor at the specified point of time, for safety reason.

Mechanical Braking Vs Electrical Braking

Mechanical Braking	Electronic Braking
The energy of the rotating parts is wasted as heat in friction.	The energy of the rotating parts can be converted to electrical energy which can be utilized or returned to the supply mains.
It require frequent maintenance like adjustment of brakes, replacement of brake linings. They are prone to tear and wear.	It require very little maintenanace because of absence of mechanical equipments.
Depending upon the conditions the braking may not be very smooth.	Braking is very smooth, without snatching.
This braking is applied to hold the system at any position	It cannot produce holding torque requires electrical energy for operation.
Brake drum, brake shoes and brake linings are needed.	Equipment of higher rating than the motor rating may be needed in Electrical braking.
Mechanical braking is low efficient method.	Electrical braking is high efficient method.

Purpose of Braking

• If a motor is disconnected from the supply at the time of its operation, the only opposing torque will be the load torque. The motor will stop only after the kinetic energy stored in its inertia is dissipated. The motor takes long time to stop, when eitheir the load torque is small or inertia is large. In application requiring frequent stops, the stopping time must be decreased by introducing additional opposing torque by use of electric braking or mechanical braking.
• In many applications where accurate stops are required such as in lifts, machine tools etc. Electric braking allows accurate steps without involving any mechanical parts to unduly large stress.
• In some important applications such as traction, rapid emergency stops are essential to prevent accidents. The electric braking is mainly used to achieveing quick and very smooth stops.