Braking of Synchronous Motor:

The methods that are employed for Braking of Synchronous Motor are:

• Regenerative braking while operating on a variable frequency supply
• Rheostatic braking
• Plugging

Regenerative braking

When the motor operates as a variable speed drive motor utilizing a variable frequency supply, it can be regeneratively braked and all the K.E. returned to the mains. As in an induction motor, regeneration is possible if the syn­chronous speed is less than the rotor speed. The input frequency is gradually decreased to achieve this at every instant. The KE. of the rotating parts is returned to the mains. The braking takes place at constant torque. With a CSI and cycloconverter, regeneration is simple and straightforward. With VSI an additional converter is required on the line side.

Rheostatic or dynamic braking

A Braking of Synchronous Motor is switched on to a three-phase balanced resistive load after disconnecting it from the mains, keeping the excitation constant. To achieve greater braking torque for effective braking, the excitation may be increased. The terminal voltage and current (change) decrease as the speed decreases. At very low speeds the resistance effect becomes considerable. The value of resistance affects the speed at which the maximum torque occurs. It can ideally be made to occur Just before the stopping of the motor.

The braking current at any instant is given by

where

E=ωL_afI_f∕√2 is the induced voltage.

In the above equations

r₁ = stator resistance per phase

L_s = synchronous inductance per phase

L_af = mutual inductance between armature and field

I_f = field current

The braking torque

The speed at which the T_br is maximum can be obtained as ω_m=r₁∕L_s.. By proper choice of r₁, the maximum braking torque can be made to occur just before stopping.

Plugging

The Braking of Synchronous Motor by plugging has serious disadvantages. Very heavy braking current flows causing line disturbances. The torque is also not effective. However, if the motor is synchronous induction type it can be braked effectively by plugging only if the machine is working as an induction motor.

Energy Relations During Braking of Electric Motors

In conventional methods of braking, such as rheostatic braking and plugging, it is necessary to know the energy wasted, so as to satisfactorily carry out the design of the braking equipment.

1.When a shunt motor is braked dynamically using a resistance the energy dissipated in the resistance is equal to the K.E. of the motor:

Where J is the moment of inertia and ω₁ is the speed at which the braking is initiated.

2.During counter current braking (plugging) the energy dissipated is 3∕2 Jω²₁.

If speed reversal is required, it is 2Jω²₁. The extra energy drawn from the mains is due to the application of the voltage in the reverse direction.

3.For a three-phase induction motor which is braked by dc in the stator

where t_br is the braking time, which can be estimated from the dynamics of the motor during braking as

where s₁ is the slip at the instant of braking.

4.When an induction motor is plugged, the energy wasted during braking is 3/2Jω²_s. If it is allowed to reverse, the energy is 2Jω²_s. If the stator losses are also added, these are 3/2 Jω²_s(1+r₁/r₂) and 2Jω²_s(1+r₁/r′₂) respectively.

5.For a Braking of Synchronous Motor the energy dissipated during rheostatic braking

Knowing the various torques occurring in the motor during dynamic braking the dynamic behaviour of the motor can be established.