Induction Motor Braking Regenerative Plugging Dynamic Braking of Induction Motor

Induction motors serve pumps, fans, conveyors, machine tools and other industrial loads. Modern Speed control of induction motors uses power converters instead of relying only on line-frequency operation. Earlier installations often chose DC motors when wide speed control was required. Variable-frequency induction motor drives now control shaft speed while managing acceleration and braking without the commutator used in DC motors. The electrical braking method depends on the required stopping time, energy path, supply and drive hardware:

1. Regenerative braking of induction motor
2. Plugging braking of induction motor
3. Dynamic braking of induction motor, further divided into:

• AC dynamic braking
• Self-excited braking using capacitors
• DC dynamic braking
• Zero-sequence braking

Regenerative Braking of Induction Motor

Using per-phase RMS values, the three-phase real power at the stator terminals is:
P_in = 3VI_scosφ_s

Here, φ_s is the phase angle between stator phase voltage V and stator phase current I_s. With this sign convention, motoring draws positive real power when φ_s < 90^o, while generating returns real power when φ_s > 90^o. On a fixed-frequency supply, regeneration begins when the rotor exceeds synchronous speed in the same direction. Slip becomes negative because the rotor conductors overtake the rotating field, and the power angle passes through 90^o. A variable-frequency drive can lower the field’s synchronous speed during deceleration, so regeneration is possible over a controlled shaft-speed range. The drive must accept the energy in its DC link and either return it to the mains or send it to another approved energy path. The operating range for regenerative braking of induction motor therefore depends on supply frequency and converter topology.

Plugging Braking of Induction Motor

Plugging induction motor braking reverses the stator phase sequence while the rotor is still turning. Plugging braking of induction motor is commonly produced by interchanging any two stator supply phases. The rotating field reverses immediately, so the machine develops plugging braking torque against the existing shaft motion. If the original motoring slip is s, the plugging slip is (2 – s).


Plugging causes high current and high thermal stress, so the switching equipment and motor must be rated for this duty. Torque remains in the reverse direction at zero speed. Disconnect the supply near standstill to stop the shaft; leaving it connected makes the motor accelerate in reverse.

Dynamic Braking of Induction Motor

Four established circuits provide dynamic braking of induction motor or rheostatic braking. Each creates opposing torque without returning energy directly to a fixed AC supply.

AC Dynamic Braking-
In this specialised method, switching changes the three-phase stator connection so the motor receives a single-phase AC supply. A pulsating stator field contains positive- and negative-sequence components. Their torques oppose one another, and the negative-sequence component can provide net braking over part of the speed range. The result depends on speed, applied voltage and rotor resistance. The circuit needs rated switching and thermal protection; unintended single-phasing is a fault, not a braking method.

Self-excited braking using capacitors

After the source is disconnected, a capacitor bank across the stator terminals can supply magnetising reactive current. Residual rotor flux and shaft motion can then build terminal voltage and make the induction machine generate. The capacitors must suit the motor, speed and connected load; voltage may fail to build or may collapse as speed falls.
Generated energy is dissipated in machine losses and any external load.
An external electrical resistance can increase braking load, but its voltage, power and energy ratings must match the duty.

DC Dynamic Braking
For this type of braking, the AC supply is removed before controlled DC current is applied to the stator. Suitable two- or three-terminal connections depend on whether the stator is connected in star or delta.

A diode bridge can rectify AC to provide the DC injection shown in the circuit.

Two Loads DC Dynamic Braking Operation
DC stator current creates a stationary magnetic field. Rotor motion through that field induces rotor current and produces torque opposite to rotation. Kinetic energy becomes heat in the rotor and motor losses rather than returning to the supply. Braking torque falls as speed approaches zero, and frequent or prolonged DC injection can overheat the motor.

Zero Sequence Braking
This specialised connection places all three stator phase windings in series and applies single-phase AC or DC, as shown. The currents are co-phasal, so they form a zero-sequence set. AC produces a field that is stationary in space and pulsates at supply frequency; DC produces a stationary field of constant polarity. All three stator windings carry the braking current, which distributes stator heating more evenly than a single-phase connection. The method requires access to the winding terminals and correctly rated switching, current control and thermal protection.

Regenerative operation, plugging and the dynamic methods above produce different energy flows and stresses. The selected induction motor breaking circuit must match the motor, drive, load and required stopping duty.