BRAKING OF THREE PHASE ASYNCHRONOUS

MOTORS

INTRODUCTION

In a large number of applications, stopping the motor is achieved simply by natural deceleration.
The deceleration time then depends solely on the inertia of the driven machine. But it is often
necessary to reduce this time. In this case, electric braking provides an effective and simple
solution. Compared to mechanical braking, it offers the advantage of being regular and of not
using any wearing parts.

The asynchronous motor is capable of operating in all four quadrants. It develops a driving
torque in both directions in quadrants Q1 and Q3 and a braking torque in quadrants Q2 and Q4.
Reversing the direction of rotation is obtained by interchanging two of the three power phases of
the motor, which has the effect of reversing the direction of the rotating field.

When safety requires it, braking must make it possible to obtain a shorter stopping time than that
obtained by simply pressing the stop button.
So, let's take the example of a circular saw machine deemed dangerous. Braking must be
immediate when pressing AU (Emergency Stop button).
There are four braking processes: Two mechanical and two electrical.
Mechanical processes:
It is a disc brake incorporated into the motor; we call the motor brake assembly: it is an action
carried out on the rotor. There are two types of braking in this process.
✓ Current inrush braking,
✓ Braking with lack of current.
Electrical processes:
It is an action carried out on the stator. There are also two types of braking.
✓ Braking Plugging,
✓ Dynamic braking.

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II. MECHANICAL BRAKING
These are motors comprising a mechanical braking device (jaw, disc, etc.): MOTOR BRAKE
controlled by an electromagnet. There are two types:

Functioning:
o A coil controls the movement of a disk blocked in rotation.
o This disc comes into contact with a disc fixed on the motor rotor.
o The friction of the two discs causes the engine to slow down.

Mechanical Braking with Brake Three-Phase Induction Motor

Shoes and Solenoid with Integral Disc Brake

Two possibilities:
✓ No-current brake.
✓ Current brake
II.1. No-current brake.
II.1.1. Principle:
The brake is mechanically actuated (spring system at rest), it is often used for safety reasons:
when the motor is not powered (normal stop or emergency stop) braking is carried out.
The electromagnet is powered by three phases, connected in parallel to the stator windings.
The advantage of this process lies in the fact that braking is always ensured, even in the event of
a power outage. This is the reason why this process is strongly recommended when safety is
paramount.
The undercurrent brake works in conjunction with the motor: it is connected in parallel with the
motor at the same level as the connection box.

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Remarks:

- Rotation is only possible if the stator windings and brake windings are powered.

- With this device, we obtain brutal braking, with a hold in the blocking position. There are
brake motors with an electromagnet supplied with rectified current via a diode bridge.

II.1.2. Control circuit:

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II.1.2. Power Circuit

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II.2. Current call brake
II.2.1. Principle:
In the case of the current call brake, it is the power supply to the brake which causes the braking
of the motor. It requires a power supply independent of that of the motor

II.2.2. Control circuit:

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II.2.3. Power control

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III. ELECTRIC BRAKING
III.1. Plugging
III.1.1. Principle:
Plugging consists of reversing two power phases, with the motor running, so that it changes
direction of rotation. The rotation frequency will then decrease rapidly and, when it becomes
zero, the power supply is cut off. The change in direction of rotation, with the motor started,
means that the motor must then overcome a greater resistive torque. It will therefore consume
much more current. To limit the latter, we must place resistors in series with the stator. Even
with the resistors which limit the current, the simultaneous closing of KM1 and KM2 would
cause a short circuit, so it is imperative to lock them electrically and mechanically. It is also
necessary to cut the power to the brake at the precise moment when the motor stops otherwise; it
will start again in the other direction. Normally a tachometer sensor will take care of this work.
Generally, an electrical cut-off device disconnects the motor from the network when the speed
changes to N=0. The average braking torque is generally greater than the starting torque.
Disadvantage of this braking mode involves significant current absorption (around 7xIn)
Remarks:
- This is a very effective braking mode.
- This braking must be interrupted as soon as the rotor stops, otherwise there is a risk of
restarting in the opposite direction.
- There is no blocking.

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III.1.2. Control circuit:

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III.1.3. Power circuit:

III.2. Dynamic braking

III.2.1. Principle:
This braking mode is used on ring and cage motors. When a three-phase voltage is applied to the
stator terminals of a three-phase asynchronous motor, a rotating field is created which causes the
rotor to rotate at a slightly lower rotation frequency due to slip (see chapter 1). If while cutting
off the three-phase power supply, we apply a DC voltage to this stator, we then create a fixed
field. This fixed field has the effect of blocking the rotation of the rotor and therefore of the
motor.

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III.2.2. Control circuit

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III.2.3. POWER CIRCUIT

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