Braking of three-phase asynchronous motors with wound rotor: by electrobrake, hypersynchronous and countercurrent

BRAKING OF THREE-PHASE ASYNCHRONOUS MOTORS WITH WOUND ROTOR

As with short-circuited rotor motors, in many applications of wound-rotor motors, it is necessary to have a device that allows the motor to be stopped at a certain time.

The most commonly used braking procedures for this type of engine are:

• Electrobrake.
• Hypersynchronous braking.
• Braking against the current.
• DC injection braking.
• DC self-excitation braking.
• Slowed down by unbalanced feeding.

Figure 1 shows the rotor and stator parts of the wound rotor motor.

ELECTRIC BRAKE

In this case, everything mentioned about the topic of short-circuited rotor motors is valid.

HYPERSYNCHRONOUS BRAKING

When an asynchronous motor operates at a speed higher than its synchronous speed (hypersynchronous operation), the motor functions as an asynchronous generator, providing a braking torque. This occurs, for example, during the lowering of a load, where, in addition to the drive motor's own speed, the acceleration due to gravity also comes into play. As a result, the load can reach a speed higher than its synchronous speed. This also happens when, during speed regulation, the motor attempts to transition from a high speed to a much lower one. In these cases, braking is regulated by rotor resistors. This braking method is widely used in lifting equipment; conversely, its use is infrequent in machine tool drives.

COUNTERCURRENT BRAKING

Similar to the short-circuited rotor motor, this braking procedure involves reversing the two-phase power supply to the stator winding once the motor is running. The braking torque can be adjusted by modifying the rotor resistance values: during braking, it is sufficient to uncircuit some or all of the rotor resistances to limit the current. In most cases, a braking torque comparable to the starting torque is chosen. Under no circumstances should a motor with short-circuited rings be coupled in reverse, as the rotor current would be too high and could seriously damage the rings and brushes. Therefore, when reversing the stator connections, a high resistance must be inserted in the rotor, which is gradually reduced as the speed decreases.

At the beginning of the braking process, the slip becomes approximately equal to 2, and the rotor voltage is almost double that measured when the motor is stopped. This circumstance necessitates special precautions for the insulation of the switchgear and the rotor resistance banks.

The figures show the explanatory diagram of a countercurrent braking system, assuming the motor rotates in only one direction. As can be seen, the power circuit is similar to that of a motor for two directions of rotation and starting by rotor resistances, with automatic short-circuiting of these resistances.

In the control circuit, it is enough to add the contactors to short-circuit the rotor resistances as the braking process takes place.

The figures show the explanatory diagram of a counter-current braking system, assuming two directions of motor rotation. Note that the power circuit is similar to that of Fig. 5.13 (single direction of rotation); however, in this case, each of the reversing contactors acts as a braking contactor when the other reversing contactor is operating in normal operation. For example, when the motor rotates counterclockwise, it is stopped by braking, by pressing the clockwise reversing button.