BRAKING AND REVERSING (Induction Motor)

3.3
In drives requiring rapid deceleration, the motor needs to develop a negative torque for braking, especially in systems with low load torque and/ or high inertia. Because the torque depends on slip, a proper change in the slip must be effected. Apart from frequency control or changing the number of poles of stator winding, there are two ways to induce a negative torque in an induction machine, plugging and dynamic braking.

Plugging consists in a reversal of phase sequence of the supply voltage,

which is easily accomplished by interchanging any two supply leads of the motor. This results in reverse rotation of the magnetic field in the motor; the slip becomes greater than unity and the developed torque tries to force the motor to rotate in the opposite direction. If only stopping of the drive is required, the motor should be disconnected from the power line at about the instant of zero speed.
Plugging is quite a harsh operation, because both the kinetic energy of the drive and input electric energy must be dissipated in the motor, mostly in the rotor. This braking method can be compared to shifting a transmission into reverse to slow down a running car. The total heat produced in the rotor is approximately three times the initial energy of the drive system. Therefore, plugging must be employed with caution to
avoid thermal damage to the rotor. Low-inertia drives and motors with high rotor resistance and, therefore, with a large high-slip torque (see Figure 2.17) are the best candidates for effective plugging.
which is only two-thirds of the rated torque, while the stator current is 6.6 times the rated value. The maximum braking torque using this method occurs at zero speed and equals the starting torque of 227 Nm (see Table 2.2). The corresponding stator current of 250 A/ph (see Section 3.2) is still very high at 6.3 times the rated current.
The load mass moment inertia is 2 X 0.4 = 0.8 kg.m2, and the energy, Ev dissipated in the rotor is three times the initial kinetic energy of the drive system. Thus,
tmp2F3-5_thumb[1]
Dynamic braking is realized by circulating direct current in stator windings. For braking, the motor is disconnected from the power line, and any two of its phases are connected to a dc voltage source. The dc stator current produces a stationary magnetic field, so that ac EMFs and
currents are induced in the rotor bars, and a braking torque is developed. The braking torque, TM bp is given by the approximate equation
tmp2F3-6_thumb[1]
where /s dc denotes the dc stator current. The relation between the braking torque and motor speed, nM, resembles that for super synchronous speeds (see Figure 2.22), with the maximum braking torque in the vicinity of nM = “syn^/^m- Indeed, with the stationary field, a braking motor can be thought of as running at a super synchronous speed. Although no energy regeneration is possible, the amount of heat dissipated in the rotor is one-third of that for plugging, being approximately equal to the initial kinetic energy of the drive system.
The dynamic-braking arrangement is illustrated in Figure 3.5. The braking dc current encounters only the stator resistance, so the dc source supplying this current must have voltage much lower than the rated ac voltage of the motor. Therefore, a step-down transformer is used, the reduced secondary ac voltage of which is converted into dc voltage by a diode rectifier. Normally, the motor operates with contacts 1 closed and contacts 2 and 3 opened. For braking, the motor is disconnected from the power line by opening contacts 1, and two of its phases are connected to the rectifier by closing contacts 2. Contacts 3 are closed simultaneously, providing power supply for the transformer. In large motors, instead of
System for the dynamic braking.
FIGURE 3.5 System for the dynamic braking.
the single-phase transformer and rectifier in Figure 3.5, their three-phase counterparts can be used.
The energy, Ep dissipated in the rotor equals the initial kinetic energy of the drive system, that is, it is only one-third of that when plugging is used. Based on results of Example 3.1, Er — 26923/3 = 8974 J. The comparison of plugging and dynamic braking has shown definite superiority of the latter method. The average braking torque is much higher than with plugging, and the heat generated in the motor, both in stator and rotor, is much lower. ■
Certain drives require prolonged stopping. For instance, too-rapid speed reduction of a conveyor belt could cause spillage, and that of a centrifugal pump may result in pipe damage due to the water-hammer effect. In such cases, power electronic soft-starters can be used to slowly reduce (ramp down) the stator voltage.
Reversing an induction motor drive involves braking the motor and restarting it in the opposite direction. The braking and starting can be done in any of the ways described above. Plugging is a good option for motors running light, while simply disconnecting the motor from the power line can be sufficient for quick stopping of drives with a high reactive load torque. In some drives, the reversing is performed in the gear train so that the motor operation is not affected.


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