Injection braking & SPEED CONTROL
Fig: 1 Torque–speed curve for d.c. injection braking of cage motor
- Injection Braking is the most widely used method of electrical braking. When the ‘stop’ button is pressed, the 3-phase supply is interrupted, and a d.c. current is fed into the stator via two of its terminals.
- The d.c. supply is usually obtained from a rectifier fed via a low-voltage high-current transformer. We saw earlier that the speed of rotation of the air-gap field is directly proportional to the supply frequency, so it should be clear that since d.c. is effectively zero frequency, the air-gap Weld will be stationary.
- We also saw that the rotor always tries to run at the same speed as the field. So, if the field is stationary, and the rotor is not, a braking torque will be exerted.
- A typical torque–speed curve for braking a cage motor is shown in Figure 1, from which we see that the braking (negative) torque falls to zero as the rotor comes to rest.
- This is in line with what we would expect, since there will be induced currents in the rotor (and hence torque) only when the rotor is ‘cutting’ the flux. As with plugging, injection (or dynamic) braking is a dissipative process, all the kinetic energy being turned into heat inside the motor.
- To operate efficiently an induction motor must run with a small slip. It follows that any efficient method of speed control must be based on varying the synchronous speed of the field, rather than the slip.
- The two factors that determine the speed of the field, are the supply frequency and the pole number. The pole number has to be an even integer, so where continuously adjustable speed control over a wide range is called for, the best approach is to provide a variable-frequency supply.
- This method is very important concerns with constant frequency mains operation, providing a choice between pole-changing, which can provide discrete speeds only, or slip control which can provide continuous speed control, but is inherently inefficient.