A wound-rotor induction motor is built for heavy load frequent starts and (or) limited speed control motoring or generating at large power loads (above a few hundred kW).
Here we insist on the classical starting method used for such motors: variable resistance in the rotor circuit (Figure 8.8). As discussed in the previous chapter, the torque/speed curve changes with additional rotor resistance in terms of critical slip Sk, while the peak (breakdown) torque remains unchanged (Figure 8.8.b.).
As expected, the stator current, for given slip, also decreases with R′ad increasing (Figure 8.8 c.). It is possible to start (S′′ = 1) with peak torque by providing SK′′ = 1.0. When R′ad increases, the power factor, especially at high slips, improves the torque/current ratio. The additional losses in R′ar make this method poor in terms of energy conversion for starting or sustained low-speed operation.
However, the peak torque at start is an extraordinary feature for heavy starts and this feature has made the method popular for driving elevators or overhead cranes.
There a few ways to implement the variable rotor resistance method as shown in Figure 8.9 a,b,c.
The half-controlled rectifier and the diode-rectifier-static switch methods (Figure 8.8 a,b) allow for continuous stator (or rotor) current close loop control during starting. Also, only a fix resistance is needed.
The diode rectifier-static-switch method implies a better power factor and lower stator current harmonics but is slightly more expensive.
A low cost solution is shown on Figure 8.9 c, where a three-phase pair of constant resistances and inductances lead to an equivalent resistance Roe (Figure 8.9 c), which increases when slip increases (or speed decreases).
The equivalent reactance of the circuit decreases with slip increases. In this case, there is no way to intervene in controlling the stator current unless the inductance L0 is varied through a d.c. coil which controlls the magnetic saturation in the coil laminated magnetic core.
from rotor from rotor brushes brushes
