Rotor rheostat speed control of slip ring motor

Principle

In a slip-ring (wound-rotor) induction motor, extra resistance can be inserted in the rotor circuit through the slip rings and brushes, so the rotor resistance becomes $R_2 + R_{ext}$. This changes the slip at which a given load torque is produced, and therefore the speed. The speed can be lowered below normal, but not raised above it.

Circuit diagram

 3-phase supply
 R ---+----+----+
 Y ---+--+-|--+-|--+
 B ---+--|-|--|-|--+
      [ STATOR ]      [ ROTOR ]
        winding  ------ winding
                         | | |  slip rings
                        brushes
                         | | |
                        [Rext]  3-phase
                        rheostat (Y)

Torque-slip characteristics

 T
 |   TL ------------------------ load line
 |   _.-*-._  _.-*-._  _.-*-._
 |  /  R2    \   R2+r1    R2+r2
 +--+----+-----+---+----+------> s
  1   s3   s2     s1  (low speed to high)

$T_{max}$ is independent of the rotor resistance, so increasing the rotor resistance only shifts the peak towards the high-slip side. For a constant load torque $T_L$ (in the low-slip region, where $T \propto s/R_2$):

$$\frac{R_2}{s_1} = \frac{R_2 + R_{ext}}{s_2}$$

(the torque is constant, so $R_2/s$ is constant). So $s_2 = s_1\frac{R_2+R_{ext}}{R_2}$, and the speed $N = N_s(1-s_2)$ decreases as $R_{ext}$ increases.

Operation

1. With $R_{ext} = 0$ the motor runs at normal speed with a small slip $s_1$.
2. When $R_{ext}$ is inserted using the rheostat, the T-S curve flattens and the operating point on the load line moves to a larger slip, so the speed falls.
3. The more resistance inserted, the lower the speed.

Merits and demerits

• Merits: simple, smooth control, good starting torque, and the same arrangement is used for starting.
• Demerits: power is lost in the rheostat (part of the rotor copper loss $sP_g$), so efficiency falls as speed is reduced; poor speed regulation (speed varies with load); only speed reduction is possible; the rheostat is bulky and expensive; not suitable for light loads (little change in speed).