Chapter 5: Speed-Torque Characteristics of Electric Motors

Direct Current Machines

S N F I V Brush F Armature windings Rotor w

I V

Separately Excited Motors

DC Machine

Speed-Torque CC of DC Motor Load w o D w } w Operating Point Motor

Starting of DC motor At Starting,  = 0, then Ea = 0

Example A dc separately excited motor has the following data: K  = 3.0 Vs (Volt second) Vt = 600.0 V Ra = 2.0 W Ia = 5.0 A (armature current at full load) Calculate the rated torque, starting torque and starting current at full voltage.

Solution

Starting of DC motor (voltage reduction)

Starting of DC motor (starting resistance)

Example Limit the starting current to 6 times the rated current

Solution Method 1: Reduce the starting voltage Method 2: Increase the starting resistance

Shunt Motor

Series Motors

Operating Series Motors in ac Circuits Assumption: small inductance When Vt is reversed, Ia is reversed When Ia is reversed,  is reversed  is maintained in the same direction

Induction Motor

El-Sharkawi@University of Washington Stator Rotor ring Rotor Stator Windings Rotor Windings El-Sharkawi@University of Washington

Rotation of Induction Motor (IM) stator ns n rotor El-Sharkawi@University of Washington

Rotating Field

i b f c i a f b  c f a  a  b i c

Rotating Field The speed of the airgap flux is one revolution per one ac cycle. The time of one ac cycle The speed of the airgap flux

Number of Poles

Number of Poles

What is the Speed of the motor? The speed of the motor depends on the magnitude of the load torque

Slip

Equivalent Circuit

Equivalent Circuit

Torque Characteristics

s 1 Torque n s max n s T st max

T Torque s 1 st Small Slip Maximum Torque s max T max Large Slip

Large slip approximation (during starting)

Small slip approximation (For steady state, full load)

Maximum Torque Set

Summary Maximum Torque region Large slip: Starting region Small slip: Steady state region Maximum Torque region

Starting of Induction Motor

Starting by Reducing Voltage

Starting by Adding Rotor Resistance

Example An induction motor has a stator resistance of 3  and the rotor resistance referred to the stator is 2 . The equivalent inductive reactance is 10 . Calculate the change in the starting torque if the voltage is reduced by 10%. Also, compute the resistance that should be added to the rotor circuit to achieve the maximum torque at starting.

Solution

Synchronous Motor

Starting of Synchronous Motor