Induction Motor Why induction motor (IM)? –Robust; No brushes. No contacts on rotor shaft –High Power/Weight ratio compared to Dc motor –Lower Cost/Power –Easy to manufacture –Almost maintenance-free, except for bearing and other mechanical parts Disadvantages –Essentially a “fixed-speed” machine –Speed is determined by the supply frequency –To vary its speed need a variable frequency supply

Construction Stator

Construction Rotor Squirrel Cage

Construction

Performance of Three-Phase Induction Motor The speed of the rotating field (Synchronous speed ) is : Slip speed is Slip, s is : Is the rotor speed, Rotor frequency in Rad/Sec. is : Rotor frequency in Hz is :

Per Phase Equivalent Circuit Per Phase Equivalent Circuit Referred to Stator

Per Phase Simplified Equivalent Circuit Referred to Stator

Per Phase Equivalent Circuit Referred to Stator

Per Phase Simplified Equivalent Circuit Referred to Stator

For regenerative breaking

(ii) Then motor speed

2. For the motor speed N rpm Then for plugging

Speed Control of Induction Motor 1- Variable Terminal Voltage Control 2- Variable Frequency Control 3- Rotor Resistance Control 4- Injecting Voltage in Rotor Circuit

1- Variable Terminal Voltage Control variable frequency controlvariable terminal voltage control Wide speed rangeLow speed range Lower & higher rated speedLower rated speed

2- Variable Frequency Control Per-Unit Frequency 1- Operation Below the Rated Frequency a <1 At rated frequency At any frequency, f Comparing of the above equations, will stay constant at a value equal to its rated value if The above equation suggests that the flux will remain constant if the back emf changes in the same ratio as the frequency, in other ward, when E/f ratio is maintained constant.

The rotor current at any frequency f can be obtained from the following equation: Angular Speed at frequency f Synchronous Speed at rated frequency Torque at frequency f At a given f and E

(6-51)

BrakingMotoring

V/f Control

(6.52) (6.53) At rated frequency (6.54) (6.55) At any frequency, f,

V/f Control

Operation above the rated frequency a>1 The terminal voltage has o be constant = Rated Volatge= (6.56) (6.57) At any frequency, f,

Ex6-3 A 3-phase, Y-connected, 60 Hz, 4 pole induction motor has the following parameters: and The motor is controlled by the variable frequency control with a constant V/f ratio. For the operating frequency of 12 Hz, calculate: 1-The breakdown torque as a ratio of its value at the rated frequency for both motoring and breaking. 2- The starting torque and rotor current in terms of their values at rated frequency. From (6.55) Motoring Breaking

At starting torque s=1 From (6.54) From Fig. 6.1(d)

Ex6-4 If the rated slip of induction motor of EX6.3 is 0.04, Calculate the motor speed for rated torque and f=30Hz. The motor is controlled with a constant V/f ratio At rated frequency,f=60Hz, From (6.52) At f=30Hz, From (6.54) [1] [2]

Equating [1] and [2] Ex 6-5 Solution:

Rated torque=

(i) At 20 Hz

At 20 Hz

(ii)

For constant flux, the