Equivalent Circuit of an Induction Motor

The corresponding poly-phase induction motor circuit is shown in Fig. 24, where

V₁ = applied voltage per phase

R₁ = stator resistance/phase

R₂ = rotor resistance/phase

X₁ = stator leakage reactance/phase

X₂ = rotor standstill leakage reactance/phase

K = turn-ratio of secondary to primary

R₀ = no-load resistance/phase

X₀ = no-load reactance/phase

Fig. 24. Equivalent circuit of an induction motor

It is illustrated in the figure that when the circuit is transferred to the left, the circuit and calculations are much simpler because of the negligible inaccuracy. Such a process is called induction motor equivalent circuit as the approximate value.

Maximum Power Output (shown in figure no. 25)

In order to obtain the maximum power output, let’s differentiate the mentioned equation by equating the first derivative to 0.

By the mentioned equation we can say that gross mechanical power output will maximum when the standstill leakage impedance is equal to the equivalent load resistance R_L of the motor Z₀₁.

Example

In a 3-phase induction motor maximum torque occurs at a slip of 15 per cent. The equivalent secondary resistance of the motor is 0.07 Ω/phase. If the gross power output is 9.5 II W. Calculate.-

(i)     Equivalent load resistance,

(ii)   Equivalent load voltage, and

(iii)Current at this slip.

Solution

Slip at maximum torque, s = 15 per cent

The equivalent secondary resistance of the motor,

R₂ = 0.07 Ω/phase

Gross power output, P_g = 9.5 kW or 9500 W

1. Equivalent load resistance, RL:

We know that

= (please calculate the value)

2. Equivalent load voltage, V:

V is a fictitious voltage drop which is equal to the consumption in the secondary load connected, i.e., rotor, V = I₂R_L

But, gross power, Pg =

= please calculate the value

Links of Previous Main Topic:-

• Current Electricity Basic Concepts
• Introduction to Alternating Current
• Introduction Three Phase A C Circuits
• Magnetic Field
• General Aspects
• General Aspects Polyphase Induction Motors
• Classification of A C Motors
• Constructional Details
• Production of Rotating Magnetic Field
• Theory of Operation of an Induction Motor
• Slip
• Frequency of Rotor Current
• Rotor E M F and Rotor Current

Links of Next Electrical Engineering Topics:-

• Starting of Induction Motors
• Factors Governing Performance of Induction Motors
• Effects of Operating Conditions
• Ratings of 3 Phase Induction Motors
• Squirrel Cage Motors Advantages Disadvantages and Applications
• Wound Rotor or Slip Ring Induction Motors Advantages Disadvantages and Applications
• Comparison of a Squirrel Cage and a Slip Ring or Phase Wound
• Comparison Induction Synchronous Motors
• Highlights in Polyphase Induction Motors
• Single Phase Motors
• Characteristics of D C Generators
• Measuring Instruments
• Power Supply System