📘 Induction Motor – Speed Control Methods
Speed control of induction motor is widely used in industries. Modern drives use frequency control. Very important for GATE.
🔹 1️⃣ Speed Equation
Rotor speed:N = N_s (1 − s)
Where: N_s = 120 f / P Therefore speed depends on:- Frequency (f)
- Number of poles (P)
- Slip (s)
🔹 2️⃣ Stator Side Control Methods
1. Frequency Control (Most Important)
From synchronous speed formula:N_s ∝ f
Reducing frequency → Reduces speed. Used in: V/f control (constant flux control).Very Important:
To keep flux constant → V/f must be constant
This is most widely used industrial method.2. Voltage Control
Torque ∝ V² Reducing voltage reduces torque. Used for small motors only.🔹 3️⃣ Rotor Side Control Methods (Slip Ring Motor)
1. Rotor Resistance Control
Increase rotor resistance: Slip increases → Speed reduces. Used for: Low speed operation.2. Cascade Control
Connect rotor to another motor. Used in large industrial drives.🔹 4️⃣ Example 1
Given: f = 50 Hz P = 4 Find synchronous speed.N_s = (120 × 50) / 4 = 1500 rpm
If slip = 4%,N = 1500 × (1 − 0.04) = 1440 rpm
🔹 5️⃣ Example 2 (Frequency Control)
If frequency reduced to 40 Hz (P = 4),N_s = (120 × 40) / 4 = 1200 rpm
Speed reduces proportionally.🔹 Important Observations
- Frequency control best method
- V/f must remain constant
- Rotor resistance control wastes power
- Voltage control reduces torque
🎯 GATE Important Points
- Remember N = N_s (1 − s)
- Most modern control = V/f control
- Slip control only for slip ring motor
- Torque ∝ V²
Best Method = Frequency (V/f) Control
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