GATE Electrical Engineering - Complete Master Library

 

📘 Induction Motor – Starting Methods

Induction motors draw very high current at starting. Starters are used to reduce starting current and control starting torque.

---

🔹 Why Starting Current is High?

At start: Slip s = 1 Rotor frequency = supply frequency Rotor impedance small → Large current flows. Starting current:

I_start ≈ 5 to 7 × I_rated

---

🔹 1️⃣ DOL (Direct On Line) Starter

Motor connected directly to supply. Advantages:

• Simple
• Cheap

Disadvantages:

• Very high starting current
• High voltage dip

Used for small motors.

---

🔹 2️⃣ Star-Delta Starter

Motor starts in star connection → Then switches to delta. In star: Phase voltage = V_L / √3 Therefore:

Starting current reduced to 1/3

Starting torque also reduced to 1/3. Very frequently asked in GATE.

---

🔹 3️⃣ Auto-Transformer Starter

Reduced voltage applied using autotransformer. If tapping = k,

Starting current = k² × I_DOL

Torque ∝ Voltage² Most flexible method.

---

🔹 4️⃣ Rotor Resistance Starter

Used in slip ring motors. External resistance added in rotor. Advantages:

• High starting torque
• Reduced starting current

---

🔹 Example

If DOL starting current = 90 A Using Star-Delta:

I_start = 90 / 3 = 30 A

---

🔹 Important Comparison

Method	Current Reduction	Torque
DOL	None	Full
Star-Delta	1/3	1/3
Auto Transformer	k²	k²
Rotor Resistance	Controlled	High

---

🎯 GATE Important Points

• Star-delta reduces current to 1/3
• Torque ∝ V²
• Slip ring motor uses rotor resistance
• DOL used for small rating motors

Starting Method = Current Control + Torque Control