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Ac Motor

The document provides an overview of AC motors, detailing their construction, working principles, types (induction and synchronous), and key equations related to their operation. It emphasizes the importance of the rotating magnetic field and slip in torque production, particularly in induction motors, which are the most widely used in industry. Additionally, it includes examples for calculating synchronous speed and determining slip.

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Abdulmalik Umar
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8 views11 pages

Ac Motor

The document provides an overview of AC motors, detailing their construction, working principles, types (induction and synchronous), and key equations related to their operation. It emphasizes the importance of the rotating magnetic field and slip in torque production, particularly in induction motors, which are the most widely used in industry. Additionally, it includes examples for calculating synchronous speed and determining slip.

Uploaded by

Abdulmalik Umar
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PDF, TXT or read online on Scribd
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# **Lecture Notes: AC Motors**

**Course:** Electrical Machines

**Topic:** Construction, Working Principle, Torque-Speed Characteristics, and Applications

---

## **1. Introduction**

An **AC motor** converts **alternating current (AC)** into **mechanical energy** (rotation). Unlike DC motors, they operate without
brushes, making them more reliable for industrial use.

### **Key Principle:**


- **Rotating Magnetic Field (RMF)**: Generated by stator windings, causing the rotor to turn.

- **Slip**: Difference between synchronous speed (\( N_s \)) and actual rotor speed (\( N \)).

---

## **2. Types of AC Motors**

### **A. Induction Motors (Asynchronous)**

- **Most common** type (90% industrial use).

- **No electrical connection** to rotor (current induced by RMF).

- **Slip** required for torque production.


### **B. Synchronous Motors**

- Runs **exactly at synchronous speed** (\( N = N_s \)).

- Requires **DC excitation** for rotor.

- Used for **precision speed control** (e.g., clocks, robotics).

---

## **3. Construction of a 3-Phase Induction Motor**

### **Main Components:**


1. **Stator**

- 3-phase windings (120° apart) to produce RMF.

2. **Rotor**

- **Squirrel Cage**: Copper/aluminum bars (cheap, robust).

- **Wound Rotor**: Slip rings for external resistance (high starting torque).

3. **Enclosure**

- Protects from dust/cooling (TEFC, ODP types).

### **Diagram (ASCII Representation):**

```
+---------------------+

| STATOR |

| (3-Phase Windings) |

+----------+----------+

+----------v----------+

| ROTOR |

| (Squirrel Cage / |

| Wound Rotor) |

+----------+----------+
|

+----------v----------+

| FAN & BEARINGS |

+---------------------+

```

---

## **4. Working Principle**

1. **Stator windings** energized by 3-phase AC **RMF** rotates at \( N_s \).


2. **RMF induces currents** in rotor (Faraday’s Law).

3. **Rotor currents** create a magnetic field **torque** via Lorentz force.

4. **Slip (\( s \))** ensures continuous torque:

\[

s = \frac{N_s - N}{N_s} \times 100\%

\]

---

## **5. Key Equations**


### **Synchronous Speed (\( N_s \)):**

\[

N_s = \frac{120 \cdot f}{P} \quad \text{(RPM)}

\]

- \( f \) = Supply frequency (Hz)

- \( P \) = Number of poles

### **Torque Equation:**

\[

T = \frac{k \cdot V^2 \cdot R_2}{R_2^2 + (s \cdot X_2)^2}


\]

- \( V \) = Stator voltage

- \( R_2 \) = Rotor resistance

- \( X_2 \) = Rotor reactance

### **Slip at Maximum Torque:**

\[

s_{max} = \frac{R_2}{X_2}

\]
---

## **6. Solved Examples**

### **Example 1: Calculating Synchronous Speed**

**Problem:**

A 4-pole induction motor runs on 60 Hz supply. Find \( N_s \).

**Solution:**

\[
N_s = \frac{120 \times 60}{4} = 1800 \, \text{RPM}

\]

---

### **Example 2: Determining Slip**

**Problem:**

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