⚡ Complete Notes on DC Motors and Transformers
This comprehensive guide covers detailed theory, formulas, working principles, losses, efficiency, tests, applications and important exam points related to DC Motors and Transformers. These notes are specially prepared for Electrical Engineering students preparing for SSC JE, RRB, GATE, PSU, Diploma, ITI and university exams.
Part 1: DC Motors
1. Working Principle of DC Motor
A DC motor works on the principle that when a current-carrying conductor is placed inside a magnetic field, it experiences a mechanical force. The direction of force is determined by Fleming’s Left-Hand Rule.
2. Construction of DC Motor
- Yoke (Frame)
- Field Poles
- Armature Core
- Armature Winding
- Commutator
- Brushes
3. Types of DC Motors
| Motor Type | Starting Torque | Speed Regulation | Application |
|---|---|---|---|
| Shunt Motor | Moderate | Excellent | Machine Tools |
| Series Motor | Very High | Poor | Cranes, Traction |
| Cumulative Compound | High | Good | Elevators |
| Differential Compound | Low | Unstable | Rarely Used |
4. Characteristics of DC Motors
Shunt Motor
- Speed remains almost constant.
- Suitable for constant speed applications.
- Used in lathes, drills, fans.
Series Motor
- Speed inversely proportional to armature current.
- Must not be started at no-load.
- Used in electric traction.
5. Losses in DC Motor
- Copper Loss (Ia²Ra)
- Hysteresis Loss
- Eddy Current Loss
- Mechanical Loss (Friction & Windage)
6. Speed Control Methods
- Field Control Method (Above rated speed)
- Armature Voltage Control (Below rated speed)
- Armature Resistance Control
- Ward-Leonard Control
7. Braking Methods
- Plugging (Highest braking torque)
- Regenerative Braking
- Dynamic Braking
8. Important Exam Points – DC Motors
- Back EMF is zero at starting.
- Starter required to limit high starting current.
- Series motor has highest starting torque.
- Shunt motor has best speed regulation.
- Torque ∝ Ia² (in series motor).
Part 2: Transformers
1. Working Principle
A transformer works on Faraday’s Law of Electromagnetic Induction. It transfers electrical energy from one circuit to another through magnetic flux.
2. Construction of Transformer
- Core (Silicon Steel Laminations)
- Primary Winding
- Secondary Winding
- Conservator
- Breather (Silica Gel)
- Buchholz Relay
- Tank & Bushings
3. Transformer Losses
| Loss Type | Depends On | Nature |
|---|---|---|
| Iron Loss | Voltage | Constant |
| Copper Loss | Load Current | Variable |
| Eddy Current Loss | Frequency | Reduced by Laminations |
4. Efficiency of Transformer
Transformer efficiency is very high (98% approx). Maximum efficiency occurs when copper loss equals iron loss.
5. Voltage Regulation
- Zero regulation at leading power factor.
- Negative regulation indicates capacitive load.
6. Transformer Tests
- Open Circuit Test → Core Loss
- Short Circuit Test → Copper Loss
- Sumpner’s Test → Temperature Rise
- Polarity Test
- Impedance Test
7. Cooling Methods
- ONAN – Oil Natural Air Natural
- ONAF – Oil Natural Air Forced
- Air Blast Cooling
8. Protection of Transformer
- Buchholz Relay – Internal fault
- Horn Gap – Lightning protection
- Breather – Moisture control
- Conservator – Oil expansion
9. Important Exam Points – Transformers
- Transformer does not change frequency.
- Rated in kVA.
- CT secondary must never be open.
- Core laminated to reduce eddy current loss.
- Maximum efficiency at 50% load (Distribution transformer).
Frequently Asked Questions (FAQs)
Why DC motor starter is required?
Because back EMF is zero at starting, resulting in very high armature current.
Why transformer is rated in kVA?
Because losses depend on voltage and current, not power factor.
Which motor is best for traction?
DC Series Motor due to high starting torque.
What is the condition for maximum efficiency?
Copper Loss = Iron Loss
Why CT secondary should never be open?
It may cause dangerously high voltage and core saturation.