Part III — Electrical Machines

Chapter 3: DC Machines, Transformers & AC Machines

3.1 DC Machines

A DC generator converts mechanical energy into electrical (DC) energy using the principle of electromagnetic induction (Faraday's Law), while a DC motor converts electrical energy into mechanical energy (motor action, based on the force on a current-carrying conductor in a magnetic field). DC machines are classified by field winding connection: series, shunt, and compound (cumulative or differential). A series motor develops very high starting torque but has poor speed regulation (speed varies significantly with load) and must never be started without load (risk of dangerously high, uncontrolled speed) — commonly used in traction applications (e.g., trains, cranes). A shunt motor has relatively constant speed regardless of load, suitable for applications requiring steady speed (e.g., lathes, fans, blowers).

3.2 Transformers

A transformer transfers electrical energy between two or more circuits through electromagnetic induction, without a change in frequency, typically used to step voltage up or down. Assuming an ideal transformer, the transformation ratio is: V2/V1 = N2/N1 = I1/I2, where V, N, and I represent voltage, number of turns, and current for the primary (subscript 1) and secondary (subscript 2) windings respectively. Transformer losses include copper losses (I²R losses in windings, varying with load) and core/iron losses (hysteresis and eddy current losses, largely constant regardless of load, occurring due to the alternating magnetic flux in the core). Maximum transformer efficiency occurs when copper losses equal iron losses. The transformer's core is laminated (thin insulated sheets stacked together) specifically to reduce eddy current losses.

3.3 Induction Motors

An induction motor (the most widely used AC motor in industry) operates on the principle of a rotating magnetic field (produced by a polyphase AC supply to the stator) inducing currents in the rotor (via electromagnetic induction, hence "induction" motor), which then interact with the stator field to produce torque. Slip (s) is defined as s = (Ns − Nr)/Ns, where Ns is synchronous speed and Nr is actual rotor speed — an induction motor's rotor speed is always less than synchronous speed (hence the name "asynchronous motor"), because relative motion between rotor and rotating field is essential for induction to occur; at synchronous speed, slip would be zero and no torque would be produced. Synchronous speed is given by Ns = 120f/P, where f is supply frequency and P is the number of poles.

3.4 Synchronous Machines

A synchronous generator (alternator) produces AC power at a speed exactly locked to supply frequency (Ns = 120f/P, with zero slip, as distinct from the induction machine), and is the standard machine used in power stations for large-scale AC power generation. A synchronous motor runs at exactly synchronous speed regardless of load (until it "loses synchronism"/pulls out of step under excessive load) and, unlike an induction motor, is generally not self-starting — requiring external methods (e.g., an auxiliary starting motor or damper windings) to bring it up to near-synchronous speed before synchronising it with the supply.

3.5 Practice Set — Electrical Machines (16 MCQs)

1. A DC generator converts mechanical energy into electrical energy based on:
   (a) Ohm's Law (b) Faraday's Law of electromagnetic induction (c) Kirchhoff's Laws (d) Coulomb's Law
2. A series DC motor is characterised by:
   (a) Very high starting torque but poor speed regulation (b) Constant speed regardless of load (c) No starting torque at all (d) Only used for constant-speed fan applications
3. A series motor must never be started without load because:
   (a) It will not start at all (b) It may reach dangerously high, uncontrolled speed (c) It will immediately burn out the windings (d) It has no practical risk
4. A shunt DC motor is best suited for applications requiring:
   (a) Very high starting torque with variable speed (b) Relatively constant speed regardless of load (c) No speed control at all (d) Only intermittent operation
5. In an ideal transformer, the transformation ratio V2/V1 equals:
   (a) N1/N2 = I2/I1 (b) N2/N1 = I1/I2 (c) N1×N2 (d) I1×I2
6. Transformer copper losses vary with:
   (a) Load current (b) Supply frequency only (c) Core material only (d) They are always constant
7. Transformer core/iron losses are:
   (a) Largely constant regardless of load (b) Directly proportional to load current squared (c) Zero at all times (d) Dependent only on the secondary winding resistance
8. Maximum transformer efficiency occurs when:
   (a) Copper losses are zero (b) Iron losses are zero (c) Copper losses equal iron losses (d) Copper losses are always twice iron losses
9. Transformer cores are laminated primarily to reduce:
   (a) Copper losses (b) Eddy current losses (c) Hysteresis losses only (d) Magnetic flux entirely
10. An induction motor's rotating magnetic field is produced by:
    (a) A polyphase AC supply to the stator (b) A DC supply to the rotor only (c) Permanent magnets in the stator (d) Mechanical rotation of the rotor alone
11. Slip in an induction motor is defined as:
    (a) (Ns + Nr)/Ns (b) (Ns − Nr)/Ns (c) Ns/Nr (d) Nr/Ns
12. An induction motor's rotor speed is always:
    (a) Equal to synchronous speed (b) Greater than synchronous speed (c) Less than synchronous speed (d) Zero under all conditions
13. Synchronous speed (Ns) is given by the formula:
    (a) Ns = 120f/P (b) Ns = f/120P (c) Ns = P/120f (d) Ns = 120P/f
14. A synchronous generator (alternator) operates with:
    (a) Non-zero slip, similar to an induction machine (b) Exactly zero slip, locked to supply frequency (c) No relationship to supply frequency (d) Variable frequency output at constant speed
15. A synchronous motor, unlike an induction motor, is generally:
    (a) Self-starting under all conditions (b) Not self-starting, requiring external starting methods (c) Unable to run at synchronous speed (d) Only usable as a generator
16. Which machine is most widely used as an AC industrial motor?
    (a) Synchronous motor (b) Induction motor (c) DC series motor exclusively (d) Stepper motor exclusively

Answer Key

1.(b)

2.(a)

3.(b)

4.(b)

5.(b)

6.(a)

7.(a)

8.(c)

9.(b)

10.(a)

11.(b)

12.(c)

13.(a)

14.(b)

15.(b) 16.(b)