Junior Lineman / Energy Assistant — ITI Electrical Quick Book · Chapter 4

Transformers, DC and AC Machines, Starters

What to remember

Transformer: V₁/V₂ = N₁/N₂ = I₂/I₁ (ideal). EMF equation: E = 4.44 f N Φm. It changes voltage and current, not frequency, and works only on AC.
Synchronous speed N_s = 120 f / P rpm; slip s = (N_s − N) / N_s. The three-phase induction motor is the workhorse of industry and is self-starting.
DC motor back EMF: E_b = V − I_a R_a. A series motor must never be started without load. A starter limits the heavy starting current of motors.

Transformer: principle and parts

A transformer is a static device that transfers electrical energy from one circuit to another by mutual induction, without change of frequency. An AC current in the primary produces alternating flux in the core. This flux links with the secondary winding and induces an EMF.

Step-up transformer: secondary turns are more than primary turns, so output voltage is higher. Step-down: secondary turns are fewer.
Turns ratio K = N₂ / N₁ = V₂ / V₁ = I₁ / I₂.
Rating is given in kVA (not kW), because losses depend on voltage and current, not on power factor.
EMF equation: E = 4.44 f N Φm volt, where Φm is the maximum flux in weber.

Core: made of thin laminations of silicon steel (CRGO, cold-rolled grain-oriented) to reduce losses. Types: core-type and shell-type. Windings: copper or aluminium, with the low-voltage winding usually nearer the core.

Losses:

Loss	Cause	Depends on	Reduced by
Iron (core) loss: hysteresis	Magnetisation reversal in the core	Voltage and frequency; constant for a given supply	Silicon steel core
Iron (core) loss: eddy current	Circulating currents induced in the core	Voltage and frequency	Thin insulated laminations
Copper loss	I²R in windings	Load current (varies with load)	Thicker conductors

Efficiency η = output / input × 100%. Maximum efficiency occurs when copper loss equals iron loss. Voltage regulation tells how much the secondary voltage falls from no load to full load.

Worked example 1: A transformer has 2000 turns in the primary and 200 turns in the secondary. On 2200 V primary, the secondary voltage = 2200 × 200 / 2000 = 220 V.

Worked example 2: A single-phase 10 kVA, 200 V transformer has full-load current = 10000 / 200 = 50 A.

Worked example 3: A transformer delivers 9.5 kW with total losses of 500 W. Input = 10 kW, efficiency = 95%.

Worked example 4: With 50 Hz, N = 100 turns and Φm = 0.01 Wb, E = 4.44 × 50 × 100 × 0.01 = 222 V.

Cooling and accessories of oil-immersed power and distribution transformers:

Transformer oil: insulating and cooling medium.
Conservator: a tank above the main tank that allows oil to expand.
Breather: holds silica gel, which absorbs moisture from air entering the conservator (blue silica gel turns pink when saturated).
Buchholz relay: a gas-operated relay between the main tank and conservator; it detects internal faults. It is used in transformers that have a conservator.
Explosion vent, temperature indicators, tap changer (to adjust voltage ratio) and bushings.

Distribution transformers in India are commonly three-phase, delta primary and star secondary with the neutral brought out (delta-star, vector group Dyn11), giving a 415 V line / 240 V phase supply. Auto-transformer: one winding is common to primary and secondary; used for starters and variac. Instrument transformers: current transformer (CT) and potential transformer (PT) are used to extend meter ranges and in protection. A CT secondary must never be left open when the primary is energised.

DC machines

A DC generator converts mechanical energy into DC electrical energy by electromagnetic induction (Fleming's right-hand rule gives the direction of induced EMF). A DC motor converts electrical energy into mechanical energy (Fleming's left-hand rule gives the direction of force).

Main parts: yoke (frame), field poles and winding, armature core and winding, commutator (converts AC in the armature to DC in the generator; reverses current in the motor), brushes (carbon) and bearings.

Armature windings: lap winding (number of parallel paths A = P, poles) suits high-current, low-voltage machines. Wave winding (A = 2) suits high-voltage, low-current machines.

Generator EMF: E = P Φ Z N / (60 A) volt.

Types by field connection: separately excited, shunt, series, and compound (cumulative or differential, short or long shunt).

Back EMF (E_b): a motor armature rotating in the field also generates an EMF that opposes the supply voltage. E_b = V − I_a R_a. At starting E_b = 0, so the armature current is very large. Hence a starter is needed.

Torque is proportional to flux and armature current (T ∝ Φ I_a).

DC motor	Characteristic	Typical use
Shunt	Nearly constant speed	Lathes, fans, pumps
Series	Very high starting torque; speed rises sharply at light load	Cranes, hoists, electric traction
Compound	High starting torque with limited runaway	Presses, rolling mills, elevators

A series motor must never be started on no load, since the speed rises dangerously (runaway). A series motor must be coupled directly or by gears, not by a belt that can slip or break. Speed control of a shunt motor: field control (above base speed) and armature-resistance or voltage control (below base speed). To reverse a DC motor, reverse either the armature connections or the field connections, not both.

Worked example 5: A motor on 220 V has armature resistance 0.5 Ω and takes 20 A. Back EMF = 220 − 20 × 0.5 = 210 V.

Starters for DC motors: the three-point and four-point starters place a variable resistance in series with the armature at starting and cut it out step by step. A no-volt (hold-on) coil releases the handle on supply failure and an overload coil trips on overload.

AC machines: three-phase induction motor

The three-phase stator winding produces a rotating magnetic field of speed N_s = 120 f / P rpm. The rotor conductors cut this field, current is induced in the rotor, and the rotor starts turning in the same direction, but always slower than N_s. The difference is the slip: s = (N_s − N) / N_s. A running motor has slip of a few percent. Rotor frequency = s × f.

Poles at 50 Hz	Synchronous speed
2	3000 rpm
4	1500 rpm
6	1000 rpm
8	750 rpm

Worked example 6: A 4-pole motor on 50 Hz has N_s = 1500 rpm. At 1440 rpm slip = 60/1500 = 0.04 = 4%. Rotor frequency = 0.04 × 50 = 2 Hz.

Types: squirrel-cage rotor (simple, rugged, cheap, low starting torque) and slip-ring or wound rotor (external resistance added through slip rings gives high starting torque and low starting current). Direction reversal: interchange any two supply lines. Speed control: change of frequency, change of number of poles, or rotor resistance (slip-ring type).

Single-phase motors are not self-starting, so they need a starting aid:

Split-phase (resistance-start) motor: for fans, small pumps.
Capacitor-start motor: high starting torque, for compressors and pumps.
Capacitor-start capacitor-run and permanent-split capacitor (ceiling fan) motors.
Shaded-pole motor: very small, low torque.
Universal motor: works on both AC and DC and has a high speed (mixers, drills).

Synchronous motor: runs only at synchronous speed and is not self-starting. It can operate at leading power factor and is used for power-factor correction (a synchronous condenser).

Alternator (AC generator): rotor carries the DC field winding; stator carries the armature winding. Frequency f = P N / 120.

Motor starters

Starters limit the starting current, protect the motor and give no-volt and overload protection.

Direct-on-line (DOL): connects the motor straight to the supply. Starting current is about 5 to 7 times the full-load current. Used for small motors.
Star-delta starter: the stator is connected in star for starting and changed over to delta for running. Starting line current and torque reduce to one-third of direct-delta starting. Used for medium squirrel-cage motors; motor must be delta-rated at the running voltage.
Auto-transformer starter: applies reduced voltage through taps.
Rotor-resistance starter: for slip-ring motors.
Soft starters and variable-frequency drives use electronics.

Protection devices: thermal overload relay (against overload), HRC fuse or MCB (against short circuit), no-volt coil.

Exam traps

Transformer rating is in kVA, not kW. Frequency does not change across a transformer.
Iron loss is constant; copper loss varies with the square of the load current.
Buchholz relay sits between main tank and conservator; the breather is on the conservator.
Fleming's right-hand rule is for a generator, left-hand rule for a motor.
Lap winding: A = P. Wave winding: A = 2.
A series DC motor races dangerously on no load; a shunt motor does not.
Induction motor speed is always below synchronous speed. Slip cannot be zero for torque.
A single-phase induction motor is not self-starting; a three-phase one is.

One-liners

1. A transformer works on the principle of mutual induction.
2. CRGO silicon steel is the transformer core material.
3. Maximum efficiency occurs when copper loss equals iron loss.
4. Silica gel in the breather absorbs moisture.
5. Buchholz relay detects internal faults in oil-filled transformers.
6. Commutator rectifies the armature AC in a DC generator.
7. Back EMF is zero at the instant of starting a DC motor.
8. Series motors have the highest starting torque among DC motors.
9. Synchronous speed is 1500 rpm for 4 poles at 50 Hz.
10. Reversing any two lines reverses a three-phase induction motor.
11. Star-delta starting reduces starting current to one-third.
12. A synchronous motor is not self-starting.

Practice questions

Which of the statements is/are correct? 1. A transformer changes the voltage level without changing the frequency. 2. A transformer works on a DC supply.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
A transformer needs changing flux, so it works only on AC.
Which of the statements is/are correct? 1. Copper loss in a transformer is constant at all loads. 2. Iron loss in a transformer remains almost constant from no load to full load.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
B. 2 only
Copper loss varies as the square of the load current.
Which of the statements is/are correct? 1. The rating of a transformer is given in kVA. 2. The rating of a transformer is given in kW.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
Losses depend on voltage and current, not on power factor, so the rating is in kVA.
Which of the statements is/are correct? 1. Fleming's left-hand rule is applied to a generator. 2. Fleming's right-hand rule is applied to a generator.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
B. 2 only
The left-hand rule gives the force direction in a motor.
Which of the statements is/are correct? 1. A Buchholz relay is fitted between the main tank and the conservator. 2. A Buchholz relay is used in dry-type transformers with no conservator.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
It needs the oil-filled pipe to the conservator to trap gas.
Which of the statements is/are correct? 1. A series DC motor can be safely started without any load. 2. A series DC motor develops a very high starting torque.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
B. 2 only
Without load the speed rises dangerously (runaway).
Which of the statements is/are correct? 1. Lap winding has as many parallel paths as the number of poles. 2. Wave winding always has six parallel paths.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
Wave winding has two parallel paths.
Which of the statements is/are correct? 1. A synchronous motor is self-starting. 2. The speed of a three-phase induction motor is always less than synchronous speed.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
B. 2 only
A synchronous motor needs a starting method.
Which of the statements is/are correct? 1. Interchanging any two supply lines reverses a three-phase induction motor. 2. Reversing the direction of a three-phase induction motor needs a change of supply frequency.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
Direction reverses by swapping two lines, not by changing frequency.
Which of the statements is/are correct? 1. Star-delta starting increases the starting torque. 2. Star-delta starting reduces the starting current to one-third.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
B. 2 only
Torque is also reduced to one-third.
Which of the statements is/are correct? 1. A split-phase single-phase motor is not self-starting without a starting winding. 2. A universal motor works only on AC.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
A universal motor works on both AC and DC.
Which of the statements is/are correct? 1. A PT secondary should be short-circuited for safety. 2. The CT secondary must not be left open while the primary carries current.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
B. 2 only
A PT secondary should not be shorted; a CT secondary should not be opened.
A transformer works on the principle of
1. Hall effect
2. electrostatic induction
3. mutual induction
4. self induction only
Answer
C. mutual induction
Flux of the primary links the secondary and induces an EMF.
The core of a power transformer is made of
1. aluminium blocks
2. solid copper
3. solid cast iron
4. laminated silicon steel
Answer
D. laminated silicon steel
Laminated CRGO silicon steel reduces hysteresis and eddy-current losses.
Laminating the core of a transformer reduces mainly the
1. windage loss
2. copper loss
3. dielectric loss
4. eddy current loss
Answer
D. eddy current loss
Thin insulated laminations break the paths of eddy currents.
A transformer has 2000 primary turns and 200 secondary turns. With 2200 V on the primary, the secondary voltage is
1. 22000 V
2. 22 V
3. 2200 V
4. 220 V
Answer
D. 220 V
V₂ = 2200 × 200/2000 = 220 V.
A single-phase 10 kVA, 200 V transformer has a full-load secondary current of
1. 20 A
2. 2000 A
3. 50 A
4. 5 A
Answer
C. 50 A
I = 10000/200 = 50 A.
A transformer delivers 9.5 kW and its total losses are 500 W. The efficiency is
1. 98%
2. 90%
3. 95%
4. 50%
Answer
C. 95%
Input = 10 kW; efficiency = 9.5/10 = 95%.
A transformer has 100 primary turns, a maximum flux of 0.01 Wb and a 50 Hz supply. The primary induced EMF is nearly
1. 444 V
2. 100 V
3. 50 V
4. 222 V
Answer
D. 222 V
E = 4.44 f N Φm = 4.44 × 50 × 100 × 0.01 = 222 V.
The maximum efficiency of a transformer occurs when
1. copper loss is zero
2. load is zero
3. copper loss equals iron loss
4. iron loss is zero
Answer
C. copper loss equals iron loss
This is the standard condition for maximum efficiency.
The breather of a transformer contains
1. transformer oil
2. sodium chloride
3. copper sulphate
4. silica gel
Answer
D. silica gel
Silica gel absorbs moisture from the incoming air.
The conservator of a transformer is meant to
1. cool the windings by air
2. step up voltage
3. allow expansion and contraction of oil
4. hold the tap changer
Answer
C. allow expansion and contraction of oil
It provides room for oil to expand with temperature.
A three-phase distribution transformer commonly has the connection
1. delta-delta
2. open delta
3. star primary, delta secondary
4. delta primary, star secondary with neutral
Answer
D. delta primary, star secondary with neutral
Delta-star (Dyn11) gives a four-wire 415/240 V supply.
The part that converts the alternating current in a DC generator armature into direct current is the
1. commutator
2. slip ring
3. interpole
4. yoke
Answer
A. commutator
The commutator rectifies the armature current.
The back EMF of a DC motor at the instant of starting is
1. equal to supply voltage
2. maximum
3. zero
4. twice the supply voltage
Answer
C. zero
The armature is at rest, so no EMF is generated.
A DC motor runs from 220 V, has an armature resistance of 0.5 Ω and takes 20 A. The back EMF is
1. 220 V
2. 10 V
3. 230 V
4. 210 V
Answer
D. 210 V
E_b = V − I_aR_a = 220 − 10 = 210 V.
Which DC motor is most suitable for cranes and electric traction?
1. Shunt motor
2. Series motor
3. Universal motor
4. Separately excited motor
Answer
B. Series motor
A series motor gives very high starting torque.
Which DC motor gives nearly constant speed for lathes and fans?
1. Shunt motor
2. Universal motor
3. Series motor
4. Differential compound motor
Answer
A. Shunt motor
Field flux is nearly constant, so speed varies little with load.
The synchronous speed of a 4-pole motor on a 50 Hz supply is
1. 3000 rpm
2. 750 rpm
3. 1500 rpm
4. 1000 rpm
Answer
C. 1500 rpm
N_s = 120 × 50/4 = 1500 rpm.
The synchronous speed of a 6-pole machine on a 50 Hz supply is
1. 1500 rpm
2. 600 rpm
3. 750 rpm
4. 1000 rpm
Answer
D. 1000 rpm
N_s = 120 × 50/6 = 1000 rpm.
A 4-pole, 50 Hz induction motor runs at 1440 rpm. Its slip is
1. 6%
2. 10%
3. 2%
4. 4%
Answer
D. 4%
s = (1500 − 1440)/1500 = 0.04.
The rotor frequency of an induction motor running at 4% slip on a 50 Hz supply is
1. 0.5 Hz
2. 2 Hz
3. 50 Hz
4. 4 Hz
Answer
B. 2 Hz
f_r = s × f = 0.04 × 50 = 2 Hz.
The induction motor that gives high starting torque with low starting current is the
1. slip-ring (wound-rotor) motor
2. single-phase shaded-pole motor
3. squirrel-cage motor
4. universal motor
Answer
A. slip-ring (wound-rotor) motor
External resistance added through slip rings improves starting torque and cuts starting current.
The single-phase motor commonly used in a ceiling fan is the
1. series DC motor
2. universal motor
3. capacitor-run (permanent-split) motor
4. synchronous motor
Answer
C. capacitor-run (permanent-split) motor
A permanent capacitor gives a starting phase shift for fans.
A motor used in mixers and drills that works on both AC and DC is the
1. shaded-pole motor
2. universal motor
3. squirrel-cage motor
4. synchronous motor
Answer
B. universal motor
A universal motor is a series motor suited to both supplies.
Direct-on-line starting current of an induction motor is about
1. equal to full-load current
2. exactly twice full-load current
3. 5 to 7 times the full-load current
4. one-tenth of full-load current
Answer
C. 5 to 7 times the full-load current
DOL starting current is several times rated current.
In star-delta starting, the motor stator windings are connected
1. in star for starting and in delta for running
2. in delta for starting and in star for running
3. in parallel for both
4. in series for both
Answer
A. in star for starting and in delta for running
Star reduces the winding voltage to 1/√3 at starting.
The synchronous motor can be used to improve the power factor because it can operate at
1. lagging power factor only
2. leading power factor
3. zero power factor only
4. very low speed
Answer
B. leading power factor
Over-excited synchronous motors draw leading current.
A three-phase motor of 4 poles, 50 Hz has a rotating field speed 1500 rpm. If the supply frequency falls to 40 Hz the field speed is
1. 1000 rpm
2. 1875 rpm
3. 1500 rpm
4. 1200 rpm
Answer
D. 1200 rpm
N_s = 120 × 40/4 = 1200 rpm.
Protection against overload for a motor is provided by a
1. breather
2. commutator
3. conservator
4. thermal overload relay
Answer
D. thermal overload relay
The relay trips when current stays above rated for too long.
The speed of a DC shunt motor above its base speed is controlled by
1. using a capacitor
2. increasing the armature resistance
3. weakening the field flux
4. reversing the supply
Answer
C. weakening the field flux
Lower flux gives higher speed.
The EMF equation of a DC generator is
1. E = PΦZN/(60A)
2. E = 4.44 fNΦ
3. E = V − IR
4. E = PΦZ/(120A)
Answer
A. E = PΦZN/(60A)
The 4.44 form is for transformers and AC windings.
A step-down transformer has a turns ratio of 10:1 (primary to secondary) and carries 2 A in the secondary. The primary current is nearly
1. 2 A
2. 20 A
3. 12 A
4. 0.2 A
Answer
D. 0.2 A
I₁ = I₂ × N₂/N₁ = 2 × 1/10 = 0.2 A.
The loss in a transformer core caused by repeated reversal of magnetisation is called
1. hysteresis loss
2. eddy current loss
3. copper loss
4. stray load loss
Answer
A. hysteresis loss
Magnetic domains reorient every half cycle, wasting energy as heat.
Which instrument transformer is used to extend the range of an ammeter on a high-current line?
1. Isolating transformer
2. Auto-transformer
3. Current transformer
4. Potential transformer
Answer
C. Current transformer
A CT steps current down to the ammeter range.

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1.Practically, the work output of a machine is _______ the work input due to the effect of friction.
2.Kidney failure is treated periodically on a kidney machine. The process is known as ________.
3.Which of the following quantities determine the rate of work performed by a machine?
4.Which of the following statements are true regarding general safety in the workshop? Statement 1: Use any available tool for the job, even if it is not the correct one. Statement 2: Keep the tools in their proper place after use. Statement 3: Clean the machine while it is still running. Statement 4: Replace worn-out or damaged tools immediately.
5.What placed between moving parts of a machine can reduce friction?
6.What are the three main lubricants used in moving parts of a machine?
7.Which machine part is used to reduce friction between hubs and axles of ceiling fans and in bicycles?
8.If lubricants are used in machine parts, how does the movement become?
9.If a machine does 500 J of work in 4 seconds, what is its power output?
10.In some machines, why is using oil as a lubricant not advisable?

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