UPDA-5 - Notes | FM Electricals

⚡ 1. WHAT IS AN ALTERNATOR?

An alternator (also called synchronous generator or AC generator) is a machine that converts mechanical energy into electrical energy in the form of alternating current (AC).

Fig.1: Basic Alternator

Working Principle:

Alternator works on the principle of electromagnetic induction (Faraday's Law). When a conductor (coil) rotates in a magnetic field, an EMF is induced in the conductor.

Input: Mechanical energy (from turbine, engine, diesel generator, etc.)
Output: AC electrical power (230V/415V, 50Hz or 60Hz)
Application: Power plants (thermal, hydro, nuclear, gas), vehicles, ships, aircraft

📌 Key Point: Alternator is also called "Synchronous Generator" because the rotor rotates at synchronous speed (N_s = 120f/P).

🔧 2. CONSTRUCTION OF ALTERNATOR

An alternator has two main parts: Stator (stationary) and Rotor (rotating).

2.1 Stator (Armature) - Stationary Part

Made of laminated silicon steel to reduce eddy current losses
Contains armature winding (where output voltage is induced)
Slots are cut on the inner periphery to hold the 3-phase windings
Windings are connected in star (Y) or delta (Δ)

2.2 Rotor (Field) - Rotating Part

Contains field winding which is excited by DC supply
Produces the main magnetic flux
Two types: Salient Pole and Cylindrical (Smooth) Rotor

Salient Pole Rotor

Cylindrical Rotor

📊 3. COMPARISON: SALIENT POLE vs CYLINDRICAL ROTOR

Parameter	Salient Pole Type	Cylindrical (Smooth) Type
Pole Shape	Poles project out from surface	Non-projecting, smooth cylinder
Air Gap	Non-uniform	Uniform
Diameter vs Length	Large diameter, short axial length	Small diameter, long axial length
Mechanical Strength	Weak	Mechanically robust
Speed	Low speed (100-1500 RPM)	High speed (1500-3600 RPM)
Prime Mover	Water turbines (hydro), IC engines	Steam turbines (thermal), gas turbines
Rating for same size	Smaller	Higher
Damper Winding	Provided (to prevent hunting)	Not necessary

📐 4. EMF EQUATION OF ALTERNATOR

When the rotor rotates, the flux linked with the stator conductors changes, inducing an EMF. The RMS value of induced EMF per phase is given by:

E_ph = 4.44 × f × Φ × T_ph × K_w

E_ph = Induced EMF per phase (Volts)
f = Frequency of induced EMF (Hz)
Φ = Flux per pole (Webers)
T_ph = Number of turns per phase
K_w = Winding factor (usually 0.95 to 0.98)

Derivation of 4.44 constant:

E_max = 2πf × N × Φ
E_rms = E_max / √2 = (2πf × N × Φ) / √2 = 4.44 × f × N × Φ

🔍 Example 1: A 3-phase alternator has 4 poles, flux per pole = 0.05 Wb, frequency = 50 Hz, turns per phase = 100. Calculate induced EMF per phase.

Solution:
E_ph = 4.44 × f × Φ × T_ph
= 4.44 × 50 × 0.05 × 100
= 4.44 × 250 = 1110 V

🔄 5. SYNCHRONOUS SPEED

Synchronous speed (N_s) is the speed at which the magnetic field rotates in the stator. The rotor must rotate at exactly this speed to generate AC power at the desired frequency.

N_s = (120 × f) / P

N_s = Synchronous speed (RPM)
f = Frequency (Hz)
P = Number of poles

Frequency (Hz)	Poles (P)	Synchronous Speed (RPM)
50 Hz	2	3000
	4	1500
	6	1000
	8	750
60 Hz	2	3600
	4	1800
	6	1200
	8	900

📌 Key Point: Maximum speed occurs when number of poles is minimum (P=2). For 50 Hz, N_s(max) = 3000 RPM. For 60 Hz, N_s(max) = 3600 RPM.

🔍 Example 2: A 3-phase alternator has 8 poles and runs at 750 RPM. Find the frequency.

Solution:
N_s = 120f / P → 750 = (120 × f) / 8 → 120f = 750 × 8 = 6000 → f = 6000 / 120 = 50 Hz

🔍 Example 3: A 60 Hz alternator runs at 1800 RPM. Find the number of poles.

Solution:
N_s = 120f / P → 1800 = (120 × 60) / P = 7200 / P → P = 7200 / 1800 = 4 poles

📝 6. KEY POINTS TO REMEMBER

Alternator = AC Generator = Synchronous Generator
Converts mechanical → electrical energy
Works on electromagnetic induction principle
Two rotor types: Salient Pole (low speed, hydro) and Cylindrical (high speed, thermal)
EMF Equation: E = 4.44 × f × Φ × T_ph
Synchronous Speed: N_s = 120f / P
For 50 Hz, maximum speed = 3000 RPM (P=2)
For 60 Hz, maximum speed = 3600 RPM (P=2)
Damper windings are provided in salient pole rotors to prevent hunting

📐 7. VOLTAGE REGULATION - DEFINITION

Voltage Regulation of an alternator is defined as the change in terminal voltage from no-load to full-load, expressed as a percentage of the rated terminal voltage, while keeping field excitation and speed constant.

% Voltage Regulation = [(E₀ - V) / V] × 100%

E₀ = No-load induced EMF (voltage when load is disconnected)
V = Rated terminal voltage at full load

📌 Key Point: Lower voltage regulation means better performance. Ideal regulation is 0%.

Fig.1: Voltage Regulation Concept

For different power factors:

Lagging PF (Inductive load): Voltage drops → Positive Regulation
Unity PF (Resistive load): Small voltage drop → Slightly Positive
Leading PF (Capacitive load): Voltage rises → Negative Regulation

📊 8. METHODS OF FINDING VOLTAGE REGULATION

Method	Also Known As	Accuracy	Test Required
EMF Method	Synchronous Impedance Method	Less accurate (overestimates)	OC & SC test
MMF Method	Ampere-Turn Method	Better than EMF	OC & SC test
ZPF Method	Potier Method	Most accurate	OC, SC & ZPF test
ASA Method	American Standard Association	Very accurate	OC, SC & ZPF test

⚡ 9. EMF METHOD (Synchronous Impedance Method)

In this method, we calculate the synchronous impedance (Z_s) from open-circuit and short-circuit tests.

Z_s = E_oc / I_sc
X_s = √(Z_s² - R_a²)

Fig.2: OC & SC Characteristics

🔍 Example: An alternator has OC voltage 400V at field current 5A, SC current 50A at same field current. Armature resistance 0.5Ω. Find Z_s and X_s.

Solution:
Z_s = E_oc / I_sc = 400 / 50 = 8Ω
X_s = √(Z_s² - R_a²) = √(64 - 0.25) = √63.75 = 7.98Ω

🎯 10. EFFECT OF POWER FACTOR ON REGULATION

Power FactorLoad TypeVoltage ChangeRegulation SignExample Lagging (cosφ < 1)Inductive (Motors, Transformers)Voltage dropsPositive (+)+5% to +8% Unity (cosφ = 1)Resistive (Heaters, Lamps)Small dropSlightly Positive+2% to +3% Leading (cosφ < 1)Capacitive (Capacitor banks)Voltage risesNegative (-)。-2% to -5%

Fig.3: Effect of PF on Terminal Voltage

📌 Important: For capacitive loads, voltage regulation is NEGATIVE because terminal voltage increases with load.

🔗 11. PARALLEL OPERATION OF ALTERNATORS

When multiple alternators are connected to a common bus bar, they operate in parallel. This is also called synchronizing.

Conditions for Parallel Operation (Must be satisfied):

Same voltage magnitude (RMS value equal)
Same frequency (both alternators at same Hz)
Same phase sequence (R-Y-B order same)
Same phase position (voltages in phase)

Fig.4: Parallel Operation of Alternators

💡 Why parallel operation?
• Continuity of supply (if one fails, others work)
• Maintenance without shutdown
• Efficiency improvement (run at optimal load)
• Future expansion possible

📝 12. KEY POINTS TO REMEMBER

Voltage Regulation = [(E₀ - V) / V] × 100%
For lagging PF → Positive Regulation (voltage drops)
For leading PF → Negative Regulation (voltage rises)
EMF method overestimates regulation (gives higher value)
ZPF (Potier) method is most accurate
Parallel operation requires: Voltage, Frequency, Phase sequence, Phase position to be same
Alternators are connected in parallel to ensure reliability and continuity

⚡ 13. WHAT IS A SYNCHRONOUS MOTOR?

A Synchronous Motor is an AC motor that runs at constant speed equal to the synchronous speed (N_s = 120f/P). It is a doubly excited machine because both stator and rotor are excited by separate sources.

Fig.1: Synchronous Motor Construction

Stator: 3-phase winding, connected to AC supply
Rotor: Field winding, excited by DC supply
Speed: Constant (does not change with load)

N_s = (120 × f) / P (RPM)

📌 Key Point: Synchronous motor runs at exactly synchronous speed from no-load to full load. It is NOT self-starting.

🔧 14. CONSTRUCTION OF SYNCHRONOUS MOTOR

14.1 Stator (Stationary Part)

Outer frame, cylindrical in shape
Made of laminated silicon steel
Has slots to carry 3-phase stator winding
Connected to 3-phase AC supply

14.2 Rotor (Rotating Part)

Rotates exactly at synchronous speed
Separated from stator by air gap
Excited by DC source (through slip rings)
Two types: Salient Pole (low speed) and Cylindrical (high speed)

Fig.2: Salient Pole vs Cylindrical Rotor

🔄 15. PRINCIPLE OF OPERATION

Synchronous motor is a doubly excited machine:

Stator winding → 3-phase AC supply → Produces rotating magnetic field
Rotor winding → DC supply → Produces constant magnetic field

Fig.3: Working Principle - Magnetic Locking

Working Steps:

Stator rotating magnetic field rotates at synchronous speed N_s
Rotor is excited with DC and becomes an electromagnet
Rotor magnetic field locks with stator rotating field
Rotor rotates at exactly N_s (synchronous speed)

📌 Important: Synchronous motor is NOT self-starting. Special starting methods are required (damper winding, pony motor, or VFD).

📊 16. V-CURVES & POWER FACTOR CONTROL

V-Curves show the relationship between armature current and field current. These curves help understand how synchronous motor power factor varies with excitation.

Fig.4: V-Curves of Synchronous Motor

Excitation	Power Factor	Motor Behavior	Application
Under-excited	Lagging PF	Absorbs reactive power (VAR)	Like an inductor
Normal-excited	Unity PF	No reactive power exchange	Normal operation
Over-excited	Leading PF	Supplies reactive power (VAR)	Power factor correction

💡 Application: Over-excited synchronous motors are used as synchronous condensers for power factor correction in industrial plants.

📐 17. EFFECT OF EXCITATION ON POWER FACTOR

Condition	Alternator (Generator)	Synchronous Motor
Over-excited	Lagging PF (supplies VAR)	Leading PF (supplies VAR to system)
Under-excited	Leading PF (absorbs VAR)	Lagging PF (absorbs VAR from system)

Fig.5: Power Factor vs Excitation

📌 Key Point: Over-excited synchronous motor = Leading Power Factor (acts like a capacitor)

🏭 18. APPLICATIONS OF SYNCHRONOUS MOTOR

Application	Reason
Power Factor Correction	Over-excited motor supplies VARs (acts as synchronous condenser)
Voltage Regulation	Used at end of transmission lines to maintain voltage
Low Speed, High Power Loads	Constant speed, high efficiency at low RPM
Air & Gas Compressors	Constant speed requirement
Crushers, Mills, Grinders	High starting torque with damper winding
Blowers, Exhausters, Fans	Constant speed operation

📝 19. KEY POINTS TO REMEMBER

Synchronous motor runs at constant speed (N_s = 120f/P)
It is a doubly excited machine (AC on stator, DC on rotor)
NOT self-starting — needs damper winding, pony motor, or VFD
Over-excited → Leading PF → Supplies VARs (like capacitor)
Under-excited → Lagging PF → Absorbs VARs (like inductor)
Used as synchronous condenser for power factor correction
V-Curves show armature current vs field current relationship
Maximum speed occurs at P = 2 → 3000 RPM (50 Hz) or 3600 RPM (60 Hz)

⚡ 20. WHAT IS AN INDUCTION MOTOR?

An Induction Motor (also called Asynchronous Motor) is an AC motor where the rotor current is induced by electromagnetic induction from the stator rotating magnetic field.

Fig.1: Induction Motor Construction

Stator: 3-phase winding, connected to AC supply
Rotor: No external supply — current induced by transformer action
Speed: Always less than synchronous speed (N < N_s)

N_s = (120 × f) / P (Synchronous Speed)
Rotor Speed N = N_s × (1 - s)

📌 Key Point: Induction motor is called "Asynchronous" because rotor never runs at synchronous speed. It always has some slip.

🔧 21. CONSTRUCTION OF INDUCTION MOTOR

21.1 Stator (Stationary Part)

Made of laminated silicon steel to reduce eddy current losses
Has slots on inner periphery to hold 3-phase winding
Windings are connected in star or delta
When energized, produces rotating magnetic field

21.2 Rotor (Rotating Part)

Cylindrical laminated core with parallel slots
Conductors (copper or aluminum bars) placed in slots
Short-circuited by end rings at both ends
Slots are skewed (not parallel to shaft) to reduce magnetic hum and prevent cogging

Fig.2: Stator & Rotor Construction

🐿️ 22. SQUIRREL CAGE ROTOR

The Squirrel Cage Rotor is the most common type of induction motor rotor. It gets its name because the rotor bars resemble a squirrel exercise wheel.

Fig.3: Squirrel Cage Rotor

FeaturesDescription ConstructionCopper/aluminum bars shorted by end rings WindingsNo windings — bars only Starting TorqueLow Starting CurrentHigh (5-7 times full load) Power FactorPoor at start Speed ControlDifficult CostLow, rugged construction ApplicationsFans, pumps, compressors, conveyors

Skewed Rotor Bars:

Bars are skewed (angled) to improve starting torque
Skewing increases bar length → increases resistance → increases starting torque
Also reduces magnetic hum and prevents cogging

🔘 23. WOUND ROTOR (SLIP RING ROTOR)

The Wound Rotor (or Slip Ring Rotor) has actual windings on the rotor, similar to the stator. The ends are brought out through slip rings and brushes.

Fig.4: Wound Rotor (Slip Ring)

FeaturesDescription Construction3-phase winding on rotor, star connected Slip RingsThree slip rings for external connections BrushesCarbon brushes to connect external resistors Starting TorqueHigh (can be controlled by external resistance) Starting CurrentLow (limited by external resistors) Speed ControlPossible by varying external resistance CostExpensive (slip rings + brushes) ApplicationsCranes, hoists, elevators, heavy loads

📊 24. COMPARISON: SQUIRREL CAGE vs WOUND ROTOR

Parameter	Squirrel Cage	Wound Rotor (Slip Ring)
Rotor Construction	Bars shorted by end rings	3-phase windings with slip rings
External Resistance	Not possible	Yes, through slip rings
Starting Torque	Low	High (adjustable)
Starting Current	High (5-7 × I_FL)	Low (2-3 × I_FL)
Speed Control	Difficult	Possible (by varying resistance)
Power Factor	Poor at start	Better (can be improved)
Cost	Low	High
Maintenance	Low	High (brushes & slip rings)
Applications	Fans, pumps, compressors	Cranes, elevators, hoists

📝 25. KEY POINTS TO REMEMBER

Induction motor = Asynchronous motor (rotor speed ≠ synchronous speed)
Rotor current induced by transformer action (electromagnetic induction)
Two rotor types: Squirrel Cage (common, rugged, low cost) and Wound Rotor (high starting torque)
Squirrel cage: Low starting torque, high starting current, no speed control
Wound rotor: High starting torque, low starting current, speed control possible
Skewed rotor bars improve starting torque and reduce humming
Slip rings and brushes are used in wound rotor motors
Wound rotor motors are used in cranes, elevators, hoists (high starting torque applications)

📐 26. SLIP IN INDUCTION MOTOR

Slip (s) is the difference between synchronous speed (N_s) and actual rotor speed (N), expressed as a fraction or percentage of synchronous speed.

s = (N_s - N) / N_s
% s = [(N_s - N) / N_s] × 100%

Fig.1: Slip Concept - Rotor always lags

ConditionRotor Speed (N)Slip (s)Explanation Rotor stationary (start)01 (100%)Maximum slip Running at no-load≈ N_s≈ 0 (0.5-1%)Very small slip Full loadSlightly less than N_s2-6%Normal operation Rotor at synchronous speedN_s0Not possible (no torque)

🔍 Example 1: A 4-pole, 50 Hz induction motor runs at 1440 RPM at full load. Find slip.

Solution:
N_s = 120f/P = (120 × 50) / 4 = 1500 RPM
s = (1500 - 1440) / 1500 = 60 / 1500 = 0.04 or 4%

🎵 27. ROTOR FREQUENCY (Slip Frequency)

The frequency of induced voltage and current in the rotor is called rotor frequency (f_r). It depends on slip.

f_r = s × f

At start (s = 1): f_r = f (supply frequency)
At no-load (s ≈ 0): f_r ≈ 0 (very low frequency)
At full load (s = 2-6%): f_r = 1-3 Hz

Fig.2: Rotor Frequency vs Slip

🔍 Example 2: A 50 Hz induction motor has 4% slip. Find rotor frequency.

Solution:
f_r = s × f = 0.04 × 50 = 2 Hz

🔌 28. INDUCTION MOTOR vs TRANSFORMER

Parameter	Transformer	Induction Motor
Primary	Primary winding (stationary)	Stator winding (stationary)
Secondary	Secondary winding (stationary)	Rotor winding (rotating)
Frequency	Same on both sides	f_r = s × f (different)
Air gap	Very small	Small but present
Working Principle	Mutual induction	Mutual induction

📌 Key Point: Induction motor is like a transformer with rotating secondary. The rotor frequency changes with speed.

📐 29. EQUIVALENT CIRCUIT OF INDUCTION MOTOR

The equivalent circuit of an induction motor is similar to a transformer, with the secondary side modified to account for varying rotor frequency.

Fig.3: Per-Phase Equivalent Circuit

R₂/s = R₂ + R₂(1-s)/s

R₁, X₁: Stator resistance and leakage reactance
R_c: Core loss resistance
X_m: Magnetizing reactance
R₂/s: Rotor resistance referred to stator (includes load)
R₂(1-s)/s: Represents mechanical load (power output)

💡 Understanding R₂/s:
• At start (s=1): R₂/s = R₂ (only rotor resistance)
• At full load (s=0.04): R₂/s = 25 × R₂ (much larger, good torque)

🔬 30. NO-LOAD & BLOCKED ROTOR TESTS

Test Procedure Parameters Measured Similar to Transformer No-Load TestMotor runs freely, rated voltage appliedCore loss, friction & windage loss, magnetizing currentOpen Circuit Test Blocked Rotor TestRotor locked, reduced voltage appliedCopper loss, stator & rotor impedancesShort Circuit Test

Fig.4: No-Load & Blocked Rotor Tests

No-Load Test Results:

Slip is very small (≈ 0.5-1%)
Rotor current negligible
Power measured = Core loss + Friction & Windage loss

Blocked Rotor Test Results:

Slip = 1 (rotor stationary)
Voltage applied to circulate rated current
Power measured = Stator copper loss + Rotor copper loss

📝 31. KEY POINTS TO REMEMBER

Slip formula: s = (N_s - N) / N_s
At start: s = 1 (rotor stationary)
At no-load: s ≈ 0 (very small, 0.5-1%)
At full load: s = 2-6% (small motors have higher slip)
Rotor frequency: f_r = s × f
Induction motor = transformer with rotating secondary
No-load test measures core loss + mechanical losses
Blocked rotor test measures copper losses
The term R₂(1-s)/s represents mechanical load in equivalent circuit

📚 ALTERNATOR & AC MOTOR • SET 1