EEE 321 Electrical Machine II LECTURE II.pptx

08/14/2025 PREPARED BY ISAH SULEIMAN @ AFUST, ALI
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DEPARTMENT OF ELECTRICAL & ELECTRONICS
ENGINEERING
ELECTRICAL MACHINES II
EEE321
LECTURE II
UG-3 SECOND SEMESTER
2024/2025 SESSION

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DISCLAIMER
These lecture notes are for educational purposes only and is provided to
support classroom learning. While every effort has been made to ensure
accuracy, neither the University nor the Department assumes responsibility for
any errors, omissions, or discrepancies that may be present in these lecture
notes. Students should verify information using recommended course materials.
For academic use only.
© 2025, Isah Suleiman.
Department of Electrical & Electronics Engineering

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Synchronous Machine
• Introduction
• A synchronous machine is a rotating electric machine that runs at a
constant speed, called the synchronous speed, when in steady operation.
• This speed matches the speed of the rotating magnetic field in the air gap.
• Unlike induction machines, where the rotor lags slightly behind the
rotating magnetic field (slip), in a synchronous machine the rotor and the
magnetic field rotate together at the same speed.

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 Main Uses
 Synchronous Generator (Alternator):
Main use of large synchronous machines.
They generate electrical power in hydro, nuclear, or thermal
power stations.
Large ratings: hundreds of MVA (Mega Volt-Amperes).
They are the main devices that convert mechanical energy (from
turbines) into electrical energy for the grid.
They will remain essential for the global power system for the
foreseeable future.
Synchronous Machine

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 Synchronous Motor:
 Operates as a motor when supplied with electrical power.
 Used when constant speed is needed:
Large motors drive big pumps in power plants.
Small motors run clocks, timers, record players, etc.
 Not widely used for variable speed industrial drives
 Linear versions (Linear Synchronous Motors) are being developed for
high-speed trains.
Synchronous Machine

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 Basic Structure
 Figure 1 shows the parts:
 Stator: Stationary part, carries the 3-phase AC winding.
 Rotor: Rotating part, has salient pole or cylindrical pole field
windings.
 Excitation:
 DC current flows in the rotor winding to create rotor poles.
 The stator winding is connected to the AC supply.
 This makes the synchronous machine a doubly excited machine (DC
for rotor + AC for stator).
Synchronous Machine

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Synchronous Machine

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Construction of Three-Phase Synchronous Machines
 Stator:
 The stator is the stationary part of the synchronous machine.
 It has a three-phase distributed winding, very similar to that of a
three-phase induction machine.
 It is sometimes called the armature winding because it is connected
to the AC supply.
 It is designed to handle high voltage and high current.
Synchronous Machine

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 Rotor
 The rotor has a field winding that carries direct current (DC).
 The rotor winding is supplied with DC through slip rings and brushes.
 The rotor is the rotating part that creates the main magnetic field
inside the machine.
Fig. 2: High-speed cylindrical
rotor synchronous generator.
Synchronous Machine

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 Basic Parts (Fig. 3):
 Stator: 3-phase AC winding.
 Rotor: Field winding supplied by DC.
 Air gap: Space between stator and rotor where the magnetic field
interacts.
Fig. 3: Low-speed salient pole synchronous generator. (a) Stator. (b) Rotor.
Synchronous Machine

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Types of Rotors
 Synchronous machines have two main types of rotors:
 Cylindrical (Non-Salient Pole) Rotor
Also called smooth rotor.
Has a uniform air gap.
Winding is distributed.
Used for high-speed machines, typically driven by steam turbines.
Power rating: hundreds of megawatts (MW).
Usually have 2 or 4 poles.
Long and small in diameter (Fig. 4).
Fig. 4
Synchronous Machine

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 Salient Pole Rotor
 Has protruding poles (salient means ‘projecting’).
 Concentrated windings on the poles.
 Non-uniform air gap.
 Used for low-speed machines, driven by water turbines in hydro
plants.
 Has many poles (up to 50) for low-speed operation.
 Power rating: tens to hundreds of MW.
 Shorter but larger in diameter (Fig. 3).
 Also used in smaller machines (50 kW to 5 MW) for emergency or
stand-alone power.
Synchronous Machine

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Fig. 5: Parts of rotor of salient pole alternator
Synchronous Machine

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 Applications of Salient Pole Machines:
 Large hydro generators.
 Stand-alone generators for emergency backup.
 Motors for pumps, cement mixers, etc.
Synchronous Machine

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Synchronous Generator: How It Works
• Refer to Fig. 5
 How excitation works:
 DC current (If) flows through the rotor field winding.
 This creates a sinusoidally distributed flux in the air gap → Excitation
Field.
 When the rotor is rotated by a prime mover (turbine, diesel engine,
motor), this field revolves.
 The rotating magnetic flux links with the stator windings (aa , bb , cc
′ ′ ′)
and induces voltages.
 These voltages are called excitation voltages (Ef) and are sinusoidal
and 120° apart in phase.
Synchronous Machine

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Fig. 5: Excitation voltage in synchronous machines.
Synchronous Machine

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Synchronous Machine

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• Open Circuit Characteristic (OCC)
• The voltage Ef depends on rotor speed (n) and
field current (If).
• Initially, increasing If increases Ef linearly.
• But at higher If, due to magnetic saturation, Ef
levels off.
• The OCC curve (Fig. 6) shows how terminal
voltage varies with field current at constant
speed.
Synchronous Machine

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Fig. 6: Open circuit characteristic (OCC) or
magnetization characteristic of a synchronous
machine.
Space phasor diagram.
Synchronous Machine

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 Armature Reaction
 When the stator is connected to a 3-phase load, current flows.
 Stator current also creates a rotating flux
 The net air gap flux is:
 The resultant flux and both components rotate at synchronous speed.
Synchronous Machine

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Phasor Diagram:
 Rotor field mmf () → creates
.
 Induced voltage lags by 90°.
 Stator current lags by an angle θ.
 Stator mmf → creates
.
 Resultant mmf is the vector sum:
Synchronous Machine

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Parallel Operation of Alternators
 Introduction
 Big power demands → better to use many generators in parallel
 One big unit is efficient but not reliable
 Modern power stations connect many alternators in parallel, often across
different stations
 Why Operate in Parallel?
 Physical Limits: One machine cannot produce all needed power
 Reliability: If one unit fails, others keep working
 Maintenance: Easier to repair small units
 Standby Units: Smaller = cheaper backup units
 Expansion: Add more units as demand grows
 Efficiency: Match output to changing load → run at best efficiency

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 Basic Requirements
 Same voltage for all units
 Same frequency (same speed) →
 Same waveform shape
 Same speed-load droop → fair load sharing
 Matching impedance triangles
Parallel Operation of Alternators

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Synchronising
 What is Synchronising?
 Synchronising = connecting a new alternator to a system already
working
 New alternator must match voltage, phase, frequency, and sequence
 Conditions for Synchronising
 Incoming voltage = bus-bar voltage
 Voltages must be 180° out of phase (no unwanted current)
 Same frequency → same vector speed
 Three-phase: same phase sequence

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 If Not Matched?
 Unequal voltages → unwanted circulating currents inside alternators
 Wrong phase angle → same problem
 Wrong frequency → voltages drift in/out of phase → internal currents change all the
time → wasted power, possible damage
Synchronising

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Infinite Bus
 What is an Infinite Bus?
 An infinite bus (or grid) is a large power supply system where many
large synchronous generators (also called alternators) are connected
together.
 Because there are so many generators working together:
 The voltage and frequency of the system remain almost constant, no
matter how many loads are connected or disconnected.
 This makes the grid very stable.
 A typical grid uses different power plants — hydro, thermal, nuclear,
oil — all connected to the same network, as shown in Figure 7.

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FIGURE 7 Infinite bus (or grid) system.
Infinite Bus

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 Why Use an Infinite Bus?
 Generators do not supply loads directly — instead, they feed power
to the grid.
 Power is generated at a lower voltage (e.g., 20–30 kV) and stepped up
to a high voltage (like 230 kV) using transformers.
 High voltage helps transmit electricity over long distances more
efficiently.
 Near the users (homes, industries), the voltage is stepped down again
to levels like 230 V, 115 V, or 600 V.
Infinite Bus

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Paralleling with the Infinite Bus
 When a power plant wants to add a generator to the grid, it must
connect it properly. This is called paralleling.
 Before connecting, the new generator must match the grid in:
 Voltage (must be equal)
 Frequency (must be the same)
 Phase sequence (the order of phases must match)
 Phase (must be in step — no angle difference)
Infinite Bus

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Checking the Conditions
 A synchroscope (see Figure 8) checks:
 The phase difference between the generator and the grid.
 If the needle moves slowly, the frequencies are close.
 When the pointer is straight up (zero phase difference), the breaker
can be closed to connect the generator.
 Synchronizing lamps (Figure 9) can also show if the generator is ready:
 If all lamps are dark at the same time, the phase and voltage match.
 If they blink in step, the frequency is slightly off.
 If they glow unevenly, the phase sequence or voltages may be
incorrect.
Infinite Bus

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FIGURE 8 Synchroscope.
FIGURE 9 Schematic diagram for paralleling a
synchronous generator with the infinite bus
using synchronizing lamps
Infinite Bus

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REFERENCES
 Electrical Machines, Drives, and Power Systems Fifth Edition (Theodore
Wildi).
 Electric-Machines-Kothari-Nagrath-4th-Edition
 B.L.Theraja & A.K.Theraja, “A Text Book of Electrical Technology”,
Volume II, S. Chand & Company Ltd.
 P. S. Bhimbra, “Electrical Machinary”, Khanna Publishers, New Delhi.

EEE 321 Electrical Machine II LECTURE II.pptx

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