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Tuesday, December 22, 2009

Synchronous Machines

Synchronous Machines
• Designed to operate at synchronous speed, ns. Hence, the name.
• Has a field circuit supplied by an external dc source.
• Can be used to operate as:
o Synchronous generator (also known as alternator)
o Synchronous motor
• Used primarily as generator

• It has 2 parts:-
a) Stator
b) Rotor
• It has 2 types of windings:-
a) Field winding
o Create magnetic field
o On the rotor
o Fed by a dc current thru’
– Slip rings & brushes, or
– Permanent magnet
b) Armature winding
o Voltage is induced on it
o On the stator
o Always connected in Y-connection. Preferred because:-
– The voltage per phase is only 1/√3 or 58% of the line voltage, permitting a reduction in the amount of dielectric insulation
– Under load, the voltage can become distorted and no longer sinusoidal due to third harmonic voltages. With a Y-connection, the 3rd harmonic voltages cancel between the line-to-line voltages but with a ∆-connection, the 3rd harmonic voltage add and appear on the line-to-line voltages
• There are 2 types of rotor:-
a) Salient pole
o Driven by low-speed hydraulic turbine
o Require large no of poles
o Posses large diameter to provide space for the poles
b) Cylindrical (non-salient/round)
o Driven by high-speed steam turbine
o No of poles cannot be less than 2
o Smaller compared to salient pole

Synchronous Generator
Principle of Operation
• A dc current is applied to the rotor winding to produce magnetic field.
• Rotor is turned by a prime mover, producing a rotating magnetic field within the air gap.
• The rotating magnetic field induced 3Ф voltage within the stator winding
• The rotating magnetic field & the rotor rotate at the same speed called synchronous speed, ns given by:

f - Freq of induced voltage (Hz)`
p - No of poles

Equivalent Circuit
• Consider only 1Ф

(For Y connection) and (For  connection)

Phasor Diagram
• To show the summation of the vectors

• Depends on type of load
a) Pure resistive load (unity power factor)

b) Inductive load (lagging power factor)

c) Capacitive load (leading power factor)

Example 1
A 3Ф star-connected generator supplies a load of 10 MW at power factor 0.85 lagging and the terminal voltage is 11 kV. The armature resistance is 0.1 ohm/phase and synchronous reactance of 0.66 ohm/phase. Calculate the line value of emf generated. Draw the phasor diagram.

Synchronous Motor
Principle of Operation
• It has 2 supplies:-
o DC supply connected to the rotor
o 3Φ ac supply connected to the stator winding
• The 3Φ ac supply produces a 3Φ current flow in the stator winding that will produce a rotating magnetic field.
• DC supply to the rotor produces a 2nd magnetic field.
• Since stator magnetic field is rotating, the rotor rotates to keep up with the moving stator magnetic field, supplying mechanical power to the load.

Equivalent Circuit
• Consider only 1Ф

(For Y connection) and (For  connection)

Phasor Diagram
• To show the summation of the vectors

• Depends on type of load
(a) Pure resistive load (unity power factor)

(b) Inductive load (lagging power factor)

(c) Capacitive load (leading power factor)

Example 2
A 2300 V 3Φ, star-connected synchronous motor has an armature resistance of 0.2 ohm/phase and a synchronous reactance of 2.2 ohm/phase. The motor is operating on 0.5 power factor leading with a line current of 200 A. Determine the value of generated or counter emf per phase. Draw the phasor diagram.

Voltage Regulation
• In general,

• For synchronous generator

• For synchronous motor

Example 3
A 200 kVA, 600 V, 50 Hz 3Φ synchronous generator is Y-connected. The generator has a synchronous reactance 0.10 ohm/phase and armature resistance of 2.0 ohm/phase. Calculate the voltage regulation if the generator is operating at 0.75 leading power factor.

Power Flow Diagram
• Synchronous Generator

From the power flow diagram:

• Synchronous Motor

From the power flow diagram:

Example 4
A 2000 V, 500hp, 3Ф Y connected synchronous motor has a resistance of 0.3 Ω and a synchronous reactance of 3.0 Ω per phase respectively. Determine the induced emf per phase if the motor works on full-load with an efficiency of 92 % and p.f = 0.8 leading.

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