Three-Phase AC machines

Three-Phase Synchronous Machines

Resource 7

Three-Phase AC Machines Resource 7

Aim

Three-Phase Synchronous Machines

• To understand the construction and operation of a three-phase synchronous machine

Objectives

Three-Phase AC Machines Resource 7

Three-Phase Synchronous Machines

• To be able to describe the construction of the stator• To be able to describe the construction of a salient pole rotor• To be able to describe the construction of a cylindrical rotor• To be able to describe the operation of a synchronous machine as a generator• To be able to calculate synchronous speed and terminals voltage• To be able to describe the operation of a synchronous machine as a motor

Stator Construction

• Stator is identical to the induction motor

• Laminated low silicon steel rings joined together

• Slots insulated with Mylar

• Example of 36 slot stator with 3 coil conductors per slot, 12 slots per phase

Stator Construction

• Slot insulator inserted by hand

• Stator is identical to the induction motor

• Laminated low silicon steel rings joined together

• Slots insulated with Mylar

• Example of 36 slot stator with 3 coil conductors per slot, 12 slots per phase

Stator frame

Stator slots with insulator

Stator Construction

• Coils inserted by hand

Coil

• Stator is identical to the induction motor

• Laminated low silicon steel rings joined together

• Slots insulated with Mylar

• Example of 36 slot stator with 3 coil conductors per slot, 12 slots per phase

• Slot insulator inserted by hand

Stator frame

Stator slots with insulator

Stator Construction

• Coils can be placed in single or double layers

Stator slot

Stator Construction

Single layer

Stator Slots

1 coil arm per slot

Coil

Stator Construction

Double layer Stator Slots

Coil

2 coil arms in each slot

Stator Construction

Stators can be very large

Rotor Construction

• Salient Pole

Two types of rotor

• Cylindrical

Rotor Construction

Salient Pole

Difference between pole face curvature and stator creates non-linear variation in flux across pole face

Non-linear variation in flux across pole face produces sinusoidal change in the induced EMF

Rotor Construction

Cylindrical

Difference in coil spacing creates non-linear variation in flux around the rotor surface

Non-linear variation in flux around rotor surface produces sinusoidal change in the induced EMF

Rotor Construction

Cylindrical

Difference in coil spacing creates non-linear variation in flux around the rotor surface

Non-linear variation in flux around rotor surface produces sinusoidal change in the induced EMF

Operation as a Synchronous Generator

Two pole cylindrical rotor example

• Rotor field is turned at 3000rpm by a prime mover

SN

A

B

B’

C

C’

A’

A

A’

B

B’

C

C’

• EMFs induced in stator coils with frequency of 50Hz

• Magnetic Flux distributed around rotor produces sinusoidal variation in induced EMF

• Phase coils separated by 120o causes delay between phase EMFs

• Field produced on rotor by dc current through slip rings

Operation as a Synchronous Generator

Two pole cylindrical rotor example

A BC

• Delay between phases = 20/3 = 6.667ms 6.667ms

Period = 20ms

• Rotor field is turned at 3000rpm by a prime mover

• EMFs induced in stator coils with frequency of 50Hz

• Magnetic Flux distributed around rotor produces sinusoidal variation in induced EMF

• Phase coils separated by 120o causes delay between phase EMFs

• Field produced on rotor by dc current through slip rings

Calculations

Synchronous speed

Induced EMF

fS = supply frequency required

p = pole pairs

Φ = flux per pole set by rotor current

z = conductor in series per phase

Volts per phase

RPM

Operation as a Synchronous Generator

Generated EMF relationship

• Rotor speed

• Rotor current

Relationship between open circuit stator EMF and rotor current is a straight line until the steel begins to saturate when it becomes non-linear.

The open circuit EMF generated depends upon

saturation

Open circuitstatorEMF

Rotor current

linear

Operation as a Synchronous Motor

Two pole cylindrical rotor example

• Rotor field must be locked on to stator field speed

• Motor runs a synchronous speed whatever the mechanical load provided rotor field is strong enough

• Stator field rotates at 3000rpm from 50Hz supply

SN

A

B

B’C’

A’

NR

NS

NR = NS

• This is impossible within an induction motor as there wound be no induced currents to cause rotation

• This motor runs at synchronous speed hence the name – SYNCHRONOUS MOTOR

Operation as a Synchronous Motor

Two pole cylindrical rotor example

• Rotor field must be locked on to stator field speed

• Motor runs a synchronous speed whatever the mechanical load provided rotor field is strong enough

• Stator field rotates at 3000rpm from 50Hz supply

NR = NS

• This is impossible within an induction motor as there wound be no induced currents to cause rotation

• This motor runs at synchronous speed hence the name – SYNCHRONOUS MOTOR

Rotor Speed (NR)

NS

Load Torque

Operation as a Synchronous Motor

The V-curve

The rotor current can be adjusted to vary the power factor of the stator

Unity power factor is achieved when stator current is at its minimum

This machine can be used to correct power factor of induction motors when connected in parallel