Three-Phase AC machines

Three-Phase Synchronous Machine Starters and Control Gear

Resource 8

Three-Phase AC Machines Resource 7

Aims

Three-Phase Synchronous Machines

• To understand the methods of starting a three-phase synchronous machine

• To understand the control gear requirements for a three-phase synchronous machine

Objectives

Three-Phase AC Machines Resource 7

Three-Phase Synchronous Machines

• To be able to explain why a synchronous machine is not self starting • To be able to describe how to start a three phase synchronous machine

with a prime mover • To be able to describe how to synchronise a three phase synchronous

machine to the mains supply • To be able to describe how to start a synchronous machine with a pony

motor

Starting Problems

A three-phase synchronous machine cannot self start as a motor

• Stator field moving at 50Hz

• Rotor field energised with DC via slip rings

• Stator field moving too quickly for rotor to lock on

Prime Mover Method

Start the machine as a generator using a prime mover and the three-lamp method

Prime Mover Method

Start the machine as a generator using a prime mover and the three-lamp method

• The frequencies must be equal• The line voltages must be

equal• The phase-relationships be the

same

Generator output must match the supply in 3 ways

Prime Mover Method

Main switch can be closed when L1 is off and L2 and L3 are equally bright

Pony Motor Method

Start the machine as a generator using a pony motor

Pony Motor Series Method

• Start the machine using a pony motor(Q, M and C3 closed)

• When near synchronous speed switch on dc supply to rotor (C2 closed, C3 open)

• Rotor synchronous motor catches up with the rotating field

• When at synchronous speed short out pony motor stator windings(C1 closed)

Pony Motor Parallel Method

• Start the machine using a pony motor(Q, M, C1 and C3 closed)

• When near synchronous speed switch on dc supply to rotor (C2 closed, C3 open)

• Rotor synchronous motor catches up with the rotating field

• When at synchronous speed short out pony motor stator windings(C1 open)

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