Three-Phase AC machines

Three-Phase Cage Rotor Induction Motor – Electronic Methods of Starting and Speed Control

Resource 4

Three-Phase AC Machines Resource 4

Aim

Three-phase Cage Rotor Induction Motor – Electronic Methods of Starting and

Speed Control

• To understand the operation of two electronic methods of starting and speed control of an induction namely the Soft-Start and the Inverter

Objectives

Three-Phase AC Machines Resource 4

Three-phase Cage Rotor Induction Motor – Electronic Methods of Starting and

Speed Control

• To be able to describe the effects of voltage-ramping on the torque speed curve of an induction motor

• To be able to describe how a soft-start ramps up the RMS stator supply voltage

• To be able to describe effects of frequency-ramping on the torque speed curve of an induction motor

• To be able to show how an inverter ramps up the frequency of the stator supply

Voltage Ramping with a Soft-Start

RMS voltage rises from V0 up to full voltagein time tS seconds

Voltage against time graph Torque against speed graph

Torque speed curve rises with voltage

Not a good method of speed control

Voltage Ramping with a Soft-Start

Thyristors TH1 and TH2 connected in anti-parallel

RMS voltage control by firing delay of gate pulse

A = anode, K = cathode

L1 – supply terminal

T1 – motor terminal

Gate terminal used to switch thyristor into conduction mode.

Thyristor switches off when current reaches zero

TH1 controls AC wave on positive half cycle, TH2 controls AC wave on negative half cycle

Voltage Ramping with a Soft-Start

Normal uncontrolled AC wave has RMS value as shown (=70.7% of peak voltage)

Voltage Ramping with a Soft-Start

Voltage Ramping with a Soft-Start

Voltage Ramping with a Soft-Start

Voltage Ramping with a Soft-Start

Full voltage supply to motor when there is no delay

Voltage Ramping with a Soft-Start

Supply terminals L1, L2 and L3 connected via isolator, fuses, main contactor and overload unit

Motor terminals T1, T2 and T3 connected to motor terminals U1, V1 and W1

Voltage Ramping with a Soft-Start

Commercial Soft-Start

Frequency Ramping with an Inverter

Frequency rises from 0 up to 50Hzin time tS seconds

Frequency against time graph Torque against speed graph

Torque speed curve rises with frequency

Frequency Ramping with an Inverter

Frequency rises from 0 up to 50Hz

Frequency Ramping with an Inverter

Inverter Block Diagram

Diodes rectify AC into DC

DC link feeds into inverter stage

Inverter stage chops up DC into 3 phase, variable frequency output to motor

Frequency Ramping with an Inverter

Inverter Output waveform

PWM = Pulse Width Modulation

Average voltage is sinusoidal

Filter is needed to remove harmonics due to chopping

Average voltage is near sinusoidal

PWM wave

Frequency Ramping with an Inverter

Commercial InverterAdjustments made on a front panel menu include:-

Maximum frequency

Ramp up time

Method of stopping

Motor can be started and stopped locally from the front panel or remotely from external signals

Speed can be adjusted using front panel or remotely from external signals