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Three-Phase AC machines

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Resource 4 Three-Phase Cage Rotor Induction Motor Electronic Methods of Starting and Speed Control Three-Phase AC machines Three-Phase AC Machines Resource 4 Aim ... – PowerPoint PPT presentation

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Title: Three-Phase AC machines


1
Three-Phase AC machines
Resource 4
Three-Phase Cage Rotor Induction Motor
Electronic Methods of Starting and Speed Control
2
Three-Phase AC Machines Resource 4
Three-phase Cage Rotor Induction Motor
Electronic Methods of Starting and Speed Control
  • Aim
  • To understand the operation of two electronic
    methods of starting and speed control of an
    induction namely the Soft-Start and the Inverter

3
Three-Phase AC Machines Resource 4
Three-phase Cage Rotor Induction Motor
Electronic Methods of Starting and Speed Control
  • Objectives
  • 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

4
Voltage Ramping with a Soft-Start
Voltage against time graph
Torque against speed graph
Torque speed curve rises with voltage
RMS voltage rises from V0 up to full voltage in
time tS seconds
Not a good method of speed control
5
Voltage Ramping with a Soft-Start
Thyristors TH1 and TH2 connected in anti-parallel
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
RMS voltage control by firing delay of gate pulse
TH1 controls AC wave on positive half cycle, TH2
controls AC wave on negative half cycle
6
Voltage Ramping with a Soft-Start
Normal uncontrolled AC wave has RMS value as
shown (70.7 of peak voltage)
7
Voltage Ramping with a Soft-Start
8
Voltage Ramping with a Soft-Start
9
Voltage Ramping with a Soft-Start
10
Voltage Ramping with a Soft-Start
Full voltage supply to motor when there is no
delay
11
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
12
Voltage Ramping with a Soft-Start
Commercial Soft-Start
13
Frequency Ramping with an Inverter
Frequency against time graph
Torque against speed graph
Frequency rises from 0 up to 50Hz in time tS
seconds
Torque speed curve rises with frequency
14
Frequency Ramping with an Inverter
Frequency rises from 0 up to 50Hz
15
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
16
Frequency Ramping with an Inverter
Inverter Output waveform
Average voltage is near sinusoidal
PWM wave
PWM Pulse Width Modulation
Average voltage is sinusoidal
Filter is needed to remove harmonics due to
chopping
17
Frequency Ramping with an Inverter
Commercial Inverter
Adjustments 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
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