# Three-Phase AC machines Three-Phase Cage Rotor Induction Motor – Electronic Methods of Starting and Speed Control Resource 4.

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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 V 0 up to full voltage in time t S seconds Voltage against time graphTorque 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

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 50Hz in time t S seconds Frequency against time graphTorque 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 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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