Transcript Three- Phase Cage Rotor Induction Motor Methods of Braking The

Three-Phase AC machines
Resource 5
Three- Phase Cage Rotor Induction Motor
Methods of Braking
Three-Phase AC Machines
Resource 5
Three- Phase Cage Rotor Induction Motor
Methods of Braking
Aim
• To understand the use of six different methods of stopping a cage rotor induction
motor coupled to a high inertia mechanical load
Three-Phase AC Machines
Resource 5
Three- Phase Cage Rotor Induction Motor
Methods of Braking
Objectives
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To be able to explain why some loads are difficult to stop
To be able to describe the operation of an electromechanical brake
To be able to describe the operation of a plug braking system
To be able to describe the operation of a DC injection braking system
To be able to describe the operation of a speed ramping system using an inverter
To be able to describe the operation of a dynamic braking system using an inverter
To be able to describe the operation of a regenerative braking system using an
inverter
The Need for a Brake
Grinding Wheel continues to rotate after the motor has been stopped
Electromechanical Braking
Power Off
Springs hold brake shoes against
drum stopping the load from
turning
Power On
Powerful electromagnets pull
the shoes off the drum allowing
the load to turn
Plug Braking
Power Schematic same as
reversing DOL starter
Two phases are swapped over
until motor stops.
A centrifugal switch, speed
sensing relay or timer used to
prevent the motor from
reversing.
Contactor C2 is for starting
Contactor C3 is for stopping
WARNING
Four times the kinetic
energy of load dissipated
in stator windings
Power Schematic
DC Injection Braking
Power Schematic requires DC available at a lower voltage e.g. 50V
This can be achieved using a transformer and 4 diodes
When required to stop, DC is connected to two of the phases
Stator field then becomes stationary
Rotor then runs faster than stator field causing a large braking torque in
the rotor
When rotor stops there is no more braking torque
Control circuit will be the same as for plug braking with
timer, centrifugal switch or speed sensing relay
Inverters often incorporate this type of braking
WARNING
DC injection braking
can burn out the
stator windings
DC Injection Braking
Run contactor C2 will
start the motor as for
DOL starter
When C3 energises,
DC is injected into 2
phases of stator for a
short time
WARNING
DC injection braking
can burn out the
stator windings if
voltage is too high
Power Schematic
DC Injection Braking
Transformer reduces
AC voltage
Transformer requires
separate fuses
Full-wave rectifier
converts AC into DC
Power Schematic – DC section
DC Injection Braking
DC current travels
through one phase
and back up through
another phase
Power Schematic – DC injection
DC Injection Braking
Press S2 to start as
for DOL starter
Press S1 to stop as
for DOL but C3 will
energise for a time
WARNING
DC injection braking
can burn out the
stator windings if
time is too long
This control circuit
can be used for PLUG
braking also
Control Circuit
Speed Ramping Down with an Inverter
Inverter required to ramp the frequency of motor supply down to zero
Stator field moves slower that rotor causing
braking torque
Ramp down time can be adjusted within
the inverter
Dynamic Braking with Inverter
Inverter required to allow kinetic energy in load to be recovered
Energy recovered is wasted in
resistor RDB switched into circuit by
transistor
Resistor RDB value determines the stopping
time
Regenerative Braking with Inverter
Inverter required to allow kinetic energy in load to be recovered
Energy recovered is fed back into the mains
This methods saves energy and money