1. Three-Phase AC machines
Resource 5
Three- Phase Cage Rotor Induction Motor Methods of Braking

2. 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

3. Three-Phase AC Machines Resource 5
Three- Phase Cage Rotor Induction Motor Methods of Braking
Objectives
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

4. The Need for a Brake
Grinding Wheel continues to rotate after the motor has been stopped

5. Electromechanical Braking
Power Off
Power On
Springs hold brake shoes against drum stopping the load from turning
Powerful electromagnets pull the shoes off the drum allowing the load to turn

6. 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

7. 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
WARNING
DC injection braking can burn out the stator windings
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

8. 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

9. Power Schematic – DC section
DC Injection Braking
Transformer reduces AC voltage
Transformer requires separate fuses
Full-wave rectifier converts AC into DC
Power Schematic – DC section

10. Power Schematic – DC injection
DC Injection Braking
DC current travels through one phase and back up through another phase
Power Schematic – DC injection

11. 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

12. 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

13. 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

14. 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