Week-7 Challenge: DC Motor Control
1.
The above figure shows the simulink model of speed control of a DC motor using BJT H-bridge.which consists of DC motor source.Bi-polar junction transistor (BJT),and diodes.the diodes are transistors are connected in the form of H contruction.the dc motor has a present model(5 HP 24V 1750 rpm).it can be changed according to our requirement.it simulates a fan type load ( where load torque is proportional to sqaure of speed).the armature mean voltage can be varied from 0 to 240V when the duty cycle.(specified in the pulse generator block)is varied from 0 to 100%.the bi polar junction transistor when used of power switching applications operates as an IGBT.when it is conducting (BJT) operating in the saturated region) a forward voltage Vf is developed between collector and emitter.because of the transistors are connected in H bridge,the back emf is neglected.
An H bridge is an electronic circuit that switches the polarity of a voltage applied to a load.These circuits are often used in robotics and other applications to allow DC motors to run forward to backward.The H bridge consists of four BJT diode pairs.two transistors are switched simultaneously.S1 and S4 or S2 and S3.
Based on the default value of DC motor,armature mean voltage ( 0 to 240 V)step time 0.5,duty cycle (specified in the pulse generator block is 75%) is varied from 0 to 100% and forward voltage (1v)& resistance(0.001 ohms) of four BJT diode pairs. the result obtained for armature current are shown below which represents sudden raise and sudden fall of current at the beginning and at step time(0.5) respectively.
To avoid the shootup marked in red circle,the duty cycle and period should vary by using trail & error method.the modified parameters are period as 1ms which inturn increasing the frequency (1000 hz) and reducing the duty cycle to 45% which is optimized. If the forward voltage altered in BJT diode pairs,there is less variation occured and the torque & speed as changed.the modified armature current as shown in output.
2.
The above model consists of 3 phase source,AC-DC converter includes rectifier and braking chopper DC motor drive and quadrant selector.
A chopper is a static device that converts fixed DC input to a variable DC output voltage directly.Essentialy, a chopper is an electronic switch that is used to interrupt one signal under the control of another.Due to this,current and voltage spikes occurs. To avoid this snubber RC circuit is used.The chopper is widely used in motor control and regenerative braking. A snubber is an electrical device that prevents voltage spikes due to sudden changes in current.these voltage spikes,or transients can damage the circuit and cause arcing and sparks.one type of electrical snubber is the RC snubber,which is composed of a resistor in parallel with a capacitor.Transients are usually caused by switches in the circuit.
4-Quadrant Chopper:
The above figure consists of voltage source 4 pairs of chopper & diode armature resistance inductance and separately excited DC motor.
First-Quadrant: forward motoring:
the chopper-1 & chopper-4 are switched ON,in which the current flows from chopper-1 through inductance (which stores energy)the load current and armature current are same and positive thus the motor rotates in forward direction.if the chopper-1 is swicthed OFF the energy stores in the inductance flows to chopper-4 which activates the D2 and forms closed loop.
Second-Quadrant:forward regenerative motoring:
the chopper-2 is switched ON D4 is activated,the current flows from D4 to chopper-2 through inductance which is in opposite direction(negative current)and the voltage is positive thus the motor stops in forward direction.if the chopper-2 is swicted OFF,the D4& D1 are conduct current.
Third-Quadrant:reverse motoring:
the chopper-2 & chopper-3 are swicted ON,in which the current flows from chopper-3 to chopper-2 through inductance the load current is negative and armature current and also the voltage is negative which is less than the back emf thus the motor rotates in reverse direction.if the chopper-3 is switched OFF,the energy stores in the inductance flows to chopper-2 which activates the D4 and forms closed loop.
Fourth-Quadrant:regenerative braking:
the chopper-4 is swicthed ON D2 is activated,the current flows from chopper-4 to D2 through inductance which is in same direction and the volatge is negative thus the motor stops in reverse direction.if the chopper-4 is swicthed OFF,the D2 & D3are conduct current.
By comparing the chopper model with H-bridge,each model as certain working operations and some applications,To control the motor in H-bridge,we need to change the polarity which turns into opposite direction.in chopper model,we need to interrupt one signal under control of another.thus it may form some spikes of current and voltage,in order to avoid this snubber RC circuit is used but in H-bridge it is not required.the H-bridge simulation is continous while the chopper is discrete which takes little more time than H-bridge.In H- bridge regenerative braking is not possible,while in chopper it is possible.The H-bridge is mainly used for robotics application,while chopper is mainly used for motor control and regenerative braking in chopper the efficiency is more and maintainance is less than H-bridge.
3.
RESULT: