Week-7 Challenge: DC Motor Control

INTRODUCTION:

BJT H bridge(Bi polar junction transistor)- it uses 2 n type semiconductors and 2 p type semiconductors connected together forming H bridge with motor in between to control the speed and direction of DC motor. Diodes are connected in parallel with each transistor to allow passing of current in one direction.

Fig 1. BJT H bridge

Swich 1 and 4 are closed when motor needs to operate in forward direction.

Switch 2 and 3 are closed when motor needs to operate in reverse direction.These switches are controlled by signals from power electronic devices setup at particular time duration.

The Four-Quadrant Chopper DC(or DC7 circuit)- This is a drive also used for speed control using 4 IGBT switches.An insulated-gate bipolar transistor (IGBT) is a three-terminal power semiconductor device primarily used as an electronic switch.I uses PI speed and Pi current controllers.

Fig 2. Four quadrant chopper circuit

It used to operate in all the 4 condition of DC motor as:

Fig 3. 4 Quadrants of DC motor.

Conditions for obtaining different quadrant working conditions:

Fig 4. 4 Quadrants switching condition

COMPARISION BETWEEN BJT H BRIDGE AND DC 7 CIRCUIT:

As discussed above, we can understand the basic components and working of BJT H bridge and DC7 circuit.

1. DC 7 can operate in all 4 quadrants of DC motor, whereas BJT H bridge circuit is designed to operate only in 2 quadrants, i.e forward and reverse motor.

2. In DC7 circuit,due to the use of high switching frequency DC-DC converters, a lower armature current ripple (compared with thyristor-based DC drives) is obtained, whereas in BJT H bridge armature current varies alot due to change in polarity of switches(which can be controlled by reducing the duty cycle of battery).

OUTPUT:

1. Speed control using BJT H bridge

Fig 5. BJT H bridge simulink model

The above simulink model is run using default conditions, we obtain the following output:

Fig 6 Output of BJT H bridge initially

As we can see, armature current is changing significantly. In order to control the variation of armature current, we had changed the duty cycle of the battery and observed the variation in aramture current.Pulse width reduces to 40%.

Fig 7 Input parameters of pulse generator

Fig 8 Final output BJT H bridge

As can be seen from Fig 8, armature current drop reduces by reducing the duty cycle.

2. EV model using 4 quadrant chopper controlled DC motor

Fig 9 EV model using 4 quadrant chopper DC drive controller

As seen from Fig 9, 4 quadrant chopper circuit is modified to form EV drive model by giving the following variation:

1. EV drive cycle is given as an input by replacing the step input

2. Torque of motor equation is applied to the block as:

3. DC input voltage is applied

Output of the model is:

Fig 10 Output of EV model using 4 quadrant chopper controller

As seen from the output, yellow and blue graph represents driving cycle and motor speed respectively.

Both curves matches significantly, so we can conclude that our model is designed correctly.