Week-7 Challenge: DC Motor Control

1.

H-bridge is a combination of four BJT and diode pairs. Two transistors are switched simultaneously depending on the spin direction of the motor required. Q2 and Q3 – reverse direction

Q1 and Q4 – forward direction.

Case 1:

When Q1 and Q4 are fired, a positive voltage is applied across the motor and diodes D2 and D3 acts as flyback diodes when Q1 and Q4 are switched off.

Case 2:

When Q2 and Q3 are fired, a negative voltage is applied to the motor and diodes D1-D4 operate as free-wheeling diodes when Q2 and Q3 are switched off.

Note: Flkyback or free-wheeling diodes reduce the flyback which is huge spikes in voltage across an inductive load when its supply current is abruptly reduced or interrupted.

With a supply voltage of 240v and duty cycle of 75%, the values of armature current, IGBT current and transistor current can be seen as follows:

It can be seen from the above picture that the armature current peaks at 50A with the set parameters and when the motor reverses its direction, it goes down to 100A in the negative axis.

This huge volte-face in the current value is due to addition of back-emf current to the armature current when the polarity is reversed.

This fluctuation can be mitigated with the help of a RC snubber or by waning the values of Duty cycle.

In this case the duty cycle is reduced to a value of 45% which in turn neutralises the spike in armature current.

the armature current value is as follows:

2. The comparison between DC7 chopper model and H-bridge model using BJT is as follows:

H-bridge model using BJT	DC7 chopper model
1. The H-bridge consists of a four IGBT-diode pairs. The maximum voltage achieved is 300-350V.	Due to the presence of DC-AC converters, DC7 model has the privilege to vary the magnitude of voltage to desired level. The maximum value of voltage that can be achieved is 400V which on phase reversal becomes 200V.
2. The torque and hence output power bandwidth are narrower as it is restricted to use single pulse generator.	It has a broader range of torque and output power compared to the former.
3. The range of armature current has lesser values comparatively since only two pairs of IGBT and diode models are functioning at a time	The use of four quadrants in DC7 model gives the armature current more degrees of freedom.

3. In order to convert the existing DC7 block into an EV model, the reference speed block was obviated and replaced with Federal Test Procedure drive cycle(FTP75). The output sampling time was later changed to 100 seconds.

The following results were obtained by the implementation of the above drive cycle: