Week-7 Challenge: DC Motor Control
Run MATLAB demo ‘Speed control of a DC motor using BJT H-bridge’. Modify the model such that armature current doesn’t shoot up when motor changes direction from forward to reverse.
About H-bridge:
Most of mini DC motors have two terminals because they are series type (armature and field windings are connected in series) DC motors.
Now if we connect terminal A with +Ve supply and terminal B with –Ve supply or ground the current will flow from motor from A to B and motor will rotate in one direction – say clockwise (CW) or forward direction. Now as shown in second figure we change the supply terminals. Now B is connected with +Ve and A is connected to ground. The current will flow from motor from B to A and motor will rotate in other direction
The arrangement is shown in right side of figure. Four switches are connected in between +Ve supply and ground and DC motor is connected in between two switches as shown. Such circuit arrangement is known as H-bridge because it looks like letter ‘H’ (H-bridge circuits are most widely used in DC motor drivers). Let us see how it gives reverse supply to motor.
If SW1 and SW4 are pressed simultaneously then current will flow from +Ve – SW1 – A – B – SW4 – Gnd. So motor will rotate in one direction. Open (release) SW1 and SW4 to stop motor. Now if SW2 and SW3 are pressed current will flow from +Ve – SW2 – B – A – SW3 – Gnd. So motor gets reverse supply and it will rotate in another direction.
Now let us move to vary the speed of DC motor. The DC motor speed varies as applied input voltage varies. As you increase applied input voltage the speed will increase and vice versa. Applying max rated voltage will rotate motor at full speed (caution: do not apply more than max rated voltage to motor otherwise motor windings may get burnt).
Now the question is how to apply variable voltage to motor?
One of the very popular methods for generating variable DC voltage is pulse width modulation (PWM). It is explained in figure given below.
Pulse width modulation means varying the width (duty) of pulse. Width means ON time Ton of pulse. The average output voltage (Vdc or Vavg) is given by equation
Vdc = [Ton / (Ton+Toff)] × Vs
It directly depends upon Ton. As shown in above figure if duty is 50% the average output voltage Vdc is exactly the half of Vs. If duty is increased to 75%, Vdc also increases to 3/4th of Vs and if duty is decreased to 25%, Vdc reduces to 1/4th of Vs. Thus as pulse width varies the average output voltage varies. So we have to apply PWM to DC motor to vary its speed
Matlab model:
On simulating the model considering 75%duty cycle.
As you can see the armature current changes it direction as soon as the speed of the motor reverses and there is a sudden drop.
2 Duty Cycle = 25 %
Here the peak value of negative current is less than 5A. Thus while changing the motor direction it is desirable that we first decrease the motor speed by adjusting the dutycycle.
2 Refer to help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with H-bridge model.
The Four-Quadrant Chopper DC Drive (DC7) block
First-Quadrant: Forward Motoring
The chopper-1 & chopper-4 are switched ON, in which the current flows from Chopper-1 to chopper-4 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 switched OFF, the energy stored in the inductance flows to chopper-4 which activates the D2 and forms closed loop.
Second-Quadrant: Forward Regenerative Braking
The chopper-2 is switched ON D4 is activated, the current flows from D4 to Chopper-2 through inductance (which stores energy) which is in opposite direction (negative current) and the voltage is positive thus the motor stops in forward direction. If the chopper-2 is switched OFF, the D4 & D1 are conduct current.
Third-Quadrant: Reverse Motoring
The chopper-2 & chopper-3 are switched ON, in which the current flows from Chopper-3 to chopper-2 through inductance (which stores energy) 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 stored in the inductance flows to chopper-2 which activates the D4 and forms closed loop.
Fourth-Quadrant: Reverse Regenerative Braking
The chopper-4 is switched ON D2 is activated, the current flows from Chopper-4 to D2 through inductance (which stores energy) which is in same direction (positive current) and the voltage is negative thus the motor stops in reverse direction. If the chopper-4 is switched OFF, the D2 & D3 are conduct current.
Visual representation:
Class E - Chopper characteristics:
1. Regenerative braking is possible.
2. Widley used in automobiles to control DC motor speed and direction.
H- BRIDGE characteristics :
1. Regenerative braking is not possible but dynamic braking can be achieved.
2. Widley used in robotics and electro-mechanical devices to control DC motor speed and direction.
3. Make a suitable EV model using DC7 block and make result report.
DC voltage source was set and a new drive cycle was introduced.
Now we will be making a subsystem which takes into account of the following equation :
Following drive cycle was created and added as input to the model.
link: https://docs.google.com/spreadsheets/d/1XMuU8BX6B11fZAjgZBw0LDlL06WivCb6SjxPkFxoA1U/edit#gid=0
output:
That motor speed tries to match the target speed.