Week-7 Challenge: DC Motor Control

1.Run MATLAB demo’ Speed control of a DC motor using BJT H-Bridge’. Modify the model such that the armature current doesn’t shoot up when the motor changes from forward to reverse?

The Bipolar Junction Transistor (BJT) when used for power switching applications. Operates as an IGBT. When it is conducting (BJT operating in the saturated region), a forward voltage vf, is developed between emitter and collector (in the range of 1v). Therefore the IGBT block can be used to model the BJT device.

The H-bridge consists of four BJT/diode pairs (BJT simulated by IGBTmodels).Two transistors are switched simultaneously, Q1 and Q4 or Q2 and Q3. When Q1 and Q4 are fired, a positive voltage is applied to the motor and diodes D2-D3 operates as freewheeling diodes when Q2 and Q3 are fired, a negative voltage is applied to the motor and diodes D1-D4 operates as free-wheeling diodes when Q2and Q3 are switched off.

Scope after running the default model to examine the ‘SHOOT UP’ in armature current when the poles are reserved.

The motor starts in a positive direction with a duty cycle of 75% (mean DC voltage of 180v). At

t=0.5sec the armature voltage is suddenly reversed and the motor runs in a negative direction.

1.On the above time variation of the rpm, armature current and load torque can be seen for the forward motoring and reverse motoring cycles.

2. Max forward speed is 950rpm between 0-0.5 sec and max reverse speed is -953 rpm and -0 to0.5 sec.
3. Similarly, the steady state current is forward rotation is 7.83A with a initial spike of 37.5A and steady state current in reverse rotation is -3.4A with a shoot up of -65.1A .This is primarily because DC motors usually require a high starting current is lower than that needed when reversing the rotation. Additionally the switching loss incurred by power converter also has an effect in increased current spike. This can be controlled by lowering the duty cycle which will be explored in the next simulation.

4.The next torque is 6.013Nm and remains the same across both the rotational cycles. ‘Scope’ shows motor speed, armature current, and load torque and ‘currents’, shows current flowing in BJT Q3 and diode D3.

2. Refer to help section of ‘The Four-Quadrant chopper DC drive (DC7) block’. Compare with H-bridge model?

Four Quadrant operations:

Quadrant 1 : Power developed is positive and the machine is working as a motor supplying mechanical energy. The 1 quadrant operation is called forward Motoring.

Quadrant 2: In this quadrant the direction of rotation is positive, and torque is negative, thus the machine operates as a generator developing a negative torque, which opposes the motion and it is called forward braking.

Quadrant 3: The motor works, in the reverse direction both speed and torque have negative values while the power is positive and it’s called reverse monitoring.

Quadrant 4: The torque is positive, and speed is negative. The quadrant corresponds to breaking the reverse and it is called Reverse Braking.

Simulation parameters:

1.Before starting the simulation, set the initial bus voltage to 515V via the GUI block (‘initial states setting ‘button and Cubs ‘variable).

2.Start the simulation you can observe the armature voltage and current, the four IGBT pulses and the motor speed on the scope. The current and speed references are shown.

3.The motor is coupled to linear load, which means that the mechanical torque of the load is proportional to the speed.

4.The speed reference is set at 500rpm at t=0sec, observe that the motor speed follows the reference ramp accurately reaches the steady state around t=1.3s.

5.The armature current follows the current reference very well, with fast response time and small ripples. Note that the current ripple frequency is 5khz.

6.At t=2s speed reference drops to -1184rpm. The current reference decreases to reduce the electromagnetic torque and causes the motor to decelerate with the help of load torque.

7.At t=2.2s the current reverses in order to produce a braking electromagnetic torque(dynamic braking mode) .This causes DC bus voltage to increase.

Comparison between H_bridge and Four quadrants DC motor:

1.control strategy –

• In H_bridge we need to reverse the polarity to change the direction but in four quadrants DC drive it uses reference speed to change the direction of motor change.
• PI Controllers are used for speed and current control in four quadrant DC drive which is absent H_bridge.

2. Armature current stability –

• The current shoot up and disturbances are high in H_bridge because of the switching losses.

3. Operation –

• Forward and Reverse monitoring are the only two operations that the H_ bridge can perform.
• The Four quadrant DC drive operates in four modes forward motoring, reverse motoring, forward braking and reverses braking. Also it can perform regenerative braking as well H_bridge.

3. Make a suitable EV model using DC7 block and make result report?

An EV model can be simulated using the DC7 example block with a couple of changes. The changes that need to be made

1.Modify energy source at input side:

The battery is source of fuel for an electric vehicle, a DC voltage source block is placed on the input with a supply voltage of 300v

2. 2. Reference speed:

A drive cycle input needs to replace the step signal block. In this case a custom drive cycle saved in excel files is imported to a drive cycle block.

3. Linear load Torque:

The linear load torque can be replaced by a subsystem model according to load characteristics given by equation.

T = 24.4 +0.0051*(w)^2

Where W is angular speed of the motor