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 armature current doesn’t shoot up when motor changes direction from forward to reverse.

Solution:

Model:

Model description:

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 collector and emitter (in the range of 1 V). Therefore, the IGBT block can be used to model the BJT device.

The IGBT block does not simulate the gate current controlling the BJT or IGBT. The switch is controlled by a Simulink® signal (1/0). The DC motor uses the preset model (5 HP 24V 1750 rpm). It simulates a fan type load (where Load torque is proportional to the square of speed). The armature means voltage can be varied from 0 to 240 V when the duty cycle (specified in the Pulse Generator block) is varied from 0 to 100%.

The H-bridge consists of four BJT/Diode pairs (BJT simulated by IGBT models). 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 operate as free-wheeling diodes when Q1 and Q4 are switched off. 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.

The output of the above model without changing any parameters,

Here we can see there is a shoot up in the armature current when the motor changes form forwarding to reversing. This can be reduced by adjusting the duty cycle of the switches connected.

changing the duty cycle to 40% from 75%(inbuilt model value).

Result after changing the duty cycle,

2. Refer to the help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model.

Model:

Model description:

The 200 HP DC motor is separately excited with a constant 150 V DC field voltage source. The armature voltage is provided by an IGBT converter controlled by two PI regulators. The converter is fed by a 515 V DC bus obtained by rectification of a 380 V AC 50 Hz voltage source. In order to limit the DC bus voltage during dynamic braking mode, a braking chopper has been added between the diode rectifier and the DC7 block.

The first regulator is a speed regulator, followed by a current regulator. The speed regulator outputs the armature current reference (in p.u.) used by the current controller in order to obtain the electromagnetic torque needed to reach the desired speed. The speed reference change rate follows acceleration and deceleration ramps in order to avoid sudden reference changes that could cause armature over-current and destabilize the system. The current regulator controls the armature current by computing the appropriate duty ratios of the 5 kHz pulses of the four IGBT devices (Pulse Width Modulation). For proper system behavior, the instantaneous pulse values of IGBT devices 1 and 4 are opposite to those of IGBT devices 2 and 3. This generates the average armature voltage needed to obtain the desired armature current. In order to limit the amplitude of the current oscillations, a smoothing inductance is placed in series with the armature circuit.

Comparison with H-Bridge:

• The H-bridge consists of 4 IGBT-diodes and the maximum voltage achieved is 300 V. In the DC7 chopper due to the presence of DC-AC converters the magnitude of the voltage can be adjusted to the desired levels. The maximum voltage that can be achieved is 400V which on phase reversal becomes 200V.
• In H-bridge the torque and the output power bandwidth are narrower as it is restricted to use a single pulse generator whereas in Dc& chopper the torque and output power has a broader range.
• The range of armature current is less due to the presence of only 2 pairs of IGBT and diodes.but the DC7 chopper provides more degree of freedom to armature current.
• The main advantage of the DC drive is it can control all the four quadrants of operation of a motor.
• The advantages of H-Bridge circuits are very high efficiency, very low loss, and the circuits can be placed in a small place

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

• A drive cycle is given instead of a speed reference.
• The AC voltage source and converter are replaced by a dc voltage of supply 100V.
• The torque equation is implemented,

T=24.7+0.0051*W^2

This torque block replaces the linear torque block.

Model:

Result: