|| DC Motor Control ||

1.

Fig.1. BJT H-Bridge

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 square of speed). The armature mean 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.

Fig.2. Shooting Up Of the Armature Current

Fig.3. Predefined Parameters

Now, Changing the pulse width % or Duty cycle with 30%

Fig.4 Reducing the pulse width with 30%

We get,

Fig.5 Armature Shoot up is been Reduced

2.

Circuit 1. DC 7 Bloskset

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 behaviour, 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.

Circuit.2 Masked Circuit

Controller Schematics

Braking Chopper

Comparison with H-Bridge:

If we compare "DC7 Blockset " With "H-Bridge" We can find out the following Differences:

i. H-Bridge works only in two quadrant. Whereas, DC 7 blocks works on Four Quadrants. That means, In addition to DC7, We can move our vehicle forward and Reverse, And also we can do forward braking and Reverse Braking too.

ii. H-bridge Can't be used in Regeneration cycle. Whereas, we can use regeneration mode in DC7.

iii. Switching losses are more in H-Bridge as compared to DC7. As a result we can see the armature current graph in which, it shoots up.

3.

The Vehicle Characteristics is given by the following Torque-Speed Equation

T_TR=24.7+0.0051w²

Drive Cycle input to the Vehicle is given by the NEDC_Drive cycle.

DC input of 515V is given.

Simulated Result