DC Motor Control
- ‘Speed control of a DC motor using BJT H-bridge’
The Bipolar Junction Transistor 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.
When the simulation is run the motor starts in the positive direction with a duty cycle ratio75%. At t = 0.5 sec the armature voltage is suddenly reversed and the motor runs in the negative direction.
Scope shows the motor speed, armature current and load torque.
As we can see from the above graph the armature current reverses and shoot up suddenly at t = 0.5 sec. so we have to modify such that the armature current doesn’t shoot p when the motor reverse its direction by changing the duty cycle ratio.
Modification:
The duty cycle ratio is changed to 50% which is given by the pulse generator block
The result obtained by this modification is as follows:
As we can see from the above result graph the armature current rise is little
Lets reduce the pulse width even more to 40
The result obtained is
Many of the rise of armature current is reduced by reducing the duty cycle ratio.
- The Four Quadrant chopper dc drive.
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.
The main advantage of this drive, compared with other DC drives, is that it can operate in all four quadrants (forward motoring, reverse regeneration, reverse motoring, and forward regeneration). In addition, due to the use of high switching frequency DC-DC converters, a lower armature current ripple (compared with thyristor-based DC drives) is obtained. However, four switching devices are required, which increases the complexity of the drive system.
- To make a EV model using the DC7 Block
The DC7 Block is modified with addition of blocks to make a suitable EV model
The Speed reference is replaced with the drive cycle.
Ac Source is replaced by the DC voltage source and a current sensor ias added in series to measure the flow of current and to view that measure of the current a scope is connected.
Linear Torque characteristic block is removed and a new subsystem is created by using the torque equation .
The input of the subsystem is the output speed of the dc chopper block, the output of the subsystem block is given to the Torque input port of the dc chopper block.
The resultant graphs obtained are as follows
The duty cycles of the IGBT’s are kept constant through out as 50% for a better armature current flow.
The armature voltage flow through is smoother and its pulse is around +50 to -50.
Without the torque characteristic equation the armature voltage goes from 0 to 500 to -500. As there will be linear load torque. When the equation is provided the voltage supplied will be as per the torque demanded hence the smoother graph and as observed the losses are also less.
Here we have used FTP75 drivecycle whose first 20seconds are at rest and the acceleration starts after 20 seconds as we can observe that the motor speed increases and the armature current also rises slightly to meet the demanded acceleration.