DC Motor Control

TARGET-

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.

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

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

SOLUTION-

1. BJT-

A bipolar junction transistor (BJT) is a type of transistor that uses both electrons and holes as charge carriers. Unipolar transistors, such as field-effect transistor, use only one kind of charge carrier. A bipolar transistor allows a small current injected at one of its terminals to control a much larger current flowing between two other terminals, making the device capable of amplification or switching.BJTs use two junctions between two semiconductor types, n-type and p-type, which are regions in a single crystal of material.

BJT H-Bridge Circuit-

Speed vs Torque all four quadrant-

Speed control of a DC motor using BJT H-bridge-

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 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.

The motor starts in the positive direction with a duty cycle of 75% (mean DC voltage of 180V)-

SIMULATION-

At t= 0.5 sec., the armature voltage is suddenly reversed and the motor runs in the negative direction.'Scope' shows motor speed, armature current and load torque and 'Currents' shows currents flowing in BJT Q3 and diode D3.

MODIFICATION-

The motor starts in the positive direction with a duty cycle of 50% (mean DC voltage of 120V).

SIMULATION-

At t= 0.5 sec., the armature voltage is suddenly reversed and the motor runs in the negative direction.'Scope' shows motor speed, armature current and load torque and 'Currents' shows currents flowing in BJT Q3 and diode D3.

MODIFICATION-

The motor starts in the positive direction with a duty cycle of 45% (mean DC voltage of 108V).

SIMULATION-

At t= 0.5 sec., the armature voltage is suddenly reversed and the motor runs in the negative direction.'Scope' shows motor speed, armature current and load torque and 'Currents' shows currents flowing in BJT Q3 and diode D3. It is first time Where armature current does not shoot up when motor changes its direction.

FURTHER MODIFICATION-

The motor starts in the positive direction with a duty cycle of 25% (mean DC voltage of 60V).

SIMULATION-

At t= 0.5 sec., the armature voltage is suddenly reversed and the motor runs in the negative direction.'Scope' shows motor speed, armature current and load torque and 'Currents' shows currents flowing in BJT Q3 and diode D3. Here current does not shoot up when motor changes its direction but torque become 0.

2. DC CHOPPER-

A chopper is basically a dc to dc converter whose main function/usage is to create adjustable or variable dc voltage from fixed dc voltage sources through the use of semiconductors.

A chopper can be said as a high speed on/off semiconductor switch. Source to load connection and disconnection from load to source happens in a rapid speed. Consider the figure, here a chopped load voltage can be obtained from a constant dc supply of voltage, which has a magnitude Vdc_. Chopper is the one represented by “S” which can be turned on or off as desired.

DC Chopper Circuit-

Let us now take a look of the output current and voltage wave forms of a chopper. During the time period Ton the chopper is turned on and the load voltage is equal to source voltage Vdc. During the interval Toff the chopper is off and the load current will be flowing though the freewheeling diode FD . The load terminals are short circuited by FD and the load voltage is therefore zero during Toff. Thus, a chopped dc voltage is produced at the load terminals. We can see from the graph that the load current is continuous. During the time period Ton, load current rises but during Toff load current decays .

Duty Cycle=[Ton/(Ton+Toff)] Vo=[Ton/(Ton+Toff)]*Vs=[Ton/T]*Vs

The Four-Quadrant Chopper DC Drive (DC7) block-

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 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.

H-Bridge-

An H-bridge is an electronic circuit that switches the polarity of a voltage applied to a load. These circuits are often used in robotics and other applications to allow DC motors to run forwards or backwards.

H-Bridge Circuit-

WORKING-

An H bridge is built with four switches. When the switches D1 and D4 (according to the first figure) are closed (and D2 and D3 are open) a positive voltage will be applied across the motor. By opening D1 and D4 switches and closing D2 and D3 switches, this voltage is reversed, allowing reverse operation of the motor.

Fig-1 Fig-2

H-Bridge block-

The H-Bridge block represents an H-bridge motor driver. The block has the following two Simulation mode options: a)PWM -The H-Bridge block output is a controlled voltage that depends on the input signal at the PWM port. If the input signal has a value greater than the Enable threshold voltage parameter value, the H-Bridge block output is on and has a value equal to the value of the Output voltage amplitude parameter. If it has a value less than the Enable threshold voltage parameter value, the block maintains the load circuit using one of the following three Freewheeling mode options: i) Via one semiconductor switch and one freewheeling diode. ii) Via two freewheeling diodes. iii) Via two semiconductor switches and one freewheeling diode. The first and third options are sometimes referred to as synchronous operation.

3. To make a suitable EV model using DC7 block- Step I Removing>> (i)380V 50Hz AC power source

(ii)AC to DC converter

(iii)Linear load torque &

(iv)converter signal block with scope as marked in the model below -

Step II Adding>>

(i)A DC Voltage source in place of AC power source

(ii)A Current sensor in series with DC Voltage source's positive terminal

(iii)A Scope with the current sensor

(iv)We have to give a torque speed relation that is Tm=24.7+0.0051Wm^2. For this we need to build a subsystem using mathematical operations of simulink. We will replace the Linear load torque with this subsytem.

AND

Using this blocks we will make a subsystem that is given below

And keeping all other parameters as default finally we will make a suitable vehicle model using DC7 block that is given below

a)Voltage amplitude- a)Simulation-

b)Voltage Amplitude-

b)Simulation-

c)Voltage Amplitude-

c)Simulation-

d)Voltage Amplitude-

d)Simulation-

It is evident that motor speed trying to match the target speed.