Week-7 Challenge: DC Motor Control

AIM: To find the characteristics of DC Motor Control.

OBJECTIVE:

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.

Dc Motor:

A DC motor is any of a class of rotary electric motors that converts direct current electrical energy into mechanical energy. The most common types rely on the forces produced by magnetic fields. Nearly all types of DC motors have some internal mechanism, either electromechanical or electronic, to periodically change the direction of current in part of the motor.

H - Bridge:

Here we the n-p-n transistors are used. The picture shown below in the left side is Forward braking and in the right side is Reverse Braking. For NPN transistor if we give +Ve input to base it will turn ON and if we give 0 input it will be turned OFF. So in this circuit if Q1 and Q4 are turned ON simultaneously the motor will rotate forward and if Q2 and Q3 are turned ON then motor will rotate reverse. Now let us move to vary the speed of DC motor. The DC motor speed varies as applied input voltage varies. As you increase applied input voltage the speed will increase and vice versa. Applying max rated voltage will rotate motor at full speed.

To generate variable DC voltage, pulse width modulation (PWM) is used. It is explained in figure given below.Pulse width modulation means varying the width (duty) of pulse. Width means ON time Ton of pulse.

Graph showing Pulse Width Modulation for controlling speed of DC Motor

Speed control of a DC motor using 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.
• The motor starts in the positive direction with a duty cycle of 75% (mean DC voltage of 180V). 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.

Armature Voltage Control :

• The step-time for the simulation is taken as 0.5 seconds.
• That means each iteration will be performed for a time of 0.5 seconds.

Pulse Generator:

• The magnitude of the pulse generator is 500 Hz.
• The amplitude is taken as 1.
• No phase delay is given.
• The pulse width is 75%.

Result:

Explanation of Plot:

Plot 1 :

• At starting when the motor is starting the torque needed is more, hence revolution per minute of the motor is increasing as a result of which the plot starts from zero and shows an increase in variation.
• Then the motor runs for the required amount of time, and when the voltage is down the rpm falls down from around 900 rpm to a negative value.
• And then the plot gets constant.

Plot 2 :

• When the motor is starting the the current required is more as a result the graph shows a increase variation at the starting.
• When the time reaches 0.5 seconds, the armature current drops suddenly in a drastic way.
• And at the same time the armature current shoots up suddenly at 0.5 seconds, which is not desired.
• We have to make changes in such a way that the armature current does not shoots up and remains constant through out.
• The value of the current goes to a value of negative approximately -70 amperes.

Plot 3 :

• At the starting of the motor the load required increases gradually and as a result the graph shows a increase variation with respect to time.
• Then with the fall of current and torque the load also decreases.
• The load is directly proportional to the current and the armature revolution .

Modification :

• In the block parameters the pulse generator is of 500 Hz.
• The amplitude is defined as 1
• The phase delay is given as Zero.
• The pulse width is taken as a value of 30.

Result:

Explanation:

• When we decrease the time period of the signal or the pulse generator, we can see that the starting of the motor is smooth without any hindrance.
• The speed of the motor increases linearly with respect to time, then at time period of 0.5 seconds it a dips very negligibly as a result the armature revolution also drops and after that the rpm remains constant.
• Beacuse of the smooth revolution of the motor the current used by the motor is also less and and constant, as the speed is constant through out the armature current consumption is linear and constant.
• As a result of which the load required during the whole process of motor operation is constant through out.

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

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

Explanation:

• In above Four-Quadrant CHopper DC drive DC7 block model, 3 Phase Voltage source and AC to DC Converter contains Rectifier bridge and Braking Chopper is added to Four-Quadrant Chopper DC drive DC7 block.
• The armature Voltage is provided by IGBT converter controlled by two PI regulators. The first regulator is Speed regulator and followed by Current regulator.
• The Four-Quadrant Chopper DC Drive (DC7) block represents a four-quadrant, DC-supplied, chopper (or DC-DC PWM converter) drive for DC motors.
• This drive features closed-loop speed control with four-quadrant operation.
• The speed control loop outputs the reference armature current of the machine.
• Using a PI current controller, the chopper duty cycle corresponding to the commanded armature current is derived.
• This duty cycle is then compared with a sawtooth carrier signal to obtain the required PWM signals for the chopper.
• 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.

Plot result:

The speed reference is set at 500 rpm at t = 0 s. Observe that the motor speed follows the reference ramp accurately (+400 rpm/s) and reaches steady state around t = 1.3 s.

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

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

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

At t = 3.25 s, the motor reaches 0 rpm and the load torque reverses and becomes negative. The negative current now produces an accelerating electromagnetic torque to allow the motor to follow the negative speed ramp (-400 rpm/s). At t = 6.3 s, the speed reaches -1184 rpm and stabilizes around its reference.

Comparision of Four-Quadrant chopper DC7 block vs H-bridge model :

• Four-Quadrant Chopper can operates in all four modes (Forward Motoring,Forward Braking, Reverse Motoring and Reverse Braking), but H-bridge can operate only in two modes (Forward and Reverse).
• In Four-Quadrant Chopper Regenerative Braking is possible, but H-bridge Regenerative Braking is not possible.
• In Four-Quadrant Chopper Two PI regulator (Speed Regulator and Current Regulator) is used to obtain electromagnetic torque needed to reach the desired speed and control the armature current and stabilize the system, but in H-bridge model PI regulator is not used.
• H-bridge is a electronic circuit that switches the polarity of voltage applied to the load and Four-quadrant Chopper is electronic switch that is used to interupt one signal under the control of another.H-bridge circuit are often used in Robotics and other applications to allow dc motor to run forward and backward.Four-Quadrant Chopper circuit is used to refer numerous type of electronic sitching devices and circuits used in power control and signal applications.
• Four-Quadrant Chopper model gives better efficiency and high energy(power) storage than H-bridge model.

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

EV model using DC7 block:

• 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.

To convert the Four-Quadrant chopper 200HP Dc drive to EV model:

• In Simulink, the library browser will provide the the Dc voltage source. That needs to be connected in the place of 380 V AC 50 Hz voltage source and followed by AC-DC converter.
• The energy source of an EV is a battery or fuel cell. Let’s take the case of a battery.
• So the model can be modified with an input of a battery and conventional measurement from the Simulink Library Browser.
• The positive of a battery is connected to the input of conventional measurement block and the negative terminal is connected to the Vcc.
• The Output terminal of the conventional terminal block is connected to the scope block to plot the results.
• You can take a generic battery model and configure it by referring to any datasheet sheet related to an EV vehicle battery model.

Plot result:

• From the above plot-1, observed that when the motor Speed increases, IGBT 1&4 duty cycle also increases.
• From the above plot-2, IGBT 2&3 is mirror image of IGBT 1&4. when the motor Speed increases, IGBT 2&3 of duty cycle decreases.
• In Plot-3, According to the motor speed Voltage level will be supplied from the DC Voltage Source. Whenever Speed increases more voltage will be required.
• In Plot-4, Observed that Voltage is proportional to Current. when Voltage increases Current increases and current in positive, when voltage decreases current also decreases and current goes in negative.
• In Plot-5, Motor Speed has shown, which was input reference speed is given in Drive Cycle Source. Motor Speed rises and reached maximum speed of 0-500 rpm approximately.