Week-7 Challenge: DC Motor Control

1. A. Speed control of a DC motor using BJT H - bridge:

• These above two models show 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 1V). Therefore, the IGBT block can be used to model the BJT device.
• The IGBT block doesn't stimulate the gate current controlling the BJT or IGBT. The switch is controlled by a simulink signal. The DC motor uses the preset model (5HP, 24V, 1750 rpm). It stimulates a fan type load( where load torque is proportional to square of speed). The armature mean voltage can be varied from 0 to 240V when the duty cycle ( specified in the pulse gen block) is varied from 0 to 100%.
• The H-bridge consists of 4 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 diode D2 and D3 operate as free wheeling diodes when Q1 and Q4 are switched off.
• When Q2 and Q3 are fired, a negative voltage s applied to the motor and diodes D1 and D4 operate as free wheeling diodes when Q2 and Q3 are switched off.
• Then we will run the model for 1sec and we will get the output results as shown here.

these two are the currents of IGBT and diode respectively

Then the output is observed in the scope.

In the scope we get the results of speed armature current and load torque.

https://drive.google.com/file/d/1ftfvUDV4Ned5SxtDFSR8chCSOKWXboWN/view?usp=sharing

Conclusion- The motor starts in the positive direction with a duty cycle of 75% ( mean DC voltage of 180V). At 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 and currents shows the currents flowing in BJT Q3 and diode D3.

B. Armature current shoot up -

• The motor starts in the positive direction with a duty cycle of 75% ( mean DC voltage of 180V). At 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 and currents shows the currents flowing in BJT Q3 and diode D3. From the graph it is clearly observed that the motor has the maximum rpm around about 950 rpm where we also read the armature current values.
• Forward motoring current of around 7.9 A of current at 0.5 seconds and after that curve falls down and indicates reverse motoring operation.
• The armature current shoot up when the motor changes direction from orward to reverse, this has to be overcome. So we need to reduce the duty cycle.

C. Four Quadrant chopper DC drive (DC7) block -

Four quadrant operation - It operates in all 4 quadrants. The power can flow either from source to load or load to source in this chopper. It consists basically four semiconductor switches as shown,

1st quadrant operation - Here switch CH4 and CH1 are ON and CH3 is OFF. and Vs = Vo. Since the CH1 is OFF, load currentfreewheels through CH4 and D2. Therefore, both Vs and Io are positive. and the operation takes place in 1st quadrant.

2nd quadrant operation - Here switch CH2 is ON and rest all CH1, CH3, CH4 are OFF. When Ch2 is ON, DC source in the load drives current through CH2, D4, E and L. and energy will be stored in L. therfore Vo is positive and Io is negative and this operation takes place in 2nd quadrant.

3rd quadrant operation - Here CH1 is OFF ans CH2, CH3 are ON and both output voltage and current are negative hence this operation takes place in third quadrant.

4rth quadrant operation - Here CH4 is ON and CH1,CH2 andCH3 are OFf. when the CH4 is ON, positive current flows through CH4, D2, L and E. L stores the energy. and when CH4 is OFF current is fed back to the source through diodes D2 and D3. Voltage is negative but current is positive hence the results in 4rth quadrant operation.

The simulink model of DC7 four quadrant chopper HP DC drive is as shown in figure,

• 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 380V, 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 5kHz pulses of the four IGBT devices (PWM).
• For proper system behaviour, the instantaneous pulse values of IGBT devices and 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.

When the above simulink model is run we get the graph as below,

Here we get different graphs like, duty cycle IGBT 1 & 4 and duty cycle 2 & 3, armature voltage, armature current, motor speed.

https://drive.google.com/file/d/1e6guXdh1crQfECh-U9yW-iO_42UH9Cq5/view?usp=sharing

comparision of four quadrant chopper DC drive with H - bridge model -

• In four quadrant chopper DC drive block, the speed reference input is used to change the direction pf motor rotation but in H - bridge model, the polarity (direction) is reversed to change the motor rotation.
• In four quadrant chopper DC drive block, the speed reference values can be changed to control the motor and in H- bridge the motor is controlled by changing pulse width of the pulse generator.
• Four quadrant chopper operates in 4 quadrants and are forward motoring, forward regenerative braking, reverse motoring and reverse regenerative braking mode. but H- bridge operates only in two modes forward and reverse.
• Speed and current control are absent in BJT H- bridge and PI controllers are used to control speed and current in this chopper.
• THe disturbances and current shoot up is high in H- bridge than four-quadrant chopper DC7 block.
• The overall performance and stability is good in four quadrant chopper DC7 block than BJT H- bridge.

2. Two quadrant chopper - In this type of chopper, the average voltage will be always positive but the average load current might be positive or negative. The two switches CH1 and CH2 should not be turned on simultaneosly as the combined action may cause a short circuit in supply lines. For regenerative braking and motoring these type of chopper configuration is used. We will aways get a positive output voltage Vo as the freewheeling diode FD is present acreoss the load. When the chopper CH1 is on the current flows from source to load. The inductor stores energy. When CH1 is turned off the freewheeling diode starts conducting and the output voltage Vo will be equal to Vs. The direction of the load current Io will be reversed. The current Io will be flowing towards the source. The load current will be negative when the CH2 is On. The current completes its path through diode D2.

The simulink model for two quadrant chopper is,

After running this code for 0.5 seconds we get the graphs as,

https://drive.google.com/file/d/15JNXbU9_Y9j5-usFLor111uqNN3KIJ22/view?usp=sharing

3. Operation of Brushless DC motor (BLDC)-

Brushless DC motors are common in industrial applications across the world. At the most basic level, there are brushed and brushless motors and there are DC and ac motors. Brushless DC motors, as you may imagine do not contain brushes and use a DC current

How does a BLDC work -

• It often helps to explain how a brushed DC motor works first, as they were used for some time before brushless DC motors were avaliable. A brushed DC motor has permanent magnets which are stationary on the outside, are called the stator. The armature which rotates and contains an electromagnet is called rotor.
• In a brushed DC motor, the rotor spins 180 degrees when an electric current is run to the armature. To go any further the poles of the electromagnet must flip. The brushes as the rotor spins make contact with the stator, flipping the magnetic field and allowing the rotor to spin a full 360 degrees.
• A brushless DC motor is essentially flipped inside out, eliminating the need for brushed to flip the electromagnetic field. In brushless DC motors, the permanent magnets are on the rotor, in the electromagnets are on the stator.

Applications -

• BLDC motors typiclly have an efficiency of 85 - 90%, while brushed motors are usually only 75-80% efficient.
• Brushes eventually wear out, sometimes causing dangerous sparking, limiting the lifespan of a brushed motor.
• BLDC motors are quite lighter and have much longer life spans.
• These are used in modern devices where low noise and low heat are required. Especially in devices that run continuosly. Includes washing machies, air conditioners and other consumer electronics.