Week-7 Challenge: DC Motor Control

AIM:

• A.To study and explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’.

B.To study the four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model.

• Develop a 2-quadrant chopper using simulink & explain the working of the same with the relevant results.
• To study and learn how the BLDC motor works and explain the operation of BLDC motor.

STUDY 1:

A. Run MATLAB demo 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 1750rpm). It simulates a fan type load (where Load torque is proportional to the square of speed). The armature means 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.

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

• The Four-Quadrant Chopper DC Drive (DC7) block represents a four-quadrant, DC-supplied, chopper drive for DC motors. This drive features closed-loop speed control with the four-quadrant operation.
• 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.

The Four-Quadrant Chopper DC Drive block uses these blocks from the Electric Drives

• Speed Controller (DC)
• Regulation Switch
• Current Controller (DC)
• Chopper

Suitable EV model using DC7 block and detail result report.

• To load an EV model using DC7. just write DC7 in MATLAB search bar.
• A Four-Quadrant Chopper 200 HP DC Drive setup will open. As shown in the figure:

• Now in this give setup, we have to modify the speed reference and linear load torque.
• Now in the subsystem, we have to create a vehicle drive condition.

Following blocks are added to create subsystem.

• Math function
• Product
• Sum
• Euler's number
• Constant
• Now we have to use 24.7+0.0051 w2 in the given block for further solution.

H-bridge model

• The H-Bridge block represents an H-bridge motor driver. The block has the following two Simulation mode options:

1. 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:

1. One semiconductor switch and one freewheeling diode
2. Two freewheeling diodes.
3. Two semiconductor switches and one freewheeling diode

2. Averaged-This mode has two Load current characteristics options:

• Smoothed
• Unsmoothed or discontinuous

C : Comparison :

Sr.No.	(DC7) Four-Quadrant Chopper DC Drive	Block H-bridge model
1	Generally used in Automotive for controlling DC motor speed and direction	It is generally used in Robotics & Electro-mechanical devices for controlling the DC motor speed and direction.
2	Regenerative braking is possible in DC7	Dynamic braking is only possible.
3	It can operate in all 4 quadrants of DC motors.	It can only operate only in forward and reverse directions.
4	Only DC-DC converters high switching frequency is possible.	It shows the simulation value in accurate mode.
5	It has a wider range of torque and output power.	t has a narrow torque and output power.

Study 2:

2-Quadrant chopper using simulink:

• 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 buck-boost converter controlled by two PI regulators. The buck-boost converter is fed by a 630 V DC bus obtained by rectification of a 460 V AC 60 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 DC6 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 two IGBT devices (Pulse Width Modulation).For proper system behaviour, the two IGBT devices have opposite instantaneous pulse values.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.

Study 3:

BRUSHLESS DC MOTOR (BLDC):

• A brushless DC motor (known as BLDC) is a permanent magnet synchronous electric motor which is driven by direct current (DC) electricity and it accomplishes electronically controlled commutation system (commutation is the process of producing rotational torque in the motor by changing phase currents through it at appropriate times) instead of a mechanically commutation system. BLDC motors are also referred as trapezoidal permanent magnet motors.
• Unlike conventional brushed type DC motor, wherein the brushes make the mechanical contact with commutator on the rotor so as to form an electric path between a DC electric source and rotor armature windings, BLDC motor employs electrical commutation with permanent magnet rotor and a stator with a sequence of coils. In this motor, permanent magnet (or field poles) rotates and current carrying conductors are fixed.
• Hall sensors or rotary encoders are most commonly used to sense the position of the rotor and are positioned around the stator. The rotor position feedback from the sensor helps to determine when to switch the armature current.
• This electronic commutation arrangement eliminates the commutator arrangement and brushes in a DC motor and hence more reliable and less noisy operation is achieved. Due to the absence of brushes BLDC motors are capable to run at high speeds. The efficiency of BLDC motors is typically 85 to 90 percent, whereas as brushed type DC motors are 75 to 80 percent efficient. There are wide varieties of BLDC motors available ranging from small power range to fractional horsepower, integral horsepower and large power ranges.

Working Principle and Operation of BLDC Motor:

• When the stator coils are electrically switched by a supply source, it becomes electromagnet and starts producing the uniform field in the air gap. Though the source of supply is DC, switching makes to generate an AC voltage waveform with trapezoidal shape. Due to the force of interaction between electromagnet stator and permanent magnet rotor, the rotor continues to rotate.

• Consider the figure below in which motor stator is excited based on different switching states. With the switching of windings as High and Low signals, corresponding winding energized as North and South poles. The permanent magnet rotor with North and South poles align with stator poles causing motor to rotate.

• When the stator coils are electrically switched by a supply source, it becomes electromagnet and starts producing the uniform field in the air gap. Though the source of supply is DC, switching makes to generate an AC voltage waveform with trapezoidal shape. Due to the force of interaction between electromagnet stator and permanent magnet rotor, the rotor continues to rotate.

  Consider the figure below in which motor stator is excited based on different switching states. With the switching of windings as High and Low signals, corresponding winding energized as North and South poles. The permanent magnet rotor with North and South poles align with stator poles causing motor to rotate.

• Based on this signal from sensor, the controller decides particular coils to energize. Hall-effect sensors generate Low and High level signals whenever rotor poles pass near to it. These signals determine the position of the shaft.