Week-7 Challenge: DC Motor Control

1. A. Explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’.

The basic operation of the H-bridge circuit diagram is given below.

• The go forward signal “positive signal” Q₁ and Q₄ turn ON, the D₂, and D₃ turn ON, Q₂ and Q₃ turn OFF, and the D₁ and D₄ turn OFF.
• The go reverse signal “negative signal” Q₂ and Q₃ turn ON, the D₁ and D₄ turn ON, Q₁ and Q₄ turn OFF, and the D₂ and D₃ turn OFF.

Simulink model:

• The BJT when used for power-switching applications, it operates as an IGBT.
• When conducting (BJT in saturation region) forward voltage V_f is developed between collector and emitter.
• So, the IGBT block can be used to model the BJT devices.
• The switch is controlled by Simulink signals 1s & 0s.

DC motor used is 5HP, 240V, 1750 RPM, and simulates a fan load.

The armature means voltage can be varied from 0 to 240V when the duty cycle is 100%

H-bridge consists of 4 IGBT models:

Positive sequence: The go forward signal “positive signal” Q₁ and Q₄ turn ON, the D₂, and D₃ turn ON, Q₂ and Q₃ turn OFF, and the D₁ and D₄ turn OFF.

Negative sequence: The go reverse signal “negative signal” Q₂ and Q₃ turn ON, the D₁ and D₄ turns ON, Q₁ and Q₄ turn OFF, and the D₂ and D₃ turn OFF.

PWM technique generates variable width pulses to represent the amplitude of an analog input signal, the length is the width of the signal, then comes the term duty cycle which represents the percentage values.

1. Comment on the armature's current shoot-up from the scope results.

-For a step time of 2 secs

We found that diode current is a value that means the diode is ON and the IGBT is OFF but after the 2 secs the IGBT is ON and the diode is OFF

@ these 2 secs

Found that the armature current is a value [pulse generator value is 75%] the positive armature current means motoring in the forwarding direction

And we have a positive torque gives us a positive RPM

Also when changing the pulse width to 50% it changed the previous curves

For the very first instant of the start, we can find the current RMS in the first part is more than the other that is because of the load starting current.

The first armature current shoot-out is when the torque is generated for the very first start only when the RPM is increased from an initial value of zero to another value.

And after these 2 seconds, the opposite happened, the same but in the opposite direction

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

A DC chopper is a switching device used to obtain a controlled variable DC voltage from a constant DC voltage

May be classified into 5 operation classes

Class A: first quadrant, I and V only positive (1^st quadrant)

Class B: V is only positive but I is only negative (2^nd quadrant)

Class C: I can be positive or negative but V is only positive (1^st and 4^th quadrants)

Class D: V can be positive or negative but I is only positive (1^st and 2^nd quadrants)

Class E: V and I both can be positive or negative (1^st,2^nd,3^rd, and 4^th quadrants)

May be classified into voltage classes

Step-up voltage: the chopper is working as a boost converter which rises the output voltage and lowers the output current

Step-down voltage: the chopper is working as a buck converter which lowers the output voltage and rises the output current

Sl.no	Four Quadrant chopper	H-bridge model
1.	The motor can be operated in all four different modes (forward motoring, reverse motoring, forward braking, and reverse braking)	In this model, we have only two modes forward and reverse by changing the direction of the motor.
2.	In this, vehicle control is better	In this, vehicle control is less
3.	Regenerative braking can be achieved	In this regenerative braking is not possible.
4.	The signals given to the four-quadrant operation are discrete.	Here the signals are continuous.
5.	The four-quadrant chopper is widely used in automobile applications for the control of DC motor speed and direction.	The H-bridge is used in robotics and electro-mechanical devices to control the speed and direction of the DC motor at low-level speed.

2. Develop a 2-quadrant chopper using Simulink & explain the working of the same with the relevant results. (Refer to the article - Multiquadrant operation of the motor)

A 2-quadrant chopper:

In the 2-quadrant chopper: Class-C.

This is a class-C Chopper the output of the voltage is positive and the output of the current is positive and negative. Type C chopper is obtained by connecting type–A and type –B choppers in parallel. We will always get a positive output voltage V₀ as the freewheeling diode FD is present across the load. When the chopper is on the freewheeling diode starts conducting and the output voltage v0 will be equal to V_s. The direction of the load current i0 will be reversed. The current I₀ will be flowing toward the source and it will be positive regardless the chopper is on or the FD conducts. The load current will be negative if the chopper or the diode D2 conducts. We can say the chopper and FD operate together as a type-A chopper in the first quadrant. In the second quadrant, the chopper and D2 will operate together as a type –B chopper.

The average voltage will be always positive but the average load current might be positive or negative. The power flow may be like the first quadrant operation i.e., from source to load or from load to source like the second quadrant operation. The two choppers should not be turned on simultaneously as the combined action may cause a short circuit in supply lines. For regenerative braking and motoring, this type of chopper configuration is used.

For the class-d chopper, the output current is positive and the output voltage is positive and negative.

The circuit diagram of the type D chopper is shown in the above figure. When the two choppers are on the output voltage V₀ will be equal to V_s. When V₀ = – V_s the two choppers will be off but both the diodes D1 and D2 will start conducting. V0 the average output voltage will be positive when the choppers turn on the time Ton will be more than the turn-off time Toff is shown in the waveform below. As the diodes and choppers conduct current only in one direction the direction of the load current will be always positive.

3. Explain in brief the operation of the BLDC motor.

BLDC motor works on a principle similar to that of a conventional DC motor, i.e., the Lorentz force law which states that whenever a current-carrying conductor is placed in a magnetic field it experiences a force. As a consequence of reaction force, the magnet will experience an equal and opposite force. In the case, of the BLDC motor, the current-carrying conductor is stationary while the permanent magnet moves.

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

Consider the figure below in which the 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 aligns with the stator poles causing the motor to rotate.

Observe that the motor produces torque because of the development of attraction forces (when North-South or South-North alignment) and repulsion forces (when North-North or South-South alignment). In this way motor moves in a clockwise direction.

Here, one might get a question that how we know which stator coil should be energized and when to do it. This is because; the motor's continuous rotation depends on the switching sequence around the coils. As discussed above Hall sensors give shaft position feedback to the electronic controller unit.

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

Advantages of BLDC Motor

BLDC motor has several advantages over conventional DC motors and some of these are

• It has no mechanical commutator and associated problems
• High efficiency due to the use of permanent magnet rotor
• High speed of operation even in loaded and unloaded conditions due to the absence of brushes that limits the speed
• Smaller motor geometry and lighter in weight than both brushed-type DC and induction AC motors
• Long life as no inspection and maintenance is required for the commutator system
• Higher dynamic response due to low inertia and carrying windings in the stator
• Less electromagnetic interference
• Quite an operation (or low noise) due to the absence of brushes

Disadvantages of Brushless Motor

• These motors are costly
• The electronic controller required to control this motor is expensive
• Not much availability of many integrated electronic control solutions, especially for tiny BLDC motors
• Requires complex drive circuitry
• Need of additional sensors

Applications of Brushless DC Motors (BLDC)

Brushless DC Motors (BLDC) are used for a wide variety of application requirements such as varying loads, constant loads, and positioning applications in the fields of industrial control, automotive, aviation, automation systems, health care equipment, etc. Some specific applications of BLDC motors are

• Computer hard drives and DVD/CD players
• Electric vehicles, hybrid vehicles, and electric bicycles
• Industrial robots, CNC machine tools, and simple belt-driven systems
• Washing machines, compressors, and dryers
• Fans, pumps, and blowers