Week-7 Challenge: DC Motor Control

I AM STUDENT OF LIVE BATCH-16

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

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

SOLUTION:

A Simulink model of 'Speed control of a DC motor using BJT H-bridge’:

MODEL EXPLANATION:

In the above 'Speed control of a DC motor using BJT H-bridge’ the input voltage is generated from the pulse generator where the amount of supply and duty cycle is assigned in the pulse generator then the switch block is connected to the pulse generator through the armature voltage control block the h Bridge consists of 4 BJT or diode pair to transistors are switched simultaneously Q1 and Q4 or Q2 and Q3.

When you and Q2 are a fired a positive voltage is applied to motor and diode D2 and D3 operators free wheeling diodes and Q1 and q4 are switched off. When Q2 and Q3 are fired a negative voltage is applied to motor and diode D1 D4 operators free wheeling diodes when Q2 and Q3 are switch off. A black at bottom of circuit splits the power supply into four parts namely P1 P2 P3 and P4.

Q1 and q4 are forward motoring Q2 and Q3 are reverse motoring the DC motor is connected to a battery and the DC Motors mechanical input is initiated as torque full stop torque for mechanical input of DC motor also switch by go to and from. The output of the DC motor is connected with the bus sector where as the speed is plotted in the scope. Conversion of speed for Radian/second to RPM is between the bus sector and the scope using the converter block.

pulse generator input

output:

B. Comment on the armature current shoot-up from the scope results.

SOLUTION:

From armature current graph we can see that at starting there is a sudden increase in current which means at transition from positive to negative direction of rotation we observe high Sergeant current this is due to the transient switching occurring in the circuit as already there is current flowing in the circuit and on changing the switches after 0.5 second the current in the circuit adds to the change in the direction of current and we observe a search in current. The surge in the current can be reduced by reducing the duty cycle

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

SOLUTION:

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.

SCOPE RESULT:

• The plot of duty cycles IGBT 1&4 are exactly opposite to that of duty cycles IGBT 2&3 as the pulse width of IGBT devices 1 and 4 are opposite to those of IGBT devices 2 and 3.
• The desired armature current is obtained from the average armature voltage.
• The armature current follows the current reference very well, with fast response time and small ripples.

COMPARISION between The four quadrant Chopper and BJT h Bridge model:

The four quadrant Chopper	BJT h Bridge model
The four quadrant Chopper works in 4 Mode · forward motoring · forward breaking · reverse motoring · reverse breaking	BJT h Bridge model works only on two modes · forward motoring · reverse motoring
to change the direction of rotation of motor from forward to reverse and vice versa the current signal gets transited from one to another switch	In BJT h Bridge model the direction of rotation of motor is changed by changing the polarity of current
the four quadrant Chopper have high switching frequency	BJT h Bridge model have low switching frequency
the armature current in four quadrant chopper have a little spike at transition from one to another switch because of high switching frequency	BJT h Bridge model have High spike of current
current the four quadrant chopper is generally used in industrial application	BJT h Bridge model used in robotics

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

SOLUTION:

Chopper-Fed DC Motor Drive- D.C motor is considered a SISO (Single Input and Single Output) system having torque/speed characteristics compatible with most mechanical loads. This makes a DC motor controllable over a wide range of speeds by proper adjustment of the terminal voltage

A simple chopper – fed DC motor drive is shown in Fig.2. The basic principle behind DC motor speed control is that the output speed of a DC motor can be varied by controlling armature voltage for speed below and up to rated speed keeping field voltage constant. The armature voltage can be controlled by controlling the duty cycle of the converter (here the converter used is a DC chopper)

SIMULINK MODEL OF 2-quadrant chopper:

The above model is the model of Type Chopper(Type A 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 simultaneously 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 always get a positive output voltage V₀ as the freewheeling diode FD is present across the load. When the chopper CH1 is on the current flows from source to load. The inductor stores energy. When Ch1 turn OFF 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 towards the source. The load current will be negative when the CH2 is on. The current completes its path through diode D2.

3. Explain in a brief about operation of BLDC motor.

SOLUTION:

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.

Construction of BLDC Motor

BLDC motors can be constructed in different physical configurations. Depending on the stator windings, these can be configured as single-phase, two-phase, or three-phase motors. However, three-phase BLDC motors with permanent magnet rotor are most commonly used.he construction of this motor has many similarities of three phase induction motor as well as conventional DC motor. This motor has stator and rotor parts as like all other motors.

Working Principle and Operation of BLDC Motor

BLDC motor works on the principle similar to that of a conventional DC motor, i.e., the Lorentz force law which states that whenever a current carrying conductor 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 case 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 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.

Observe that 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). By 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. This is because; the motor continuous rotation depends on the switching sequence around the coils. As discussed above that Hall sensors give shaft position feedback to the electronic controller unit.

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.

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 commutator system
• Higher dynamic response due to low inertia and carrying windings in the stator
• Less electromagnetic interference
• Quite operation (or low noise) due to absence of brushes

Disadvantages of Brushless Motor

• These motors are costly
• Electronic controller required 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 equipments, 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