Week-7 Challenge: DC Motor Control

Question 1]

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

Answer:-

• Speed control of a DC motor

The relationship given below gives the speed of a D.C. motor

The above equation shows that the speed depends upon the supply voltage V, the armature circuit resistance R_a, and the field flux Ф, which is produced by the field current. In practice, the variation of these three factors is used for speed control. Thus, there are three general methods of speed control of D.C. Motors.

1. Resistance variation in the armature circuit :- This method is called armature resistance control or Rheostat control.
2. Variation of field flux Ф :- This method is called field flux control.
3. Variation of the applied voltage :- This method is also called armature voltage control.

• BJT :-

A bipolar junction transistor is a three-terminal semiconductor device that consists of two p-n junctions which are able to amplify or magnify a signal. It is a current controlled device. The three terminals of the BJT are the base, the collector, and the emitter. A signal of a small amplitude applied to the base is available in the amplified form at the collector of the transistor. This is the amplification provided by the BJT. Note that it does require an external source of DC power supply to carry out the amplification process.

• H-bridge:-

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 then it goes negative.

S1	S2	S3	S4	Result
1	0	0	1	Motor moves right
0	1	1	0	Motor moves left
0	0	0	0	Motor coast
1	0	0	0	Motor coast
0	1	0	0	Motor coast
0	0	1	0	Motor coast
0	0	0	1	Motor coast
0	1	0	1	Motor brakes
1	0	1	0	Motor brakes
X	X	1	1	Short circuit
1	1	X	X	Short circuit

Model of Speed control of a DC motor using BJT H-bridge :-

In this model H-bridge is used to control DC motor speed. In this the bipolar junction transistor (BJT) when it used for power switching applications it operates an Insulated gate bipolar transistor (IGBT). When it is conducting (BJT operating in saturating region) a forward Voltage Vf is developed between collector and emmitter. Therefore the IGBT block is 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 signal. Tye DC motor uses the preset model (5HP 24V 1750RPM) It simulates the fan type load ( where load 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 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 motpor runs in the negative direction.

Scope shows motor speed, Armature current and load torque and currents shows the current flowing in BJT Q3 and diode D3.

In pulse genertaor block gives 75% duty cycle. they are shown below

Result Duty cycle 75% :-

As we see in armature current the current was go in up direction quickly so it is a Overshoot curve and then it goes to constant.

Overshoot/Undershoot:-

Overshoot occurs when the transitory values exceed final value. When they are lower than the final value, the phenomenon is called Undershoot.

Now we have change the duty cycle in battery and observed the armature current, speed and torque tehn lets see the overshoot reduce or not.

Duty cycle 60%:-

Result Duty ccycle 60% :-

Duty cycle 50%:-

Result Duty cycle 50%:-

As we see decreasing the Duty cycle speed, Load torque decrease and the armture current also decrease but the armature current is goes in constant. there are no overshoot available. and the IGBT current and diode current it also decreased.

BJT H-Bridge Voltage 200V and DC Motor Voltage 100V Duty cycle 75% :-

Result :-

The overshoot is also reduce reducing the voltages of BJT H-bridge and reducing the DC motor input Voltage increase the Armature Current without decreasing Duty cycle.

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

Four Quadrant Operation of any drives or DC Motor means that the machine operates in four quadrants. They are Forward Braking, Forward motoring, Reverse motoring and Reverse braking. A motor operates in two modes – Motoring and Braking. A motor drive capable of operating in both directions of rotation and of producing both motoring and regeneration is called a Four Quadrant variable speed drive.

In motoring mode, the machine works as a motor and converts the electrical energy into mechanical energy, supporting its motion. In braking mode, the machine works as a generator and converts mechanical energy into electrical energy and as a result, it opposes the motion. The Motor can work in both, forward and reverse directions, i.e., in motoring and braking operations.

The figure below shows the four-quadrant operation of drives:

In the first quadrant power developed is positive and the machine is working as a motor supplying mechanical energy. The first quadrant operation is called Forward Motoring. second quadrant operation is known as Braking. In this quadrant, the direction of rotation is positive, and the torque is negative, and thus, the machine operates as a generator developing a negative torque, which opposes the motion. The third quadrant operation is known as the reverse motoring. The motor works, in the reverse direction. Both the speed and the torque have negative values while the power is positive.In the fourth quadrant, the torque is positive, and the speed is negative. This quadrant corresponds to the braking in the reverse motoring mode.

Function	Quadrant	Speed	Torque	Power Output
Forward Motoring	I	+	+	+
Forward Braking	II	+	-	-
Reverse Motoring	III	-	-	+
Reverse Braking	IV	-	+	-

Model:-

Comparison Four-Quadrant Chopper DC Drive (DC7) block and H-bridge model:-

Four-Quadrant Chopper DC Drive:-

• The armature voltage is provided by IGBT controllerrs is controlled by PI regulations.
• A braking chopper ha sbeen added between the diode and motor.
• This model contains speed and current regulations.
• The speed regulations represnets armature current used by the current controller in order to obtain the electromagnetic torque needed to reach the desired speed.
• The chopper converts the fixed DC voltage to variable DC voltage.
• It requires current for each sector and consist of four quadrant forward motoring, reverse motoring, forward braking and reverse braking.
• Current signal is interupted with one other to chang ethe direction of the motor.
• The change in direction in BJT is done by changine the polarity.
• PID controller s used in DC7 model.

H-bridge:-

• It consists two operations forward motoring and reverse motoring.
• It is used in robotics technology.
• PID controllers are not used in this model.
• It operates in two quadrants.
• In this model the current changes the change in polarity in switches.
• this model is continuos.
• Speed controller and current controller sampling times are present.
• The current shoot up and disburces are high in this due to switching.

Question 2] Develop a 2-quadrant chopper using simulink & explain the working of the same with the relevant results.

Two quadrant DC drive block:-

The two quadrant Chopper DC drive (DC6) block represent a two quadrant DC supplied Chopper Drive for DC motors. This drive features closed loop speed control with two quadrant operation. The speed control loop outputs the reference armature current of the macxhine. Using a PI 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 are its implementation simplicity and that it can operatein two quadrants (forward motoring and reverse generation). In addition due to the use oh high switching frequency DC-DC converters a lower armature current ripplle (compared with thyristor based DC drives) is obtained. However for all two quadrants DC drives reversibe and regenerative operations (reverse motoring and forward regeneration ) which are required in most DC drives cannot be obtained.

The two quadrant chopper DC drive block uses these blocks from the electric drives/ fundamental drive blocks library.

• Speed Controller
• Regulation Switch
• Chopper

Model:-

Result:-

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

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.

The armature coils are switched electronically by transistors or silicon controlled rectifier at the correct rotor position in such a way that armature field is in space quadrature with the rotor field poles. Hence the force acting on the rotor causes it to rotate. 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.

Construction of BLDC Motor:-

It 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.

The 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.

Stator of a BLDC motor made up of stacked steel laminations to carry the windings. These windings are placed in slots which are axially cut along the inner periphery of the stator. These windings can be arranged in either star or delta. However, most BLDC motors have three phase star connected stator. Each winding is constructed with numerous interconnected coils, where one or more coils are placed in each slot. In order to form an even number of poles, each of these windings is distributed over the stator periphery.

The stator must be chosen with the correct rating of the voltage depending on the power supply capability. For robotics, automotive and small actuating applications, 48 V or less voltage BLDC motors are preferred.

Working Principle and Operation Of 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.

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 Motor:-

1] It has no mechanical commutator and associated problems.

2] High efficiency due to the use of permanent magnet rotor.

3] High speed of operation even in loaded and unloaded conditions due to the absence of brushes that limits the speed.

4] Smaller motor geometry and lighter in weight than both brushed type DC and induction AC motors.

5] Long life as no inspection and maintenance is required for commutator system.

6] Higher dynamic response due to low inertia and carrying windings in the stator.

7] Less electromagnetic interference.

8] Quite operation (or low noise) due to absence of brushes.

Disadvantages of Motor:-

1] These motors are costly.

2] Electronic controller required control this motor is expensive.

3] Not much availability of many integrated electronic control solutions, especially for tiny BLDC motors.

4] Requires complex drive circuitry.

5] Need of additional sensors.

Applications of motors:-

It is used 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.

Thank you

Dharmesh Bonde...