Challenge

Week-7 Challenge: DC Motor Control

SSatish Idnur19 Dec 2022

Q.1) -A -Explain a MATLAB demo model named ‘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 1750 rpm). It simulates a fan type load (where Load torque is proportional to square of speed).

The term H-bridge is derived from the typical graphical representation of such a circuit. An H-bridge is built with four switches

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 motor is rotate in forward direction that mode of operation is known as Forward motoring.

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 in this mode of operation the motor act as Generator.

iii)

In a bridge, you should never ever close both Q1 and Q2 (or Q3 and Q4) at the same time. If you did that, you just have created a really low-resistance path between power and GND, effectively short-circuiting your power supply.

Simulation

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 motor runs in the negative direction.

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

During the period of (0-0.1sec) the motor speed is increasing gradually and motor torque also increasing gradually,
During the period of (0.5-0.6sec) the motor speed is decreasing gradually it gives negative up to- 1000 and motor torque also decrease gradually it comes to zero and go down to negative value.
But the armature current has positive shoot-up or spikes during (0-0.1) sec and negative during (0.5-0.6) sec, this spike occurs because of inrush current in the armature winding.

What is meant by inrush current?

Inrush current or input surge current, or switch-on surge is the maximal instantaneous input current drawn by Electrical device when the switches are (Q1, Q4) and (Q2, Q3) are turning ON or OFF.

Alternating-current electric motors and transformers may draw several times their normal full-load current when first energized, for a few cycles of the input waveform.

During the period of (0-0.5sec) the motor speed is increasing gradually and motor torque also increasing gradually.
During the period of (0.5-1sec) the motor speed is decreasing gradually it gives negative up to -200 voltage and motor torque also decrease gradually it comes to zero and go down to negative value.
But the armature current has positive shoot-up or spikes during (0-0.5) sec and negative during (0.6-1) sec, the spikes will reduce with the reduce the pulse width 75% to 35%.

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

The Four-Quadrant Chopper DC Drive (DC7)

The Four-Quadrant Chopper DC Drive (DC7) block represents a four-quadrant, DC-supplied, chopper (or DC-DC PWM converter) drive for DC motors.
This drive features closed-loop speed control with four-quadrant operation.
The speed control loop outputs the reference armature current of the machine.
Using a PI current 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, is that it can operate in all four quadrants

1) forward motoring

2) reverse regeneration

3) reverse motoring

4) forward regeneration

5) 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

Limitation of this Drive:

There are four switching devices are required, which increases the complexity of the drive system.

The machine is separately excited with a constant DC field voltage source. There is thus no field voltage control. By default, the field current is set to its steady-state value when a simulation is started.

The Four-Quadrant Chopper DC Drive block uses these blocks from the Electric Drives/Fundamental Drive Blocks library:

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

Compare Four-Quadrant Chopper DC Drive (DC7) with the H-bridge model.

Sr.No	Four Quadrants Chopper DC Drive	H-Bridge Model
1	It can operate four quadrants	It can operate only two Quadrant
2	forward motoring, reverse breaking, reverse motoring, forward breaking	Forward Motoring and Reverse Motoring
3	More complex	Less complex
4	Discrete and simulation time is less	Continuous and simulation time is more
5	Lower armature current ripple due to high switching of DC-DC converters	Higher armature current ripple
6	Widely used in Automobiles	Widely used in Robotics

Q.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)

What is Chopper?

The conversion of one level of DC voltage to another DC voltage, then that type of power converter is known as Chopper.

we can classify chopper circuits according to their working in any of these four quadrants as type A, type B, type C, type D and type E

1. Type A Chopper or First–Quadrant Chopper

2. Type B Chopper or Second-Quadrant Chopper

3. Type -C chopper or Two-quadrant type-A Chopper

4. Type D Chopper or Two-Quadrant Type –B Chopper

5. Type –E chopper or the Fourth-Quadrant Chopper

Type -C chopper or Two-quadrant type-A Chopper

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 i₀will be reversed. The current i₀ will be flowing towards 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 is or the diode D2 conducts.

Circuit diagram of Two quadrant chopper

V-I Characteristics of Two quadrant DC-Chopper

We can say the chopper and Freewheeling Diode operate together as type-A chopper forward Motoring or Generative Mode in first quadrant.

In the second quadrant, the chopper and D2 will operate together as type –B chopper or forward Breaking or Regenerative Mode.

The average voltage will be always positive but the average load current might be positive or negative. The power flow may be life 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 my cause a short circuit in supply lines. For regenerative braking and motoring this type of chopper configuration is used.

Simulink of Two-quadrant chopper using :

Frist of all we click on the Symbol of Library Browser and search for Powergui block which is especially needed for simulation to take place without this block simulation will not take place. We need voltage measurement and current measure block. We will require a DC voltage source it will in bottom ensure that should be in black which is used in Power electronics applications, if they are Blue in colour that for various purpose like digital signal processing and a lot of other application. We will be requiring MOSFET, Pulse Generator for requiring pulse to trigger, Diode choose black one, we will require RLC series Branch and scope add this block because its necessary to see how the waveform.

Disable the measurement port of MOSFET and DIODE block rotate Drain should be up and Source should be down. we have to make the connection according to circuit diagram of Two-quadrant chopper. Positive port of DC voltage source connects with the drain of Q1 and source is connected to Drain Q2 and source is connected to negative of DC Voltage source and negative of Diode. If you have a current source I this so connect the current measurement block don’t connect in between the MOSFETs (source and Darin). Connect the current measurement block positive input in between the Diodes (D1 D2) and negative to inductance block. Here we have to measuring the inductance Current and Voltage across the Inductance. There is another source is required to generate the voltage in the inductance.

So now double click on the source and se the voltage to be 24v and change the Second Voltage source equal to 12v which is half of the source voltage. Set the voltage Amplitude of the pulse is 10, time= 0.02sec and pulse width= 50. Logical Operator (NOT) is connect before the Pulse generator, when pulse generator send pulse to one of the MOSFET to ON other MOSFET can be OFF.

It is very simple when 50% it is ON, once after 50% it is OFF another. Set Simulation time to be 1sec. and run it. Double click on scope blocks to see how the waveform looks like which is shown below:

Here, we see that the voltage waveform is not less than 0 that is the minimum value is always positive direction. However, if you see current increase sometimes and decrease sometimes and it goes to negative as well. This Positive and negative happens based on the switching action take place.

The Conclude is that voltage always positive and current can be either negative or positive. This type of circuit is called as C-type Chopper.

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

What is BLDC Motor?

A brushless DC motor (known as BLDC) is a permanent magnet synchronous electric motor which is driven by direct current (DC) electric supply. 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.

Construction of BLDC Motor:

This motor has power converter is nothing but Electronically commutator. In this power converter we have the arrangement of the switches which will excite the different phases turn by turn. We cannot connect the terminal battery to three terminal BLDC Motor, because it will not rotate therefore the Electronically controlled commutator is required.

This motor are different configuration as:

Single-Phase Motor
Two-Phase Motor
Three-Phase Motor

* three-phase BLDC motors with permanent magnet rotor are most commonly used.

Rotor.

The rotor part of the BLDC Motor is made up of permanent magnets

The Permanent Magnets are usually, rare earth alloy magnets like

Neodymium (Nd),
Samarium Cobalt (SmCo)

alloy of Neodymium, Ferrite and Boron (NdFeB)).
Based on the application, the number of poles can vary between two and eight with North (N) and South (S) poles placed alternately.

The following image shows three different arrangements of the poles.

In the first case, the magnets are placed on the outer periphery of the rotor.
The second configuration is called magnetic-embedded rotor, where rectangular permanent magnets are embedded into the core of the rotor.
In the third case, the magnets are inserted into the iron core of the rotor.

Stator

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. For industrial applications and automation systems, 100 V or higher rating motors are used.

The stator winding is further divided into Trapezoidal and sinusoidal Motors.

In a trapezoidal motor, both the drive current and the back EMF are in the shape of a trapezoid (sinusoidal shape in case of sinusoidal motors). Usually, 48 V (or less) rated motors are used in automotive and robotics (hybrid cars and robotic arms).

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.

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

The arangement of the DC motor there are Hall signals H1, H2 and H3 and the phase A, phase B and phase C. Here high, OFF and low are that at perticular states are achived so it is called as Inverter circuit. So here V+ is comming from the battery and ground is comming negative of the battery, the Red colour shows that tha hall sensors are Activated.

The Hall senor H3 under the effect of s-pole is going to Generate a high signal and Hall sensor H1 now was under the effect of n-polewhich is low this effect to Rotor Rotates. So Consider H1 to become high signal value and this is Zero degree of Rotation.

Now the exicitation of the current which phase are excited decoded from six Power BJT or Transistors which are become ON and OFF acordingly Phase changing. Here, RED wire is positive path B wire is connected to one of the pole, the current is flowing this RED wire another RED wire also is connected to Opposite pole. These two poles are opposing or Repulsion each other in direction there will be magnetic field established.

The GREEN wire is the return path wire which is negative one pole of the other pole BLUE so and BLACK one is remains Idle. so RED is the Incomming and BlUE is outgoing.
Phase B is high, then we can see that the current is flowing into the positive terminal and Phase C is low, Phase-A is inactive or OFF state. Phase-B is positive, Phase-C is negative and Phase-A is left open.

Position Sensors (Hall Sensors)

there are no brushes in a BLDC Motor, the commutation is controlled electronically. In order to rotate the motor, the windings of the stator must be energized, a the motor continuous rotation depends on the switching sequence and the position of the rotor (i.e. the North and South poles of the rotor) must be known to precisely energize a particular set of stator windings.

A Position Sensor, which is usually a Hall Sensor (that works on the principle of Hall Effect) is generally used to detect the position of the rotor and transform it into an electrical signal.

Most BLDC Motors use three Hall Sensors that are embedded into the stator to sense the rotor’s position of the shaft. Based on this signal from sensor, the controller decides particular coils to energize.

The output of the Hall Sensor will be either HIGH or LOW depending on whether the North or South pole of the rotor passes near it.

Hall sensors are used for proximity sensing, positioning, speed detection, and current sensing

Brushless DC Motor Drive

As described above that the electronic controller circuit energizes appropriate motor winding by turning transistor or other solid state switches to rotate the motor continuously. The figure below shows the simple BLDC motor drive circuit which consists of MOSFET bridge (also called as inverter bridge), electronic controller, hall effect sensor and BLDC motor.

Here, Hall-effect sensors are used for position and speed feedback. The electronic controller can be a microcontroller unit or microprocessor or DSP processor or FPGA unit or any other controller. This controller receives these signals, processes them and sends the control signals to the MOSFET driver circuit.

In addition to the switching for a rated speed of the motor, additional electronic circuitry changes the motor speed based on required application. These speed control units are generally implemented with PID controllers to have precise control. It is also possible to produce four-quadrant operation from the motor whilst maintaining good efficiency throughout the speed variations using modern drives.

Advantages of BLDC Motor

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

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
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
Need of additional sensors

Application:

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.For industrial applications and automation systems, 100 V or higher rating motors are used.

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