Week-7 Challenge: DC Motor Control

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

WORKING:

In the above circuit, 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 pulse generator through an armature voltage control block.

The H - bridge consists of 4 BJT/ Diode pairs. Two transistors are switched simultaneously: Q1 and Q4 or Q2 and Q3.

When Q1 and Q4 are fired a positive voltage is applied to 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 motor and diodes D1-D4 operate as free-wheeling diodes when Q2 and Q3 are switched off.

A switch block at bottom of circuit splits the power supply into 4 parts namely are p1, p2, p3, and p4.

SWITCHES:

Q1 and Q4 - Forward motoring

Q2 and Q3 - Reverse motoring

The DC motor is connected to a battery, and the DC motor's mechanical input is Initiated as torque.

Torque for mechanical input of DC motor also switched by goto and from.

The output of the DC motor is connected with the BUS sector whereas the speed is plotted in the scope. A conversion of speed for red/sec to rpm is between the BUS sector and the scope using the converter block.

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

In the current, the amount of circuit acted is shown as the graphical representation during the period of duty cycle. The scope shows the IGBT current and diode current passed in the quadrant Q3 and D3.

Where Q3 represent as IGBT current and D# is represented as Diode Current in scope

The graph shows that when the switch Q3 is OFF on the starting stage of current flow and diode current is on that stage.

During the 0.5 sec of time D3 switch is ON that are given according to the pulse generator and after the time period of 0.5 sec D3 switch is OFF and IGBT current is acted after 0.5 sec and Q3 switch remains ON.

The speed and armature current is obtained by the DC motor and Load torque is calculated by torque proportional to the square of speed.

In the starting point, there is a sudden shoot-up in act.

This scope shows that speed is increased in a certain period of time and at that time armature current is positive whereas Load torque also moves forward.

After the time period of 0.5 sec which is given in pulse generator, sudden shoot up acted and speed gets decreased in that time armature current is in negative terminal whereas torque act as reverse action.

Here the sudden shoot-up is acted while armature current is processed at a positive and negative terminal.

BY CHANGING THE PULSE GENERATOR INPUT VALUE

By opening the pulse generator we can change the pulse width i.e the input value to whatever desire. So here we change the value to 50%.

Here we see that the sudden shoot-up is not acted by comparing with 75 of the pulse width. same cycle time is followed in this experiment also.

In this observation starting current is generated as diode current for time period of 0.5 sec, then IGBT current is acted while diode current is being stopped.

This graph shows when the switch Q3 is OFF on the starting stage of current flow and Diode current is ON at that stage.

During the 0.5 sec of time D3 switch is ON that is given according to the pulse generator and after the time period of 0.5 sec D3 switch is OFF. An IGBT current is acted after 0.5 sec and Q3 switch remains ON.

There will be little variation as shown in graph at the starting point of current but the complete reduction of shoot up is not held here.

In this above scope, a flow of armature current is displayed and speed load torque is also displayed.

The change is made in pulse width as 50% from the pulse generator and results are shown that voltage fluctuation is held in starting point of forwarding reverse state.

The sudden fluctuation is reduced in this experiment by reducing the pulse width of the input supply

A change is also held speed and load torque where the constant level of speed and torque is acted in above scope compare with the previous 75% of the pulse width.

A time period of forwarding and reverse action is maintained as same as the previous Duty cycle.

In this observation, the shot up is reduced when motor changes direction from forward to reverse compared to 75% of pulse width.

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

Four Quadrant Operation of DC motor means that the machine operates in four quadrants. They are forward motoring, Forward braking, reverse motoring and reverse braking.

QUADRANT - I

When the motor draws power from supply, the voltage and current are in same direction.

Now the motor is said to be in motoring operation, its operation corresponds to the 1st quadrant.

In the 1st Quadrant power developed in positive and machine is working as motor supplying mechanical energy.

The 1st quadrant operation is called Forward motoring.

QUADRANT - II

If there is a possibility of reversing the current direction keeping the voltage polarity same, the motor is said to be operating in 2nd quadrant.

This happens when regenerative braking is applied to motor.

During regenerative braking operation, reverse power flow will happen i.e power will be feedback to supply system.

@nd 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 that opposes the motion.

The kinematic energy of the rotating part is available as electric energy which may be supplied back to mains. In dynamic braking energy may be supplied back to mains. In dynamic braking energy is dissipated in the resistance.

QUADRANT - III

If the voltage polarity and load current are reversed simultaneously, the motor operation will be in the opposite direction, but the power will be consumed by the load.

The operation is corresponding the 3rd Quadrant and even it is said to be in motoring operation.

The 3rd quadrant operation is known as reverse motoring.

The motor works, in the reverse direction. Both the speed and torque have negative values while the power is positive.

QUADRANT - IV

If the voltage polarity is reversed maintain the current direction source to load the power is reversed and the operation corresponding to 4th quadrant.

Now the motor is said to be in reverse Regenerative operation.

In the 4th quadrant, the torque is positive and speed is negative. This quadrant corresponds to braking in the reverse regenerative braking mode.

BJT H - Bridge Drive

1. It is used for changing the direction of the Motor.
2. Regenerative Braking cannot be available.
3. The source is of fixed DC supply.
4. The armature current shoots up during the phase transition.
5. Less control of the Vehicle
6. Signals are continuous

Four quadrant chopper drive

1. Here the motor can be operated in 4 different modes.
2. Regenerative braking can be achieved.
3. The source is an AC supply which can be rectified to DC varying supply.
4. The armature current doesn't shoot up to the value of BJT H-bridge but for low voltage value, it's negligible.
5. Better control of the vehicle
6. Signals are not continuous and they are discrete.

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 )

• Class - C chopper is a Two quadrant Chopper
• Its functions in the 1st and 2nd quadrants
• In class C chopper, the output voltage is always positive and output current can be positive or negative.
• This type of chopper obtains by connecting class-A and class - B choppers in parallel

1. Power GUI
2. Voltage measurement Block
3. Current measurement block.
4. DC voltage source
5. Mosfet
6. Pulse generator
7. Diode
8. Series RLC band
9. Logical Operator

The Two - quadrant chopper DC drive block represents a two-quadrant, DC - supplied, chopper drive for a DC motor. This drive features closed-loop speed control with a two-quadrant operation. The speed control loop output 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 advantages of this drive, compared with other DC drives, are its implementation simplicity and the that it can operate in two quadrants. In addition due to the use of a high switching frequency DC-DC converter, a lower armature current ripple is obtained. However, for all two-quadrant DC drives reversible and regenerative operations, which are required in most DC drives cannot be obtained.

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

A brushless DC electric motor (BLDC motor or BL motor), also known as an electronically commutated motor (ECM or EC motor) or synchronous DC motor, is a Synchronous motor using a direct current (DC) electric power supply. It uses an electronic close loop control to switch DC currents to the motor winding producing magnetic fields which effectively rotate in space and which the permanent magnet rotor follows. The controller adjusts the phase and amplitude of the DC pulses to control the speed and torque of the motor. This control system is an alternative to the mechanical commutator (brushes) used in many conventional electric motors.

The construction of a brushless motor system is typically similar to a permanent magnet synchronous motor (PMSM), but can also be a switched reluctant motor or an induction motor They may also use a neodymium magnet and be outrunners (the stator is surrounded by the rotor), in runners (the rotor is surrounded by the stator), or axil (the rotor and stator are flat and parallel).

The advantages of a brushless motor over a brushed motor are high power-to-weight ratio, high speed, nearly instantaneous control of speed (rpm) and torque, high efficiency, and low maintenance. Brushless motors find applications in such places as computer peripherals (disk drives, printers), hand-held power tools, and vehicles ranging from model aircraft to automobiles. In modern washing machines, brushless DC motors have allowed the replacement of rubber belts and gearboxes by a direct-drive design.

Construction of BLDC Motor

The main design difference between brushed and brushless motors is the replacement of a mechanical commutator with an electric switch circuit. Keeping that in mind, a BLDC motor is a type of synchronous motor in the sense that the magnetic field generated by the stator and the rotor revolve at the same frequency.

Brushless Motors are available in three configurations: single-phase, two-phase, and three-phase. Out of these, the three-phase BLDC is the most common one.

The following image shows the cross-section of a BLDC Motor

As you can see in the image, a BLDC motor consists of two main parts: a stator and a rotor.

Stator

The structure of the stator of a BLDC motor is similar to that of an induction motor. It is made up of stacked steel laminations with axially cut slots for winding. The winding in BLDC is slightly different than that of the traditional induction motor

Generally, most BLDC motors consist of three stator windings that are connected in star or ‘Y’ fashion (without a neutral point). Additionally, based on the coil interconnections, the stator windings are 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).

Rotor

The rotor part of the BLDC Motor is made up of permanent magnets (usually, rare earth alloy magnets like Neodymium (Nd), Samarium Cobalt (SmCo), an 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 the 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.

Position Sensors (Hall Sensors).

Since there are no brushes in a BLDC Motor, the commutation is controlled electronically. To rotate the motor, the windings of the stator must be energized in a 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.

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. By combining the results from the three sensors, the exact sequence of energizing can be determined.

Working Principle

Consider the following setup of three windings in the stator designated A, B, and C. For the sake of understanding, let us replace the rotor with a single magnet.

We know that when a current is applied through a coil, a magnetic field is generated and the orientation of the field lines i.e. the poles of the generated magnet will depend on the direction of the current flowing through the coil.

Using this principle, if we supply current to coil A it will generate a magnetic field and attract the rotor magnet. The position of the rotor magnet will shift slightly clockwise and will align with A.

If we now pass current through coils B and C one after the other (in that order), the rotor magnet will rotate in the clockwise direction.

To increase efficiency, we can wind the opposite coils using a single coil so that we get a double attraction. Further increasing the efficiency, we can energize two coils at the same time so that one coil will attract the magnet and the other coil will repel it. During this time, the third will be idle.

For a complete 3600 rotation of the rotor magnet, six possible combinations of coils A, B, and C are applicable and are shown in the following timing diagram.

Based on the above diagram, we can confirm that at any time, one phase is positive, one phase is negative and the third phase is idle (or floating). So, based on the inputs from the Hall Sensors, we have two switches in the phases as per the above diagram

Driving Brushless DC Motors

If stator and rotor are essential parts of a BLDC Motor that are integral to it, then the driving electronics are equally essential. A Block diagram of a typical Brushless DC Motor control or drive system is shown in the following image.

This drive circuitry is often known as Electronic Speed Controller System or simply an ESC. One common setup is called the Full-Bridge Drive Circuit. It consists of an MCU with PWM outputs, six MOSFETS for the three phases of the stator windings, feedback from the Hall sensors, and some power supply-related components.

The MCU can be programmed to appropriately switch the MOSFETS based on the data from the Hall Sensors.

Advantages of BLDC Motors

Since BLDC Motors are electronically commutated, there are several advantages over traditional Brushed DC Motors. Some of them are

• No wear and tear (due to absence of brushes).
• High efficiency
• Better speed vs torque characteristics.
• Long life.
• Less noise or noiseless operation
• Significantly higher RPM

Applications of Brushless DC Motors

• Some of the areas of applications of BLDC Motors are mentioned below:
• Single-speed applications
• Adjustable speed applications
• Position control.
• Low noise applications
• High-speed applications