Week-7 Challenge: DC Motor Control

OBJECTIVE:

1. a) Explain a MATLAB demo model’s speed control of a DC motor using BJT H-Bridge.

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

    c) Refer to the help section of ‘The four-quadrant chopper DC drive (DC7) block’. Compare it with the H-bridge model.

2. Develop a 2-quadrant chopper using Simulink and explain the working of the same with the relevant results.

    (Refer to the article – Multi quadrant operation of the motor)

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

OBJECTIVE-1:

BJT (Bipolar Junction Transistor):

A bipolar junction transistor is a three-terminal semiconductor device that is used for switching or amplification. The three terminals of the BJT are the Base, Collector, and Emitter. And it consists of two P-N Junctions which are able to amplify or magnify a signal. Small amplitude of a signal which is given to BJT will amplify that signal, for this amplification process does not require any external source.

The BJT is operating in three regions, Active Region, Saturation Region, and the Cut-off region.

Active Region:

In this region transistors (BJT) acts as an amplifier.

Saturation Region:

In this region, the transistor (BJT) is fully on and operates as a switch such that the collector current is equal to the saturation current.

Cut-off Region:

In this region, the transistor is fully off and the collector current is equal to zero.

There are two types of BJT.

1. PNP BJT
2. NPN BJT

PNP BJT:

In PNP BJT, the n-type semiconductor is sandwiched between the two p-type semiconductors. The two p-type semiconductors act as emitter and collector respectively while the n-type semiconductor acts as a base. The current enters the transistor through the emitter such that the emitter-base junction is forward biased and the collector-base junction is reverse biased.

The PNP BJT is shown below figure.

NPN BJT:

In an NPN BJT, the p-type semiconductor is sandwiched between the two n-type semiconductors. The two n-type semiconductors acct as emitters and collectors respectively while the p-type semiconductor acts as a base. Current entering the emitter, base, and collector has the sign convention of positive while the current that leaves the transistor has the sign convention of negative.

The NPN BJT is shown below figure.

Equivalent circuit of BJT:

BJT characteristics:

Applications of BJT:

BJT is used as a switch, amplifier, filter, and oscillator.

BJT is used as a detector or also known as a demodulator.

BJT is used in clipping circuits so that the waves can be shaped.

Logic circuits and switching circuits use BJT.

H-Bridge:

H-bridge is a rather simple circuit, containing four switching elements, with the load at the center, in an H-like configuration.

The switching elements (Q1, Q2, Q3, and Q4) are usually bi-polar or FET transistors, in some high-voltage applications IGBTs.

Integrated solutions also exist but whether the switching elements are integrated with their control circuits or not is not

relevant for the most part for this discussion. The diodes (D1, D2, D3, and D4) are called catch diodes and are usually of a

Schottky type. The top end of the bridge is connected to a power supply (battery for example) and the bottom end is

grounded. In general, all four switching elements can be turned on and off independently, though there are some obvious

restrictions.

The H bridge circuit is shown below figure.

                                                               

a) Explain ‘Speed control of a DC motor using BJT H-bridge

Firstly open the Matlab and then enter the command power_Hbridge to open the Demo model of Speed control of a DC motor using BJT H-bridge.

after giving the command power_Hbridge, the demo model simulation window will open.

OPERATION:

If the switches Q1 and Q4 are turned ON the left lead of the motor will be connected to the power supply.

Current starts flowing through the motor which energies it in the forwarding direction and the motor starts spinning. 

                                                                     

If Q2 and Q3 are turned ON, then the reverse will happen.

The motor is energized in the reverse direction and the shaft will start spinning in the backward direction.

                                                                   

In a bridge, we should never ever close both Q1 and Q2 or Q3 and Q4 at the same time.

If we did that, a low resistance path is created between power and GND effectively short-circuiting the power supply.

This condition is called Shoot through the condition. It will quickly damage or destroy the bridge.

                                                                    

Adding diodes across the switches is protect the electricity that may flow backward from the motor which makes damage to the transistor. When we disable the transistors to stop running the motor the energy needs to be released. If diodes are connected in the reverse direction to the transistors, then it will give a path for the current to release this energy. Without them, the voltage rises and damages the transistors.

From the demo Simulink model, The IGBT Switch is controlled by a Simulink signal that is 1 or 0. The DC motor uses in the pre-set model which is 5HP, 24V, 1750RPM. It simulates the load as fan type and where the load torque is directly proportional to v^2. The armature voltage is varied from 0-240V when the duty cycle is varied from 0% to 100%. The duty cycle value is specified in the Pulse Generator block.

This demo Simulink model is run for 1 second with default values. 

Total Cycle Time =1 second

Duty Cycle = 75%

The result is shown below plots.

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

In this demo model, Pulse Generator is used to change the input of Amplitude and Duty Cycle, according to the input values signal is generated and given to the Switches. The above demo Simulink model is run with a total cycle time of 1 second. and Duty Cycle at 50%. Remain parameters values are the same. After the simulation run through 1 second. then the results are as follows in the below figures.

Case-1:

Total cycle time = 1

Duty Cycle = 50%

From 0 to 0.5 sec the IGBT Switches Q1, and Q4 are turned ON and the Q2 and Q3 are turned OFF. In this condition, the motor will run in a forward or clockwise direction.

Then 0.5 to 1 sec the IGBT Switches Q2, and Q3 are turned ON and Q1, and Q4 are turned OFF. In this condition, the motor will run in the reverse or anti-clockwise direction.

In the above figure, we can see the IGBT and DIODE Current plots. From this scope, we can observe the diode currents will shoot up a spike after 0.5 seconds to 1 second in the plot.

From 0 to 0.5 seconds diode currents are does not shoot up any spikes.

The IGBT switches Q1, and Q4 are turned ON in the forward biased condition and then Q2 and Q3 are turned ON in the reverse biased condition. When the circuit is in forward biased condition from 0 to 0.5 seconds armature current shoots up, and speed and torque are also raised along with armature currents in a positive direction. After 0.5 to 1 second, the motor would run in a negative direction, speed, torque, and armature currents are run in the negative region.

Case-2:

In this case, the Duty Cycle is changed from 50% to 30% in the pulse generation block. then run the simulation.

Total Cycle Time = 1

Duty Cycle = 30%

When the duty cycle is 30% the sudden shoot-up in the armature current at the 0 to 0.5-sec start of the motor is reduced

after 0.5 seconds. The armature current is constant at 5A in positive and negative directions.

c) Refer to the help section of ‘The four-quadrant chopper DC drive (DC7) block’. Compare it with the H-bridge model.

What is a chopper:

A chopper is a basically static power electronics device that converts fixed DC voltage/power to variable DC voltage or power.

It is nothing but a high-speed switch that connects and disconnects the load from the source at a high rate to get variable or chopped voltage at the output.

                                               

Low power applications: GTO, IGBT, Power BJT, Power MOSFET, etc.

High power application: Thyristor or SCR.

The four-quadrant chopper DC drive (DC7):

A four-quadrant chopper is a chopper that can be operated in all four quadrants. The power can flow either from source to

load or load to source in this chopper. In the first quadrant, a Class-E chopper acts as a Step-down chopper whereas in the

second quadrant it behaves as a Step-up chopper. This type of chopper is also known as Class-E or Type-E chopper.

 Block diagram of four-quadrant chopper DC drive (DC7):

To open the dc7 reference model enter the command dc7_example in the command window.

Then the simulation model will open.

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 regulatory 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 behavior, 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.

Quadrant -1 operation when switch s1 turned on:

When the switch s1 is operated, switches s1 and s4 conduct, the output voltage Vo and the output current io both are positives, power flows from source to load and inductor stores energy, the motor rotates in the forward direction hence called forward motoring.

Quadrant -1 operation when switch s1 turned off:

Switch s1 turned off but switch s4d and diode D2 conducts, output current io is positive and the output voltage Vo becomes zero, inductor release energy and freewheeling action using diode D2 takes place, the motor rotates in the forward direction hence called forward motoring.

Quadrant -2 operation when switch s2 turned on:

Let us assume that the motor is running in the forward direction. Switch s2 operated, switch s2 and diode D4 conducts, the output voltage Vo is zero and Eb is responsible for the negative output current io, machine behave as generator and inductor stores energy.

Quadrant -2 operation when switch s2 turned off:

Switch s2 turned off, diode D1 and diode D4 conducts, output voltage Vo becomes positive and the output current io is negative, inductor release energy using diodes D1 and D4, power flows from load to source and hence called as reverse braking.

Quadrant -3 operation when switch s3 turned on:

The polarity of back emf Eb must be reversed. Here switch S3 operated, switches S3 and S2 conducts, the output voltage Vo and the output current io both are negatives, power flows from source to load and inductor stores energy the motor rotates in the reverse direction hence called as reverse motoring.

Quadrant -3 operation when switch s3 turned off:

The polarity of back emf Eb must be reversed. Switch s3 turned off but switch s2 and diode D4 conducts, output current io is negative and the output voltage Vo becomes zero, inductor release energy and freewheeling action using diode D4 takes place, the motor rotates in the reverse direction hence called as Reverse motoring.

Quadrant -4 operation when switch s4 turned on:

The polarity of back emf Eb must be reversed. Let us assume that the motor is running in the reverse direction. Here switch s4 operated, switches s4 and diode D2 conduct, the output voltage Vo is zero and Eb is responsible for the positive output current io, machine behave as generator and inductor stores energy.

Quadrant -4 operation when switch s4 turned off:

The polarity of back emf Eb must be reversed. Switch s4 turned off, diode D2 and diode D3 conducts, output voltage Vo becomes negative and output current io is positive, inductor release energy using diodes D2 and D3, power flows from load to source and hence called as reverse braking.

Comparison of H-bridge vs Four-Quadrant chopper DC7:

Four-Quadrant chopper (DC7):

1. Four-quadrant chopper can operate in all four modes (forward motoring, forward braking, reverse motoring, and reverse braking).

2. In four-quadrant chopper regenerative braking is possible.

3. In a four-quadrant chopper two PI regulator (speed regulator and current regulator) is used to obtain the electromagnetic torque needed to reach the desired speed and control the armature current and stabilize the system

4. a Four-quadrant chopper is an electronic switch that is used to interrupt one signal under the control of another

5. four-quadrant chopper circuit is used to refer to numerous types of electronic switching devices and circuits used in power control and signal applications.

6. Four-quadrant chopper model gives better efficiency and high energy(power) storage.

H-bridge:

1. H- bridge can operate in two modes (forward and reverse).

2. But h-bridge regenerative braking is not possible.

3. H-bridge model PI regulator is not used.

4. H-bridge is an electronic circuit that switches the polarity of the voltage applied to the load.

5. H-bridge circuits are often used in Robotics and other applications to allow the dc motor to run forward and backward.

OBJECTIVE-2:

A two-quadrant chopper is a category of chopper that can operate in the first as well as a second quadrant. This basically means that the power can either flow from source to load or load to source in this chopper. This kind of chopper is also known as the Two quadrant Class-A chopper.

Class-A and Class-B choppers operate in the first and second quadrant respectively. we connect both these types of choppers in parallel then it is possible to have a chopper operation in the first as well as a second quadrant.

Type-C chopper is obtained by the parallel connection of Class-A and Class-B chopper.

Chopper CH1, free-wheeling diode (FD), and load are forming Class-A Chopper whereas chopper CH2, D2, and Load are forming Class-B chopper. Both these choppers are connected in parallel. To obtain the first quadrant operation we should switch ON chopper CH1 and for getting the second quadrant operation we should switch ON chopper CH2.

Case-1: When CH1 is switched ON / OFF

When chopper CH1 is switched ON, source V_s directly gets connected to the load and hence, load voltage V_o is equal to source voltage. The direction of load current is from source to load as shown in the circuit diagram which is assumed positive.

When CH1 is switched OFF, the free-wheeling diode FD comes into the circuit as it gets forward biased and hence shorts the load. Therefore, the output voltage V_o becomes zero. However, the i_o continues to die down through the FD and L in the same direction as shown in the circuit diagram. Thus, the average output voltage V_o and current I_o are positive and hence the operation of the chopper is in the first quadrant. In fact, this is the Class-B mode of operation.

Case-2: When CH2 is switched ON / OFF

When chopper CH2 is switched ON, load DC source E drives current through CH2 and load. The direction of this current i_o will be opposite to that shown in the circuit diagram and hence is assumed negative. Output voltage V_o is zero during this time. When CH2 is made OFF, diode D2 gets forward biased and hence the current into the source from the load. The output voltage is V_s in this time as the load is connected to the source through D2 during the OFF time of chopper CH2. Thus, the load current is always negative i.e., the operation of the chopper is within the second quadrant. In fact, this is the Class-B mode of operation.

From the above two cases, we can conclude the following points:

• The output voltage V_o is zero when chopper CH2 is ON or free-wheeling diode FD conducts.
• The output voltage V_o is equal to source voltage V_s when chopper CH1 is ON or diode D2 conducts.
• The load current flows in the direction shown in the circuit diagram i.e.i_ois positive when CH1 is ON or FD conducts.
• The load current flows opposite to the direction shown in the circuit diagram i.e.i_ois negative when CH2 is ON or D2 conducts.

The average load voltage is always positive but the average load current may be positive or negative. Therefore, power flow may be from the source to load (first quadrant operation) or load to the source (second quadrant operation). The operating region of Class-C or Type-C chopper is shown below by hatched area.

Two-Quadrant Class-A Chopper Simulink model:

Two-Quadrant chopper is developed in Simulink as shown below figure.

To develop the Two-Quadrant chopper we use the DC VOLTAGE SOURCE’S, INDUCTOR, MOSFET, DIODE, NOT GATE, PULSE GENERATOR, VOLTAGE MEASUREMENT, CURRENT MEASUREMENT, and SCOPE blocks.

MOSFET:

When we open the MOSFET block all parameters are needed to be unchanged and then the measurement port should be unchecked.

DIODE:

When we open the DIODE block all parameters are needed to be unchanged and then the measurement port should be unchecked.

PULSE GENERATOR BLOCK:

In the Pulse Generator Block, Amplitude, time period, and pulse width parameters are changed as shown below.

Amplitude = 10,

Time Period = 0.02 seconds,

Pulse Width = 50

The main DC voltage source is taken as 24v amplitude, Load side DC voltage source is 12v amplitude. A series RLC branch is taken and select an inductor. The Inductor is connected in series with load side DC voltage source. a voltage measurement block is connected to the load voltage source negative terminal and inductor positive terminal. The current measurement block is connected to the diodes and MOSFET bridges to measure the current rating, and then the scope is having three input connections, pulse generator, current measurement, and voltage measurement. to analyze the results of input and output.

Main DC Voltage source:

Load side DC Voltage source:

SCOPE:

From the above scope, we can observe the Pulse generator, current, and voltage measurement blocks output data.

When the chopper ch1 is switched on source Vs directly gets connected to the load and load voltage Vo is equal to source voltage.

The direction of load current is from source to load shown in the circuit diagram get froward biased and hence shorts the load. Therefore, the output voltage Vo becomes zero.

When chopper ch2 is switched on load dc source E drives current through ch2 and load.

The direction of this current ratio will be opposite to that shown in the circuit diagram and hence is assumed negative.

Output voltage vo is zero during this time.

When ch2 is made off, diode d2 gets forward biased and hence the current into the source from the load.

OBJECTIVE-3:

What is a Brushless DC motor?

A brushless DC motor is a permanent magnet synchronous electric motor. which is driven by direct current (DC) electricity and it accomplishes an electronically controlled commutation system instead of a mechanical commutation system. BLDC motors are also referred to as trapezoidal permanent magnet motors.

wherein the brushes make the mechanical contact with a 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 a permanent magnet rotor and a stator with a sequence of coils. In this motor, a permanent magnet rotates and current-carrying conductors are fixed.

The armature coils are switched electronically by transistors or silicon-controlled at the correct rotor position in such a way that the 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 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:

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. The three-phase BLDC motors with permanent magnet rotors are most commonly used.

The construction of this motor has many similarities to a three-phase induction motor as well as a conventional DC motor. This motor has stator and rotor parts like all other motors.

The stator of a BLDC motor is made up of stacked steel laminations to carry the windings. These windings are placed in slots that 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 a 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

Rotor:

BLDC motor incorporates a permanent magnet in the rotor. The number of poles in the rotor can vary from 2 to 8 pole pairs with alternate south and north poles depending on the application requirement. In order to achieve maximum torque in the motor, the flux density of the material should be high. A proper magnetic material for the rotor is needed to produce the required magnetic field density.

Ferrite magnets are inexpensive; however, they have a low flux density for a given volume. Rare earth alloy magnets are commonly used for new designs. Some of these alloys are Samarium Cobalt (SmCo), Neodymium (Nd), and Ferrite and Boron (NdFeB). The rotor can be constructed with different core configurations such as the circular core with a permanent magnet on the periphery, a circular core with rectangular magnets, etc.

 

Hall Sensors:

Hall sensor provides the information to synchronize stator armature excitation with rotor position. Since the commutation of the BLDC motor is controlled electronically, the stator windings should be energized in sequence in order to rotate the motor. Before energizing a particular stator winding, acknowledgment of the rotor position is necessary. So, the Hall Effect sensor embedded in the stator senses the rotor position.

Most BLDC motors incorporate three Hall sensors which are embedded into the stator. Each sensor generates Low and High signals whenever the rotor poles pass near to it. The exact commutation sequence to the stator winding can be determined based on the combination of these three sensors’ responses.

 

 

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 is placed in a magnetic field it experiences a force. As a consequence of the 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 the 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 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. 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 it. These signals determine the position of the shaft.

Brushless DC Motor Drive:

    

The electronic controller circuit energizes appropriate motor winding by turning transistors or other solid-state switches to rotate the motor continuously. 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. The switching for a rated speed of the motor, and additional electronic circuitry change the motor speed based on the 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

• 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 an operation (or low noise) due to the absence of brushes

Disadvantages of Brushless Motor:

• These motors are costly
• 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 for additional sensors