Week-7 Challenge: DC Motor Control

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

B. Comment on the armature 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.

Answer:-

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

H Bridge

The term H-bridge is derived from the typical graphical representation of such a circuit. An H-bridge is built with four switches (solid-state or mechanical). When the switches S1 and S4 (according to the first figure) are closed (and S2 and S3 are open) a positive voltage is applied across the motor. By opening S1 and S4 switches and closing S2 and S3 switches, this voltage is reversed, allowing reverse operation of the motor. Using the nomenclature above, the switches S1 and S2 should never be closed at the same time, as this would cause a short circuit on the input voltage source. The same applies to switches S3 and S4. This condition is known as shoot-through.

Fig:- H bridge

Changing the polarity of the power supply to the DC motor is used to change the direction of rotation. Apart from changing the rotation direction, the H-bridge can provide additional operation modes, "brake" and "free run until frictional stop". The H-bridge arrangement is generally used to reverse the polarity/direction of the motor, but can also be used to 'brake' the motor, where the motor comes to a sudden stop, as the motor's terminals are shorted. In shorted case, the kinetic energy of a rotating motor is consumed rapidly in form of electrical current in the shorted circuit. The other case, is to let the motor 'free run' to a stop, as the motor is effectively disconnected from the circuit.

Fig:-Operation modes of H bridge

# Model Description

Fig:-Simulink Model

• The DC motor uses the preset model (5 HP 24V 1750 rpm). It simulates a fan type load (where Load torque is proportional to the square of speed).
• The armature means 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 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. 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.
• A Switch block at bottom of the circuit Splits the Power Supply into 4 parts namely they are [(p1) (p2) (p3) (p4]]. These inputs are in the Goto form block.
• That block is used to send a signal to "From Block" that have the specified tag. If the tag visibility is "scoped, then a Goto tag visibility block must be used to define the visibility of that particular tag. The block icon displays the selected tag name (local tag are enclosed in bracket " and Scoped tag names are enclosed in braces" 0)
• Therefore these 4 types of Input are connected to transistor Ql Q2, Q3, and Q4 through "From Block"; An "From block" Receives. the Signal from the "Goto Block" with the specified tag The block icon displays the selected tag name (local tags are enclosed in [] brackets and Scoped tag names are enclosed in brace{} brackets).

SWITCHES

Q1 and Q2=Forward Motoring

Q2 and Q3=Reverse Motoring.

• This formation is called an H-Bridge. An H-bridge is an arrangement of transistors that allows a circuit full control over a standard Electric DC Motor
• The DC Motor is connected to the battery and the Mechanical Input of the DC Motor is Initiated as torque.
• Load Torque Proportional to Square of Speed

T=k* ω^2

Where,

T= Load torque

ω = Speed

• 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 from rad/sec to rpm is held between the BUS sectors to scope using a converter block.

Simulation Results

Fig:-Simulink Results

• In the armature current graph, we can see that at starting there is a sudden spike increase in current at 0sec and also at 0.5 sec.
• AS voltage changes the speed also changed.
• In the transition from positive to the negative direction of rotation we observe a high surge in current, this is due to the transient switching occurring in the circuit.

B)Armature current shoot-up from the scope results

• In 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 (step time), sudden shoot-up is 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.

Fig:-With 75% duty cycle

By changing the Duty cycle we can reduce the shoot up

• One simple and easy way to control the Speed of a Motor is to Regulate the Amount of Voltage across its Terminals and this can be achieved using "Pulse Width Modulation
• For Experimental Section we will reduce the Pulse width (Le refer as DUTY CYCLE) of the Pulse generator block by 50% at the same time period.
• The Inputs of Pt. P2, P3 and P4 are also changed according to the drive cycle. When the Qland Q4 switch is ON the Motor is generated as a Forwarding direction that state Q3 switch and Q2 the switch is in OFF.
• When the Q2 and Q3 switch is ON the Motor is generated as the reverse direction that state Q1 and Q4 switch is in OFF Condition
• The change made as 50% of Pulse width is Shown as below.

Fig:-With 50% Duty cycle

C)The Four-Quadrant Chopper DC Drive (DC7) block

Fig:-Four Quadrant chopper drive

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. 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 to obtain the electromagnetic torque needed to reach the desired speed. The speed reference change rate follows acceleration and deceleration ramps 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. To limit the amplitude of the current oscillations, a smoothing inductance is placed in series with the armature circuit.

Working of Four-Quadrant Chopper DC Drive

Fig:-Quadrant chopper

The Four-Quadrant chopper circuit diagram is shown in Figure 1. Here the motor load is assumed, R. and La armature resistance and inductance of the motor respectively. Eb is the back emf of the motor.

Quadrant I operation when Switch S1 turned on

Here switch S1, operated, Switches S1, and S4 conduct, output voltage Vo and the output current to 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 I operation when Switch S1, turned off

Switch S1, turned off but switch S4, and diode D2, conducts, the output current is i0 positive and the output voltage Vo becomes zero, inductor release energy and free-wheeling and action using diode D2 takes place, the motor rotates in the forward direction hence called forward motoring

Quadrant II operation when Switch S2, turned on

Let us assume that the motor is running in the forward direction. Here switch S2, operated Switch S2, and diode D4, conducts, the output voltage Vo is zero and Eb, is responsible for the negative output current (machine behave as gen generator and inductor stores energy)

Quadrant II 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 III 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 the inductor stores the motor rotates in the reverse direction hence called reverse motoring.

Quadrant III 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 lo is negative and the output voltage Vo becomes zero, inductor release energy and free-wheeling action using diode D4, takes place, the motor rotates in the reverse direction hence called Reverse motoring

Quadrant IV 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 conducts, 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 IV operation when Switch S4, turned off

The Four-Quadrant chopper equivalent circuit diagram for Quadrant is shown in Figure 10. The polarity of hack emf Eb must be reversed Switch S4 turned off, diode D2, and diode D3 conducts output voltage Vo becomes negative and output current io positive, inductor release energy using diodes D2, and D3, power flows from load to source and hence called a reverse braking

COMPARISON OF 4 QUADRANT CHOPPER AND BJT H-BRIDGE

1) The 4 quadrant chopper works in all four modes like

• forward motoring
• forward breaking
• reverse motoring
• reverse breaking

but the BJT h bridge model

• forward motoring
• reverse motoring

2)To change the direction of rotation of the motor from forward to reverse and vice versa, the current signals get transited from one to another switch while in the h bridge model, the direction of rotation of the motor is changed by changing the polarity of the current.

3)The 4 quadrant chopper has a high switching frequency while the h bridge model has a low switching frequency.

4)The armature current in the 4 quadrant chopper has a little spike at the transition from one to another switch because of the high switching frequency while the h bridge model has a high spike of current.

5)The 4 quadrant chopper is generally used in an industrial application while the h bridge model is 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 )

Answer:-

2)quadrant Chopper

A 2-quadrant Chopper is a category of the 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. In this article, we will discuss A 2-quadrant chopper, its working principle with the help of a circuit diagram and its application.

Operation / Working of A 2-quadrant Chopper

As we know that, a Class-A and Class-B chopper operates in the first and second quadrants respectively. Therefore, if we connect both these types of the chopper in parallel then it is possible to have chopper operation in the first as well as a second quadrant.In fact, A 2-quadrant chopper is obtained by the parallel connection of Class-A and Class-B choppers. The figure below shows the circuit diagram of this type of chopper.

Fig:-2 quadrant Chopper

Carefully observe the circuit diagram. You will notice that 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 forgetting the second quadrant operation we should switch ON chopper CH2. Let us discuss this in detail.

Case-1: When CH1 is switched ON / OFF

When chopper CH1 is switched ON, source Vs directly gets connected to the load and hence, load voltage Vo 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 Vo becomes zero. However, the io continues to die down through the FD and L in the same direction as shown in the circuit diagram. Thus, the average output voltage Vo and current Io 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 io 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. The output voltage is Vs 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. 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 Vo is zero when chopper CH2 is ON or free-wheeling diode FD conducts.
• The output voltage Vo is equal to source voltage Vs when chopper CH1 is ON or diode D2 conducts.
• The load current flows in the direction shown in the circuit diagram i.e.io is positive when CH1 is ON or FD conducts.
• The load current flows opposite to the direction shown in the circuit diagram i.e.io is 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 A 2-quadrant chopper is shown below by hatched area.

2-quadrant chopper using the Simulink model

Fig:-simulink model of 2 quadrant chopper

• let us take two MOSFET blocks and diodes blocks shown in the circuit diagram.The dc voltage source is connected as shown. let the voltage DC voltage source be 24 volts.

Fig:-Supply Voltage

• let us consider a series RLC branch block and select inductor L.DC voltage source of 12 volts is also connected in series with an inductor.

Fig:-Voltage Source

• The pulse generator block is used to create a gate pulse.
• The not gate is used in the model so that the two MOSFET will not get closed at the same time otherwise short circuit will occur due to the use of not gate, when one MOSFET is ON the other MOSFET will get off and when another MOSFET is ON the first MOSFET will get off
• The amplitude of pulse generator is kept 10 so that we can clearly see the pulses at the scope and let period be 0.02 seconds and let pulse width be 50%

Fig:-Pulse Generator

SIMULATION RESULT:

Fig:-Simulation result

• The voltage waveform is not going less than zero the minimum value of voltage is zero and the maximum value is always the positive direction.
• However, in the current waveform, the current increases and decreases the current may be positive as well as negative and the pulse amplitude is 10 thus from the pulse waveform we can see that the pulse amplitude is 10.
• When chopper CH1 is switched ON, source Vs directly gets connected to the load and hence, load voltage Vo 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 Vo becomes zero.
• When chopper CH2 is switched ON, load DC source E drives current through CH2 and load. The direction of this current io 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.

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

Answer:-

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

Unlike conventional brushed type DC motor, wherein the brushes make the mechanical contact with a commutator on the rotor 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 (or field poles) rotates and current-carrying conductors are fixed. The armature coils are switched electronically by transistors or silicon-controlled rectifiers 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 per cent, whereas brushed type DC motors are 75 to 80 per cent efficient. There are wide varieties of BLDC motors available ranging from small power range to fractional horsepower, integral horsepower and large power ranges.

Fig:- BLDC

Construction of Brushless DC motor

In this motor, the permanent magnets are attached to the rotor. The current-carrying conductors are located on the stator. They use electrical commutation to convert electrical energy into mechanical energy.The main design difference between brushed and brushless motors is the replacement of a mechanical commutator with an electric switch circuit. 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. A Brushless motor does not have any current-carrying commutator The field inside a brushless motor is switched through an amplifier which is triggered by the commutating device like an optical encoder.The layout of a DC brushless motor can vary depending on whether it is in “Outrunner” style or “Inrunner” style. Outrunner – The field magnet is a drum rotor that rotates around the stator. This style is preferred for applications that require high torque and where high rpm isn’t a requirement.In runner – The stator is a fixed drum in which the field magnet rotates. This motor is known for producing less torque than the outrunner style but is capable of spinning at very high rpm. Brushless DC Motors Introduction for Next-Generation Missile Actuation Systems Outline | Analog Devices

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 rotors are most commonly used. The construction of this motor has many similarities to the three-phase induction motor as well as the conventional DC motor. This motor has stator and rotor parts as like all other motors.

• Stator

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. 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.

• 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. 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, 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 to rotate the motor. Before energizing a particular stator winding, acknowledgement 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 sensor’s 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, 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 the permanent magnet rotor.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 align with stator poles causing the motor to rotate.

Fig:-Working of BLDC

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 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 to it. These signals determine the position of the shaft.

Brushless DC Motor Drive

As described above that the electronic controller circuit energizes appropriate motor winding by turning transistors or other solid-state switches to rotate the motor continuously. The figure below shows the simple BLDC motor drive circuit which consists of a MOSFET bridge (also called an 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.

Fig:-Driving of BLDC

In addition to the switching for a rated speed of the motor, additional electronic circuitry changes 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 Brushless DC motor

Less overall maintenance due to absence of brushes

Reduced size with far superior thermal characteristics

Higher speed range and lower electric noise generation.

It has no mechanical commutator and associated problems

High efficiency and high output power to size ratio 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

Low noise due to absence of brushes

Limitations of Brushless DC motor

These motors are costly

The electronic controller required to control this motor is expensive

Requires complex drive circuitry

Need of additional sensors

Applications of Brushless DC motor

Brushless DC motors (BLDC) use 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 equipment etc.

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