Week-7 Challenge: DC Motor Control

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

 

 

 

This model shows the simulation of an H-bridge used to generate a chopped voltage and to control the speed of a DC motor.

 

Here the power BJT switches act as chopper & by varying the duty cycle of the switches the speed of DC motor is controlled.

 

 

 

Description:

 

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 pulse generator block operating the switches with a duty cycle of 75%. 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 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 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.

 

Using Pulse generator block the signal input is divided into 4 inputs(P1,P2,P3,P4) which will be sent to 4 switches to operate them.

 

The fan load to the DC motor is created using square law block(where torque is directly proportional to square of the speed of motor).

 

The torque output is again given as a feedback to the motor for closed loop control of speed & torque supplied by the motor.

 

Simulation:

 

 

 

From the plot we can see that the speed of the motor(w) reverses its direction(becomes negative) after 0.8 seconds this is because the direction of current supplied to the motor is also reversed & the load torque also reverses it direction as the speed of the motor reverses.

 

Now we observe the IGBT & diode currents for Switch Q3 & Diode D3:

 

 

 

When Q3 & Q2 are conducting then the direction of motor rotation is reversed.

 

as from the plot we can see the current flows from battery to the Load(current is positive) & when the Switches Q1 & Q4(for forward rotation of motor) are switched off the diodes D2-D3 conduct as freewheeling diodes & in the above plot the diode current can be seen as positive.

 

Till 0.5 seconds the switch Q3 is OFF & Diode D3 is ON.

 

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

 

 

 

As the switches are operated at 75% duty cycle at an high frequency of 500Hz.The average voltage applied across the armature will be more & due to the transients between ON-OFF & OFF-ON states in high frequency there is spike in the armature current at the start of switching.

 

This spike of Armature current can be reduced by lowering the Duty cycle of switches.

 

Here i have reduced the Duty cycle to 60% & we can observe the spike current is lowered by half from previous observation

 

 

 

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

 

Four Quadrant operation of DC motors:

 

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 product of angular speed and torque is equal to the power developed by a motor. For the multi-quadrant operation of drives, the following conventions about the signs of torque and speed are used. When the motor is rotated in the forward direction the speed of the motor is considered positive. The drives which operate only in one direction, forward speed will be their normal speed.

 

In loads involving up and down motions, the speed of the motor which causes upward motion is considered to be in forward motion. For reversible drives, forward speed is chosen arbitrarily. The rotation in the opposite direction gives reverse speed which is denoted by a negative sign.

 

The rate of change of speed positively in the forward direction or the torque which provides acceleration is known as Positive motor torque. In the case of retardation, the motor torque is considered negative. Load torque is opposite to the positive motor torque in the direction.

 

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

 

 

 

 

 

In the I quadrant power developed is positive and the machine is working as a motor supplying mechanical energy. The I (first) quadrant operation is called Forward Motoring. II (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 kinetic energy of the rotating parts is available as electrical energy which may be supplied back to the mains. In dynamic braking, the energy is dissipated in the resistance. The III (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 IV (fourth) quadrant, the torque is positive, and the speed is negative. This quadrant corresponds to reverse braking mode.

 

4 quadrant operation in terms of Voltage & Current:

 

                                            

 

1st quadrant: voltage & current are positive & current flows from source to load anf the power is also positive.

 

2nd quadrant: voltage is positive but current is negative(current flos from load to source) and the power is negative. 

 

3rd quadrant: voltage and current both are negative but power is positive.

 

4th quadrant: Voltage is negative but current is positive and the power is negative.

 

 

 

4 quadrant chopper circuit:

 

 

 

A four quadrant chopper is a chopper which can operated in all the four quadrants. The power can flow either from source to load or load to source in this chopper. In first quadrant, a Class-E chopper acts as a Step-down chopper whereas in second quadrant it behaves as a Step-up chopper. This type of chopper is also known as Class-E or Type-E chopper.

 

The circuit of a four quadrant chopper or class-E chopper basically consists of four semiconductor switches CH1 to CH4 and four diodes D1 to D4. The four diodes are connected in anti-parallel. The circuit diagram of this type of chopper is shown below.

 

First Quadrant Operation:

For first quadrant operation, CH4 is kept ON, CH3 is kept OFF and CH1 is operated. When both CH1 & CH4 are ON simultaneously, the load gets directly connected to the source and hence the output voltage becomes equal to the source voltage. This essentially means that vo = vs. It may be noted that the load current flows from source to load as shown by the direction of io.

 

When CH1 is switched OFF, the load current free wheels through CH4 and D2. During this period, the load voltage and current remains positive.

 

Thus, both the output voltage vs and load current io are positive and hence, the operation of chopper is in first quadrant. It may be noted that, Class-E chopper operates as a step-down chopper in this case.

 

 

 

Second Quadrant Operation:

To obtain second quadrant operation, CH2 is operated while keeping the CH1, CH3 & CH4 OFF. When CH2 is ON, the DC source in the load drives current through CH2, D4, E and L. 

When CH2 is turned OFF, current is fed back to the source through D1, D4. It should be noted at this point that (E+Ldi/dt) is more than the source voltage Vs. As load voltage Vo is positive and Io is negative, it is second quadrant operation of chopper. Since, the current is fed back to the source, this simply means that load is transferring power to the source. Kindly read Step-up chopper for detailed analysis and better understanding.

 

For second quadrant operation, load must contain emf E as shown in the circuit diagram. In second quadrant, configuration operates as a step-up chopper.

 

 

 

Third Quadrant Operation:

To obtain third quadrant operation, both the load voltage and load current should be negative. The current and voltage are assumed positive if their direction matches with what shown in the circuit diagram. If the direction is opposite to what shown in the circuit diagram, it is considered negative. One important thing to notice is that the polarity of emf E in load must be reversed to have third quadrant operation. Circuit diagram of Class-E chopper for third quadrant operation is shown below.

 

For third quadrant operation, CH1 is kept off, CH2 is kept ON and CH3 is operated. When CH3 is ON, load gets connected to source and hence load voltage is equal to source voltage. But carefully observe that the polarity of load voltage vo is opposite to what shown in the circuit diagram. Hence, vo is assumed negative. Let us now see what is the status of load current io. It may be seen that io is flowing in the direction opposite to shown in the circuit diagram and hence negative.

 

Now, when CH3 is turned OFF, the negative load current free wheels through the CH2 and D4. In this manner, vo and io both are negative. Hence, the chopper operates in third quadrant.

 

 

 

Fourth Quadrant Operation:

To obtain fourth quadrant operation, CH4 is operated while keeping CH1, CH2 and CH3 OFF. The polarity of load emf E needs to be reversed in this case too like third quadrant operation.

 

When CH4 is turned ON, positive current flows through CH4, D2, L and E. Inductance L stores energy during the time CH4 is ON. When CH4 is made OFF, current is fed back to the source through diodes D2, D3. Here load voltage is negative but the load current is always positive. This leads to chopper operation in fourth quadrant. Here, power is fed back to the source from load and chopper acts as a step-up chopper.

 

                            

 

 

 

 

 

Description:

 

 

 

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 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. In order to limit the amplitude of the current oscillations, a smoothing inductance is placed in series with the armature circuit.

 

Comparing 4 Quadrant chopper Drive & H-Bridge for controlling DC motor:

 

  H-Bridge drive 4 quadrant chopper DC motor drive 

1 It operates the motor in only 2 quadrants(forward motoring & reverse motor[1/3, 8:42 PM] VIJAYS REALME C11 S: It operates the motor in only 2 quadrants(forward motoring & reverse motoring(It can only control the direction & speed of the motor) It operates the motor in only all 4 quadrants(forward motoring , forward braking, reverse motoring & reverse braking.

2 Regenerative braking cannot be achieved  Regenerative braking can be achieved

3 Less control of motor & vehicle Wide range control of motor & vehicl

4 The power source  is fixed DC supply The supply source is AC supply which is converted to DC using rectifie

5 Motor speed reference cannot be given to the power converter circuit & control Motor speed reference can be given to the power converter circuit & control for better following of the ideal speed & torque characteristic

6 Armature current shootup is more during switching transitions Armature current shootup is less during the transitions & for low voltages shootup is negligible

 2. 2-Quadrant chopper operation & simulation with Simulink

 

 

 

 

Circuit Operation

 

A 2 quadrant chopper operates in 1st quadrant(forward motoring) & 2nd quadrant(forward braking) modes & the current direction can be from source to load as well as load to source but the voltage accross the load is always maintained positive due to the presence of freewheeling diode D

 

This 2 quadrant chopper circuit is used for motoring & regenerative braking of DC motor

 

1. When CH2 is ON or Diode D2 conducts, the output voltage=0 & in case CH1 is ON or diode D1 conducts, the output voltage = Source voltag

 

2. Output current (load current) is positive when CH1 & D2 conduct & Output current is negative when CH2 & D1 condu

 

3. CH1 & D2 together operate as 1st quadrant chopper & CH2 & D1 together operate as 2nd quadrant choppe

 

4. The pulse generator produces periodic pulse with a duty cycle of 80% & period of 2sec.The pulse generator helps in switching the Choppers CH1 & CH2 periodically & NOT gate is used in order to prevent simulataneous switching ON of CH1 & CH2 because it will short circuit the load.When CH1 is ON CH2 is turned OFF & vice vers

 

Source voltage = 10

 

Load EMF = 5

 

Result

 

 

 

 

 

 

 

 

 3. Construction & operation of BLDC motors

 

Brushless DC motors (BLDC) are increasingly the preferred choice in many applications, especially in the field of motor control technology. BLDC motors are superior to brushed DC motors in many ways, such as ability to operate at high speeds, high efficiency, and better heat dissipatio

 

They are an indispensable part of modern drive technology, most commonly employed for actuating drives, machine tools, electric propulsion, robotics, computer peripherals and also in for electrical power generation. With the development of sensorless technology besides digital control, these motors become so effective in terms of total system cost, size and reliabilit

 

What is a Brushless DC motor (BLDC

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 fixe

 

The armature coils are switched electronically by transistors or silicon controlled rectifiers 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 

[1/3, 8:45 PM] VIJAYS REALME C11 S: 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 range

 

Construction of BLDC Mot

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 magne

 

rotor are most commonly use

 

 

                            

 

 

The construction of this motor has many similarities of three phase induction motor as well as conventional DC motor. Thi

 

motor has stator and rotor parts as like all other motor

 

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 stato

 

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 peripher

 

 

                                       

 

                                     

 

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 use

 

 

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 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 permanent magnet on the periphery, circular core with rectangular magnets, et

 

Hall Sensor

Hall sensor provides the information to synchronize stator armature excitation with rotor position. Since the commutation of 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 rotor position is necessary. So the Hall Effect sensor embedded in 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 respons

 

 

Working Principle and Operation of BLDC Moto

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 o

[1/3, 8:46 PM] VIJAYS REALME C11 S: 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 rotat

 

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 rotat

 

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 directio

 

 

 

 

                                 

 

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 that Hall sensors give shaft position feedback to the electronic controller uni

 

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 shaf

 

 

Brushless DC Motor Driv

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 circui

 

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 drive

 

Advantages of BLDC Mot

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

 

It has no mechanical commutator and associated proble

High efficiency due to the use of permanent magnet roto

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

Smaller motor geometry and lighter in weight than both brushed type DC and induction AC motor

Long life as no inspection and maintenance is required for commutator syste

Higher dynamic response due to low inertia and carrying windings in the stato

Less electromagnetic interferenc

Quite operation (or low noise) due to absence of brushe

Disadvantages of Brushless Moto

These motors are costl

Electronic controller required control this motor is expensiv

Not much availability of many integrated electronic control solutions, especially for tiny BLDC motor

Requires complex drive circuitr

Need of additional sensor

 

 

Applications of Brushless DC Motors (BLD

Brushless DC Motors (BLDC) are used 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 equipments, etc. 

Some specific applications of BLDC motors ar

 

Computer hard drives and DVD/CD playe

Electric vehicles, hybrid vehicles, and electric bicycle

Industrial robots, CNC machine tools, and simple belt driven system

Washing machines, compressors and dryer

Fans, pumps and blower.

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