Week-7 Challenge: DC Motor Control

Aim1:

Introduction:

Bipolar Junction Transistor

A bipolar junction transistor (BJT) is a type of transistor that uses both electrons and electron holes as charge carriers. In contrast, a unipolar transistor, such as a filed-effect transistor, uses only one kind of charge carrier. A bipolar transistor allows a small current injected at one of its terminals to control a much larger current flowing between two other terminals, making the device capable of amplification or switching.

BJTs use two junctions between two semiconductor types, n-type and p-type, which are regions in a single crystal of material. The junctions can be made in several different ways, such as changing the doping of the semiconductor material as it is grown, by depositing metal pellets to form alloy junctions, or by such methods as diffusion of n-type and p-type doping substances into the crystal. The superior predictability and performance of junction transistors soon displaced the original point contact transistor. Diffused transistors, along with other components, are elements of integrated circuits for analog and digital functions. Hundreds of bipolar junction transistors can be made in one circuit at very low cost.

Bipolar transistor integrated circuits were the main active devices of a generation of mainframe and mini computers, but most computer systems now use integrated circuits relying on filed effect transistors. Bipolar transistors are still used for amplification of signals, switching, and in digital circuits. Specialized types are used for high voltage switches, for radio-frequency amplifiers, or for switching heavy currents.

Power MOSFET (Metal Oxide Semiconductor Field-Effect Transistor)

A power MOSFET is a specific type of Metal Oxide Semiconductor Field-Effect Transistor (MOSFET) designed to handle significant power levels. Compared to the other power semiconductor devices, such as an insulated gate bipolar transistor (IGBT) or a thyristor, its main advantages are high switching speed and good efficiency at low voltages. It shares with the IGBT an isolated gate that makes it easy to drive. They can be subject to low gain, sometimes to a degree that the gate voltage needs to be higher than the voltage under control.

The design of power MOSFETs was made possible by the evolution of MOSFET and CMOS technology, used for manufacturing integrated circuits since the 1960s. The power MOSFET shares its operating principle with its low-power counterpart, the lateral MOSFET. The power MOSFET, which is commonly used in power electronics, was adapted from the standard MOSFET and commercially introduced in the 1970s.

The power MOSFET is the most common power semiconductor device in the world, due to its low gate drive power, fast switching speed, easy advanced paralleling capability, wide bandwidth, ruggedness, easy drive, simple biasing, ease of application, and ease of repair. In particular, it is the most widely used low-voltage (less than 200 V) switch. It can be found in a wide range of applications, such as most power supplies, DC to DC converter, low-voltage motor controllers, and many other applications.

 

IGBT (Insulated Gate Bipolar Transistor)

An insulated-gate bipolar transistor (IGBT) is a three-terminal power semiconductor device primarily used as an electronic switch, which, as it was developed, came to combine high efficiency and fast switching. It consists of four alternating layers (P–N–P–N) that are controlled by a metal oxide semiconductor (MOS) gate structure.

Although the structure of the IGBT is topologically the same as a thyristor with a "MOS" gate (MOS-gate thyristor), the thyristor action is completely suppressed, and only the transistor action is permitted in the entire device operation range. It is used in switching power supplies in high-power applications: variable-frequency drives (VFDs), electric cars, trains, variable-speed refrigerators, lamp ballasts, arc-welding machines, and air conditioners.

Since it is designed to turn on and off rapidly, the IGBT can synthesize complex waveforms with pulse width modulation and low-pass filters, so it is also used in switching amplifiers in sound systems and industrial control systems. In switching applications modern devices feature pulse repetition rates well into the ultrasonic-range frequencies, which are at least ten times higher than audio frequencies handled by the device when used as an analog audio amplifier. As of 2010, the IGBT is the second most widely used power transistor, after the power MOSFET.

H Bridge

An H-bridge is an electronic-circuits that switches the polarity of a voltage applied to a load. These circuits are often used in robotics and other applications to allow DC motors to run forwards or backwards.

Most DC-to-AC converters (power inverters), most AC/AC converters, the DC-to-DC push-pull converter, isolated DC to DC converter most motor controllers, and many other kinds of power electronic use H bridges. In particular, a bipolar stepper motor is almost always driven by a motor controller containing two H bridges.

H-bridges are available as integrated circuits, or can be built from discrete components.

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 the switches S3 and S4. This condition is known as shoot-through.

Static Operation:

The basic operating mode of an H-bridge is fairly simple: if Q1 and Q4 are turned on, the left lead of the motor will be connected to the power supply. While the right lead is connected to the ground. Current starts flowing through the motor which energizes the motor in the forward direction and the motor shaft starts spinning.

If Q2 and Q3 are turned on, the reverse will happen, the motor gets energized in the reverse direction and the shaft will start spinning backwards.

In a bridge never close both Q1 and Q2 (or Q3 and Q4) at the same time. If did then, a really low-resistance path between power and GND is created, resulting in the short-circuiting the power supply. This condition is called shoot through.

Q1) MATLAB demo model ‘Speed control of a DC motor using BJT H-bridge’:

Blocks Used:

IGBT

The IGBT block implements a semiconductor device controlled by the gate signal. The IGBT is simulated as a series combination of a resistor Ron, inductor Lon, and a DC voltage source Vf in series with a switch controlled by a logical signal (g>0 or g = 0). The IGBT block contains a series Rs-Cs snubber circuit, which is connected in parallel with the IGBT device (between terminals C and E)

When V_CE > 0, V_CE > V_f, g > 0 → On state

When, V_CE > 0, g = 0 → OFF state

The Bipolar Junction Transistor (BJT) when used for power switching applications, operates as an IGBT. When it is conducting a forward voltage Vf is developed between collector and emitter (in the range of 1V). Therefore, the IGBT block can be used to model the BJT device.

Diode

The diode block models a piecewise linear diode. If the voltage across the diode is bigger than the forward voltage parameter value, then the diode behaves like a linear resistor with low resistance, given by the On resistance parameter value, plus a series voltage source. If the voltage across the diode is less than the forward voltage, then the diode behaves like a linear resistor with low conductance given by the Off conductance parameter value.

DC Machine

The DC machine block implements a wound-field or permanent magnet DC machine. This block consists of 3 segments-armature circuit, field circuit and a torque-measurement signals output. The motor used here is 5HP, 240V, 1750 RPM. It simulates a fan type model, proportional to the square of speed. The armature voltage can be varied from 0 to 240 V by charging the duty cycle. The duty cycle can be changed by a pulse width generator block.

Here the DC machines is used as a DC motor. We need to control the DC motor’s rotation in such a way that when to spin ACW and when to spin CW and variable speeds. For that, we use a H-bridge configuration of circuits. The name H-bridge is given as the circuit resembles the letter H

BE	BC	Mode	Operation
Reverse	Reverse	Cut off	Switch (off)
Forward	Forward	Forward Active	Amplifier
Forward	Forward	Saturation	Switch (on)

From the above table it is clear that, when the BJT is in saturation or conducting state, the Base-Emitter and Base-Collector is forward bias. Hence the BJT act in On state. The BJT is controlled by SIMULINK signals as 1 and 0. Logic ‘1’ represents a higher voltage, such as 5 volts, which is commonly referred to as a high value, while a logic ‘0’ represents a low voltage, such as 0 volts or ground, and is commonly referred to as low value. But to make these 5 volts as 1 volt, we have used IGBT block instead of BJT. The H-bridge consists of 4 transistors, namely Q1, Q2, Q3 and Q4. Each one has a snubber circuit connected in parallel. The H bridge works in such a way that at a time 2 IGBT and 2 diodes are in On state. When the IGBT’s Q2 and Q3 are in conducting state and the diodes D1 and D4 act as freewheeling diodes, makes the motor spin in one direction. When the IGBT’s Q1 and Q4 are in conducting state and the diodes D2 and D4 act as freewheeling diodes, makes the motor spin in opposite direction. The motor speed can be controlled by the self-excitation voltage provided across the rotor windings (i.e. 0 to 300V)

Case 1:

• Total cycle time = 1 sec
• Duty cycle = 75 %
• Forward motoring time = 0.5 sec
• Reverse motoring time = 0.5 sec

Output:

 

Q1-B) Comment on the armature current shoot up from the scope results

In the above plot, the armature current has a sudden rise and drop when the motor changes direction from forward to reverse motion.

A pulse width modulation signal is a method for generating analog signal using a digital source and a frequency. By cycling a digital signal OFF and ON at a fast enough rate, and with a certain duty cycle, the output will appear to behave like a constant voltage and current analogue signal when providing power to devices.

We can see the first plot of speed, starts from the positive side then runs constantly at that rpm and then suddenly drops and spin in opposite direction. Simultaneously the armature current also spikes up initially at the start and at the reversal of armature voltage causing the rotor to spin in opposite direction. This happens because, by default the armature voltage control is set at a step time of t = 0.5. It means at t = 0.5s the motor rotation will change in the reverse direction as the armature voltage will reverse.

To minimize the sudden surge in the armature current at the start and when the motor runs in reverse, we have to modify the duty cycle. The default value is 75 %. This means that the voltage is supplied 75% of the time to the motor and the rest 25 %, the motor receives no voltage from the source and it is in a OFF state. This sudden change in OFF and ON state causes the armature current to spike up abruptly.

Case2:

• Total cycle time = 1 sec
• Duty cycle = 60 %
• Forward motoring time = 0.5 sec
• Reverse motoring time = 0.5 sec

 

Output:

When the duty cycle is 60 %, there is a sudden spike of armature current.

 

Case3:

• Total cycle time = 1 sec
• Duty cycle = 50 %
• Forward motoring time = 0.5 sec
• Reverse motoring time = 0.5 sec

 

Output:

When the duty cycle is 50 % there is again a sudden spike in armature current when the armature voltage reverses at t = 0.5s. but it is not at the start of the motor.

Q1-C) Refer to the help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H bridge model.

The ‘DC7’ motor drive in Simscape ‘Power Systems’ software, referred to as the four quadrant chopper DC drive block in MATLAB/SIMULINK. It represents a four-quadrant, DC-supplied chopper drive for DC motors, featuring closed-loop speed control with the four-quadrant operation. This closed-loop speed control gives the armature current of the machine as an output. By using a PI current controller and adjusting the chopper duty cycle, we get the command armature current as per out needs. The duty cycle is then compared with a sawtooth carrier signal to obtain the required PWM signals for the chopper.

Compared to the DC drives that are available, the main advantage of using the four-quadrant chopper DC drive is that it can operate in all 4 quadrants.

Function	Quadrant	Speed	Torque	Power Output
Forward motoring	I	+	+	+
Forward braking	II	+	-	-
Reverse motoring	III	-	-	+
Reverse braking	IV	-	+	-

 

Applications of 4-quadrant operation:

• Compressor, pump and fan type load requires operation in I quadrant only. As their operation is unidirectional, they are called one quadrant drive system.
• Transportation drives require operation in both direction
• If refrigeration is necessary, application in all four quadrants may be required. If not then the operation is restricted to quadrant I and III, and thus dynamic braking or mechanical braking may be required.
• In hoist drives, a four-quadrant operation is needed.

 

OUTPUT:

The scope shows the motor armature voltage and current, the four IGBT pulses and the motor speed on the scope. The current and speed references are also shown. The motor is coupled to a linear load, which means that the mechanical torque of the load is proportional to the speed.

Comparison:

DC drive using BJT H-bridge	DC drive using 4-quadrant chopper
Power switches are powered using 240 V DC supply	Power switches are powered by a 380 V 50 Hz AC 3-phase supply which is converted to a 515 V DC supply
Field windings are separately excited by a 300 V DC supply	Field winding s are separately excited by a 150 V DC supply
Works only in forward motoring and reverse motoring	It operates in all 4 quadrants.
It is just controlling the DC motor speed	It is a drive that symbolizes the mechanical and power electronics components
Efficiency is less	Efficiency is more due to being a drive
Here the torque is proportional to the square of the speed	Here the motor is subjected to linear load. Hence the torque is just proportional to speed

 

Aim2: 

Q2) Develop a 2-quadrant chopper using Simulink and explain the working of the same with the relevant results

 Circuit diagram of 2-quadrant chopper

SIMULINK Model:

Construction:

• Arrange the blocks as per the circuit diagram
• Double click on MOSFET and DIODE block to disable (uptick show measurement port), and rotate the diode in upward direction as per our requirement (source should be facing in downward direction for MOSFET and for DIODE source should be in upward direction)
• Open series RLC branch block and change the branch type to (L) with default value 1 mill H, once done arrange according to circuit diagram.
• Open logic operator and select NOT in operation options, connect its input to output of PULSE Generator block. The reason we use NOT gate to ensure both the switches does not conduct simultaneously if they do so supply will be short-circuited
• We will be giving the tapping for the pulse gate terminal for 1 switch MOSFET and for NOT gate we are giving it to other switch MOSFET1. So, if MOSFET is conducting MOSFET1 will not conduct because we are using the NOT gate.
• Once done we will be measuring the voltage by connecting its 1^st port at starting of inductor and other to DC2 supply which is connected adjacent.
• And output is connected to 1^st of the 3 port of scope input. Current measurement port is also connected to 2^nd port of scope and 3^rd port is taken from output of pulse generator
• Double click on main DC voltage supply and set amplitude (V) to 24 V and for adjacent DC2 supply set amplitude as 12 V.
• Open pulse generator block set amplitude to 10 for better viewing, time period set to 1sec, pulse width is 50 (because we need 50 % to ON and 50 % to OFF) i.e., NOT gate is ON when 50% OFF.

Working:

Type C chopper is obtained by connecting type-A and type-B choppers in parallel. We always get a positive output voltage V₀ as the freewheeling diode FD is present across the load. When the chopper is on the freewheeling diode starts conducting and the output voltage V₀ will be equal to V_s. The direction of the load current i₀ will be reversed. The current i₀ will be flowing towards the source and it will be positive regardless of the chopper is on or the FD conducts. The load current will be negative if the chopper is or the diode D2 conducts. We can say the chopper and FD operate together as a type-A chopper in the first quadrant. In the second quadrant, the chopper and D2 will operate together a type-B chopper.

The average voltage will be always positive but the average load current might be positive or negative. The power flow may be like the first quadrant operation i.e. from source to load or from load to source as the second quadrant operation. The two choppers should not be turned on simultaneously as the combined action may cause a circuit in supply lines. For regenerative braking and motoring, this type of chopper configuration is used.

Let the voltage of the DC voltage source be 24 volts

For second DC voltage source block, set amplitude to 12 V.

Block parameter of Pulse Generator are as,

 

OUTPUT:

 

Aim3:

Q3) 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 closed loop controller to switch DC currents to the motor windings 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 current 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 reluctance motor, or an induction (synchronous) motor. They may also use neodymium magnets and be outrunners (the stator is surrounded by the rotor), inrunners (the rotor is surrounded by the stator), or axial (the rotor and stator are flat and parallel).

The advantages of a brushless motor over brushed motors 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 replacement of rubber belts and gearboxes by a direct-drive design.

Construction:

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 rotor are most commonly used.

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

Stator of a BLDC motor made up of stacked steel laminations to carry the windings. These windings are placed in slots which are axially cut along the inner periphery of the stator. These windings can be arranged in either star or delta. However, most BLDC motors have three phase star connected stator.

Each winding is constructed with numerous interconnected coils, where one or more coils are placed in each slot. In order to form an even number of poles, each of these windings is distributed over the stator periphery.

The stator must be chosen with the correct rating of the voltage depending on the power supply capability. For robotics, automotive and small actuating applications, 48 V or less voltage BLDC motors are preferred. For industrial applications and automation systems, 100 V or higher rating motors are used.

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, etc.

Hall Sensors

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 response.

 

Working Principle and Operation of BLDC Motor

BLDC motor works on the principle similar to that of a conventional DC motor, i.e., the Lorentz force law which states that whenever a current carrying conductor placed in a magnetic field it experiences a force. As a consequence of reaction force, the magnet will experience an equal and opposite force. In case BLDC motor, the current carrying conductor is stationary while the permanent magnet moves.

When the stator coils are electrically switched by a supply source, it becomes electromagnet and starts producing the uniform field in the air gap. Though the source of supply is DC, switching makes to generate an AC voltage waveform with trapezoidal shape. Due to the force of interaction between electromagnet stator and permanent magnet rotor, the rotor continues to rotate.

Consider the figure below in which motor stator is excited based on different switching states. With the switching of windings as High and Low signals, corresponding winding energized as North and South poles. The permanent magnet rotor with North and South poles align with stator poles causing motor to rotate.

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 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 that Hall sensors give shaft position feedback to the electronic controller unit.

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 shaft.

 

Brushless DC Motor Drive

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

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

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

 

Advantages of BLDC Motor

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

 

Disadvantages of Brushless Motor

• These motors are costly
• Electronic controller required control this motor is expensive
• Not much availability of many integrated electronic control solutions, especially for tiny BLDC motors
• Requires complex drive circuitry
• Need of additional sensors

 

Applications of BLDC Motor

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 are

• 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