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

An H-bridge is a simple circuit that lets us to control a DC motor to go backward or forward.

In general an H-bridge is a rather simple circuit, containing four switching element, with the load at the center, in an
H-like configuration:

The switching elements (Q1..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..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.
Though the load can in be anything we want, by far the most pervasive application if H-bridges is with a brushed DC
or bipolar stepper motor (steppers need two H-bridges per motor) load. In the following we will concentrate on
applications as a brushed DC motor driver.

Static Operation:
The basic operating mode of an H-bridge is fairly simple: if Ql 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 ground. Current starts flowing through the motor
which energizes the motor in (let's say) 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, you should never ever close both Ql and Q2 (or Q3 and Q4) at the same time. If you did that, you just
have created a really low-resistance path between power and GND, effectively short-circuiting your power supply.
This condition is called 'shoot-through' and is an almost guaranteed way to quickly destroy your bridge, or
something else in your circuit.

Matlab demo model named 'Speed control of a DC motor using BJT H-bridge’.

WORKING:
• In the above circuit, the input voltage is generated from the pulse generator where the amount of supply and duty cycle is assigned in the pulse generator. • Then the switch block is connected to pulse generator through an armature voltage control block. • The H - bridge consists of 4 BJT/ Diode pairs. Two transistors are switched simultaneously: Ql and Q4 or Q2 and
Q3.
• When QI and Q4 are fired a positive voltage is applied to motor and diodes D2-D3 operate as free-wheeling diodes when QI and Q4 are switched off.
• When Q2 and Q3 are fired a negative voltage is applied to motor and diodes DI-D4 operate as free-wheeling diodes when Q2 and Q3 are switched off.
• A switch block at bottom of circuit splits the power supply into 4 parts namely are pl, p2, p3, and p4.

SWITCHES:
Q1 and Q4 - Forward motoring
Q2 and Q3 - Reverse motoring • The DC motor is connected to a battery, and the DC motor's mechanical input is Initiated as torque. Torque for mechanical input of DC motor also switched by goto and from.
• The output of the DC motor is connected with the BUS sector whereas the speed is plotted in the scope. A conversion of speed for red/sec to rpm is between the BUS sector and the scope using the converter block.

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

answer:

 simulation results:

pulse width is set to 75

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

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

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

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

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

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

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

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

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

Now by changing pulse generation:

simulation results:

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

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

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

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

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

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

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

ANSWER:

The Four-Quadrant Chopper DC Drive (DC7) block represents a four-quadrant, DC-supplied, chopper (or DC-DC PWM converter) drive for DC motors. This drive features closed-loop speed control with four-quadrant operation. The speed control loop outputs the reference armature current of the machine. Using a PI current controller, the chopper duty cycle corresponding to the commanded armature current is derived. This duty cycle is then compared with a sawtooth carrier signal to obtain the required PWM signals for the chopper.

The main advantage of this drive, compared with other DC drives, is that it can operate in all four quadrants (forward motoring, reverse regeneration, reverse motoring, and forward regeneration). In addition, due to the use of high switching frequency DC-DC converters, a lower armature current ripple (compared with thyristor-based DC drives) is obtained. However, four switching devices are required, which increases the complexity of the drive system

The machine is separately excited with a constant DC field voltage source. There is thus no field voltage control. By default, the field current is set to its steady-state value when a simulation is started.

The armature voltage is provided by an IGBT converter controlled by two PI regulators. The converter is fed by a constant DC voltage source. Armature current oscillations are reduced by a smoothing inductance connected in series with the armature circuit.

The model is discrete. Good simulation results have been obtained with a 1-µs time step. In order to simulate a digital controller device, the control system has two different sampling times:

The speed controller sampling time has to be a multiple of the current sampling time. The latter sampling time has to be a multiple of the simulation time step.

• 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 PL regulators.

• The converter is fed by a 515 V DC bus obtained by rectification of a 380V AC 50Hz 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 output the armature
current reference 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 regulatory control the armature current by computing the appropriate duty ratio of the 5 Hz pulse
of the four IGBT devices.

• For proper system behavior, the instantaneous pulse values of IGBT devices 1 and 4 are opposite to those of
IGBT 2 and 3.
• This generates the average armature voltage needed to obtain the desired armature current. To limit the
amplitude of the current oscillation, a smoothing inductance is placed in series with the armature circuit.

 Compare it with the H-bridge model:

S.NO	BJT H - Bridge Drive	Four quadrant chopper drive
1	It is used for changing the direction of the Motor.	Here the motor can be operated in 4 different
2	Regenerative Braking cannot be available.	Regenerative braking can be achieved.
3	The source is of fixed DC supply.	The source is an AC supply which can be rectified to DC varying supply.
4	The armature current shoots up during the phase transition	The armature current doesn't shoot up to the value of BJT H-bridge but for low voltage value, its negligible.
5	Signals are continuous	Signals are not continuous and they are discrete.

2. Develop a 2-quadrant chopper using simulink & explain the working of the same with the relevant results. (Refer to article - Multiquadrant operation of motor )

ANSWER:

Chopper- A dc to dc converter, also known as dc chopper, is a static device which is used to obtain a variable dc voltage from a constant dc voltage source. Choppers are widely used in trolley cars, battery-operated vehicles, traction motor control, control of large numbers of dc motors, etc. They are also used as dc voltage regulators.

Choppers are of two types: (1) Step down choppers, and (2) Step up choppers.

In step down choppers, the output voltage will be less than the input voltage, whereas in step up choppers output voltage will be more than the input voltage

                                                                                  

Chopper-Fed DC Motor Drive-  D.C motor is considered a SISO (Single Input and Single Output) system having torque/speed characteristics compatible with most mechanical loads. This makes a DC motor controllable over a wide range of speeds by proper adjustment of the terminal voltage

A simple chopper – fed DC motor drive is shown in Fig.2. The basic principle behind DC motor speed control is that the output speed of a DC motor can be varied by controlling armature voltage for speed below and up to rated speed keeping field voltage constant. The armature voltage can be controlled by controlling the duty cycle of the converter (here the converter used is a DC chopper)

                                                               

                                                                    

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

WORKING
Blocks use for the model
1. Power GUI
2. Voltage measurement Block
3. Current measurement block
4. DC voltage source
5. MOSFET
6. Pulse generator
7. Diode
8. Series RLC band
9. Logical Operator
• The Two - quadrant chopper DC drive block represents a two-quadrant, DC - supplied, chopper drive for DC
motor.

• This drive features closed-loop speed control with a two-quadrant operation. The speed control loop output
thereference armature current of the machine. Using a PI current controller, the chopper duty cycle
corresponding to the commanded armature current is derived. This duty cycle is then compared with a
sawtooth carrier signal, to obtain the required PWM signals for the chopper.

• The main advantages of this drive, compared with other DC drives, are its implementation simplicity and the
that it can operate in two quadrants. In addition due to the use of a high switching frequency DC-DC converter,
a lower armature current ripple is obtained. However, for all two-quadrant DC drives reversible and
regenerative operations, which are required in most DC drives cannot be obtained.

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

ANSWER:

• A brushless DC electric motor (BLDC motor or BL motor), also known as an electronically commutated motor
(ECM or EC motor) or synchronous DC motor, is a Synchronous motor using a direct current (DC) electric power
supply.

• It uses an electronic close loop control to switch DC currents to the motor winding producing magnetic fields
which effectively rotate in space and which the permanent magnet rotor follows.

• The controller adjusts the phase and amplitude of the DC pulses to control the speed and torque of the motor.
This control system is an alternative to the mechanical commutator (brushes) used in many conventional
electric motors.

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

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

The layout of a DC brushless motor cn vary depending on wheather it is an 'Outrunner' or 'Inrunner' style.

• Outrunner- In outunner type BLDCM the field magnet is a drum rotor which rotates around the stator. This style is preferred for applications that requires high torque and where high speed of motor is not required.
• Inrunner-  In inrunner type BLDCM the stator is a fixed drum in which the field magnet rotates inside, inside the rotor. This style is usually preferred for applications that requires less torque and is capable of spinning at high speed.

But the basic fundamental principle for both type of BLDC motor is same. The images for inrunner and outrunner type BLDC motor is given below. 

           

         Outrunner BLDC motor                                                                               Inrunner BLDC motor 

BLDC motor works on the principle similar to that of a brushed DC motor which is based on 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 the BLDC motor, the current carrying conductor is stationary  and the permanent magnet is moving.

Construction of BLDC

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.

1. Stator- 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.

Similar to an Induction AC motor, the BLDC motor stator is made out of laminated steel stacked up to carry the windings. Windings in a stator can be arranged in two patterns; i.e. a star pattern (Y) or delta pattern (∆). The major difference between the two patterns is that the Y pattern gives high torque at low RPM and the ∆ pattern gives low torque at low RPM.  This is because in the ∆ configuration, half of the voltage is applied across the winding that is not driven, thus increasing losses and, in turn, efficiency and torque.

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.

          

Steel laminations in the stator can be slotted or slotless as shown in Figure 2. A slotless core has lower inductance, thus it can run at very high speeds.  Because of the absence of teeth in the lamination stack, requirements for the cogging torque also go down, thus making them an ideal fit for low speeds too (when permanent magnets on rotor and tooth on the stator align with each other then, because of the interaction between the two, an undesirable cogging torque develops and causes ripples in speed). The main disadvantage of a slotless core is higher cost because it requires more winding to compensate for the larger air gap.

                                                                         

Proper selection of the laminated steel and windings for the construction of stator are crucial to motor performance. An improper selection may lead to multiple problems during production, resulting in market delays and increased design costs.

2. 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. 

The rotor of a typical BLDC motor is made out of permanent magnets. Depending upon the application requirements, the number of poles in the rotor may vary. Increasing the number of poles does give better torque but at the cost of reducing the maximum possible speed. Another rotor parameter that impacts the maximum torque is the material used for the construction of permanent magnet; the higher the flux density of the material, the higher the torque.

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.

                                                                          

Working Principles and Operation

The underlying principles for the working of a BLDC motor are the same as for a brushed DC motor; i.e., internal shaft position feedback. In case of a brushed DC motor, feedback is implemented using a mechanical commutator and brushes. With a in BLDC motor, it is achieved using multiple feedback sensors.  The most commonly used sensors are hall sensors and optical encoders. Note: Hall sensors work on the hall-effect principle that when a current-carrying conductor is exposed to the magnetic field, charge carriers experience a force based on the voltage developed across the two sides of the conductor.

If the direction of the magnetic field is reversed, the voltage developed will reverse as well. For Hall-effect sensors used in BLDC motors, whenever rotor magnetic poles (N or S) pass near the hall sensor, they generate a HIGH or LOW level signal, which can be used to determine the position of the shaft.

In a commutation system – one that is based on the position of the motor identified using feedback sensors – two of the three electrical windings are energized at a time as shown in figure 4.

In figure 4 (A), the GREEN winding labeled “001” is energized as the NORTH pole and the BLUE winding labeled as “010” is energized as the SOUTH pole. Because of this excitation, the SOUTH pole of the rotor aligns with the GREEN winding and the NORTH pole aligns with the RED winding labeled “100”. In order to move the rotor, the “RED” and “BLUE” windings are energized in the direction shown in figure 4(B). This causes the RED winding to become the NORTH pole and the BLUE winding to become the SOUTH pole. This shifting of the magnetic field in the stator produces torque because of the development of repulsion (Red winding – NORTH-NORTH alignment) and attraction forces (BLUE winding – NORTH-SOUTH alignment), which moves the rotor in the clockwise direction.

    

This torque is at its maximum when the rotor starts to move, but it reduces as the two fields align to each other. Thus, to preserve the torque or to build up the rotation, the magnetic field generated by stator should keep switching. To catch up with the field generated by the stator, the rotor will keep rotating.  Since the magnetic field of the stator and rotor both rotate at the same frequency, they come under the category of synchronous motor.