Week-7 Challenge: DC Motor Control

1) Aim:

A) To explain the 'Speed Control Of A DC Motor Using BJT H-Bridge' from the MATLAB demo and view the results.

B) To comment on the armature current shoot-up.

C) To refer the 'Four-Quadrant Chopper DC Drive(DC7) block' and compare it with the H-bridge model.

A) H-bridge:

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

• A DC motor spins either backward or forward, depending on how you connect the plus and the minus.
• If you close switch 1 and 4, you have plus connected to the left side of the motor and minus to the other side. And the motor will start spinning in one direction.
• If you close switch 2 and 3, you have plus connected to the right side and minus to the left side. And the motor spins in the opposite direction.

Transistorized model of H-bridge:

• In this circuit all transistors are wired as switches. An NPN transistor (Q3 and Q4) will be ON when we give HIGH to it and a PNP transistor (Q1 and Q2) will be ON when we give LOW to it. So when (A = LOW, B = HIGH, C = LOW, D = HIGH), transistors Q1 & Q4 will be ON and Q2 & Q3 will be OFF, so the motor rotates in clockwise direction.
• Similarly when (A = HIGH, B = LOW, C = HIGH, D = LOW), transistors Q2 & Q3 will be ON and transistor Q1 & Q4 will be OFF, thus the motor rotates in anticlockwise direction.
• To avoids problems due to negative voltage produced by the back emf the dc motor. Resistors R1 – R4 are used to limit the input current of transistors and are designed in such a way that transistor will work as a switch.

Different types of transistors are BJT's, MOSFET's, IGBT's, SCR's etc.,

BJT (Bipola Junction Transistor):

• A bipolar junction transistor is a three-terminal semiconductor device that consists of two p-n junctions which are able to amplify or magnify a signal. It is a current controlled device. The three terminals of the BJT are the base, the collector, and the emitter. A signal of a small amplitude applied to the base is available in the amplified form at the collector of the transistor. This is the amplification provided by the BJT. Note that it does require an external source of DC power supply to carry out the amplification process.

• There are three operating regions of a bipolar junction transistor:

  • Active region: The region in which the transistors operate as an amplifier.
  • Saturation region: The region in which the transistor is fully on and operates as a switch such that collector current is equal to the saturation current.
  • Cut-off region: The region in which the transistor is fully off and collector current is equal to zero.

PNP Transistor NPN Transistor

MOSFET (Metal Oxide Semiconductor Field Effect Transistor):

• A metal–oxide–semiconductor field-effect transistor a is field-effect transistor (FET with an insulated gate) where the voltage determines the conductivity of the device. It is used for switching or amplifying signals.
• The ability to change conductivity with the amount of applied voltage can be used for amplifying or switching electronic signals. MOSFETs are now even more common than BJT's in digital and analog circuits.

• MOSFETs are particularly useful in amplifiers due to their input impedance being nearly infinite which allows the amplifier to capture almost all the incoming signal. The main advantage is that it requires almost no input current to control the load current, when compared with bipolar transistors. The various types of MOSFET are given below:

  • Depletion Type: The transistor requires the Gate-Source voltage (VGS) to switch the device “OFF”. The depletion-mode MOSFET is equivalent to a “Normally Closed” switch.
  • Enhancement Type: The transistor requires a Gate-Source voltage(VGS) to switch the device “ON”. The enhancement-mode MOSFET is equivalent to a “Normally Open” switch.
• The working of a MOSFET depends upon the MOS capacitor. The MOS capacitor is the main part of MOSFET. The semiconductor surface at the below oxide layer which is located between source and drain terminals. It can be inverted from p-type to n-type by applying positive or negative gate voltages.
• When we apply positive gate voltage the holes present under the oxide layer with a repulsive force and holes are pushed downward with the substrate. The depletion region populated by the bound negative charges which are associated with the acceptor atoms. The electrons reach the channel is formed. The positive voltage also attracts electrons from the n+ source and drain regions into the channel. Now, if a voltage is applied between the drain and source, the current flows freely between the source and drain and the gate voltage controls the electrons in the channel. If we apply negative voltage, a hole channel will be formed under the oxide layer.

P-Channel MOSFET N-Channel MOSFET

Model: Speed Control Of A DC Motor Using BJT H-Bridge

• 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 Simulink® signal (1/0). 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.
• Power = 5HP, Rated Volatge = 240V, Rated Speed = 1750RPM
• Motor starts in positive direction with duty cycle of 75% => DC volatge = 180V.

• At t =0.5 sec the voltage gets reversed as a result the speed also gets reversed and starts to reduce.
• As armature current Ia = (V-Eb)/Ra , Since at initial when speed is zero the Eb back emf is also zero so the armature current at starting is Ia = V/Ra which is the max current at the starting.
• As the speed increases the back emf gets increased, the difference between the terminal voltage and back emf reduces V-Eb reduces which inturn reduces the armature current.
• Similarly it also shoots out in the negative direction when the voltage is reversed it starts to rotate in reverse direction which has to start from zero speed , high armature current in reverse direction.
• Plot 1 => The speed increases gradually and reaches the top speed of 950.6 RPM, as the voltage terminals are reversed the speed also increases in the opposite direction.
• Plot 3 => Load torque Vs Time, the torque increaces during the normal rotation, highest torque of 6Nm and during opposite rotation the torque also becomes negative -6Nm.
• Plot 2 => Armature current Vs Time, as explained above the armature current initially has the highest current as the speed increases the armature current reduces, when the direction of rotation is changed then also the armature current will be high in opposite direction.

B)

• To reduce the spike of the armature current we need to reduce the duty cycle as the result the input terminal volatge will be reduced.

When duty cycle is 70%.

When duty cycle is 50%.

When duty cycle is 30%.

• We can notice that as the duty cycle is reduced the spike of the armature current is reduced but it also limits the speed and torque.

C) Four-Quadrant Chopper DC Drive (DC7):

• Operation in all four quadrants
  • Va and Ia can be controlled in magnitude and polarity.
  • Power flow can be in either direction.
• When a switch is on (i.e. ‘ON state’) it may or may not conduct current depending on the direction of ia
  • If a switch conducts current, it is in a conducting state
  • Converter has two legs (Leg A & Leg B)
• Both switches in each leg, are alternately switched
  • If T1 = ON, T4 = OFF
  • If T4 = ON, T1 = OFF
• Positive Current (Ia > 0)
  Va = Vdc when T1 and T2 are ON, Current increases, Q1 operation.
  Va = 0 when current freewheels through T2 and D4, Current decreases.
  Va = -Vdc when D3 and D4 conducts current, Current decreases=> Energy returned to supply, Q4 operation.

• Negative Current (Ia > 0)
  Va = -Vdc when T3 and T4 are ON, Current increases in negative direction, Q3 operation
  Va = 0 when currentfreewheels through, T4 and D2, Current decreases
  Va = Vdc when D1 and D2 conducts current, Current decreases => Energy returned to supply, Q2 operation

• The armature current Ia is either positive or negative (flow in to or away from armature) the armature voltage Va is also either positive or negative. This is known as four quadrant chopper drive. Two type – C choppers can be combined to form a class – E chopper.

First Quadrant – Forward motoring mode

• For first quadrant operation, thyristor S4 is kept on, thyristor S3 is kept off and thyristor switch S1 is operated. With S1, S4 ON, armature voltage Va = Vs and armature control Ia begins flow. Here both Va and Ia are positive giving first quadrant operation, when S1 is turned off, positive current freewheels through S4, D2. In this manner, Va, Ia can be controlled in this first quadrant, and operation gives forward motoring mode.

Second Quadrant – Forward braking mode

• Here thyristor S2 is operated and S1 , S3 and S4 are kept off. With S4 on, reverse or negative current flows through La, S2, D4 and Eb. During the operation time of S2, the armature inductance ‘La’ stores energy during the time S2 is on. When S2 is turned off, current is fed back to source through diodes D1 , D4 . Note that here ( E+L(di/dt)) is more than the source voltage Vs.As the Vs is positive and Ia is negative, it is a second quadrant operation gives forward braking mode. In that power is fed back from armature to source.

Third Quadrant – Reverse motoring mode

• For third quadrant operation, thyristor S1 is kept off, S2 is kept on and S3 is operated, polarity of armature back emf Eb must be reversed for this quadrant operation. With thyristor S3 is on, armature gets connected to source V. So that both Va, Ia are negative, leading to third quadrant operation. When S3 is turned off,negative current free wheels through S2,D4. In this manner only Va and Ia can be controlled in the third quadrant.

Fourth Quadrant – Reverse Braking mode

• Here thyristor S4 is operated and other devices kept off, back emf Eb must have its polarity reversed as in third quadrant operation. With S4 on, positive current flows through S4,D2,La and Eb (armature). Armature inductance La stores energy during the time S4 is on. When S4 is turned off, current is fed back to source through diodes D2,D3.Here armature voltage Va is negative, but Ia is positive, leading to the chopper drive operation in the fourth quadrant. Also power is fed back from armature to source.

Model: The Four-Quadrant Chopper DC Drive (DC7) block:

• This drive features the 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. The 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 four quadrants. In addition, due to DC-DC converters, a lower armature current ripple is obtained. Due to four switching devices complexity of the drive system will be increased.
• 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.

• Mechanical Torque is proportinal to the speed since, motor is coupled to linear load.
• Armature current follows the current reference, with fast response time and small ripples.

Comparision Of DC7 and H-Bridge:

H-Bridge	DC7 Drive
Regenerative breaking is not possiblle.	Regenartive Breaking is possible due to its four quadrant operation.
Current values are continuous.	Current values are chopped not continuous.
Widely used in Robotics, Elecrto-Mechanical decice speed & direction control of DC motor	Widely used in Automobile for speed control of DC motor.
Only polarity of the current is to be changed for changing the direction of motor.	Interruption of current signals with one another to change the direction of motor.

Conclusion:

• Hence the Speed Control of a DC Motor Using BJT H_Bridge is explained and results are viewed.

• Hence the armature current shoot-up results is discussed i.e., by reducing the duty cycle we reduce the input voltage by which the armature current shoot-up reduces along with that speed and torque will also get reduced.

• Hence referred the 'Four-Quadrant Chopper DC Drive(DC7) block' and compared it with the H-bridge model.

2) Aim: To develop a 2-quadrant chopper using simulink and explain the workinf with relevant results.

Type -C chopper:

• Type-C Chopper is a category of chopper which can operate in first as well as 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 Two Quadrant Class-A chopper.
• If we connect Type A(first quadrant) and Type B(second quadrant) choppers in parallel then it is possible to have chopper operation in first as well as second quadrant.

• 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 first first qaudrant we should switch ON chopper CH1 and for getting second quadrant operation we should switch ON chopper CH2.

Case 1: When CH1 is ON:

• When chopper CH1 is switched ON, source V_s directly gets connected to the load and hence, load voltage V_o is equal to source voltage. The direction of load current is from source to load.
• When CH1 is switched OFF, the free-wheeling diode FD comes into the circuit as it gets forward biased and hence shorts the load. Therefore, the output voltage V_o becomes zero.
• However, the i_o continues to die down through the FD and L in the same direction as shown in circuit diagram. Thus, the average output voltage V_o and current I_o are positive and hence operation of chopper is in first quadrant.

Case 1: When CH2 is ON:

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

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

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

Model: Two Quadrant Chopper:

• MOSFETs are used switching devices which supplies gate pulses.
• The series RLC branch is connected, R & C terms are made zero in the parameter table.
• Diodes are connected in order for the continuos flow of current.
• Pulse generator is connected to the MOSFETs as gate signals.
• Connect a Powergui block and mention it as continuous flow.
• Save and run the model to view the results

Pulse Generator:

• The Simulink model is created, pulse geneartor is fed with NOT gate inorder not to trigger both the MOSFET's at one time.
• The pulse generator is given 10 units amplitude with 0.02 as time period with 50% duty cycle.
• The input is supllied with 240V from source side and 120V from load side.

Output:

• From the Voltage output we can observe that voltage never becomes negative though current is negative. The volatge only varies between maximum and zero .
• From current output we can observe a negative value when the voltage is not negative.
• This ensures that Voltage operates only with positive values and current taking the positive and negative values which is the case of operation of two quadrants.

Conclusion: Hence the two quadrant chopper model is made in Simulink and the outputs have been plotted.

3) Aim: To explain about operation of BLDC motor.

• 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.
• 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 fixed.
• The armature coils are switched electronically by transistors or SCR's 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 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 ranges.

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.
• 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 application and automation systems, 100 V or higher rating motors are used.

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 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 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 above 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.
• 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.

Advnatges 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 BLDC 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:

• 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.

Conclusion: Hence we have discussed about the construction,working opeartion of BLDC motor.