Week-7 Challenge: DC Motor Control

• Aim:

1. Explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’.
2. Comment on the armature current shoot-up from the scope results.
3. Refer to the help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model.
4. Develop a 2-quadrant chopper using simulink & explain the working of the same with the relevant results. (Refer to article – Multi quadrant operation of motor) .
5. Explain in a brief about operation of BLDC motor.

• Solution 1 :

Part A :

We have taken Demo model named ‘Speed control of a DC motor using BJT H-bridge’ from the MATLAB documentation. Also as the H-Bridge produces a chopped voltage of the input voltage and it controls the speed of the DC motor. It uses a BJT (Bipolar Junction Transistor which is a type of transistor that uses both electrons and holes as charge carriers, hence the name bipolar. Other transistors like FETs (Field Effect Transistors) are unipolar transistors meaning that they only use one type of charge carrier (either electrons or holes). BJTs are used for switching and amplifying applications. BJTs have 3 terminals which are: the collector (C), the base (B), and the emitter (E). Current flows in only the emitter and the collector and the amount of current that flows through them is controlled by the base. BJTs use semiconductors in their work, and so there are 2 types of BJT:

1. NPN type
2. PNP type.

The circuit symbols are shown below:

BJT H-Bridge: The H-bridge consists of 4 BJT and Diode pairs. Two transistors are switched on simultaneously: Q1 and Q4 or Q2 and Q3. When Q1 and Q4 are fired, a positive voltage is applied to the dc motor, and the diodes, D2 and D3 operate as free wheeling diodes when Q1 and Q4 are switched off. Similarly, when Q2 and Q3 are fired, a negative voltage is applied to the dc motor, and the diodes, D1 and D4 operate as free wheeling diodes when Q2 and Q3 are switched off at that moment.

The Bipolar Junction Transistor (BJT) when used for power switching applications, operates as an IGBT. When it is conducting i.e. in the saturation region (BJT operating in the saturated region), 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. The Insulated Gate Bipolar Transistor also called an IGBT, is something of a cross between a conventional Bipolar Junction Transistor, (BJT) and a Field Effect Transistor, (MOSFET) making it ideal as a semiconductor switching device. The IGBT Transistor takes the best portions of these two types of common transistors i.e. the high input impedance and high switching speeds of a MOSFET with the low saturation voltage of a bipolar transistor and combines them to produce another type of transistor switching device that is capable of handling large collector-emitter currents with virtually zero gate current drive. 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 present model (5 HP 24V 1750 rpm). It simulates a fan-type load (where Load torque is proportional to the square of speed). The armature means voltage can be varied from 0 to 240 V when the duty cycle (specified in the Pulse Generator block) is varied from 0 to 100%.

Below image shows the Simulink model for the above operation

From the above model we can describe :

Characteristics from the Dc Motor Block:

IGBT Parameters:

Pulse generator:

1. And the following Results can be obtained from the Default Case-

The input parameter, which is the Pulse Generator is provided with,

Amplitude = 1, Period = 2e^-3 and Pulse width or Duty Cycle = 75%

• Part B : Comment on the armature current shoot-up from the scope results.
• The Armature Current Shoot-up can be controlled by controlling the input parameters. Since the model is using the pulse generator, it is possible to vary the armature current by varying the duty cycle.
• For a 100% Duty cycle, the armature current is spontaneously varying at High value
• As we lower the duty cycle, the armature current also become stable
• By changing the duty cycles we will see the following observations

1. At duty cycle 60%:

2. At Duty cycle 50%

• Results : Hence observations from the output results, we can see that the armature current does not shoot anymore or don't have a rapid change in value from froward to reverse operating directions with a 50% duty cycle and as the duty cycle is decreased from 75% 60% 50%; the armature current shoot in transition occurs in a very less amount. Also the simple reason is that with the lesser duty cycle, the voltage is supplied to the DC motor terminals for a less amount of time, and hence the current value of the armature will be less, and since the change in armature current values will not be rapid and this the reason the current it did not shoot up.

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

Observing above Section;

Simulink block diagram

• 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.

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.

• Forward Motoring: Here motor rotates clockwise also the controller applies clockwise drive.
• Reverse Motoring: Here motor rotates anti-clockwise also the controller applies an anti-clockwise drive.
• Forward Braking: Here motor rotates clockwise also the controller applies an anti-clockwise drive.
• Reverse Braking: Here motor rotates anti-clockwise also the controller applies a clockwise drive.

Advantages:

1. 4 quadrant choppers have more control over the deceleration of the motor and can more accurately brake the motor. This will make the deceleration smoother.
2. There is a reduction to the risk of current spikes, which can damage the motor and the controller itself because this can occur when the motor is made to brake while running at a high speed. So the controller life is generally longer with 4 quadrant choppers, by mitigating those sudden surges or spikes from the output.
3. Also the 4 quadrant choppers are more efficient because lesser energy is wasted as heat and lesser current spikes occur which means lesser power dissipation over a complete. iv. In the case of a vehicle, the four quadrant chopper makes regenerative braking possible so that when the motor is made to brake, the kinetic energy of the rotor moving is transferred back as electrical energy which can then be used to charge a battery.

• Results : Observations from scope

• Comparison to H-Bridge and Chopper Drive:

1. In Four Quadrant Chopper DC Drive, there are total of 4 modes – Forward Motoring, Forward Braking, Reverse Motoring and Reverse Braking, while In BJT H- bridge Model, there are only 2 modes – Forward and Reverse Motoring
2. Both BJT H-bridges and 4 quadrant choppers have their advantages and disadvantages. H-Bridges like the 4 quadrant choppers are also capable of the braking operation without using air resistance or friction or physical brakes to bring the motor to rest. 4 quadrant DC choppers are more commonly used in automobiles whereas BJT H-bridges are commonly used in robotics and electromechanical devices.
3. Unlike 4 quadrant DC choppers, BJT H-bridges are not capable of performing regenerative braking but they can perform dynamic braking. It is this difference that makes 4 quadrant DC choppers a clear choice in hybrid and electric vehicles.
4. In 4 quadrant DC choppers, the current signals are interrupted with one another to change the direction of rotation of the motor. This is done by a speed reference or a drive cycle from which the speed values are converted to current signals which interrupt one another to change the rotation of the direction of the motor. However, in BJT H-bridges, the polarity of the current is reversed (depending on the duty cycle) to change the direction of rotation of the motor.
5. In the four-quadrant DC choppers, a Pl controller is used to control the speed and the current by comparing them with the reference values; but in the H-bridge, this is not implemented.
6. In Four Quadrant Chopper DC Drive, the overall performance and efficiency is much better compared to BJT H- bridge Model

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

• SIMULINK MODEL for Two Quadrant Chopper:

Two-quadrant type-A chopper, or Type-C Chopper

This type of chopper is obtained by connecting type-A and type-B choppers in parallel z . (a). The output voltage V, is always positive because of the presence a freewheeling diode FD across the load. When chopper CH2 is on, or freewheeling diode ti conducts, output voltage vo = 0 and in case chopper CH1 is on or diode D2 conducts, output voltage v, = V. The load current i, can, however, reverse its direction. Current i, flows in the arrow direction marked in Fig. 7.10 (a), i.e. load current is positive when CH1 is on or FT conducts. Load current is negative if CH2 is on or D2 conducts. In other words, CH1 and FT operate together as type-A chopper in first quadrant. Likewise, CH2 and D2 operate together as type-B chopper in second quadrant.

Simulink Blocks Used :

1. Diode: here Two Diodes are used :

2. Mosfet: Metal Oxide Field Effect Transistor
3. Pulse Generator:
4. Serios RLC load : Here in the load block we have used values for R and C = 0

Also A DC supply voltage is provided as supply Load

5. Logical Operator NOT: An NOT logical gate is used to fed signal accordingly to Q1 and Q2 from the Pulse generator. When high, signal is fed to Q1 and when low, it is fed to Q2

• Results :

Observations from the Scope :

Following results for the waves Shoeing for The Pulse generator, current measurement, Voltage measurement.

Solution 3: Brief about operation of BLDC motor.

THEORY:

A brushless DC electric 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.

Also 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 (asynchronous) motor. They may also use neodymium magnets and be outrunners (the stator is surrounded by the rotor), in runners (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:

The motor, the permanent magnets attach to the rotor. The current-carrying conductors or armature windings are located on the stator. They use electrical commutation to convert electrical energy into mechanical energy. The main design difference between a brushed and brushless motors is the replacement of mechanical commutator with an electric switch circuit. A BLDC Motor is a type of synchronous motor in the sense that the magnetic field generated by the stator and the rotor revolve at the same frequency. Brushless motor does not have any current carrying commutators. The field inside a brushless motor is switched through an amplifier which is triggered by the commutating device like an optical encoder. The layout of a DC brushless motor can vary depending on whether it is in “Out runner” style or “In-runner” style.

1. Outrunner –The field magnet is a drum rotor which rotates around the stator. This style is preferred for applications that require high torque and where high rpm isn’t a requirement.
2. In runner– The stator is a fixed drum in which the field magnet rotates. This motor is known for producing less torque than the out-runner style but is capable of spinning at very high rpm.

Below shows the circuit Diagram:

Also explains the working Principle;

1. BLDC motor works on the principle like that of a Brushed DC motor.
2. The Lorentz force law which states that whenever a current carrying conductor placed in a magnetic field it experiences a force. Because of reaction force, the magnet will experience an equal and opposite force.
3. In the BLDC motor, the current carrying conductor is stationary and the permanent magnet is moving.
4. When the stator coils get a supply from 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.
5. 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 aligns with stator poles which causes the motor to rotate.
6. In brushless DC motors, an electronic servo system replaces the mechanical commutator contacts.
7. An electronic sensor detects the angle of the rotor and controls semiconductor switches such as transistors which switch current through the windings, either reversing the direction of the current or, in some motors turning it off, at the correct angle so the electromagnets create torque in one direction. The elimination of the sliding contact allows brushless motors to have less friction and longer life; their working life is only limited by the lifetime of their bearings.

Advantages of Brushless DC motor

1. Less overall maintenance due to absence of brushes
2. Reduced size with far superior thermal characteristics
3. Higher speed range and lower electric noise generation.
4. It has no mechanical commutator and associated problems
5. High efficiency and high output power to size ratio due to the use of permanent magnet rotor
6. Smaller motor geometry and lighter in weight than both brushed type DC and induction AC motors.
7. Long life as no inspection and maintenance is required for commutator system
8. Higher dynamic response due to low inertia and carrying windings in the stator
9. Less electromagnetic interference
10. Low noise due to absence of brushes
11. High speed of operation even in loaded and unloaded conditions due to the absence of brushes that limits the speed

Applications of Brushless DC motor

The cost of the Brushless DC Motor has declined since its presentation, because of progressions in materials and design. This decrease in cost, coupled with the numerous focal points it has over the Brush DC Motor, makes the Brushless DC Motor a popular component in numerous distinctive applications. Applications that use the BLDC Motor include, yet are not constrained to:

• Consumer electronics
• Transport
• Heating and ventilation
• Industrial engineering
• Model engineering

For examples Computer hard drives and DVD/CD players

Electric vehicles, hybrid vehicles, and electric bicycles

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

Speed Control

Speed control of BLDC motor is essential for making the motor work at desired rate. Speed of a brushless dc motor can be controlled by controlling the input dc voltage. The higher the voltage, more is the speed. When motor works in normal mode or runs below rated speed, input voltage of armature is changed through PWM model. When motor is operated above rated speed, the flux is weakened by means of advancing the exiting current.

The speed control can be closed loop or open loop speed control.

1. Open Loop Speed Control – It involves simply controlling the dc voltage applied to motor terminals by chopping the dc voltage. However this results in some form of current limiting.
2. Closed Loop Speed control – It involves controlling the input supply voltage through the speed feedback from the motor. Thus the supply voltage is controlled depending on the error signal.