DC Motor Control

CONTROL OF DC MOTOR

AIM:

• To run the MATLAB demo script “Speed Control of DC Motor using BJT H-Bridge” & evaluate the model such that Armature current doesn’t shoot-up when the Motor changes its direction of rotation.
• To compare 4-Quadrant Chopper DC Drive (DC7) block with BJT H-Bridge Model.
• To build EV Model using DC7 block.

INTRODUCTION:

Power Electronics

Power electronics is the application of solid-state electronics to the control and conversion of electric power.

In modern systems, the conversion is performed with semiconductor switching devices such as diodes, thyristors, and power transistors such as the power MOSFET and IGBT. In contrast to electronic systems concerned with transmission and processing of signals and data, in power electronics substantial amounts of electrical energy are processed. The power range is typically from tens of watts to several hundred watts. In industry a common application is the variable speed drive (VSD) that is used to control an induction motor. The power range of VSDs start from a few hundred watts and end at tens of megawatts.

Switching Technique is adopted from Power Electronics to control the Voltage and Current for Electric Motor. This avoids high amount of losses by replacing Resistor with and Power Module/Switch. And these Power Modules are operated at high Frequency where the Rated Battery Voltage (Vbat) can be reduced to desired voltage by ON-OFF of Modules.

Switching Frequency will be in such a way that it utilizes Vbat & switches low and High to maintain Average desired Voltage.

The principle behind the Power Modules is not controlling the Magnitude of the Voltage but the duration of High & Low Value of Voltage resulting the desired Voltage level.

This ON-OFF Switching Frequency is referred in terms of Duty Ratio

Duty Ratio (d) = (ON) time/(ON+OFF) time

By changing the Duty Ratio of the Power Modules, we can vary the Voltage from one level to another level.

Vavg = Vbat * d

Power Modules/Switches

When it comes to EV application, controlling the Speed of the Motor mainly depends on controlling the Voltage & Current flowing to the Motor. To ensure a rotating field in the motor continuously, flux has to be switched continuously by controlling the direction and timing of the Current in to the coil.

In order to achieve this Controllers/Inverters are used to adjust the Voltage in the coil and to control the magnitude of current.

PWM (Pulse Width Modulation) is the technique incorporated in Power switches which alters the Voltage level by lengthening or reducing the Pulse ON time.

Power Modules used in Controllers/Inverters

Transistor

A transistor is a semiconductor device used to amplify or switch electronic signals and electrical power. It is composed of semiconductor material usually with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals controls the current through another pair of terminals. Because the controlled (output) power can be higher than the controlling (input) power, a transistor can amplify a signal.

Power Signal acts across Collector-Emitter (CE) when there is a Control Signal across Base-Emitter (BE).

Bipolar Junction Transistor

A Bipolar junction transistor (BJT) is a type of transistor that uses both electrons and holes as charge carriers. Unipolar transistors, such as field-effect transistors, use 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 collector–emitter current can be viewed as being controlled by the base–emitter current (current control), or by the base–emitter voltage (voltage control). These views are related by the current–voltage relation of the base–emitter junction, which is the usual exponential current–voltage curve of a p–n junction.

MOSFETs

The Metal Oxide Semiconductor Field Effect Transistor is a type of insulated-gate field-effect transistor that is fabricated by the controlled oxidation of a semiconductor, typically silicon. The voltage of the covered gate determines the electrical conductivity of the device; this ability to change conductivity with the amount of applied voltage can be used for amplifying or switching electronic signals.

IGBTs

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 without regenerative action. 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.

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1. To run the MATLAB demo script “Speed Control of DC Motor using BJT H-Bridge” & evaluate the model such that Armature current doesn’t shoot-up when the Motor changes its direction of rotation.

power_Hbridge

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

Source: https://in.mathworks.com/help/physmod/sps/examples/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 pre-set 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.

Motor & Simulation Specs:

• Power = 5 HP
• Rated Voltage = 240 V
• Rated Speed = 1750 RPM
• The motor starts in the positive direction with a duty cycle of 75%
  • a. DC voltage of 180V
• At t= 0.5 sec., the armature voltage is suddenly reversed and the motor runs in the negative direction.

PLOTS

Discussion

• Plot 3 from above figure shows the Load Torque against time
  • Torque of 6 Nm is applied to the Motor initially and at time t=0.5s, direction of rotation is reversed and a Negative torque of same Magnitude can be observed.
• Plot 1 from above figure shows the Motor Speed against time
  • Speed is proportional to the applied Load Torque.
• Plot 2 shows the Armature Current drawn at each instant.
  • Initial spike of current is observed in the armature (1^st Shoot up)

In motor, armature current (I) is given by: -

I=(V-E)/R.

Where, V is the voltage supplied, E is the back emf and R is the armature resistance.

Thus, I depends upon E and R when V is kept constant. When the motor starts its speed remains zero i.e. E also remains zero (since, back emf is ∝ speed of the armature).

Thus, initial starting armature current: -

I=(V-0)/R= V/R. (as, E =0)

Since, the armature resistance of a motor is very small, generally less than one ohm, therefore the starting current would be very large.

As the motor speed increases, the back emf increases and the difference (V-E) goes on decreasing. This results in the gradual decrease of armature current until the motor attains its stable rated speed. Under this condition the armature current reaches its desired rated value.

• Another Shoot up can be observed in the Armature Current when motor changes direction from forward to reverse.

In order to avoid this, Duty ratio can be optimized

Duty Ratio changed from 75% to 45 %.

• By reducing the Duty Ratio, Armature Current Shoot-Up can be completely optimized.
• But there will be considerable fall in the Load Torque and hence the Speed.
• No load Torque of the Motor has been reduced from 6 Nm to 2 Nm.
• No load Speed is reduced from 900 RPM to 400 RPM

• Conclusion
  • Reducing the Duty Ratio of the Power Modules, ultimately reduces the effect of shooting Armature current but reduces the Supply Voltage in to the Motors and results in the reduction of the Rated Speed and Torque of the Motor.

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2. Comparison of 4-Quadrant Chopper DC Drive (DC7) block with BJT H-Bridge Model.

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.

dc7_example

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.

Comparison with H-Bridge Model

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
• The current controller sampling time

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 DC7 is capable of operating in all 4 Quadrants of Motoring Operation. They are Forward Braking, Forward motoring, Reverse motoring and Reverse braking. A motor operates in two modes – Motoring and Braking. A motor drive capable of operating in both directions of rotation and of producing both motoring and regeneration is called a Four Quadrant variable speed drive.

In motoring mode, the machine works as a motor and converts the electrical energy into mechanical energy, supporting its motion. In braking mode, the machine works as a generator, and converts mechanical energy into electrical energy and as a result, it opposes the motion. The Motor can work in both, forward and reverse directions, i.e., in motoring and braking operations.

In the I quadrant power developed is positive and the machine is working as a motor supplying mechanical energy. The I (first) quadrant operation is called Forward Motoring. II (second) quadrant operation is known as Braking. In this quadrant the direction of rotation is positive, and the torque is negative, and thus, the machine operates as a generator developing a negative torque, which opposes the motion.

The kinetic energy of the rotating parts is available as electrical energy which may be supplied back to the mains. In dynamic braking dissipated the energy is dissipated in the resistance. The III (third) quadrant operation is known as the reverse motoring. The motor works, in the reverse direction. Both the speed and the torque have negative values while the power is positive

In the IV (fourth) quadrant, the torque is positive, and the speed is negative. This quadrant corresponds to braking in the reverse motoring mode.

Simulation Plots with 200 HP DC Drive

• The motor is coupled to a linear load, which means that the mechanical torque of the load is proportional to the speed.
• The armature current follows the current reference very well, with fast response time and small ripples.
• When the motor reaches 0 rpm and the load torque reverses and becomes negative. The negative current now produces an accelerating electromagnetic torque to allow the motor to follow the negative speed ramp.
• 4 Quadrant DC Drive uses reference Load and Speed to cope up with the Motor direction of rotation and Supply Current.
• The current Shoot up are minimized here when compared that of with H-Bridge Model.

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3. Building EV Model using DC7 Block

Using the above DC7 example Model, building an Electric Vehicle Model.

The DC7 Model has to be modified in order to simulate the EV Model.

Drive Cycle

Replace the Speed reference Block with any Drive Cycle of Short duration by calling the Speed-Time values into Workspace Variables or by attaching .xls file.

Modifying the Energy Source at Input

Use a Simple DC Source as Energy Source and give it a suitable Voltage value as in EV Application.

Replace this with AC-DC Converter Block and finally add a Current measurement Scope and connect to Motor Block.

Vehicle Load Characteristics

Linear Load Torque block is replaced with Sub System Model according to the Load Characteristics

This Sub system takes Speed as input from the Motor Block and calculates for the Load Torque.

Electric Vehicle Final Model

Results

Discussion

• The above figure shows Plots of Supply Voltage, Armature Current, IGBT duty Ratios & Motor Speed.
• Speed values are based on the Input Duty Cycles.
• Battery Voltage is capable enough to provide the required Armature Current throughout the Cycle.
• The Supply Voltage has been found Proportional to the demanding Load and Speed of the Motor.
• During deceleration after t=20s, a negative Armature Current can be observed which shows the Regenerative Braking where the Battery is being Charged.
• IGBT 1 & 4 are used for Positive Cycles and IGBT 2&3 for Negative Cycles.
• It can be observed that Duty Ratio of IGBT 1&4 is inversely proportional to IGBT 2&3.
• This duty ratios will control the Supply Voltage and in turn the Armature Current.

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