Speed Control of BLDC Motor

Aim:

To study the open loop and closed loop speed control methods of a BLDC motor

Apparatus Required:

1. BLDC motor

2. DSP 2812

3. IPM (intelligent power module)

4. Hall sensor signal conditioner

5. Connectors

BLDC Motor Operation:

A permanent magnet AC motor, which has a trapezoidal back-emf, is referred to as

brushless DC motor (BLDC). The BLDC drive system is based on the feedback of the rotor

system at fixed points for communication of the phase currents. The BLDC motor requires quasi-

rectangular shaped currents to be fed into the machine. Alternatively, the voltage may be applied

to the motor every 120 degrees, with current limit to hold the current within motor capabilities.

Because the phase currents are excited in synchronism with constant part of the back-emf,

constant torque is generated.

The electromagnetic torque of the BLDC motor is related to the product of a phase, back

emf and current. The back-emf in each phase are trapezoidal in shape and are 120 degrees

electrical phase shifted with respect to each other. A rectangular current pulse is injected into

each phase, so that current coincides with back-emf waveform hence the motor develops almost

constant torque.

Hall effect sensor provides the information needed to synchronize the motor excitation

with rotor position in order to produce constant torque. It detects the change in magnitude field.

The rotor magnets are used as triggers to hall sensor. A signal conditioning circuit integrated

within the hall switch provides a TTL compatible pulse with sharp edges.Three hall-effect

sensors placed at 120 degree apart are mounted on the stator frame. The three hall sensor digital

signals are used to sense the rotor position.

The functional block diagram, circuit diagram and the waveforms are as follows,

1. Hall sensor signal conditioner

2. connectors

Block diagram of the BLDC Drive:

Circuit Diagram:

Three Phase Inverter

Typical Waveforms of a BLDC Motor

DSP TMS320F2812

Micro-2812 is a 16-bit (data lines) fixed point DSP trainer, based on Texas instruments

TMS320F2812 DSP Processor. This trainer enables the user to learn the basics of digital signal

processing & digital control along with basic DSP functions like filtering, PWM generation,

calculation of spectral characteristics of input analog signals. The trainer helps to perform real

time implementation of very complex algorithms, such as adaptive control, vector control, etc.,

The TMS320F2812 contains a C28xx DSP core along with useful peripherals such as ADC

Timer, PWM Generation are integrated onto a single piece of silicon. The Micro-2812 trainer

can be operated in two modes. In the mode:1 (serial mode) the trainer is configured to

communicate with the PC through serial port. In the mode:2 (stand alone mode), the user can

interact with the trainer through the IBM PC keyboard and 16 × 2 LCD display. From the DSP

processor, eight PWM waveforms (outputs) can be generated simultaneously by each event

manager, three independent pairs (six outputs) by the three full-compare units with

programmable dead bands, and two independent PWMs by the GP-timer compares

PWM Generation:

The PWM switching signals of the inverter switches are obtained by comparing a DC

reference signal with a fixed frequency triangular carrier wave. The DC reference will regulate

the average voltage applied to the motor by changing the duty ratio of the PWM signals to

control the speed of the motor.

Open loop speed control:

Connection Procedure:

1. Connect the U,V, W terminals of the IPM (intelligent power module) to motor input

2. Hall sensor is to find the position of the rotor, the sensor output which is given through

34 pin FRC cable one end to the DSP 2812 capture peripheral and other end to IPM gate

input peripheral

3. Connect the serial port of computer to serial port of DSP 2812 for communication

4. Connect the feedback terminals of IPM to A/D peripherals of DSP processor

Experimental procedure:

1. Open the Vi BLDC_2812 software,select the port and baud rate and click on “connect”

button to connect the trainer with PC

2. Select open loop control option and download the source code and execute the program

3. Select “CW/CCW/Brake”, the screen displays the speed plot along with the speed display

4. Change the set speed of the motor and observe the change in duty cycle of the Pulse

Width Modulation signal, Hall Sensor signals and phase current waveforms

5. Tabulate the duty cycle of the PWM signal for speed 1000, 2000 and 3000 RPM

Sl. No. Ton Toff PWM Duty Cycle Speed in RPM

Closed loop speed control:

1. Select the closed loop control option from the BLDC GUI window

2. Set the speed 2000 RPM, then execute the program

3. Apply the belt load to BLDC drives, then note down the current waveforms and observe

whether the actual speed and set speed of the motor are same

Block diagram of the closed loop speed control of BLDC drive and the typical experimental

waveforms are shown in the following figures.

Block Diagram for Closed loop Speed control

Set speed

Actual speed

Experimental Waveforms :

Terminal Voltage, Back-EMF, Switching Signals and Phase Current waveforms of Phase A

CLOSED LOOP CONTROL OF DC SERVO MOTOR AIM To study the Torque – Speed characteristics of DC servo motor while loading in open loop and maintain constant specified speed even under loaded condition in closed loop. EQUIPMENTS REQUIRED

1. DC Servo Motor. 2. Control Circuitry. 3. CRO, DMM

SPECIFICATIONS:

1. Input to the DC servo motor control unit is 230V ±10%, 50Hz, AC, single phase. 2. DC Power supply to the motor is 12V, by a PWM power converter. 3. DC Motor: 12V, Permanent Magnet DC motor

Max. Current : 1.5Amp. Max Torque : 1.5 Kg-cm Max Speed : 1500 rpm at rated voltage (12V) and current (1.5A)

4. Optical speed sensor for sensing speed: 800 pulse / 100 revolution 5. One 3½ Digit display of set speed / actual speed: 6. Armature resistance Ra = 1.2 ohm 7. Armature inductance La = 2.3 mH 8. Back emf constant Kb = 5.66 V/Krpm 9. Torque constant Kt = 5.54 Ncm/A 10. Damping constant(eata) = 0.3 Ncm/Krpm 11. Moment of inertia J = 0.39 Kg cm2 12. Viscous coefficient of friction B = 1.5 N cm

Theory In control systems a servo motor is used to convert the final control element into mechanical displacement, velocity, torque etc. as the desired output. Servomotors can be either DC or AC. The commonly used servomotors are separately excited DC motors and squirrel cage or drag-cup type induction motors. Important requirements of servo motor are low moment of inertia of the rotor, linear T-N characteristic with negative slope, and capacity to withstand frequent starting and stopping. DC servomotor speed control is similar to that of DC shunt motor. The DC servo motor speed control system consists of the following,

1. Regulated DC Power supply which supplies to the control circuits, power amplifier and the chopper.

2. MOSFET based single quadrant chopper (PWM Power Converter), through which the armature voltage is controlled.

3. Speed sensor and Speed feedback circuitry for measurement of speed and closed loop control of the motor.

4. Speed Sensor - Optical sensor is provided to sense the speed. 5. Speed controller - Either proportional (P) or Proportional plus Integral (PI) controller can

be selected. The system / plant to be controlled is the DC motor. The objective is to vary the speed

through a

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The inputs to the comparator are Ramp signal and Control voltage signal Vc. The output of the comparator goes high whenever V C is greater than the ramp signal. By varying the control voltage VC , the duty cycle ratio D = T ON /T and hence the output voltage of the amplifier is controlled. The control voltage V C to the comparator is the output of the controller. The controller adjusts the control voltage depending on the output of the error amplifier. Dynamic modeling of DC servomotor

The DC motor can be modeled as a linear system, if the magnetic saturation is neglected and the field flux is assumed to be constant. For this purpose, a permanent magnet DC motor is used. Here the flux is produced by the permanent magnets which are constant. The DC motor can be represented by the equivalent circuit shown below. The armature resistance and inductance are represented as lumped parameters as R and L. The field current is assumed to be constant. This sets the constant flux in the machine.

Ra - Armature resistance (ohms) La - Armature inductance (Henrys) Va - Voltage applied to the armature (volts) Ia - Armature current (Amps.) eb - back emf (volts) if - field current (amps) - Assumed as constant for wound field motor. ω - angular speed of the motor in Rad/sec. N - angular speed of the motor in RPM J - Equivalent moment of inertia of the motor and load (kg-m2) B - Equivalent viscous friction coefficient of the motor and the load (Nm/rad/sec) The equations governing the behaviour of the motor are given below. The electromagnetic torque developed by the motor is

Te = k1 kf if ia -------- (1) If the flux is assumed to be constant Te = kt ia N.m -------- (2) Where kt = k1 kf if , Torque constant N.m / amp The back emf developed is eb = kb ω -------- (3) where kb - back emf constant volts / rad / sec. The differential equation governing the armature circuit is La (dia / dt) + Ra ia + eb = Va -------- (4) The differential equation governing the mechanical system comprising armature and load is

J (dω / dt) + Bω + TL = Te -------- (5) where TL = Load torque . N.m In the closed loop control system block diagram, the amplifier is represented as the gain G, which is constant. The output of the chopper will be maximum, when the control voltage V C equals the peak value of the Ramp V pst , since the MOSFET will be ON for the whole period and V O = V dc i.e. when the duty cycle ratio is one. The output voltage varies linearly with the control voltage VC as shown below. . = =

Where G - Gain the power amplifier Vpst - peak value of the Ramp (Saw tooth) VC - Control voltage

In the DC Motor servo controller, 24 , 10 PROCEDURE: Open loop speed control

1. Before Switching ON the unit, a. EXT/INT switch should be in INT mode. b. Integral open/close switch in open (OL) mode. c. Signal conditioner switch in open (OL) mode. d. Interface the motor supply sensor with module.

2. Initially, pulse ON/OFF switch should be in OFF mode. 3. Set the proportional gain kp at minimum. 4. Switch ON the unit, and keep the pulse ON/OFF to ON mode. 5. Run the motor at 1500 rpm by suitably adjusting the Vref and Kp. Note down the armature

voltage (Va) armature current (Ia) and Speed (N). 6. Apply load by brake magnet close to the disc. Apply the load in steps such a way that the

current is increased by 0.25A in each step. Note the armature current, voltage and speed of the each step. Tabulate the readings.

7. It may be noticed that the motor armature current may not be increased when the speed drops below a certain value. This is because the speed is low the eddy current induced in the disc becomes less, which reduces the load torque.

8. Decrease the load and reduce the gain to minimum. 9. Switch OFF the power supply.

Calculate the torque using T = Kt Ia Plot the Speed Vs Torque and Speed vs armature voltage characteristics. Closed loop speed control

1. Before Switch ON the unit, a) EXT/INT switch should be in INT mode. b) Integral open/close switches in open (CL) mode. c) Signal conditioner switch in open (CL) mode. d) Interface the motor supply sensor with module.

2. Initially, pulse ON/OFF switch should be in OFF module. 3. Set the proportional gain kp at minimum.

4. Switch ON the unit, and keep the pulse ON/OFF to ON mode. 5. Set the speed at 1500 rpm by suitably adjusting the Vref and Kp. Note down the armature

voltage (Va) armature current (Ia) and Speed (N). 6. Apply load by brake magnet close to the disc. Apply the load in steps such a way that the

current is increased by 0.25A in each step. Note the armature current, voltage and speed of the each step. Tabulate the readings.

7. It may be noticed that the speed maintains constant for increasing load current. 8. Decrease the load and reduce the gain to minimum. 9. Switch OFF the power supply. 10. Calculate the torque as T = Kt Ia

TABULATION:

Sl .No Va(V) Ia(Amps) N (rpm) T = kt × Ia( N-m)

RESULT

Study of Four Quadrant Operation of DC Drive

Aim:

To study the four quadrant operation of separately excited DC drive.

Apparatus required:

PEC 16HV3 Module

VPET -106A Module

Separately Excited DC machine

Block Diagram of a typical DC Drive:

Fig. 1: Block Diagram of the DC Drive

Theory:

A typical block diagram of a DC chopper drive is shown in Fig.1, which can operate in all four quadrants of the V0 and I0 plane as shown in Fig. 2 i.e. the output voltage and current can be controlled both in magnitude and direction. Therefore the power flow can be in either direction.

1st Quadrant: In the first quadrant, switch T1 and T2 are switched on, the power flows from source to load and is positive, i.e. both voltage and current are positive as indicated in four quadrant diagram. This mode of operation is known as Forward motoring.

2nd Quadrant:

After switching off T1 and T2, before switching on the T3 and T4, the diodes D3 and D4 conducts, and during this period the power flow is from load to source. In this case, the voltage is negative and current is positive and hence power is negative as indicated in the four quadrant diagram. This mode of operation is known as Regenerative braking.

3rd Quadrant:

In the third quadrant, switch T3 and T4 are switched ON, then both voltage and current are negative, therefore the power flow is positive, but in reverse direction and the power flows from source to load, this mode of operation is known as Reverse motoring.

4th Quadrant:

In the fourth quadrant, the switch T3 and T4 are switched off, the diode D1 and D2 conducts, the power flows from load to source. In this case, the voltage is positive and current is negative and hence power is negative as indicated in four quadrant diagram. This mode of operation is known as Regenerative braking.

Four quadrant operation of DC Motor:

Fig. 2: Four Quadrant Operation Diagram

Regenerative Braking

Regenerative Braking

Forward Motoring

Reverse Motoring

1st 2nd

3rd

4th

D1 & D2 ON

T1 & T2 ON

T3 & T4 ON

D3 & D4 ON

Chopper circuit diagram for four quadrant operation:

Fig. 3: Chopper circuit diagram for four Quadrant operation

Procedure:

• Connect the power module and controller module to AC supply mains • Connect the PWM output of the controller module to the PWM input of the power module

using pulse cable • Connect the armature and field circuit terminals of the DC motor to the DC motor power

module kit • Connect the motor feedback cable to the motor feedback input of the controller module

Selection of switch and potentiometer position:

1) First select the switch S2 at SCM speed control mode, then select switch S1 at open loop

2) Initially, keep the armature pot is minimum and pulse release switch S3 at ON position, unless the program will not execute

3) Keep the field pot in the minimum position, and reset the controller module using switch S4

4) Now the following panels are shown in Fig. below

I. Forward II. Reverse

Select the forward option using I quadrant, after that, the following display as shown below

Vary the armature duty cycle pot, so that the motor runs in selected direction and at a speed corresponds to the duty cycle.

In the open loop condition, keep the motor runs at 1500 RPM condition and gradually vary the load

Select the “reverse option “using a II quadrant switch, now the display will be

Closed Loop control of DC drive:

For closed loop operation, select the forward motoring option using I quadrant switch, then enter the PI controller parameters Kp and Ki values (For example Kp as 0.02 and Ki value as 0)

Set the speed for example 600 RPM and load the machine, verify whether the set speed and actual speed of the motor are same.

Select the reverse motoring option using III quadrant switch and follow the same procedure as mentioned above.

Conclusions:

D.C drive (CW) D.C.Y field = 60% D.C.Y .Armature=50% Actual speed = 0

D.C drive (CW) D.C.Y field = 80% D.C.Y .Armature=56% Actual speed = 2

D.C drive (CCW) D.C.Y field = 60% D.C.Y .Armature=50% Actual speed = 0

Study of Half Bridge and Full Bridge Inverter

Aim:

To study the half bridge and full bridge square wave inverter operation with R and R-L load

Apparatus Required:

Power MOSFETs - IRF740

Driver ICs – IR2110

Capacitors - 1000 µf, 25V

Resistor - 150Ω, 3W

Inductor

Theory:

A single phase full bridge inverter shown in Fig. 4 consists of four switching devices T1, T2, T3, T4 and the four inverse parallel diodes D1, D2, D3, D4. The diodes are essential to conduct the reactive current, to feedback the stored energy in the inductor to the DC source. These diodes are known as feedback diodes. The switching devices may be any one of the full controlled power switching devices like MOSFET.

Square wave switching of single phase Full Bridge inverter

For square wave operation, the switches, T1, T4 and T3, T2 are operated as two pairs with a duty cycle ratio of 0.5. Each of the switches is ON for one half cycle (180 degree) of the desired output frequency. This results in an output voltage waveform as shown in Fig. 5. The voltages VAO and VBO are the potentials of the point A and B with respect to the fictitious midpoint O, respectively. The capacitance ‘C’ must be sufficiently large to assume that the potential at point ‘O’ remains essentially constant with respect to the negative dc bus N.

The load voltage VAB = VAO -VBO

The voltage and current waveforms show that the full bridge inverter operates in all the four quadrants of the Vo-Io plane. When the load is inductive the load current cannot change instantaneously with the output voltage. i.e., if T1 and T2 are turned off at at, t = T/2, the load current would continue to flow through D3, the DC source and D4. Similarly, when the devices T3 and T4 are turned off t = T, the load current flows through D1 the DC source and D2.

The RMS Value of the output voltage is

T/2

Vo = [(2/T) ∫ Vs/2 dt]1/2 = Vs

0

Fourier series representation of instantaneous output voltage, Vo (t)

∞

Vo (t) = ∑ (4A / nП) sin nWt

n =1, 3, 5….

A = Vs Full Bridge Inverter

= Vs / 2 Half bridge Inverter

The rms value of the fundamental component (n =1) is

V1 = 4 A/ (√2 П) = 0.9 Vs

The instantaneous load current with inductive load is

∞

io(t) = ∑ (4A / nП √(R2 + (nwL)2) sin (nwt-Φn)

n =1, 3, 5….

Φn = tan-1 (nwL/R)

w = fundamental frequency

For square wave operation the output voltage magnitude can be controlled by controlling the input dc voltage and output frequency is controlled by varying the switching frequency of the inverter switches. The advantage of the square wave operation is that each inverter switch changes its state only twice per cycle, which is important at very high power levels when the power switching devices have slower turn-on and turn-off speeds.

One of the serious disadvantages of the square wave operation is that the inverter is not capable of regulating the output voltage magnitude. Therefore, the DC input voltage Vs must be adjusted in order to control the magnitude of inverter output voltage. Also the output load voltage is square wave; it contains much of the harmonic components, which is undesirable for most of the applications.

Circuit Diagram: Half Bridge Inverter

Fig. 1 Circuit Diagram of Half Bridge Inverter

Load

C1, 1000µf

C2, 1000µf,

P1 Vdc P2

T1

T2

Typical waveforms of Half Bridge Inverter (R Load):

Fig. 2 - Typical waveforms of Half Bridge Inverter (Resistive Load)

Typical waveforms of Half Bridge Inverter (R-L Load)

Fig. 3 - Typical waveforms of Half Bridge Inverter (R-L Load)

T1 T1 T2 Vdc

Vdc/2

- Vdc/2

Vdc/2

P2

Vo

P1

Vdc/2 Vdc/2

Vdc/2R

Io

T T/2

T/2 T

T/2 T

- Vdc/2R

t

t

T1 T1 T2 Vdc

Vdc/2

- Vdc/2

Vdc/2

P2

Vo

Vdc/2 Vdc/2

Io

T/2 T

T

T/2

T

T/2

t

t

t

t

t

Circuit Diagram: Full Bridge Inverter

Fig. 4 Circuit Diagram of Full Bridge Inverter

Typical waveforms Full Bridge Inverter (R-Load):

Fig. 5 - Typical waveforms of Full Bridge Inverter (Resistive Load)

T1 T1 T4

T3 T2 T2

Vs/2

Vs

- Vs

VBO

VAB = VAO - VBO

IL

Vs/R

-Vs/2

Vs/2

-Vs/2

T

T/2 T

t

t

t

t

A

T4

T1

Load B 0

C

C

D1

D2

D4

D3

T3

T2

VS

VAO

-Vs/R

Typical waveforms Full Bridge Inverter (R-L Load):

Fig. 6 - Typical waveforms of Full Bridge Inverter (R-L Load)

T1 T1 T4

T3 T2 T2

Vs/2

Vs

- Vs

VBO

VAB = VAO - VBO

IL

-Vs/2

Vs/2

-Vs/2

T1,T2 T3,T4

t

t

t

t

T/2 T

VAO

D1,D2 D3,D4

Speed control of 3-phase induction motor using V/F Control

Aim:

To study the speed control of a 3-phase induction motor by V/F technique in open loop and closed loop.

Apparatus required:

1. Chopper inverter/PWM controller

2. Power module

3. 3-phase Induction motor

4. Patch chords

Circuit diagram: Three Phase Inverter

Fig. 1

Theory of V/F Control:

The AC induction motor is a workhorse of an adjustable speed drive systems. The most popular type is

the 3-phase, squirrel-cage AC induction motor. It is maintenance-free, lower noise and efficient motor.

The stator is supplied by a balanced 3-phase AC power source. The synchronous speed ns of the motor is

given by Equation (1)

sn = sf120 / p ……………………. (1)

Where, fs is the synchronous stator frequency in Hz, and p is the number of stator poles. The load torque

is produced by slip frequency. The motor speed is characterized by a slip s, given by the Equation (2).

The relation between fs, s and p is given in Equation (3)

s = ( sn - rn ) / sn = sln / sn ………….... (2)

rn = sf120 s1 / p ……………. (3)

Where, nr is the rotor mechanical speed and nsl is the slip speed, both in rpm.

Principle of Volts per Hertz (V/F) Control

Changing the supply frequency or the number of poles as given by the Equation (1) can vary speed of an

induction motor. Changing the number poles is cumbersome procedure and hence it is not in practice.

Changing the supply frequency is the best way to vary the speed but as per the Equations (5) and (7)

changing the frequency alone will affect the air-gap flux, which ultimately affects the torque production

capability.

Volt per Hertz control methods is the most popular method of Scalar Control; it controls the

magnitude of the variable like frequency, voltage or current. The purpose of the volt per hertz control

scheme is to maintain the air-gap flux of AC Induction motor constant in order to achieve higher run-time

efficiency and also to maintain the torque at all speeds. In steady state operation the machine air-gap flux

is approximately related to the ratio V/fs, where V is the amplitude of motor phase voltage and fs is the

synchronous electrical frequency applied to the motor. The base point of the motor defines the

characteristic as shown in Figure 8. Below the base point the motor operates at optimum excitation

because of the constant V/f ratio. Above this point the motor operates under-excited because of the DC-

Bus voltage limit. A simple close-loop volts/hertz speed control for an induction motor is the control

technique targeted for low performance drives. This basic scheme is unsatisfactory for more demanding

applications where speed precision is required.

Since the development of power-electronics and microcontrollers a new way of controlling the motor-

speed has been introduced. A three-phase inverter topology is shown in Figure 9. This topology can be

used for changing the frequency and / or the amplitude voltage applied to the motor 3-phase stator

windings. With this controllable frequency and voltage it is possible to achieve a high efficient speed-

controller for induction motor. One thing to consider is the torque on the shaft. If the voltage applied to

the motor changes, the frequency also has to change to ensure sufficient torque on the shaft. Looking at

the torque, expressed from the power (P) and the speed the following equations are derived:

Power 60/.2 TnP r

Torque rn

PT

55.9* =

sf

pVI

s

1.120

55.9**cos..3* ……………. (4)

= sf

Vk * ………………. (5)

E = msf 44.4 ……………….. (6)

m = sfEk /* ………………. (7)

Equation (7) implies that if the ratio between the applied stator voltage and the frequency are kept

constant the torque also stays constant. These terms are the ones used to implement the speed controller

for the induction machine.

Fig. 2 Volts Frequency Relation of V/F control

To ensure maximum torque capability at all time it is therefore necessary to maintain the

magnetic flux at its rated value at any frequency. The flux can be maintained constant at its rated value by

maintaining the ratio sfE / constant. At high speed, where the induced back-EMF is large, the drop

across the stator impedance is negligibly small. Therefore sfE / is maintained constant by maintaining

V/f constant. However at low speed, the back-EMF is low and also the drop is significant. Thus the flux is

reduced below rated torque, capability is also reduced. The performance at low speed can be improved by

boosting the voltage at low frequency as shown in Fig. 2. Three induction motor is fed from a three phase

PWM inverter whose switching is controlled by V/F technique as shown in Fig. 3.

Block diagram of V/F control:

Fig. 3

Connection procedure:

1. Connect 1φ AC input supply to the power module

2. Connect PWM output from the controller module to the power module

3. Connect the R, Y, B terminals of the AC motor to their respective terminals of the power module

4. Connect the motor speed feedback cable to the controller unit

Experiment procedure:

1. Switch on the power ON/OFF switch in the IGBT based power module and the controller module

2. Switch ON the MCB in the power Module

3. Vary the VARIAC from minimum to maximum position and supply AC voltage gradually to the

power module

For open loop:

1. Using decrement/increment key select the drive DC-AC inverter

2. Switch on the PWM pulse

3. Vary the frequency

4. Observe the motor speed by varying input frequency

5. Tabulate the observed readings

3-ph Induction Motor

Tabular column:

S.no Frequency, Hz Modulation index (ma) Actual speed, rpm

For closed loop:

Select the Closed loop option.

1. Enter the desired Kp and Ki values.

2. Set a desired speed.

3. Observe the actual speed of the motor by increasing the load

4. Tabulate the observed readings

Tabular column:

S.no Kp Ki Set Speed, rpm Actual speed, rpm

Conclusions:

Study of PWM Inverter

Aim:

To study the operation of sinusoidal pulse width modulation (SPWM) switching single phase PWM

inverter.

Apparatus Required:

1. Single phase PWM Inverter Control Module

2. MOSFET power module

3. DSO

4. R - L load

5. 9 pin cable

6. Power chord

7. Patch chords

Circuit diagram:

Fig -1

Pulse Width Modulation (PWM) techniques:

1. Sinusoidal PWM

2. Trapezoidal PWM

Sinusoidal Pulse Width Modulation:

The switching sequence for the inverter switches is obtained by comparing a sinusoidal reference

signal, of adjustable amplitude and frequency with a fixed frequency triangular carrier wave. The

sinusoidal reference frequency decides the fundamental frequency of the inverter output voltage and

is also called as modulating frequency. The inverter output voltage will contain fundamental

frequency voltage component and voltage components at harmonic frequencies of reference signal.

Sine Triangular PWM Generation

Waveforms of Sinusoidal Pulse Width Modulation (Unipolar Voltage Switching)

In Unipolar voltage switching, two reference sinusoidal waveforms with 180 degrees phase shift are

compared with high frequency triangular wave as shown in the Figure below to generate PWM signals.

Figure: Typical Unipolar Voltage Switching Waveforms

Output voltage Amplitude (Fundamental):

Vo,1 = Ma * Vdc

Vdc - Input DC voltage

Amplitude Modulation ratio:

Ma = Ar / Ac

Ar - Reference Sine wave Amplitude

Ac - Carrier wave Amplitude

Frequency Modulation ratio:

Mf = fc / fr

fc - Carrier wave frequency

fr - Reference wave frequency

Connection Procedure:

1. Connect P1 of 24V DC input +ve terminal to P2 of Mosfet.

2. Connect P8 of 24V DC input -ve terminal to P9 of Mosfet.

3. Connect P4 to P7 of mosfet.

4. Connect the 9-pin connector of inverter control module to that of power module.

5. Connect the external load between P4 and P6.

Experiment Procedure:

1. Switch on the Inverter control module and MOSFET power module & reset it initially.

2. Set the reference sine wave using reference wave selection switch and set its amplitude and

frequency.

3. Set the PWM pulse amplitude and frequency of carrier wave.

4. Switch on the SPDT switch to release PWM pulse to the power module.

5. Check the test waveform in every test points using DSO.

6. Connect the R-load first and observe the output AC voltage.

7. Then connect the inductive load in series with the resistive load.

8. An output AC voltage obtained across the load is observed.

9. Also, observe the output voltage harmonic spectrum using FFT mode in DSO

10. Calculate the amplitude of the output voltage.

Tabular column:

Sl. No. Sine wave

Amplitude (V)

Ar

Carrier wave

Amplitude (V)

Ac

Modulation index

(Ma)

Measured output Vo,1 (V)

Conculsions: