TRIAC Controlled AC Motor Drive

core technologies

CORE TECHNOLOGIESSUITE NO.8A, 3RD FLOOR184, LENIN SARANIKOLKATA 700 013PHONE: 91-33-22126863, 65FAX: 91-33-22124167EMAIL: [email protected]: www.core-technologies.com

1. Aim of the Experiment

Study of TRIAC Controlled Single Phase AC Motor Drive

2. Apparatus Required

1) Drive Unit2) 0.5 HP AC Motor3) C. R. O.4) Connecting wires

3. Operating Panel

Operating Panel

4. Theory of Operation

Schematic Diagram

Signal Waveforms

The above diagram consists of the following blocks:

4.1 Zero Crossing

The A.C. signal from the secondary of a step-down transformer is full-wave rectified and applied to the non-inverting terminal of an op-amp configured as a comparator. The inverting terminal of the op-amp is connected to a reference D.C. voltage. The resulting waveform from the comparator is a square-wave of 100Hz frequency and remains low during the zero-crossing period.

4.2 Ramp-Generator

The IC 555 is used as an astable multivibrator in the ramp generator configuration by using a current mirror. The current mirror starts charging the capacitor towards Vcc at a constant rate. When the voltage across the capacitor equals to 2/3 Vcc, the internal comparator turns on the internal transistor and the capacitor rapidly discharges through the transistor. However, when the discharge voltage across the capacitor equals to 1/3 Vcc, the internal comparator switches the internal transistor off, and then the capacitor starts charging up again. The charge-discharge cycle keeps on repeating. The Reset terminal of IC 555 is connected to the output of the zero-crossing detector, thereby synchronizing the output of the ramp-generator with the input waveform. Therefore, the output goes low during the zero-crossing period and ramps up till the next zero crossing is encountered.

4.3 Phase Control

A DC voltage is compared using a comparator, with the ramp-generator output to provide pulses for phase-control. The more the DC voltage, the further away the ramp cuts from zero-crossing, thus giving an output pulse width which starts further away from the zero-crossing. The more the DC voltage, the more the phase angle. (e.g. 0 ~ 5V corresponds to 0 ~ 180).

For open-loop control, a potentiometer is used to control the phase-angle. For closed-loop control, the output of the PID Controller is used to control the phase angle.

4.4 TRIAC firing

An IC 555 is used to generate a 50% duty cycle, 5 KHz square wave to be used as a pulse generator for the pulse transformer. The reset terminal of the IC is connected to the output of the phase-control. The resulting waveform is a 5 KHz waveform enveloped in the phase control waveform. One 1:1 pulse transformer is used for providing the TRIAC with isolated pulses.

4.5 PID Controller

A proportional–integral–derivative controller (PID controller) is a generic control loop feedback mechanism widely used in industrial control systems. A PID controller attempts to correct the error between a measured process variable and a desired setpoint by calculating and then outputting a corrective action that can adjust the process accordingly.

The PID controller calculation (algorithm) involves three separate parameters; the Proportional, the Integral and Derivative values. The Proportional value determines

the reaction to the current error, the Integral determines the reaction based on the sum of recent errors and the Derivative determines the reaction to the rate at which the error has been changing. The weighted sum of these three actions is used to adjust the process via a control element such as the position of a control valve or the power supply of a heating element.

By "tuning" the three constants in the PID controller algorithm the PID can provide control action designed for specific process requirements. The response of the controller can be described in terms of the responsiveness of the controller to an error, the degree to which the controller overshoots the setpoint and the degree of system oscillation. Note that the use of the PID algorithm for control does not guarantee optimal control of the system or system stability.

Some applications may require using only one or two modes to provide the appropriate system control. This is achieved by setting the gain of undesired control outputs to zero. A PID controller will be called a PI, PD, P or I controller in the absence of the respective control actions. PI controllers are particularly common, since derivative action is very sensitive to measurement noise, and the absence of an integral value may prevent the system from reaching its target value due to the control action.

Block Diagram of a PID Controller

Proportional term

The proportional term makes a change to the output that is proportional to the current error value. The proportional response can be adjusted by multiplying the error by a constant Kp, called the proportional gain.

Change of response for varying Kp

The proportional term is given by:

Pout = Kp e(t)

WherePout : Proportional output Kp : Proportional Gain, a tuning parameter e : Error = SP − PVt : Time or instantaneous time (the present)

A high proportional gain results in a large change in the output for a given change in the error. If the proportional gain is too high, the system can become unstable. In contrast, a small gain results in a small output response to a large input error, and a less responsive (or sensitive) controller. If the proportional gain is too low, the control action may be too small when responding to system disturbances.In the absence of disturbances, pure proportional control will not settle at its target value, but will retain a steady state error that is a function of the proportional gain and the process gain. Despite the steady-state offset, both tuning theory and industrial practice indicate that it is the proportional term that should contribute the bulk of the output change.

Integral term

The contribution from the integral term is proportional to both the magnitude of the error and the duration of the error. Summing the instantaneous error over time (integrating the error) gives the accumulated offset that should have been corrected previously. The accumulated error is then multiplied by the integral gain and added to the controller output. The magnitude of the contribution of the integral term to the overall control action is determined by the integral gain, Ki.

Change of response for varying Ki

The integral term is given by:

t

Iout = Ki e(τ) dτ 0

WhereIout : Integral output Ki : Integral Gain, a tuning parameter e : Error = SP − PVτ : Time in the past contributing to the integral response

The integral term (when added to the proportional term) accelerates the movement of the process towards setpoint and eliminates the residual steady-state error that occurs with a proportional only controller. However, since the integral term is responding to accumulated errors from the past, it can cause the present value to overshoot the setpoint value (cross over the setpoint and then create a deviation in the other direction).

Derivative term

The rate of change of the process error is calculated by determining the slope of the error over time (i.e. its first derivative with respect to time) and multiplying this rate of change by the derivative gain Kd. The magnitude of the contribution of the derivative term to the overall control action is termed the derivative gain, Kd.

Change of response for varying Kd

The derivative term is given by:

Dout = Kd de/dt

WhereDout : Derivative output Kd : Derivative Gain, a tuning parameter e : Error = SP − PV t : Time or instantaneous time (the present)

The derivative term slows the rate of change of the controller output and this effect is most noticeable close to the controller set point. Hence, derivative control is used to reduce the magnitude of the overshoot produced by the integral component and improve the combined controller-process stability. However, differentiation of a signal amplifies noise and thus this term in the controller is highly sensitive to noise in the error term, and can cause a process to become unstable if the noise and the derivative gain are sufficiently large.

The output from the three terms, the proportional, the integral and the derivative terms are summed to calculate the output of the PID controller. Defining u(t) as the controller output, the final form of the PID algorithm is:

t

u(t) = MV(t) = Kp(t) + Ki e(τ) dτ + Kd de/dt 0

and the tuning parameters are

Kp: Proportional GainLarger Kp typically means faster response since the larger the error, the larger the Proportional term compensation. An excessively large proportional gain will lead to process instability and oscillation.

Ki: Integral GainLarger Ki implies steady state errors are eliminated quicker. The trade-off is larger overshoot: any negative error integrated during transient response must be integrated away by positive error before we reach steady state.

Kd: Derivative GainLarger Kd decreases overshoot, but slows down transient response and may lead to instability due to signal noise amplification in the differentiation of the error.

4.6 Speed Measurement

The shaft of the motor is connected an optical encoder. The encoder has a circular plastic disc having 100 alternate black and transparent stripes. The disc is passed through an opto-interrupter MOC7811. Pulses are generated when the disc turns with 100 pulses per revolution.The pulses from the sensor are shaped using an op-amp configures as a comparator. The pulses are fed into a microprocessor-based display for indication of the speed in RPM.

MOC7811 has four connections, i.e. D+ & D- for Detector and E+ & E- for emitter. The connections from the motor setup to the drive unit should be made one to one. On rotation of the motor, if the tachometer fails to display the speed, the connections should be checked. In case of sensor failure, do not run the drive unit in closed loop.

4.7 Load Setup

The load setup for the motor is a flat pulley and friction belt braking system. A friction belt goes around a flat pulley with a spring balance on one side and a nut-bolt arrangement on the other. On tightening the nut the friction between the rope and pulley increases to create a larger load on the motor.

5. Operation of Motor Controller

5.1 Introduction

The Digital Motor Controller is a PID Controller with some additional features required to operate DC/AC1Ø/AC3Ø motors. The control unit consists of a four-digital digital display unit for displaying various parameters, some

indicating LED’s and some buttons for setting of parameters and running of the motor.

The motor speed is sensed using an optical encoder. The speed, set point, error and control parameters are displayed on the digital display panel one by one by selecting the viewing parameter using the SET button. The parameters may be changed and save by using the edit buttons.

The motor controller provides the facility to run the motor in open or closed loop. Different faults are indicated on the display unit.

The controller also incorporates safety features like soft-start and open/closed loop transfer through zero speed.

5.2 Speed Measurement

The motor speed is sensed using an optical encoder that generates 100 pulses per revolution. These pulses are counted and the frequency in Hz is calculated every 100 msec. The PID algorithm is executed and the output is used to drive the signal section of the specific drive.

5.3 Open Loop Operation

In the open loop operation, the output to the drive section may be varied from 0 to 100%. No feedback or control action is taken. The output and the speed or process variable may be displayed using the SET button.

5.4 Closed Loop Control

In the closed loop, the output to the drive section is controlled using a PID algorithm. The PID section describes in detail the operation and functioning of the PID algorithm. The PID parameters may be changed while the motor is running. The set point may be varied between 0.0 to 50Hz. The various parameters like set point, process variable (speed), error, proportional gain, integral gain, derivative gain and output are displayed on the digital display. The parameter required to be observed/modified may be selected using the SET button.

5.5 Operation

5.5.1 Operating Panel

The front fascia consists of 4 7-segment displays, 14 LED’s and 8 switches. The 7-segment display indicates the various parameters. The LED’s indicate the mode of operation, motor status and the parameter being viewed. The switches are used to set different parameters and switch between different modes.

5.5.2 Operating Panel Layout

5.5.3 Description of Switches

REMOTE/LOCAL – Remote / Local SwitchSelect between Remote and Local modes of operation. Change over only possible when the motor is stopped.

OPEN/CLOSED – Open / Closed Loop switchSelect between Open Loop and Closed Loop modes of operation. Change over only possible when the motor is stopped.

RUN – Motor Run switchUsed to run the motor.

STOP – Motor Stop switchUsed to stop the motor.

SET – Select / Exit switchIn normal mode, it is used to select the display mode. In editing mode, it is used to quit without saving the parameter.

INC – Increment switchUsed to increase the value of the parameter to be edited.

DEC – Decrement switchUsed to decrease the value of the parameter to be edited.

ENT – Enter switchUsed to save the parameter that is being edited.

5.5.4 Description of LED’s

RE – Remote Mode IndicatorLO – Local Mode IndicatorOP – Open Loop Mode IndicatorCL – Closed Loop Mode IndicatorRUN – Run Operation IndicatorSTOP – Stop Operation IndicatorSP – Set Point IndicatorPV – Process Value IndicatorER – Error Value IndicatorKP – Kp Value IndicatorKI – Ki Value IndicatorKD – Kd Value IndicatorOU – Output Value IndicatorFA – Fault Indicator5.5.5 Open-Loop Operation

When the unit is powered on, it is in the Open Loop mode by default. The motor is stopped and the set point = 0.0Press SEL to toggle between PV and OUT displays.Press O/C to toggle between Open Loop and Closed Loop modes.

In the open loop mode, only the output can be set. The value of output can be from 0.0 ~ 100.0%. Please note that the motor may not rotate below a certain

minimum output. The output may be set by pressing the INC or DEC buttons and confirmed by pressing ENT. If the motor is on, the output value is instantaneously transferred to the soft-start algorithm so that it is slowly reflected on the output. The output value is not saved on power off.

5.5.6 Closed-Loop Operation

Press O/C to toggle between Open Loop and Closed Loop modes.Press SEL to display the various parameters in the following sequence.

In the closed loop mode, the following parameters may be set.

PARAMETER RANGESP – Set Point 0.0 ~ 50.0 HzKP – Proportional Gain 0.0 ~ 20.0KI – Integral Gain 0.0 ~ 70.0 s-1

KD – Derivative Gain 0.0 ~ 20.0 s

The value of set point can be varied from 0.0 ~ 50.0Hz. Please note that the motor may not if adequate gain is not selected. All parameters may be set by pressing the INC or DEC buttons and confirmed by pressing ENT. The effect is instantaneous. The values of Kp, Ki and Kd are saved on power failure.

Additionally, the following parameters may be displayed.

PVERR

OUTSPKP

KI

KD

PARAMETER RANGEPV – Process Value 0.0 ~ 50.0 HzERR – Error -50.0 ~ 50.0 HzOUT – Output 0.0 ~ 100.0 %

5.6 Fault Indications

Faults are indicated on the digital display. The various faults, their indications and sources are given below:

Fault Display Reason Applicable Drives

Comments

Field Failure

FFAU Field not connected/faulty

CP-DL-1CP-DL-2

Requires Power On.

DC Bus Not Ready

nrdy Internal DC Bus Not Ready.

CP-DL-2CP-DL-4

At startup only.

Over Current

oCur Armature Current has gone above the limit.

CP-DL-1CP-DL-2CP-DL-3CP-DL-4

Requires Power On.

6. Procedure

AC MOTOR0.5 HP

A

AA

A

AA

L

N220 V

ACDRIVE UNIT

ARMATURE

Connection Diagram

Experiment 1: Open Loop Operation

1) Use connecting wires to connect the motor, drive unit and mains according to the connection diagram.

2) Connect the A.C. mains cord to the power socket, but do not switch on the M.C.B.3) Switch on the M.C.B. and see if the voltmeter, ammeter and tachometer come on.

If they do not turn on, then switch of the M.C.B. and report the problem to the instructor.

4) Remove load from the loading arrangement.5) Set the output to 10% using the buttons. Put the motor in RUN mode. Observe the

voltage, current and speed readings on the displays. The motor may not rotate at 10% output. This is normal.

6) Increase the output is steps of 10% and observe the readings. If the motor does not rotate after 50V, switch of the drive unit and check the motor connections.

7) Note the readings in observation table.8) Repeat the above experiment with variable load by applying different loads using

the loading system in steps of 0.5kg.9) Draw a graph of Voltage vs. Speed for different load values.10) Draw a graph of Current vs. Speed for different load values.

SN Speed (RMP) Voltage (V) Current (I)

SPEEDSENSOR

Observation Table

Experiment 2: Closed Loop Operation

1) Use connecting wires to connect the motor, drive unit and mains according to the connection diagram.

2) Connect the A.C. mains cord to the power socket, but do not switch on the M.C.B.3) Switch on the M.C.B. and see if the voltmeter, ammeter and tachometer come on.

If they do not turn on, then switch of the M.C.B. and report the problem to the instructor.

4) Apply a minimum load (non-zero) using the loading arrangement.5) Set the operation to closed loop mode.6) Check the default settings. (P = 5.3, I = 1.7, D = 0.0).7) Set the set point at 5.0Hz (300 RPM) using the buttons. Put the motor in RUN

mode. Observe the voltage, current and speed readings on the displays.8) If the motor does not rotate, switch of the drive unit and check the motor

connections.9) Note the readings in observation table.10) Repeat the above experiment with variable load by applying different loads using

the loading system in steps of 0.5kg. Please check if the motor is drawing current in excess of 90% of its rating.

11) Draw a graph of Voltage vs. Speed for different load values.12) Draw a graph of Current vs. Speed for different load values.13) Repeat the above experiment for speeds of 10.0Hz (600RPM), 15.0Hz (900

RPM), 20.0Hz (1200 RPM), 25.0Hz (1500 RPM), 30.0Hz (1800 RPM), 35.0Hz (2100 RPM) and 40.0Hz (2400 RPM).

14) Draw a graph of Voltage vs. Load for different Speed values.15) Draw a graph of Current vs. Load for different Speed values.

SN Load (Kg) Voltage (V) Current (I)

Observation Table