- 1 Introduction to soft starter
- 2 why we need soft starter for induction motor
- 3 Soft starter types/techniques
- 4 TYPICAL SOFT STARTER TECHNIQUES
- 5 Soft starter block diagram using microcontroller
- 6 COMPONENTS of soft starter
- 7 circuit diagram of Soft starter using microcontroller
- 8 Complete PCB design of soft starter
- 9 CONCLUSION on soft starter project
Introduction to soft starter
Soft starter becomes a very important part of induction motors.Starting an induction motor smoothly has always been a difficult task because induction motor draws initial high current and torque that may burn the windings of induction motor. Engineers with the passage of time proposed and implemented several techniques to overcome this problem of controlling initial high current and torque value. Some techniques, for example, star-delta, direct online, auto transformer etc are currently being used in different industrial setups to run induction motors smoothly but every technique has some limitations or constraints.
Based on the previously known old techniques our proposed project work is to control the initial high current and torque by triggering of the triacs. Triggering of triac is controlled through the firing angle and firing angle is adjusted through the knob of potentiometer. By varying the knob of potentiometer firing angle increases due to which voltage reduces which in turn reduces the current and torque value. Firing angle (delayed pulse) set by the knob of potentiometer goes into the gate of triac to start induction motor smoothly with reduced voltage. Once induction motor attains the desired RPM (revolution per minute) provided by the user through keypad, micro-controller sends the signal to relay for switching from NC (normally closed) path where reduced voltage feed to induction motor into NO (normally open) path where induction motor runs on full speed without any burning of windings or extra heat up.
By using this technique initial high current and torque controlled during the starting of an induction motor and after getting desired speed induction motor run at full speed. This proposed work is an extension to new engineering design technique, a modern and state of the art technique which will run an induction motor smoothly
An induction motor will run only because of the phase difference between the stator’s magnetic field and rotor’s magnetic field. If phase difference is same induction motor will not run. So phase difference is necessary and because of this difference, rotor’s magnetic field tries to catch stator’s magnetic field and draws current. When the induction motor runs at full speed, current increases to a high value which in turn increases the value of torque. As a result of initial high current, the windings of induction motor damaged and cause overheating.
So to prevent induction motor from damage, motor soft starter is needed. A soft starter is a device that reduces the torque applied to the induction motor. There are different techniques of soft starters and every technique has some limitations. In proposed technique solid state soft starter method is used to start induction motor smoothly.
In solid state soft starter technique, the applied voltage is controlled through triac by adjusting the firing angle of triac. The firing angle is adjusted through the potentiometer, by varying the knob of potentiometer value of resistance changes, which reduces the voltage, current and further torque value. By raising the knob of potentiometer, value of resistance increases which reduces voltage and in the same way current and torque also. Once the induction motor start smoothly with low voltage and acquires desired RPM provided by user, then micro-controller sends the signal to bypass the triac circuitry through relays. After bypassing is done, the induction motor will run smoothly and safely at its full speed.
A feedback path is used to measure the speed of the induction motor through encoder. Encoder counts the RPM and displayed it on LCD (liquid crystal display). After desired RPM provided by the user through keypad reaches, the micro-controller sends signal to bypass the triac circuitry. This soft starting technique increases the efficiency of induction motor because it produces no current spikes. It is also a low cost technique as compared to other techniques. Not only this after smooth start, induction motor automatically bypassed the circuit and attains the full speed without damaging the induction motor.
why we need soft starter for induction motor
The aim of this project is to design of a soft starter for three phase induction motors that will protect induction motors from initial high current and torque that develop due to slip in induction motor rotor.
The objectives of this project are to design a soft starter with following salient features:
- Reduced starting current
- Reduced electrical power cost
- Wide range of torque and current ratings
- Surge current protection for starting conditions
Starting current in induction motor is so high that it damages the induction motor windings and increases the cost of electricity. To avoid this problem a soft starter is required to run the induction motor smoothly. Current technologies used for soft starting induction motor have certain limitations which necessitates the need to develop a soft starter to limit the current.
Soft starter is used to start an induction motor smoothly and protect the motor from initial high current and torque. To run an induction motor smoothly is the need of today technology otherwise we can always have another induction motor because it will damage the windings of induction motor and cause overheating. So, to avoid burning of winding and other issues like voltage and current spikes etc soft starter is used. It is mostly used in:
- Power sectors (Industrial Applications)
- Controlling high torque
- Reducing starting current.
Soft starter types/techniques
To start an induction motor smoothly different techniques are implemented in different times which are given below:
A SOFT STARTING METHOD FOR WOUND–ROTOR INDUCTION MOTOR
This method is used to start an induction motor smoothly with reduced torque and starting current. In this method additional elements are added in the rotor circuit. Additional elements are in parallel combination of resistors, self-inductors and capacitors in rotor circuit.In figure below and are the parameters of the Thevenin’s equivalent circuit for stator and and are rotor parameters. Other parameters, and represent the added resistor, self inductor and capacitor respectively. In this circuit, the impedance of rotor equivalent circuit is R+jX.
In this technique a resistor is used between supply and motor. In the start of the motor some voltage drop across the resistor which in turn reduces the current and torque value. With increase in the speed of the motor, resistance manually decreases and when motor reaches at its desired speed then resistance completely removed. The disadvantage of this technique is that resistor is to be removed manually with respect to increment in the speed of the motor.
INSULATED-GATE BIPOLAR TRANSISTOR INDUCTION MOTOR SOFT STARTER
A large amount of current is drawn by three phase induction motor. In order to deal with the adverse effect of initial high currents in induction motor a technique is used which electronically control the soft starting with IGBTs. Initial high current and torque are reduced by triggering back-to-back-connected IGBT at proper points on the first supply voltage cycle. This circuit is used during starting to protect the motor from high current and torque. Once motor start smoothly and run at desired speed then it is disconnected through main system because now motor is protected.
VARIABLE FREQUENCY DRIVE SOFT STARTER
Through VFD (variable frequency drive) speed of induction motor can be control which reduces the voltage and current. By changing the frequency, different speed levels are achieved.
The frequency depends upon the expression given below :
N=120 * F * P
N= Speed (RPM)
F= Frequency (Hz)
P= Number of motor poles
PART WINDING soft starter TECHNIQUE
The lowest cost method to reduce the starting initial high current, torque and voltage is part winding technique. In this technique voltage is given to only half-winding of the circuit to start it smoothly while remaining windings get voltage after motor started smoothly. Initially voltage is applied to half-winding circuit. By using this current and torque reduces. This technique offer better performance as compared to the wye delta technique.
DIRECT TORQUE CONTROL soft starter FOR THREE-PHASE INDUCTION MOTOR
DTC (direct torque control) technique is used to prevent the induction motor from initial high current. However, this technique requires accurate system parameters, inner current loop and complex coordinate transformation. The DTC method provides strong and fast torque response without such coordinate transformations, PWM pulse generation and current regulators. Moreover, DTC minimizes the use of motor parameters.The DTC strategy is based on a predictive algorithm and is implemented in a synchronous reference frame aligned with the machine stator flux vector. The advantages of the this strategy are constant inverter switching frequency, good transient and steady-state performance, low distortion of machine current with respect to DSC (direct self control) and other DTC schemes with variable switching frequency.
TYPICAL SOFT STARTER TECHNIQUES
In soft starter device, controlling the applied voltage and current will control the acceleration of an electric induction motor.Several soft starting techniques are being used for starting the induction motors smoothly.
The five main techniques are :
- Primary resistor.
- Auto transformer.
- Direct-on-line starting.
- Wye delta.
- Solid state.
PRIMARY RESISTOR soft starter
The first technique to start an induction motor is primary resistor technique. High torque can be established by the addition of external resistance in rotor circuit. The addition of external resistance reduces starting torque down to 2-2.5 times the full load torque and the starting current down to 1-1.5 times the full load current.
“L” denotes line and in figure 2-2 there are three lines from Line 1 to Line 3.
“T” denotes transformer in figure 2-2 there are three transformers from T1 to T3.
AUTO TRANSFORMER soft starter
In auto transformer technique, starting current and torque can be maintained by reducing the initial voltage. The motor that is connected permanently in star or delta, is first switched on reduced voltage from a 3-phase tapped auto-transformer and when it has accelerated sufficiently, it is switched to the running (full voltage) position. The advantage of this method is that by correct taping in autotransformer the current and torque can be adjusted to required value. This method is more expensive because of the additional autotransformer. This technique is similar to star/delta starting with similar limitations.
“L” denotes line and in figure 2-3 there are three lines from Line 1 to Line 3.
“T” denotes transformer in figure 2-3 there are three transformers from T1 to T3.
Direct-on-line starting is the simplest and cheapest method for starting the induction motor because in this method, the induction motor is connected and switched on directly to the power supply. It involves no circuitry between the power supply and induction motor.But there is one big disadvantage of this method. By switching on induction motor directly with the power supply, initial high currents and high torque is generated which damages the induction motor. So this method is not safe for large motors. This method can be applied on small motors because small motors have low voltage ratings.
WYE DELTA soft starter
This technique is applicable to motors designed for delta connection in normal running conditions. Connect both ends of each stator to 3-phase change-over switch. Initially stator windings are connected for starting the motor and when motor start running the switch is thrown quickly to the running position and connecting the motor in delta for normal operation. In star connection the phase voltages and phase currents of the motor reduce to 1/3 of the direct-on-line values in delta. One disadvantage of this method is that the starting torque (which is proportional to the square of the applied voltage) is also reduced to 1/3 of its delta value.
“T” denotes the terminal in figure 2-4 there are six terminals form terminal 1 to 6.
SOLID STATE soft starter
Solid State based soft starter is the advanced technique among all the techniques discussed before. In solid state soft starter, solid state devices, like thyristors or SCRs, are used and replaced with the mechanical components. The speed of the induction motor is controlled by the thyristors. The thyristors limit the applied voltage to the induction motor which in turn reduces the initial currents and torque. This will start motor smoothly and after the induction motor is started smoothly, the thyristor circuitry will be by-passed and then motor will be connected directly with the power source and induction motor will run at its full speed safely and smoothly.
The steps through which solid state soft starter starts the induction motor are:
- In the first step, contact (C1) is closed.
- After the switch is closed, thyristors turn on and motor starts accelerating.
- When the thyristors are fully on, then contact C2 is closed.
- After the closing of contact C2, the motor will be connected directly with power source and will run at its full speed.
“L” denotes line and in figure 2-5 there are three lines from Line 1 to Line 3.
On the basis of above described techniques, a technique is proposed which is basically a modification of solid state technique. Proposed solid state soft starter technique is used not only to start an induction motor smoothly but also disconnect it automatically through feedback path when induction motor acquires desired speed. This is modern, efficient and low cost technique as compared to the other techniques like direct-on-line technique (in which chances of burning of windings increases) and primary resistor technique (in which manually resistance added or removed) etc.
Soft starter block diagram using microcontroller
Figure below represents zero cross detector circuit which detects the zero at each phase and after the zero detection this signal goes into the 555 timer IC which generates delay (at different firing angle depend upon potentiometer). Delayed pulse then goes into gate of triac to turn it on through opto-coupler, once triac get triggered they turn on induction motor smoothly. Encoder counts the RPM of induction motor, once induction motor gets the desired RPM provided by user micro-controller will send a signal to bypass triac circuit through relays. Now induction motor operates on normal condition and disconnect from triac path. Encoder not only used to detect speed in terms of RPM but also detect error in running of induction motor it is just like a feedback path which send feedback at desired speed and in case of any error.
COMPONENTS of soft starter
On the basis of above methodology following components selected:
- Relay (JQC3F (T73))
- Triac (BT136)
- Arduino (Mega 2560)
- Opto-coupler (MOC3021)
- 555 timer IC
- Induction motor (four-pole squirrel-cage)
- Encoder (ENC005)
- Keypad (4×3)
- LCD (16×2)
- Other components (voltage regulating IC’s, resistors etc).
RELAY (JQC3F (T73))
A relay is a device that opens or closes the contacts to another circuit through a low powered electrical signal.Relays come in different types. In this project JQC3F (T73) type of relay is used. Reason for using this 6V relay is that it switched on low coil voltage and there is no need of additional circuit for switching of relay. This voltage easily taken from regulated IC of LM7808 using in zero cross detector otherwise if, 12V or any other relay is used there is need for an extra circuit to operate it. The purpose of this relay is to bypass the triac circuitry. Each relay is connected to every phase of the system. Relay is connected with AC source, arduino mega 2560 and triac. Arduino will send a signal to relay, to bypass the triac circuitry when induction motor attains desired RPM which is detected by encoder. After getting the signal from arduino, relay bypasses the triac circuitry and connects the induction motor direct with the power source.
Triac is a bidirectional semiconductor device with three terminals which is used for bidirectional current both in positive as well as in negative cycle. Triac can be triggered by both positive and negative current applied to its gate.There are many different types of triac. The type of triac that is used in this project is BT136. Reason for choosing this triac is that it is used in motor control application with high voltage and current rating which is suitable with induction motor as compared to other the triacs. It is easy available and inexpensive. The purpose of using triac in the project is to lower the voltage supplied to the induction motor by giving delayed pulse to its gate. This prevents the induction motor from initial high currents. For each phase one triac is used. Gate terminal of the triac is connected with the 555 timer IC through opto-coupler, by varying the knob of potentiometer value changes which generates different delayed pulses that further goes into the gate terminal of triac.
ARDUINO (MEGA 2560)
Arduino mega 2560 is a microcontroller board based on the ATmega2560. Reason for choosing arduino is that it has fast processing it can do multiple task at a time. It is an intelligent device and its execution is fast.Arduino Mega 2560 is used in soft starter for different purposes:
- When the induction motor attains the desired speed (RPM), arduino sends a signal to a relay to acquire by pass path for continuously running of induction motor without abruption.
- In order to sense the speed of motor a feedback is used to check whether induction motor is running at desired speed or not.
- For LCD interface.
- For keypad interface.
MOC3021 also called opto-coupler or opto-isolator that uses light to transfer the electrical signals between two isolated circuits.Purpose of MOC3021 in this project is to prevent the high voltage or voltage transients. This is used as a safety for 555 timer IC to prevent it from burning or heating up.
A potentiometer is a variable resistance with three terminals out which two are variable and one is fixed, by using one fixed and variable terminal different values of resistance can be achieved.Purpose of potentiometer in this project is to reduce the voltage which further reduce the current and torque. By varying the knob of potentiometer value of resistance increases which reduced the voltage (three phase voltage). So, with the help of potentiometer any firing angle can be set and will be vary. By varying knob in such a way that resistance increases which means firing angle also increases which reduces voltages, currents and torques values calculations are mentioned in equation (4) to equation (8). In this project potentiometer up to of 5k ohm is used.
555 timer IC as its name indicate it is used for timer (time delays), pulses, oscillator applications etc.Purpose of 555 timer IC in this project is to generate delayed pulse at different levels. At pin 2 zero crossing detector pulse is connected which detects zero at each level and pin 3 generates the delayed pulse depending upon potentiometer connected with pin 7. Resistance changes by varying the knob of potentiometer which reduces voltage, because of this width of pulse changes that show different delay at different value of resistance. After this, delayed pulse goes into triac to trigger it.
INDUCTION MOTOR (FOUR-POLE SQUIRREL-CAGE)
An induction is a type of motor which operate on the principle of electromagnetic induction. Unlike synchronous motor, induction motor does not needs any exciter for the excitation because the current in the rotor’s winding is induced from the magnetic field of stator’s winding by electromagnetic induction. Induction motors are used more than synchronous motors because they are more economical in cost as compared to synchronous motors. The induction motors come in two types, single phase and three phase induction motor. The rotor of the induction motor can be squirrel cage type or wound type.Induction motor takes a high torque current for starting which can be harmful for induction windings of the motor, so a reliable soft starter for wide range of induction motors is required.
Encoders are the devices that are used to read or detect speed of the rotation of object.Encoders convert rotational speed into analogue voltage signal which can be used in motor’s speed indication.There are multiple kinds of speed sensors but in this project incremental optical rotary encoder is use as a speed sensor. The purpose of encoder in this project to measure the speed by counting the RPM (coding of rpm mentioned in Appendix A: source code of LCD, keypad and encoder) when motor attains desired RPM enter by user via keypad then arduino send a signal to relay to acquire by pass circuitry through micro-controller. It is like a feedback path to detect error also whether motor is working or not.
Keypad is a user input device with it user can enter any value. It is user friendly device.There are different types of keypad but in this project numeric keypad of 4×3 is used to enter the different RPM. User can enter any RPM by using keypad at which they want to bypass the circuitry (coding of keypad mentioned in Appendix A: source code of LCD, keypad and encoder). So, it is user defined attached with arduino mega 2560
LCD is used for displaying the number and values. So, on run time LCD display the numbers etc.LCD comes in different sizes to display various things but in this project we use 16×2 LCD to display the values. LCD display the RPM entered by user and display RPM of induction motor continuously (coding of LCD mentioned in Appendix A: source code of LCD, keypad and encoder). It is also attached with arduino mega 2560.
circuit diagram of Soft starter using microcontroller
ZERO CROSS DETECTION
When signal changes from positive to negative point i.e. crossing axis value become zero this is known as zero cross detection. At this point no voltage and current is present. This zero cross detection is important when dealing with in phase circuitry and when desired delayed has to set for signals.
To start three phase induction motor direct source can’t send to the gate of triac because of high current and torque rating so there is need of some delay in it before sending it to the triac gate. To generate delay first it is necessary to detect zero at each phase. After detecting zero at each phase, 555 timer IC generates the delay. Delayed pulse goes into the gate of triac through opto-coupler to prevent 555 timer IC from high voltages. At each phase one zero cross detector is used with same connection as given in figure below.
TIMER IC CIRCUITRY for soft starter
To generate delayed pulse with reduced voltage 555 timer is used at each phase. Zero cross pulse goes into pin 2 of 555 timer IC, output at pin 3 depend upon knob of potentiometer. By varying the knob of potentiometer resistance increase which further reduces the voltage. Reduced voltage goes into the gate of triac through opto-coupler. Pin 2 activated on negative pulse from zero cross detector. Pin 3 operates on 1 and 0 (logic states). When it is 0 then “sinking the current” which means it absorb current into its output and when it is 1 then “sourcing the current” which means it can give out the current from output. Due to this property it is used in digital applications. 555 timer IC is very stable and give accurate time delay its duty cycle and frequency control through external elements variable resistance and capacitor. Value of resistance directly affects its delay (duty cycle).
In this project monostable mode of 555 timer IC is used whose time period can be change using external RC (resistor and capacitor) elements.
Here is the calculation of time delay and firing angle.First there is need to calculate the (time delay) through following expression:
τ = [ 1.1(R1 + R2 ) * C ] ms Equation (4)
R2= 5K (potentiometer which can be vary and value of τ depend upon it)
Note: only R2 changes through knob of potentiometer of 5k ohm while other parameters are fixed.After getting value of τ from equation (4) firing angle can be calculated by following expression:
θ = ( τ × 360 × f )° Equation (5)
τ= delay in ms
f= frequency 50 Hz
After this duty cycle has to be calculated through following expression:
Duty cycle( D) = On time / on time + Equation (6)
Off time is equal to τ obtained through equation (4).
On time= [Total time – (Off time*2)]. Equation (7)
Off time multiply by 2 shows this off time is for both positive and negative cycle
Total time = 20ms.
Total time comes from:
Time period = 1 / frequency
Time period = 20ms
So, Time period= Total time= 20 ms.
After calculating duty cycle from equation (6), now calculate reduced voltage through following expression:
Vo = (Vs * D) volt Equation (8)
Vs= Supplied voltage (total voltage in single phase 240)
D= Duty cycle.
So, with the help of above expressions mathematically/theoretically reduced voltage can be calculated.At each phase one 555 timer IC is used with same connection as shown in figure below.
TRIAC AND OPTO-COUPLER CIRCUITRY for soft starter
To prevent the motor from burning and from high voltage, current and torque there is need to reduce the voltage and this can be done through triac (Three terminal device having two main terminal called anodes and one gate). By giving delayed signal to the gate of triac voltage will be reduced according to delay which in turn reduces the current and torque. Triac, opto-coupler and relay is connected to each phase with the same connection as shown in figure below.
From pin 3 of 555 timer IC pulses at different delay will achieved that goes into the resistor attached to the base of transistor of pin 2 of moc3021. Then from pin 4 of moc3021 delayed signal goes into the gate of triac. By using moc at each phase, 555 timer IC saved from high voltages or voltage transient. Whenever base of the transistor gets the signal it goes into the gate of triac and triggers the triac. While from remaining two terminals of triac one is connected to the neutral wire of phase voltage while other is connected to the load through NC of relay as shown in figure 4-3. So, before by pass circuitry triac follow the path through NC of relay, and motor starts slowly and smoothly because of low voltage, current and torque.
soft starting BYPASS CIRCUITRY
Once induction motor attains the desired RPM provided by the user (detected by optical incremental encoder) then arduino mega 2560 sends signal to resistor attached to the base of transistor which is further attached to relay to switch it from NC to NO path as shown in figure below and now there is direct path between three phase AC source and induction motor. Relay is attached with 6v including diode which protects it from back emf. At each phase one relay is connected. Whenever relay switches LED turned on, LED is used as an indicator.
INCREMENTAL ROTARY ENCODER CIRCUITRY
Here in soft starter speed sensor is used as feedback path to determine the RPM of motor and to detect error in induction motor whether induction motor is running or not. This rotary encoder can count the pulses as well as RPM of the motor. It has 400 PPR (pulse per revolution) which is fixed for ENC005 type of encoder. It has four wires red, black, yellow and green. Red and black connected with 5v and Gnd of arduino uno while yellow and green attached with pin 2, 3 of arduino uno which are interrupts as shown in figure 4-5. It can count pulses for both CW (clockwise) and ACW (anti clockwise), here yellow and green both are used for this purpose for CW and ACW values. RPM can be calculated by following expression:
RPM = pulse frequency * frequency / PPR Equation (9)
Pulse frequency calculated via programming in arduino mega 2560. For pulse frequency first time calculated of each pulse then by taking inverse of time, frequency will be achieved.
Rotary encoder counts the RPM using equation (9) and displayed RPM on LCD (via arduino coding). When RPM equals to the value entered by the user arduino send signal to relay to attain the bypass circuitry. The flow chart of coding (in arduino) involved in encoder is given in figure below.
In this project 4×3 numeric keypad is used, which is attached with the arduino mega 2560. It has four rows and three columns. It is attached with the pin 22, 24, 26, 28, 30, 32, 34 of arduino mega 2560. Through programming it can get values from user. User can enter value of RPM, after pressing value enter # to execute it further. In this way this project is reliable and user friendly. User can enter any value at which they want to bypass
LCD is used in this project to display RPM entered by user and also to display RPMs of induction motor continuously. LCD is connected with the arduino mega 2560. A variable resistor of 10K ohm is attached with LCD. While some of the pins of arduino is connected to the pin 7, 8, 9, 10, 11, 12 and Gnd of arduino mega 2560
Complete PCB design of soft starter
CONCLUSION on soft starter project
A soft starter for three phase induction motor is designed and developed to limit the high current, voltage and torque value during the start of an induction motor, otherwise it will damage the motor by burning the windings or overheating.The proposed technique of soft starter reduced the voltage which further reduces the initial high current and torque. The voltage is reduced through the knob of potentiometer. By varying the knob of potentiometer value of the resistance increases (firing angle increases and delay increases) due to which voltage decreases which in turn reduces the current and torque values.
With reduced current and torque value induction motor start smoothly with no current spikes. Once induction motor start smoothly with less voltage and attains desired RPM provided by the user through the keypad then arduino sends signal to relays to bypass the circuit. Now induction motor runs at full speed with full voltage free from any burring or overheating. The method developed could be beneficial in industrial and commercial applications.