INTRODUCTION:
As we discussed earlier that an induction motor needs some methodology for its starting. In this article we are going to discuss the second method of starting of an Induction motor known as START-DELTA STARER. Let us discuss that why and how this method supports to start an Induction motor.
Basic logic in Star-Delta method is that first we have to start the motor in Star connection and when the motor gains full speed at Star i.e. the change is speed becomes zero, then we have to switch the connection to Delta, then motor attains its rated r.p.m and can bear its full load. As we know that in Star connection Voltage is under root three times less than the voltage in Delta. So as the power of the motor is constant and and voltage is less in star connection, the motor will draw less amount of current when started in star.We have to switch the connection from start to delta when the motor starts running smoothly and motor will behave normal and will bear full load.
In older days, the switching from start to delta was done manually by changeover switches and now we are now using contactors for auto switching from star to delta by designing a proper control circuit. The control and power circuit of Star-Delta is more complex then that DOL starter, as we have to use three contactors in a power circuit (Main Contactor, Star Contactor and Delta Contactor), we have to use timer and also we will introduce electrical interlocking between star and delta contactor. The reason of interlocking is that at a time only one contactor with Main contactor remains ON, either Star or Delta.
What will happen if at a time both star and delta contactors switches ON? Comment your answer.
COMPONENTS:
Basic components are the same as we used in DOL starter and almost the calculation of the components is also same except the contactors.
- Fuses
- Breakers
- Contactors
- Timer
So,
Power = 10 x 746 = 7460W or 7.5KW
Now we have to calculate the full load current of the motor.
Full Load Current = 7500/(1.732 x 400)
=10.8A
=11A
SELECTION OF FUSE:
We Know that for three phase induction motors if we want to use time delay fuse we have to make it 300% of the full load current.
Maximum Size of Time Delay Fuse =(300x11)/100
=33A
SELECTION OF BREAKER:
We know that we have to install a breaker which have the tripping current equal to eight times of full load current.
So,
Maximum size of circuit Breaker=800x11/100
=88A
Now we have to select a breaker which have the current rating most closest to that of our desired calculated value, we may get a breaker of 80A.
SELECTION OF CONTACTORS:
The third and the most important and technical thing is selection of contactors for a star delta circuit, as we know that in a Star-Delta circuit, There are three contactors.
One is the main contactor Second is the Star Contactor and the third one is Delta Contactor. We have to select three contactors now.
Delta & Main
The main and Delta contactors are smaller as compared to the single contactor used in DOL circuit because the current in this case current will be divided between both the contactors. As we know that the current in delta is 1/√3 times the total current applied.Now the delta and main contactor must have the current rating equal to 58% of the full load current.
Ampere Rating of Delta & Main Contactor
= 58x88/100
=51.4A
Star
We used similar contactors for Main and Delta but for Star we will use a different contactor which may be smaller in rating as compared to both Delta and Main. This is because of the fact that in Star the motor will draw 1/√3 times the current in delta which is 58% of Delta. So this contactor could be 1/3 of the motor rating i.e. 33%.
Ampere Rationg of Star Contactor = (33x88)/100
=29.4A
OVERLOAD RELAY:
First of all we have to keep in mind that we will install the Overload (Thermal or Electronic) on the Main Contactor.
What will happen if we install our overload on the Star Contactor? comment your answer
Now as we discussed in DOL circuit designing (https://electrical1991.blogspot.com/2020/08/direct-online-dol-motor-starting-circuit.html) that we are following the American standard and we will adjust the overload ta 80% of full load. So the relay I am going to select is an EOCR (Electronic Over Current Relay). My full load current is 11A so I am going to select EOCR 0~30A.
Ampere Setting of EOCR = 80 x 11 / 100
= 8.8A
TIMER
The new component we are using here in Star-Delta circuit is a Timer. Timer is used because we want our Star contactor to automatically stop and Delta to switch ON after some time which will be adjusted according to the nature of the load. We have different timers, ON Dealy Timers, OFF Delay Timers, Flip Flop etc. Here we are interested in OFF Delay Timer.
The timer we are using here is an Off Delay Timer and ranging from 0sec to 10minutes.
The timer will always be adjusted according to the nature of load. If a 30KW Star-Delta Motor is connected to an Impeller and another 30KW Star-Delta Motor is connected to a conveyor belt, both have different timer settings.
We have the details of all the components which needs to be calculated and selected for our Star Delta circuit. Now we are in the condition to design our Star Delta Circuit to start a motor of 11KW.
(Figure 1.1)
BASIC LOGIC:
Here we can see that we have three contactors, One is the main contactor which will remain ON until we stop the motor. There are two more contactors one is Star and other is Delta. From these two, only one contactor will remain ON at a time. Intitally Main contactor and star contactor will be ON so the motor will get less voltage at its terminals. As in Star connection the voltage is Under Root Three times less than the voltage in Delta. So the motor will start smoothly and slowly and will speeds up after some time. When we observe that the motor is now stopped speeding up and has constant speed, we will then switch the Star Contactor OFF and simultaneously we will switch ON the Delta Contactor and motor will then speeds up to its full r.p.m and will be ready to bear its full rated load.
SCHEMATIC EXPLANATION:
If we see Figure 1.1 we can divide your schematic diagram in two parts, one is the Power Circuit and Control circuit. As our circuit is a bit complex, so we will first look at power circuit and then control.
Power Circuit: In power Circuit, first of all we have Fuse Fu, then we installed a three phase Breaker Q1 to protect our motor under sever fault conditions. After breaker our supply divides into two. The contactor KM3 is the main contactor and contactor KM2 is the Delta contactor. As we can see that the KM3 and KM2 are connected in such a way that if both the contactors are ON, the motor will be in Delta connection. Overload relay is also connected before the main contactor KM3.
Remember, overload relay must be on the main contactor or on the delta contactor, preferably on the main contactor, but never on the star contactor.
Now we have another contactor wich is a star contactor and connected in parallel with delta contactor. The other three ends of this contactor are short circuited to create a star point. Which means when this contactor is ON, the other three terminals of Motor will be short circuited making a Star point and motor will be in Star Connection.
Control Circuit: In control circuit first of we have the normally closed contact of the Overload relay, and our supply will only passes through it if the relay is normal and not trip. Then supply passes through OFF push button and reaches ON push button, we have to push the ON push button to start the motor.
Starting In Star As we push the start button, timer K1T and KM1(Star Contactor) energizes and as KM1 contactor energizes, The NO(Normally Open) auxiliary contact of KM1 closes which allows the supply to switch on the main contactor KM3. When KM3 is ON, its auxiliary contact closes, which will provide holding for the contactor.
Switching to Delta K1T will operate when the timer is completed, which cuts off the supply (Opens the Normally closed auxiliary contact of K1T) to KM1 (star contactor) and closes the Normally open contact of K1T, which will provide the path to switch ON the KM2(Delta Contactor). Hence switching the motor from start to delta.
Electrical Interlocking As we can see that the supply to KM1 passes through a NC contact of KM2 and supply to KM2 passes through the NC contact of KM1. It means KM1(star) and KM2(Delta) contactors are electrically interlocked with each other. When KM1 is ON, KM2 can never be ON simultaneously and vice versa. This interlocking provides us safety that only one of the contactor either Star or Delta must be ON at a time not both.
Points To Remember:
- Overload Relay must be installed on Delta or Main contactor.
- Make sure that you made accurate Delta otherwise motor may flash/reduce r.pm/produce highly abnormal sound.
- The time delay of switching from star to delta depends on the nature of load connected to the motor.
- Both the star and delta contactors must be electrically interlocked.
- Only squirrel cage motors can be started using Star-Delta starting method.
If you have any questions regarding the topic you are free to ask in comments or e-mail me or inbox me on my facebook.
e-mail : anwaarahmad31@gmail.com
facebook id : www.facebook.com/anwaar.ahmad.9275
Comments
Post a Comment