INTRODUCTION:
Induction motor is now a basic electric machine which is widely used domestically and obviously in industries all over the world. If we talk about the induction motor, we must know that all the induction motors need some methodology for their starting. For single phase motors we use capacitors for their starting to produce a phase difference between Voltage and Current. If we talk about the three phase motors, their are several methods to start them like DOL, STAR-DELTA, RHEOSTAT,SOFT STARTERS and the latest one is by using VFD. So, here we are going to discuss the simplest method of starting an Induction motor (single or three phase).
COMPONENTS:
- Fuses
- Breakers
- Contactor
- Over Load Relays
Now we will discuss each component and its power rating according to the load.
Let us design a DOL circuit for a three phase 2HP motor working at 400V, so we have to calculate the ampere rating of fuses, circuit breakers and contactor.
First of all we have to convert the motor rating from HP to KW and we know that 1HP=746W
So the motor power rating will be 746*2 = 1492W we will take it as 1500W = 1.5KW
Motor power rating P=1.5KW.
Full load current I = P/1.732xV
I= 1500/1.732x400 = 2.16A
FUSE:
We must know that we have two types of fuses Time Delay Fuse and Non-Time Delay Fuse.
Maximum Size of Time Delay Fuse = 300% x Full Load Current.
= (300x2.16)/100
Maximum Size of Time Delay Fuse=6.48A
CIRCUIT BREAKER:
Now we have to select a suitable circuit breaker for our motor. We must know that we have a standard that for three phase induction motors, we have to choose the circuit breaker having current rating 8 times of motor's full load current. This is because of the fact that an Induction motor draws current upto 6 to 7 times of its full load current initially while starting. So we have to select a breaker which can hold the starting peak in the current.
Maximum Size of Circuit Breaker = 800% x Full Load Current
= (800 x 2.16)/100
Maximum Size of Circuit Breaker= 17.28A
As 17.28A is not a standard Ampere rating of breaker we find in market, we can select the nearest possible match, could be 16A or 18A according to availability.
CONTACTOR:
Now the third main component is a contactor and selection of a contactor for a DOL circuit is very simple, we have to choose the contactor of Ampere rating equal to that of full load including 10 to 20 percent of overload.
Maximum Size of Contactor = 20% x 2.16
=2.16+ (2 x 2.16) / 100
Maximum Size of Contactor = 2.6A
As we don't have contactors having exactly the same value of Ampere rating in market, so we will select the most closest.
OVERLOAD RELAY:
The fourth and the most important component in DOL circuit is the selection and the setting of a thermal overload (or electronic overload relay). Selection of overload relay is not that technical, just select the relay having current setting near to the full load current of your motor.
The full load current of our motor is 2.16A so the relay of Ampere rating from 0A to 5A is enough for us.
Overload Relay = 0-5A
Now main thing is the adjustment of this overload relay so that when our motor is overloaded, the relay must operate and stops the motor before damage. We have two standards, American and Chinese.
According to American standard we have to set our overload relay at 80% of motor's full load current.
So according to this standard or Overload Relay Setting would be 0.8x2.16 =1.723A
According to Chinese standard we have to set the motor overload relay setting at 120% of full load, which means that Chinese runs there motors at 120% efficiency because they have perfect motors, perfectly installed and well maintained, so they can take more output from motors.
In our case we will use the American Standard and hence.
Overload Relay Setting = 1.732A
After all this calculation, we are now in the condition to design the circuit and implement the DOL circuit.
(Figure 1.1)
The above circuit diagram is a DOL circuit which has two parts. one is Power Circuit and other one is the control circuit.
Power Circuit includes fuses, overload relay and contactor. The power to the motor is delivered through this circuit.
Control Circuit is the main logical thing is all the motor starting circuits. Remember my words, To start a motor is not a difficult task, the main thing is to stop it under fault conditions (overload). For this purpose we design control circuits.
Starting The Motor Here when we push the ON push button at point A the current starts flowing and reaches point B, which is already closed so it travels to C point. If the motor was normal and overload relay was not operated, then Normally Closed Contact of Overload relay will be closed and current will flow through it to point D and energizes the coil of Contactor. Once the coil gets supply, Contactor operates and our motor will start running. When contactor closes, then its auxiliary contact K will also operate providing holding to the circuit. Now if we release the Start Push Button, even then the motor will operate.
Stopping The Motor If we press the OFF push button at point B, then the supply voltage to the coil of the contactor will be removed and hence the contactor will be opened, also its auxiliary contact will be open and motor will be stopped.
Stopping Motor Under Overload Condition Now we have to se what will happen when motor is overloaded and current of motor crosses the 80% of the full load (limit at which we set the overload realy).
When the current crosses 1.732A, the Overload relay will operate and its contact at point C in control circuit will open, cutting off the supply voltage to the coil of contactor and the contactor will open. So the motor will be stopped.
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