STARTING INDUCTION MOTOR USING RHEOSTAT.

 INTRODUCTION:

Among several methods of induction motor starting, one and the oldest method is using Rheostat. It is a very old method and still in use  all over the world. A Rheostat is used to rise or lower the resistance of the circuit, to decrease or increase the current flowing through the circuit. We have two types of rheostats Wire wound rheostat and Liquid Rheostat. The logic behind the rheostat starting method is that we connect our rheostat in series with the rotor of the induction motor to reduce the initial torque of the motor i.e. limit the current flowing in the rotor. We will discuss both the rheostats one by one now.

Wire Wound Rheostat:

 It is just like a variable resistor and used to limit the current flowing through the circuit by changing its own resistance without any interruption. 

(Figure 1.1)

Figure 1.1 shows a wire wound rheostat used to start small slip ring induction motors. We can see that similar to potentiometers, rheostat is also a three terminal device. We use only two of its terminals, one is a constant and other is movable (to change the resistance).  For a three phase induction motor, we have to install three rheostats in series which must be mechanically interlocked.

                                                                        (Figure 1.2)

In figure 1.2 three rheostats are connected to three different phases and the dotted line shows that the three rheostats are mechanically interlocked to each other such that if we move one rheostat, the other two will be automatically follow that one immediately.

Note that the three rheostats must be of same power rating, same resistance.

Liquid Rheostat: Liquid rheostat works on the same principle as wire wound rheostat. The difference is that, in liquid rheostat we are not using wire to produce resistance. We are using liquid electrolyte to produce resistance is the path of current. We use simple water and add Soda Ash in it to make a suitable solution of electrolyte as we need.

Here, a question arises that why we add soda ash in water in a liquid rheostat? Comment your answer if you know, I am going to explain it in the end.

(Figure 1.3)

Figure 1.3 shows us the internal structure of Liquid Rheostat. We can see three cylinders in a big rectangular box filled with electrolyte (solution of water and soda ash). each cylinder has two terminals one is stationary and other is movable. Initially, the movable terminals are at maximum height i.e. lifted and as we start the motor, the movable terminals starts lowering down simultaneously with a constant speed. The lowering neither be very slow nor very fast. We have to keep in mind that when the movable contacts comes close to stationary contacts (Lowering completes) the motor must attained 80% of its maximum rated r.p.m. 

STARTING THE MOTOR:

Wire Wound Rheostat: To understand the mechanism of starting induction motor with the help of wire wound rheostat, we have to look at the Figure 1.2. Initially the rheostats will be at maximum position i.e. rheostat will be showing maximum resistance.When we start the motor, the rheostat will also start decreasing the resistance of rheostat and when the motor attains 80% speed of its rated speed, we have to by pass the rheostats and motor will run smoothly at full r.p.m.

Liquid Rheostat:

When we start the motor, Initially the movable contacts of the liquid rheostat will be at full lifting condition.We have to put a limit switch which will operate when the movable contact gets very close to the stationary contact. As the limit switch operates, the short contactor will switch ON and all the three phases will be short circuited. The induction motor will be then start acting like a squirrel cage motor whose rotor is short circuited.

Summary The rheostat is used in series with the rotor of an induction motor in order to produce and extra resistance for the induced current in rotor. As the starting torques is very high, so we introduce high resistance in series with the rotor, but when motor starts running, we slightly decrease the resistance due to which the r.p.m of motor increases. When motor gets its 80% of rated speed, we short the three phases of rotor, and motor will then act as a squirrel cage motor having a short circuited rotor.

Points to Remember:

•  Rheostat only works with the Slip Ring Motors.
• We add Soda Ash in Water to reduce the Resistance of water.
• If by mistake someone adds more Soda Ash, motor draws huge amount of current, as the rheostat resistance will become more less.
• If we add very low amount of Soda Ash in our rheostat, the motor may not reach the 80% of rated speed even when the lowering of contacts completed. When the short circuit contactor will operate, the motor may behave abnormally or Trip due to huge amount of current. 

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