To understand motor thermal overload protection in induction motor we can discuss the operating principle of three phase induction motor. There is a cylindrical stator and three phase windings are distributed in parallel in the inner circle of the stator. Due to such symmetric distribution, when a three-phase power supply is applied to the stator winding, a rotating magnetic field is generated. This field rotates at a synchronous speed. The rotor in an induction motor is basically made up of numbers of solid copper bars which are shortened at both ends in such a way that they form a cylindrical cage like structure. This is the reason why this motor is also called squirrel cage induction motor. Anyway let’s come to the basic point of three phase induction motor – which will help us clearly understand about motor thermal overload protection.
When the rotating magnetic flux cuts each bar conductor of the rotor, an induced eddy current will flow through the bar conductor. When starting the rotor is stationary and the stator field is rotating at synchronous speed, the relative motion between the rotating field and the rotor is maximum.
Therefore, the rate of flux cut along the rotor bars is maximum, in which case the induced current is maximum. But as the induced current is due to this relative speed, the rotor will try to reduce this relative speed and hence it will start rotating in the direction of rotation of the magnetic field to catch the synchronous speed. As the rotor approaches synchronous speed this relative velocity between the rotor and the rotating magnetic field will become zero, so no more flux cutting will occur and consequently no current in the rotor bars. As the excitation current becomes zero, there is no longer any need to maintain zero relative velocity between the rotor and the rotating magnetic field, so the rotor speed drops.
As the rotor speed decreases, the relative velocity between the rotor and the rotating magnetic field again attains a non-zero value, causing current to be generated in the rotor bars again, allowing the rotor to regain synchronous speed. will try and this will continue until the motor is switched. But due to this tendency the rotor will never achieve a synchronous speed and also it will never stop running during normal operation. The difference between the synchronous speed with respect to the synchronous speed of the rotor is called the slip of the induction motor.
The slip in a normally operating induction motor usually varies from 1% to 3% depending on the loading condition of the motor. Now we will try to draw the speed current characteristics of an induction motor – let us give the example of a large boiler fan.
In the characteristic Y axis is taken as time in seconds, X axis is taken as % of stator current. When the rotor is stationary which is in the initial state the slip is maximum so the induced current in the rotor is maximum and due to the transformation action the stator will also draw heavy current from the supply and it is about 600% of rated. Will be. Full load stator current. As the rotor is being accelerated, the slip decreases, resulting in the rotor current therefore dropping to about 500% of the full load rated current within 12 seconds when the rotor speed reaches 80% of the synchronous speed. . The stator current then rapidly drops to the rated value as the rotor reaches its normal speed.