Induction Motor Starting

There are various methods of starting of an induction motor:

(a)    Direct on line starting (DOL)

(b)   Star – Delta starting

(c)    Auto transformer starting

(d)   Soft-start

(e)    Part-winding starting

As most of the squirrel-cage induction motors draw 6 to 7 times Full-Load current while starting direct on line, it is desirable to have some mechanism to bring down this starting current. Y- Δ starting reduces the starting line current as well as torque to one-third of the DOL value. Soft-start employs back to back connected thyristor pairs in between the supply lines and the motor so that reduced voltage is applied depending upon the firing angle chosen. This experiment observes first the starting performance of the induction motor at various loads on DOL starting and using a soft-start (at different firing angles).

CIRCUIT DIAGRAM:

The direct on line starting is done as follows: Rated supply voltage is applied to the induction motor with the specified R1, x1, R2, x2 values. (R1=0.294 Ohms; L1 =1.39mH R2 =0.156 Ohms; L2 =0.74 mH Lm = 41 mH Moment of inertia =0.4 kgm² Vrated =400 V  I rated = 250 A Speed =970 rpm)

CIRCUIT DIAGRAM FOR DOL

CIRCUIT DIAGRAM FOR SOFT-START

Speed and current transients are observed for both types of starters by using the tacho and current sensors. Record these transients, observe how long it takes for the machine to come up to its rated speed using each of these methods when it is running on no-load. Repeat this for different loads and also for different firing angles for the soft-start.

                    The equation governing induction motor dynamics are as follows:

 where V_a  =  V_a  -  ½ V_b   -  ½ V_c

                        V_a  =   0  + Ö3/2V_b  - Ö3/2 V_c 

                        V_d  =   0 

                      V_q  =   0                        

                                       where     V_{a ,} V_b, V_c  are the stator applied voltages( per phase) instantaneous values  &  V_q,, V_d  are rotor voltages.       

 This result is obtained by transforming 3-phase stator and rotor to their 2-phase equivalents and then transforming all the rotor quantities to stationary reference frame. (Please refer to Unified Theory of Electrical Machines by C. V. JONES)

              L₁ = M + l₁

                     Mutual + leakage inductance of stator

               L₂ = M + l₂

                     Mutual + leakage inductance of rotor

              w_r = instantaneous rotor speed

These four simultaneous differential equations allow us to solve for currents for the given m/c parameters and i/p voltages.

                        T =  -M i_a i_q  +  M i_b i_d  =  Electromagnetic torque

                        T_e  -  T_L  =  J dw_r / dt

                          From which w_r (instantaneous rotor speed) can be solved for.

Thus all these five simultaneous differential equations can be solved and hence the response of the m/c can be obtained.

The waveforms of line-to-line voltage, three-phase currents and the speed are shown at a firing angle of 110 degrees on no-load condition of the induction motor are shown below.

Induction Motor Braking

  Induction motor braking can be done by various methods like regenerative braking, plugging and dynamic braking (AC dynamic and DC dynamic brakings and capacitor braking). In this experiment capacitor braking will be explored.

             In capacitor braking, the induction machine will be disconnected from the supply and connected to a capacitor bank connected in either Star or Delta. So, now the excitation would be provided by the capacitor and the machine will start acting like a generator (Why?). The generated electric power will be dissipated in the inherent resistance of the windings and thus braking is accomplished.