**6. Experimental validation**

120 Induction Motors – Modelling and Control

0

0.2

0.4

**Efficiency**

0.6

0.8

1

**Figure 13.** Efficiency

supported in Figure 13.

0

0

0

10

**Current (pu)**

10

10

The dynamic behaviour of a compensated and uncompensated induction machine is compared in Figure 12. The uncompensated machine (dashed waveform) has a low power factor when starting and settles at a power factor of just more than 0.7. The compensated machine (solid waveform) has a higher power factor when starting and settles at a power factor close to unity. This shows how effective this concept is in power factor correction.

<sup>0</sup> 0.5 <sup>1</sup> 1.5 <sup>2</sup> 2.5 <sup>3</sup> -0.2

**Time (seconds)**

Uncompensated Compensated

The earlier statement that the improvement in power factor will improve the efficiency is

The inrush current of the machine is shown in Figure 14. This machine has a transient state

<sup>0</sup> 0.5 <sup>1</sup> 1.5 -10

Ib

<sup>0</sup> 0.5 <sup>1</sup> 1.5 -10

Ic

<sup>0</sup> 0.5 <sup>1</sup> 1.5 -10

**Time (Seconds)**

Ia

when starting where the current can reach eight times rated current.

The current of the auxiliary winding is shown in Figure 15.

**Figure 14.** Phase Currents – Main Stator Winding

In order to validate the theoretical model with the practical model, three capacitor values of 10, 20 and 30µF are used for the three phase auxiliary winding.

**Figure 16.** The Experimental set up

The stator current of the motor is observed for both uncompensated and compensated windings. It is seen that the starting current for the uncompensated winding is high as compared to the compensated. The current at steady state also identifies the stator current for the uncompensated to be lower as compared to the compensated. These results are shown in figures 17 and 18.

Modelling and Analysis of Squirrel Cage Induction Motor with Leading Reactive Power Injection 123

\*

\*

\*

**Figure 19.** Experimental results of active power and power factor versus capacitance values

\*

\* \*

0 9 18 27 36 45

Capacitance (Micro F)

0 9 18 27 36 45

Capacitance (Micro F)

Firing angle (0

0 36 72 108 144 180

0 36 7 10 144 18

Firing angle (

\*

)

\*

\*

\*

0 ) \*

\* \*

\* \* \*

\*

\*

\*

\* \* \* \*

1.2

1.1

Active Power (pu)

1.0

\*

1. 0.9 0.8 0.70 0.60 0 5

\*

0.8 0. 0.7 0.7

\*

0.6 0.6

1 2

\*

1 1

1 0

Active

power (p

u)

Power factor

Power factor

\*

\*

\* \*

\*

\*

**Figure 20.** Experimental results of power factor and active power versus firing angle

\* \* \* \* \*

**Figure 17.** Stator current of uncompensated winding

**Figure 18.** Phase Currents-Stator current of the compensated winding

Other experimental results such as the active power versus the capacitance and power factor versus capacitance are shown in figure 19. These results conform to the theoretical simulations.

shown in figures 17 and 18.

**Figure 17.** Stator current of uncompensated winding

**Figure 18.** Phase Currents-Stator current of the compensated winding

simulations.

Other experimental results such as the active power versus the capacitance and power factor versus capacitance are shown in figure 19. These results conform to the theoretical

The stator current of the motor is observed for both uncompensated and compensated windings. It is seen that the starting current for the uncompensated winding is high as compared to the compensated. The current at steady state also identifies the stator current for the uncompensated to be lower as compared to the compensated. These results are

**Figure 19.** Experimental results of active power and power factor versus capacitance values

**Figure 20.** Experimental results of power factor and active power versus firing angle

Modelling and Analysis of Squirrel Cage Induction Motor with Leading Reactive Power Injection 125

with experimental set up has shown a great improvement compared to the uncompensated machine. Another very important improvement is the supply current decreasing with increasing capacitance. This is not the case with conventional power factor correction techniques because the reactive power needed is still drawn through the only stator winding set. This advantage of the modified machine may potentially reduce installation costs as

Despite its good performance it has certain drawbacks. The machine would be bigger in structure than a conventional machine. More copper is needed for the additional winding and more insulating material is needed. This would make the machine much more expensive than the conventional three phase machine. Another drawback of this concept is that the capacitors have to be sized for a specific load. When there is an application with varying load, the machine might not always operate at optimum power factor. A possible solution to this is to implement a PWM controller between the capacitors and the auxiliary winding; this will make the capacitance and therefore power

This modified induction machine has a research potential with the recent focus on energy efficiency. Further research needs to be carried out on the performance behaviour of this

el-Sharkawi, M. A., Venkata, S. S., Williams, T. J. & Butler, N. G. (1985) An Adaptive Power Factor Controller for Three-Phase Induction Generators. *Power Apparatus and Systems,* 

Jimoh, A. A. & Nicolae, D. V. (2007) Controlled Capacitance Injection into a Three-Phase Induction Motor through a Single-Phase Auxiliary Stator Winding. *Electric Machines &* 

Lipo, T. A. & Novotny, D. W. (1996) Vector Control and Dynamics of AC Drives. In Hammond, P., Miller, T. J. E. & Kenjo, T. (Eds.). New York, Oxford Science Publications. Muljadi, E., Lipo, T. A. & Novotny, D. W. (1989) Power factor enhancement of induction machines by means of solid-state excitation. *Power Electronics, IEEE Transactions on,* 4**,**

Park, R. H. (1929) Two-Reaction Theory of Synchronous Machines Generalized Method of Analysis-Part I. *American Institute of Electrical Engineers, Transactions of the,* 48**,** 716-727.

smaller supply cables can be used.

factor controllable.

**Author details** 

**8. References** 

409-418.

Adisa A. Jimoh, Pierre-Jac Venter and Edward K. Appiah *Tshwane University of Technology, Pretoria, South Africa* 

*IEEE Transactions on,* PAS-104**,** 1825-1831.

*Drives Conference, 2007. IEMDC '07. IEEE International.* 

Krause, P. C. (1986) *Analysis of Electric Machinery,* New York, Mcgraw-Hill.

machine.

**Figure 21.** A p.u plot of torque versus p.u. loading for various angles

Since the effective capacitance varies with load when there is an application with varying load, the machine might not always operate at optimum power factor. A possible solution to this is to implement a thyristor controlled static switch or a PWM controller between the capacitors and the auxiliary winding; this will make the capacitance and therefore power factor controllable. Finally, further experiment is carried out on the active power per phase, the power factor versus firing angle and the per unit torque versus per unit loading where the switched series capacitor is connected to the auxiliary winding. The static switching is such that only the required level of reactive compensation is allowed. Figures 20 and 21 are the experimental results obtained based on this analysis.

## **7. Conclusion**

A study has been conducted on a 0.75 KW machine with 380V, 50Hz supply for an effective power factor correction. This has been achieved by connecting the main winding to the three phase supply and the auxiliary winding to the fixed capacitors for reactive power injection. The modified machine with reactive power injection has potential compared to the conventional three-phase machines. It is seen from the waveform analysis that the machine has capability of reducing the starting current. Simulation results have shown a good improvement on both power factor and efficiency when introducing the reactive power injection with increase of capacitor value. Both steady-state and dynamic analysis together with experimental set up has shown a great improvement compared to the uncompensated machine. Another very important improvement is the supply current decreasing with increasing capacitance. This is not the case with conventional power factor correction techniques because the reactive power needed is still drawn through the only stator winding set. This advantage of the modified machine may potentially reduce installation costs as smaller supply cables can be used.

Despite its good performance it has certain drawbacks. The machine would be bigger in structure than a conventional machine. More copper is needed for the additional winding and more insulating material is needed. This would make the machine much more expensive than the conventional three phase machine. Another drawback of this concept is that the capacitors have to be sized for a specific load. When there is an application with varying load, the machine might not always operate at optimum power factor. A possible solution to this is to implement a PWM controller between the capacitors and the auxiliary winding; this will make the capacitance and therefore power factor controllable.

This modified induction machine has a research potential with the recent focus on energy efficiency. Further research needs to be carried out on the performance behaviour of this machine.
