Starting characteristics of induction machine

An induction machine consumes a very high inrush current during start-up, which can be potentially damaging to the machine itself and to the appliances connected. In the past, various studies focusing on analytical and experimental techniques for direct on line start, star delta starting and V/Hz control methods have been proposed. However, this paper provides and compares computational simulations of these methods, investigates techniques to avoid high starting current and develop the starting characteristics of an induction machine. For the V/Hz control relationships, the paper shows modeling of a six-step inverter by simulating the system with different values of ramp rates for the excitation frequency. Governing relations between the DC link voltage source amplitude and frequency are formulated and constant V/Hz control is used to maintain the magnetizing flux at its rated value for each steady-state excitation frequency and load. A comparison between all the proposed methods shows that constant V/Hz control operation is capable of producing a softer starting procedure than direct on-line starting and star-delta starting. However, it is not suitable for very high frequency ramp rate.

INTRODUCTION

An induction machine (IM) with direct online start, draws up to 5 to 8 times current of its rated value, while the transient starting torque oscillations can produce torque peaks up to 3 times higher than the rated torque ^[1]. The power source keeps the frequency fixed but the voltage and the current can change during the starting of the IM. During the IM acceleration, this increase in the current causes voltage to drop. Lower voltage with constant supply frequency means lower V/Hz ratio and lower flux, which affects the torque ^[2]. In addition, this high amount of initial current can affect other appliances connected in line with the IM. There are several starting methods of an IM but each has its own advantages and disadvantages. This paper lays down the computational foundation of two approaches, which can minimize the initial current transients.

METHODS UNDER CONSIDERATION

1. Direct on-line starting

Direct on-line starting (DOL) starting is a process with full voltage supplied to an IM from the supply grid. This high voltage leads to production of high torque and very high inrush current. This method is not suitable for commercial applications due to two reasons. Firstly, since the inrush current can reach up to 5 times the rated current, the IM will damage itself because of the significantly developing strains thus causing long-term damage to the IM. Secondly, the high inrush current causes a dip of voltage in the supply line thus affecting the other appliances connected in line with the IM.

2. Star Delta starting

In the case of star-delta starting, the IM first connects in a star configuration, which ensures that the supplied voltage per phase to each winding is 58% (or) of the delta or direct online connection as seen in Fig 1. If the input impedance of phase winding is kept constant, a 33% reduction in the current can be observed. With this smoother start, when the IM speed reaches about 50% of its rated speed, the supply connection changes back to delta configuration ^[3]. Although it reduces the starting current but it comes at a cost of reduction of starting torque, which poses to be a disadvantage. Moreover, star-delta starter faces a common problem of the current spike that occurs during the transition from star to delta ^{[4] [5]}.

3. Constant V/Hz operation

Constant V/Hz operation is another approach that can start the induction machine. In this method, a six-step inverter connects the IM whose frequency is ramped from zero to its rated value ^[6]. Fig 2 represents the schematic of this operation. To maintain the constant V/Hz relation, the dc link voltage is also ramped proportionally to the frequency as shown in Fig. 3.

The inverter equation (21) relates the dc link voltage with the IM input voltage and uses it as a ramping parameter in the simulation model. Similarly, equation (24) brings up a working relationship between dc link current and induction machine current. This developed relationship performs a check on the dc link current ensuring the IM to operate in the safe limit i.e. the inrush current does not exceed twice the rated current.

GOVERNING CIRCUIT AND EQUATIONS

The main analysis of this paper is to investigate how efficiently constant V/Hz control works as compared to the direct on line start and star- delta starting. The computational simulations were performed using MATLAB SIMULINK. The IM model along with the six step inverter model were build using the following equations and equivalent circuits (Fig 4)

EQUATIONS

SIMULINK MODEL

The abc to αβ transformation were performed using equations (16) and (17) and implemented as shown in Fig. 5. Rotor and stator currents were transformed using the similar method.

Table 2. Comparison of maximum values for different frequency ramp rates

Fig. (17 to 20) and Table 2 showed that constant V/Hz gives very high values of starting current and torque for very large values of ramp rates. However, this can be controlled by putting a check and limiting the values of dc link current to the desired value of safe limits. Since our investigated maximum dc link current is 50.61 Amps, the values in the Fig.(21) which are lesser then this threshold will give us the IM current running within the safe range of the rated current.

CONCLUSIONS

The analysis shows that both star and delta starting and constant V/Hz operations are capable of decreasing the starting current. Among the two, constant V/Hz method turns out to be more valuable as the starting values of current and torque can be limited to values close to IM rated values. The simulation shows that as the ramp rate decreases, the starting values get close to the desired values. However, this method poses a disadvantage that the time needed to reach the rated speed increases as we decrease the ramp rates. Conclusively constant V/Hz method can be used to reduce the starting currents but the time taken to reach the rated conditions increases.

APPENDIX

All the performed simulations uses MATLAB SIMULINK for investigation.

REFERENCES

[1] M. Habyarimana and D. G. Dorrell, "Methods to reduce the starting current of an induction motor," 2017 IEEE International Conference on Power, Control, Signals and Instrumentation Engineering (ICPCSI), Chennai, 2017, pp. 34-38.

[2] G. Musgrove, B. Bauer, K. Hall, M. Hinchliff, C. Meher-Homji, R. Kurz, B. Pettinato, D. Ristanovic, and M. Taher, “Chapter 7 - Drivers,” K. Brun and R. B. T.-C. M. for O. and G. Kurz, Eds. Gulf Professional Publishing, 2019, pp. 309–372.,

[3] K. Pillay, M. Nour, K. H. Yang, D. N. Datu Harun and L. K. Haw, "Assessment and comparison of conventional motor starters and modern power electronic drives for induction motor starting characteristics," 2009 IEEE Symposium on Industrial Electronics & Applications, Kuala Lumpur, 2009, pp. 584-589.

[4] “Traditional Electromechanical Starters”, [Online]. Available:http://www.lmphotonics.com/m_start.htm#StarDelta, (26/11/2008)

[5] B. Omrane, Induction Motor Lecture Handouts, 2008. “Star-delta starters with overload relay”, [Online]. Available:http://www.wiringmanual.com/motor039.html, (26/11/08

[6] A. Shaltout and O. E. M. Youssef, "Speed control of induction motors using proposed closed loop Volts/hertz control scheme," 2017 Nineteenth International Middle East Power Systems Conference (MEPCON), Cairo, 2017, pp. 533-537