An improved delta-star switching scheme for reactive power saving in three-phase induction motors

doi:10.1007/s11708-014-0324-8

Front. Energy

2014, Vol. 8

Issue (3) : 364-370 https://doi.org/10.1007/s11708-014-0324-8

RESEARCH ARTICLE

An improved delta-star switching scheme for reactive power saving in three-phase induction motors

P. RAJA¹,N. KUMARESAN^1,^*(

),M. SUBBIAH²

1. Department of Electrical and Electronics Engineering, National Institute of Technology, Tiruchirappalli, Tamil Nadu 620015, India
2. Department of Electrical and Electronics Engineering, Rajalakshmi Engineering College, Chennai 602105, India

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Abstract

It is proposed that a capacitor can be connected permanently across each phase winding of a three-phase induction motor along with the conventional delta-star switching, for further saving in VARh at reduced loads on the motor. The method of choosing a suitable value for the capacitor and the criteria to be adopted for calculating the power output at which the star to delta switching is to be made are also explained. The experimental results on a 3-phase, 4-pole, 415 V, 50 Hz, 3.3 kW induction motor verify the advantages in adding the capacitor to the phase winding of the motor. Compared to using only a single delta connected stator winding or a delta-star switching, the advantages of the proposed addition of a capacitor, are also demonstrated through a case study conducted on a 400 V, 250 kW motor. Any further improvement in grid side power factor can be achieved by employing a static synchronous compensator (STATCOM) of reduced VAR rating.

Keywords delta-star switching induction motor power factor steady-state analysis

Corresponding Author(s): N. KUMARESAN

Issue Date: 09 September 2014

Cite this article:

P. RAJA,N. KUMARESAN,M. SUBBIAH. An improved delta-star switching scheme for reactive power saving in three-phase induction motors[J]. Front. Energy, 2014, 8(3): 364-370.

URL:

https://academic.hep.com.cn/fie/EN/10.1007/s11708-014-0324-8
https://academic.hep.com.cn/fie/EN/Y2014/V8/I3/364

Fig.1 Three-phase induction motor using star-delta switching with permanently connected capacitor

A₂, B₁, B₂, C₁, C₂ — stator phase winding terminals; S₁ to S₅ — anti-parallel thyristor units; R, Y, B — supply terminals

Fig.2 Equivalent circuit of three-phase induction motor

(a) Exact equivalent circuit; (b) Thevenin-equivalent of (a) with respect to terminals X and Y

Fig.3 Predetermined performance characteristics of the test motor

(a) Mechanical power outpfut vs. power factor and phase current of the motor; (b) mechanical power output vs. torque and efficiency of the motor

Fig.4 Mechanical power output vs capacitor required to maintain unity power factor

Fig.5 Mechanical power output vs. motor line current

Fig.6 Experimental performance characteristics of the test motor

(a) Mechanical power output versus power factor of the motor; (b) Mechanical power output vs. line current of the motor

Fig.7 Voltage (v_a) and current (i_a, i_b and i_c) waveforms while closing the motor to the grid in the star connection with capacitor (Voltage axis: 500 V/div, current axis: 5 A/div and time axis: 10 ms/div)

Fig.8 Voltage and current waveforms while switching the stator winding from star connection to delta connection with capacitor (Voltage axis: 500 V/div, current axis: 5 A/div and time axis: 100 ms/div)

(a) No-load condition; (b) loaded condition

Tab.1 Loading patterns considered for the motor over a day

Tab.2 Comparison of kWh and kVARh for two-stage with, without capacitor and single setting stator connections for the different loading patterns for the 250 kW motor

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