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

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

Introduction

It is known that three-phase squirrel-cage induction motors are commonly used in electrical drives and constitute a large percentage of total electrical loads in industries. Many of these motors operate at a much less than full-load condition for many hours in the daily duty cycle with low efficiency and power factor, even if they are designed with high performance values at full-load [1]. Ammasai Gounden et al. [2] described, with examples, the design optimization of single-winding, pole-changing, two-speed induction motors, the objective function being the minimization of the operating cost of the motor. The starting current and torque, maximum torque, full-load slip and power factor and tooth and core flux densities were taken as the performance constraints. These constraints resulted in obtaining high full-load efficiency at both pole settings. In these machines, in certain applications, the low speed setting with a lower power rating was also found to be useful, at times when only reduced power output was required. This resulted in a considerable energy saving. To improve the efficiency and power factor of operation of motors at reduced loads, the motors could also be operated at a suitable lower voltage at lower load, thereby decreasing the core loss and magnetizing current. To obtain such suitable reduced voltages, the use of three-phase, phase control circuits employing triac and anti-parallel thyristor units were suggested [3–5]. But with these circuits, a non-sinusoidal voltage was applied to the stator of the induction machine, which resulted in a non-sinusoidal current, leading to undesirable current harmonics in the stator of the motor. In recent times, owing to the ever increasing demand for electrical energy and escalating energy cost, even with single- speed motors, adopting energy efficient methods of operation have become essential. In this context, Kostic et al. [6] have advocated a changeover of a YY to Δ connection when the load on the motor was reduced below 75% of the rated load. Karthigaivel et al. [7] have shown that a motor designed with an unconventional delta-star parallel (ΔY) winding, which provides a three-stage switching, namely, ΔY, Δ and Y, can be used for saving Wh (energy drawn from the grid) and VARh (reactive volt-ampere hours drawn from the grid). Kumaresan et al. [8] have also secured an Indian patent for a four stage solid state switching controller for achieving improved energy efficiency and saving in reactive power in the operation of three-phase induction motors and wind-driven induction generators. This controller consists of 14 sets of anti-parallel thyristor units. Certain specified thyristor units are turned on, for securing ΔΔ, YY, Δ and Y connections in the stator of the machine at suitable reduced loads. Ferreira et al. [9] have proposed a stator winding with two sets of coils for each phase which can be connected in series or parallel in many different combinations of Δ and Y to form the three-phase connection. Each connection has been demonstrated to be suitable for a particular percentage of rated loads, thereby improving the efficiency and power factor of operation. However, these arrangements require 12 terminals to be taken out from the motor, for effecting the changeover from one connection to the other. Earlier, Ferreira et al. [1,10] have suggested only a delta to star changeover of the stator winding connection, when the load on the motor comes down to less than approximately 40%, requiring only six terminal control gears, similar to that of a star-delta starter. They have also devised a microcontroller based smart switch for an automatic changeover of the connection by sensing the stator current.

Recently, Raja et al. have proposed a Δ to Y switching of the stator winding of grid connected induction generators, with each phase being permanently connected to a capacitor across the winding. It has been shown that the combined effect of switching from Δ to Y at an appropriate wind speed, together with the added capacitor of suitable fixed value, reduces the reactive power taken by the generators [11]. This configuration requires only six terminals to be taken out from the stator and five sets of anti-parallel thyristor units for the switching operation. This simple version is now adopted in the present paper for the saving of both Wh and VARh drawn by induction motors from the supply. A brief description of the configuration of the delta-star switch, the switching criterion to be adopted, along with an illustrative example and a case study of the annual performance of a 250 kW, 400 V motor are presented in succeeding sections.

Delta-star switch with permanently connected capacitor for induction motors

Figure 1 shows the configuration of a three-phase motor connected to the supply through the controller. The stator of the motor is designed with delta connection for delivering the rated mechanical power output, when supplied with rated voltage. The star connection of the stator could also be used to reduce the inrush current while starting the motor.

The star connection is obtained by firing units S₁ and S_2. When the mechanical load increases to a certain value, S₁ and S₂ are deactivated and S₃, S₄, and S₅ are fired, thereby changing the stator connection to delta.

The switching from delta to star or vice-versa should be made at an appropriate mechanical load so that the best overall saving of Wh and VARh can be obtained. To determine the appropriate switching point, the following procedure is adopted:

For each of the two connections, using the steady-state equivalent circuit of the motor, the variation of the following performance quantities are predetermined and plotted with respect to the mechanical power output of the motor: power factor, phase current, efficiency, and developed torque. Then, the output rating up to which the star connection can be used is taken as

(1)P_{\mathrm{m}\mathrm{y}}=\min \left\{P_{\eta },P_{\mathrm{P}\mathrm{F}},P_{\mathrm{T}},P_{\mathrm{c}}\right\},

where P_η and P_PF are the values of the highest mechanical power output at which the efficiency and power factor, respectively, are higher in the star setting, compared to the corresponding values in the delta setting. P_T and P_c are the mechanical power output that corresponds to the 50% of the maximum torque and 110 % rated current respectively, at the star setting.

This criterion thus guarantees that the motor operates with optimum efficiency and power factor and within allowable percentage of maximum torque and full load current. The application of this criterion is illustrated with an example of a laboratory size motor in the succeeding section.

Illustrative example

To demonstrate the application of the switching criterion, a 3-phase, delta connected, 3.3 kW, 415 V, 4-pole, 50 Hz squirrel-cage induction machine available in the laboratory is considered. The per phase equivalent circuit parameters of the machine are R₁ = 14.64 Ω, R₂ = 8.95 Ω, X₁ = 11.94 Ω, X₂ = 11.94 Ω, R_m = 2880.0 Ω and X_m = 281.0 Ω. The peak winding current at full-load is 9.0 A. Since the delta to star switching in the lower output range causes reduced volts/turn, the no-load current and the iron loss are reduced from 1.42 A to 0.80 A and 180.5 W to 59.2 W, respectively. The equivalent circuit of the induction motor is given in Fig. 2(a) and its Thevenin-equivalent with respect to the terminals X and Y is shown in Fig. 2(b) [11]. The expressions for the Thevenin voltage (V_th), Thevenin impedance (Z_eq), rotor current referred to stator (I₂), induced EMF (E), no-load current (I_n), stator current (I₁) and power input to the motor (P_e) are listed in the Appendix. The mechanical power output of the motor is then

(2)P_{\mathrm{m}}={\displaystyle \frac{\mathrm{3}{I}_{\mathrm{2}}^{\mathrm{2}}{R}_{\mathrm{2}}\left(\mathrm{1}-s\right)}{s}}.

Using the expressions given in the Appendix and Eq. (2), the predetermined performance characteristics of the motor for the star and delta settings are illustrated in Fig. 3. Using Fig. 3 and the switching criterion stated in Eq. (1), the output power rating of the motor up to which star connection of the stator can be used is

P_{\mathrm{m}\mathrm{y}}=\min \left\{990,1350,1170,1170\right\}=990 \mathrm{W}

Choice of capacitor

Since a single fixed capacitor across each phase winding of the stator is proposed in this work, the capacitor value has to be chosen suitably such that the power factor of the motor is maintained above 0.95, irrespective of mechanical loading as well as the type of connection. So, considering the same 3.3 kW, 415 V test machine, the capacitance values thus calculated at various mechanical power output, both for star and delta connections are depicted in Fig. 4. The required capacitor value increases with the increase in the mechanical power output of the motor. With delta connection, this value varies from 10.7 µF to 11.4 µF. Figure 5 demonstrates the variation of line current of the motor with and without the capacitor. It is seen that from the point of securing a lower source current and hence reduced transmission line losses, a capacitor of 11 µF connected across each phase winding, would be suitable. It is noted that as per the switching criterion, the motor will operate in star setting for light loads.

Experimental results

Six terminals, i.e., the two ends of each phase, were taken out from the stator winding of the motor, a 11 µF capacitor was connected across each phase, and the star-delta switching controller built in the laboratory was incorporated as presented in Fig. 1. A load test was conducted with delta connection from no load to full load and with star connection up to 990 W output, whose results are given in Fig. 6. Both these tests were also conducted without the capacitor across the phase winding.

The experimental results shown in Fig. 6 reveal that increased power factor and decreased line current could be achieved by operating the motor in the two stator winding settings with a single capacitor per phase.

Transients during closing to grid

To study the transient behavior of the motor with fixed capacitor, the rated voltage was applied directly to the motor in star connection. The per phase voltage and the transient currents in all the three phases with capacitor connection were observed and recorded as exhibited in Fig. 7. The peak line current in one of the phases reaches 16 A, which is less than two times the rated winding peak current.

Transients during star to delta switching with capacitor

Figure 8 shows the experimentally obtained waveforms of a phase voltage and stator line currents of the motor, when the stator connection is changed from star to delta during no load and in a loaded condition. Again, during the changeover, the line current in the delta connection is limited to less than two times the rated peak winding phase current.

Estimation of annual Wh and VARh—a case study of a 250 kW three-phase induction motor

The usefulness of this proposed controller is further illustrated with a case study on a three-phase, delta connected 400 V, 50 Hz, 250 kW, 4-pole, squirrel-cage induction motor. The parameters for this machine are R₁ = 0.033 Ω, R₂ = 0.025 Ω, X₁ = 0.150 Ω, X₂ = 0.150 Ω, R_m = 40.0 Ω and X_m = 5.250 Ω. The performance characteristics were predetermined. Using Eq. (1), the power output up to which the star setting could be set was calculated as P_my = 0.496 pu= 146 kW. The capacitor to be connected across each phase of the stator winding was also obtained as 619 µF.

To demonstrate, the advantage of the delta-star switching with the parallel capacitor across each phase winding, the four loading patterns given in Table 1 were considered.

With each of these loading patterns given in Table 1, the kWh and kVARh taken by the motor in a day were calculated for the following cases:

1) Motor operated only in delta connection at all loads (M₁);

2) Motor operated with star connection up to 146 kW and then switched to delta connection without the capacitor (M₂);

3) Motor operated with star connection up to 146 kW and then switched to delta connection with capacitor across each phase (M_2C).

Taking the M₁ operation as the reference, the percentage saving in kWh and kVARh with the proposed switching, are shown in Table 2. It can be concluded that, compared to the fixed stator connection or the two-stage operation without capacitor connection, the proposed two-stage operation with permanently connected capacitor gives an increased saving in kVARh. Consequently, in industries employing a number of medium or large size three-phase induction motors, there will be a reduction in the overall kVA demand and hence in the kVA tariff. This increase in saving becomes greater in the case of motors working at light loads over a greater part of the duty cycle in a day. In other words, if the two-stage operation with permanently connected capacitor is adopted, the input power factor of the motor is close to unity at any load above approximately 10% of the rated load.

It may be noted from Table 2, that the proposed configuration M_2C gives the same kWh saving as in the simple two-stage setting but saves approximately 43€% more in kVARh consumption compared to that of M₂. More importantly, the configuration of the controller is much simpler for M_2C in which only six terminals have to be taken from the stator winding against 12 terminals in the earlier proposals [7–9]. Besides, only five sets of anti-parallel thyristors as the power devices are required for the proposed M_2C configuration.

Conclusions

It has been shown that adding a fixed capacitor across each phase winding of a three-phase delta connected induction motor and switching it to star connection at reduced loads give an increased saving in VARh consumed by the motor. Since such a switching causes only a reduction in the magnitude of the stator applied voltage, without altering the sinusoidal waveform, no current harmonics are introduced in the supply. Experimental results on the motor indicate that the capacitor addition results in high power factor even at 10€% rated load. It has been demonstrated that the input and output performance of the motor can be easily predetermined using Thevenin’s network theorem for the exact equivalent circuit of the motor. A case study has also been conducted on a 400 V, 250 kW motor operated with various duty cycles. The results reveal a substantial additional VARh saving with the inclusion of a capacitor, compared to the adoption of only a conventional delta-star switching.

References

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