Impact of Three-Phase Voltage Dips on the Induction Motors – An Experimental Study

Helerea Elena, Lepădat Ionel, Ciobanu Anca Faculty of Electrical Engineering and Computer Science

Transilvania University of Brasov Brasov, Romania

helerea@unitbv.ro, ionellepadat@gmail.com, cbn_anca@yahoo.com

Abstract—The paper deals with the investigation of the effects of three-phase voltage dips on the induction motors. The experimental set-up comprises a dip generator, a three-phase analyzer and a control system. As model, a three-phase squirrel cage induction motor of 1.5 kW / 0.4 kV is tested with balanced three-phase individual voltage dips of different magnitudes and durations, and of different rectangular and non-rectangular forms. A comparative analysis is done regarding the wave form and shape of rms three-phase currents and voltage dips curves. The decreasing of current and power of induction motor was correlated with voltage dip magnitudes. The experimental determinations of the effects of balanced three-phase voltage dips are compared with characteristics of induction motor operating in normal conditions.

Keywords—experimental set-up; induction motor; rms shape of three-phase current curves; voltage dip; testing method

I. INTRODUCTION In present, the increasing of the density of distribution

networks, diversification of their topology, and the introduction of distributed systems led to an intensification of problems connected to voltage variations, especially to those generated by the voltage dips [1], [2].

Voltage dips influence the functionality of equipment as: motors, adjustable-speed drives, another type of power electronic equipment, discharge lamps, computers, programmable logic controller and contactors, with direct effects on the efficiency of production [2]-[4]. The actual studies regarding the sensitivity of consumers in operation regime with voltage dips have concentrated to the analysis of performances of IT equipment. But, new researches on the classical consumers – power transformers and induction motors – have been also developed.

The goal of this paper is to analyze the effect of voltage dips on the induction motor characteristics, through theoretical and experimental investigations.

II. VOLTAGE DIPS-CHARACTERISTICS, PROPAGATION AND EFFECTS

The researches in the domain of energy quality substantiated the international regulations, as the IEC 61000-4-

30 [5], in which the majority of energy quality problems are expressed by a set of indicator of voltage variations. In conformity with this standard, a voltage dip is a temporary reduction of the voltage in a node of electric network under a specified threshold. The threshold is 90 % of rated voltage and the duration is ranges between 10 ms and 1 minute.

Voltage dips are determined by: faults of type short-circuit in electric transmission and distribution networks or in electric networks of the consumers; connection of large loads (transformers, induction motors, etc.).

A. Balanced and non-balanced voltage dips In function of the type of faults, the voltage dips can be

either balanced or unbalanced. Thus, a three-phase short-circuit or a large motor starting can produce balanced voltage dips, in which the individual phase voltages are equal. Single-line to-ground or phase-to-phase faults due to lightning, accidents, etc. can cause non-balanced voltage dips, in which the individual phase voltages are different or the phase relationship is other than 120°[6], [7]. A voltage dip is characterized by magnitude, duration, and phase angle shift [8]-[10]. For non-symmetrical faults, the symmetrical component method is applied to obtain the characteristics of non-balanced voltage dips. In [8] and [9] the voltage dips are experienced and classified into four types, denoted as A, B, C and D.

B. Rectangular and non-rectangular voltage dips The shape of voltage dip keeps the rectangular form (Fig.

1) if the system impedances remain constant in time.

Fig. 1. Ideal rectangular voltage dip.

For the case of the variation in time of impedances, which the case of the induction motor of various load characteristics, the voltage dip has a non-rectangular shape. In the case of non-rectangular voltage dip, the duration of voltage recovery is higher that the decay duration of the dip. This conducts to an incorrect assessment of the dip effect on the sensitive load [11].

To the propagation of voltage dips in the power networks, the voltage dips change their characteristics in function of a multitude of factors: network topology, line and cables impedances, transformer winding connections, protection and grounding systems, load connection and dynamics.

C. Impact on the sensitive loads The voltage dips, as deviation of supplying voltage from

the rated one, can cause important damages to end user equipment. The voltage dip impact on the power network and on the sensitive consumers depends on the specific characteristics of the dip: magnitude, duration, phase-angle jump, dip shape, etc.

The growth of the number of devices sensitive to voltage dips and the possibility of miss-operation of the electronic devices present in the control systems imposed new researches to establish the voltage dip acceptability curves for LV loads. For each equipment the limit curve amplitude – duration of the perturbation can be established, like CBEMA curve [4], [6], and [12].

There are other types of tolerance curves [13] as SEMI F47 curve, which indicates the tolerance requirements to the voltage variations for IT equipment, in the same way ITIC curve, which indicates the duration in which an equipment withstand to a voltage dip with a certain level. But, the tests for obtaining the voltage dip tolerance curves considered mainly the case of rectangular dip [8], [11], [14], and [15].

In this paper the effect of individual balanced three-phase voltage dips on the induction motor is established through experimental investigations.

III. VOLTAGE DIPS IMPACT ON THE INDUCTION MOTOR In connection with induction motor, the voltage dip issues

are complex, including two aspects:

• the modality in which the large power induction motors influence the shape and characteristics of voltage dips;

• the modality in which the voltage dips influence the performance of induction motor.

Regarding the second aspect, the analyses of the voltage dip impact on the performances of induction motors was done having in view the three-phase rectangular and non-rectangular dips, mainly by analytical methods and simulations [15], [16].

Thus, [6] and [16] justify the decreasing of induction motor performances with the relationship between the motor torque and applied voltages. The motor torque is directly proportional to the square of the applied voltage; therefore, as the supply voltage to the induction motor decreases, the motor torque will decrease drastically. The speed also decreases with a decrease in torque.

Depending on the depth and the duration of the voltage sag, the motor speed, respectively the motor slip, may recover to its normal value as the voltage amplitude recovers.

The decreasing the motor speed, and the presence of picks in current and motor torque, which modify the dynamics of motor, have the effects on the power network, and the protection systems too [16], [17].

Various algorithms have been proposed for calculation of current, maximum torque and losses of speed due to the voltage dips [7], [15]. The calculus considers only the critical points of transitory regime: drop voltage point and recovery voltage point. The effect of voltage balanced and non-balanced dips on the induction motor of 5.5 kW, 380 V, 50 Hz, 1450 rpm [18] is study by simulations. The experimental determinations and simulations indicated an increasing of dissipation energy due to the increasing of current in transitory regime, different in the cases of mono-phase, bi-phase and three-phase voltage dips. Some non-concordances are found between theoretical approaches and experiments.

In this paper the effect of voltage dips on the performance of induction motor is done by experimental investigations, in the case of individual three-phase balanced voltage dips.

IV. EXPERIMENTAL TESTS The experimental tests have been done on the induction

motor supplied in a regime with tree-phase individual balanced voltage dips.

A. Description of set-up The set-up from Advanced Electrical System Laboratory of

the Research & Innovation Institute of Transilvania University of Brasov allows the equipment testing to the various regimes of voltage dips. The configuration of testing stand is shown in Fig. 2.

Fig. 2. Configuration of set-up.

The set-up has the following components:

• Dip generator consisting of a three-phase controlled source, of type Netwave 30, with power of maximum 30 kVA, which generate the voltage dips of various types;

• Three-phase digital power analyzer of type DPA 503N, which allows the measurement of rms voltage for each phase, current, phase angles, power.

• Calculus system, having the software dpa.control for control, measurement and processing the data – voltage variations, harmonics and flicker. The software netwave.control allows the parameters control of the multifunctional three-phase source.

B. Sample and voltage dip testing regime The equipment under test (EUT) was an induction motor

having the following parameters: Pn =1 kW, Un = 400 V, n = 380 rpm, star connection, In =4.4 A, squirrel cage. The load was a three-phase synchronous generator, loaded by variable resistors.

Two types of dips have been created and applied; one with gradual shape (non-rectangular), and another with abrupt shape (rectangular).

For the non-rectangular voltage dip applied gradual (Fig. 3), the rms voltage is decreased during a decay time tdc, it is maintained constant during the dip time td and it is increased during the rise time tr.

Fig. 3. Characteristics of gradual voltage dip.

The total duration of dip is considered Dd= tdc+td + tr. In Fig. 4 the interface of Netwave dip generator is shows, with the data of gradual voltage dip (0%, 1.5 s) for line L1.

Fig. 4. The interface of Netwave with gradual dip (0%, 1.5 s), line L1.

Fig. 5. Interface of Netwave with abrupt dip: (40 %, 1,5 s), line L1.

In Table I the parameters of gradual voltage dips applied on induction motor are presented. The magnitudes for voltage dips were: 0; 20; 40; 70; 80; 90 [%].

TABLE I. THE GRADUAL VOLTAGE DIP PARAMETERS

Test Ud [%] tdc [ms] td [ms] tr[ms] Dd [ms] GR 1 0 - 90 50 100 50 200 GR 2 0 - 90 100 200 100 400 GR 3 0 - 90 500 1500 500 1500

The testing with abrupt three-phase voltage dip is considered more severe. In Table II the parameters of abrupt voltage dips applied on induction motor are presented. The magnitudes for voltage dips were: 0; 20; 40; 70; 80; 90 [%].

TABLE II. THE ABRUPT VOLTAGE DIP PARAMETERS

Test Ud [%] td [ms] AB 1 0 - 90 200 AB 2 0 - 90 400 AB 3 0 - 90 1500

In Fig. 5 the interface of Netwave is shows, illustrating the wave shape for an abrupt dip (40 %, 400 ms, AB2 test) for line L1.

C. Measurements and results With test set-up described in IV-A, three tests with gradual

dips (GR1, GR2, GR3) and other three tests with abrupt dips (AB1, AB2, AB3) have been applied on the three phase motor.

For a measurement time of 20 seconds 100 cycle-tests have been registered.

Three-phase digital power analyzer registers the rms values of voltages and currents for each supply lines of the testing motor. The rms values are obtained for averaging on 10 cycles.

For each cycle-test, a set of data have been registered: active power for each phase P, in W; reactive power for each phase Q, in VAR; power factor, between phase and earth; harmonic content for voltages and currents.

V. EFFECT OF VOLTAGE DIPS ON INDUCTION MOTOR

A. Testing with balanced gradual voltage dips For each gradual dips test GR1, GR2 and GR3 (Table I),

the data corresponding to the 100 test-cycles have been processed and the curves of rms currents and voltages have been obtained. A comparative analysis of wave form of rms current and of motor power during the gradual voltage dip was done.

1) Shape of rms current curves Fig. 6 shows the shave of rms voltage dip (magnitude of 40

% and 0 %, duration 1.5 s), and rms wave current curves in the line L1.

a)

b)

Fig. 6. The shape of rms current in line L1 for GR3 test (1.5 s) with magnitude: a) 40 %; b) 0 %.

Remarks: The minimum rms current is obtained at a time interval of order 1.5 s, corresponding to the end of voltage dip; Recovery of rms a current takes place in a time interval of approx. aprox. 5 Dd; A shift angle between voltage deep and current is observed. The measurements indicate non-symmetry of rms currents on the three phases, due to construction of induction motor.

2) Variation of active power of induction motor Fig. 7 illustrates the comparison between the shape of rms

gradual voltage dip curve and active power on the line L1 in GR3 test - three phase voltage dip with duration of 1.5 s and magnitude: 92 V (40 %), and 0 V (0 %).

In Table III the data for minimum rms current and active power of the induction motor under three-phase balanced gradual voltage dips of different magnitudes in GR3 test (total duration 1.5 s) are noted. Also, in the Table III, significance of the notations are: Pn – nominal power of induction motor (Pn = 1.5 kW); Pd – active power corresponding to the duration td of voltage dip; Pd-rec – active power including the time of recovery

for the current; Imin – rms minimum current, corresponding to minimum voltage dip.

a)

b)

Fig. 7. The shape of active power curve in line L1 for GR3 test (1.5 s) with magnitude: a) 40 %; b) 0 %.

TABLE III. MINIMUM CURRENT AND POWER IN GR3 TEST

Ud

[%] Imin [A]

PdGR [W]

PdGR/Pn [%]

PdGR-rec

[W] PdGR-rec/Pn

[%] 0 2.612 201.14 13.41 457.92 30.53

20 2.81 229.13 15.28 462.72 30.85

40 3.078 232.81 15.52 466.99 31.13

70 3.619 310.71 20.71 478.45 31.90

80 3.837 344.60 22.97 485.31 32.35

90 4.081 390.05 26.00 499.16 33.28

Fig. 8. Dependence of active power absorbed by motor in GR3 test in function of magnitude of gradual voltage dip.

In Fig. 8 the dependence of active power corresponding to the duration td of voltage dip corresponding to the duration td of voltage dip, and active power including the time of recovery for the current is done in function of magnitude of voltage dip in GR3 test.

a)

b)

Fig. 9. The shape of rms current in line L1 for AB3 test (1.5 s) with magnitude: a) 40 %; b) 0 %.

The active power absorbed by the induction motor characterizes the motor dynamics: a large power correspond to a small decreasing of the torque and speed of motor. Calculus of active power taking in account only the dip duration can introduce errors in estimation of the effect of voltage dips on the performances of motors.

B. Testing with balanced abrupt voltage dips For each abrupt dips test AB1, AB2 and AB3 (Table II), the

data corresponding to the 100 test-cycles have been processed and the curves of rms currents and voltages have been obtained.

1) Shape of rms current curves Fig. 9 shows the comparison between the voltage dip and

current shape form for two magnitudes of gradual voltage dips: 0 %, and 40 % voltage dip, duration being 1.5 seconds.

2) Variation of active power of induction motor Fig. 10 illustrates the comparison between the shape of rms

abrupt voltage dip curve and active power on the line L1 in GR3 test - three phase voltage dip with duration of 1.5 s and magnitude: 92 V (40 %), and 0 V (0 %).

In Table IV the data for minimum average rms current and active power of the induction motor under three-phase balanced abrupt voltage dips of different magnitudes (total duration 1.5 s) are noted.

C. Comparison of the effects In Table V and Table VI the minimum rms current of

induction motor and recovery duration of current for with abrupt shape (AB3 test) and graduate shape (GR3 test) of voltage dip (40 %, 1.5 s).

a)

b)

Fig. 10. The shape of active power curve in line L1 for AB3 test (1.5 s) with magnitude: a) 40 %; b) 0 %.

TABLE IV. MINIMUM CURRENT AND POWER IN AB3 TEST

Ud

[%] Imin

[A] PdAB [W]

PdAB/Pn [%]

PdAB-rec

[W] PdAB-rec/Pn

[%] 0 2 139.45 9.30 422 28.13

20 2.48 207.93 13.86 439.35 29.29

40 2.72 223.93 14.93 443.17 29.54

70 3.39 309.47 20.63 460.10 30.67

80 3.66 342.46 22.83 470.56 31.37

90 3.98 392.03 26.14 488.03 32.54

TABLE V. MINIMUM RMS CURRENT AND RECOVERY DURATION FOR AB3 TESTS OF 40 % DIP MAGNITUDE

Ud [%]

AB3 dip

IavAB [A] trev [ms] IL1 IL2 IL3 Iav

0 2.003 2.013 2.109 2.04 6200

20 2.487 2.518 2.615 2.54 6200

40 2.725 2.751 2.878 2.78 6200

70 3.395 3.408 3.59 3.46 5600

80 3.668 3.677 3.876 3.74 5000

90 3.988 3.992 4.212 4.06 4200

A strong correlation is between the current and amplitude of the dip, and between the shape of voltage dip and characteristics of induction motor (Fig. 11).

TABLE VI. MINIMUM RMS CURRENT AND RECOVERY DURATION FOR GR3 TESTS OF 40 % DIP MAGNITUDE

Ud [%]

GR3 dip

IavGR [A] trev [ms] IL1 IL2 IL3 Iav

0 2.612 2.637 2.769 2.67 7600

20 2.81 2.838 2.979 2.88 7600

40 3.078 3.104 3.262 3.15 7400

70 3.619 3.635 3.829 3.69 6000

80 3.837 3.849 4.058 3.91 5400

90 4.081 4.086 4.312 4.16 3800

Fig. 11. Dependence of average rms current on voltage abrupt (AB3 test) and gradual (GR3 test) dip magnitude for induction motor.

An abrupt shape of the dip determines a value of current more small that the case of a gradual dip. For abrupt dip, the time of current recovery is larger than for gradual dip. The active power absorbed by the induction motor during the abrupt dip is smaller than in the case of gradual dip. This corresponds with smaller values of the torque and speed of motor.

VI. CONCLUSIONS AND FUTURE WORKS In this paper the theoretical and experimental investigations

are done regarding the effect of abrupt and gradual shape three-phase balanced voltage dips on the induction motors. A test bench was used which allows to generate the voltage dips with various magnitudes, dip duration, abrupt and gradual shape, of type individual or repetitive, and supplying the induction motors of power between 0.5 – 30 kW. The registered curves of current and power show the displacement of minimum values of rms current, in correlation with the magnitude of voltage dip. Abrupt shape of voltage dips is a more severe test of the motor to the voltage dip. New investigations will be done for non-balanced three-phase voltage dips. The knowledge of the impact of voltage dips on the induction motor is useful for the establishing the efficient testing methods for obtaining the tolerance curves, for adequate control of protection systems, and for the substantiating the studies of stability and load flow in electrical networks, in which the complex consumer is associated with an induction motor.

ACKNOWLEDGMENT This paper is supported by the Sectoral Operational

Programme Human Resources Development (SOP HRD), ID76945 financed from the European Social Fund and by the Romanian Government.

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