Modernization of a frequencycontrolled asynchronous electric drive system
Abstract
Keywords
For citation:
Shestakov I.V., Safin N.R. Modernization of a frequencycontrolled asynchronous electric drive system. Journal of «Almaz – Antey» Air and Space Defence Corporation. 2019;(2):2533. https://doi.org/10.38013/25420542201922533
A modern frequencycontrolled asynchronous electric drive (FCAD) is widely used both in the products of military equipment (ME) and in those converted for civilian use (CE). Products of the first category must comply with severe requirements to operating conditions (in accordance with the set of state military standards “Klimat6” and “Moroz7”). In the ME products, electric drive often functions under conditions of a thermally loaded enclosure, which complicates the task of reducing heat emission and providing heat dissipation. The requirements for ensuring warranty performance of the driving/working mechanism are conditioned, in particular, by FCAD energy efficiency improvement. Such a task is dependent, first of all, on the degree of minimisation of losses in the drive components, as such losses result in efficiency deterioration and increased power consumption. Proceeding from this, improving FCAD energy efficiency is a vital task in these circumstances.
The object of this study is a frequency controlled AC electric drive whose power circuit employs a threephase asynchronous motor (AM) with a squirrelcage rotor powered from frequency converter (FC) of a power controller with pulse width modulation (PWM).
The purpose of the work is to investigate feasibility of improving energy efficiency of FCAD used in electrohydraulic transmission of a selfpropelled hoisting machine. The paper is a continuation of works [1][2].
One of the options for solving the task is refinement of the existing electric motor types and semiconductor converters and development of new ones, with improved energy characteristics.
In the field of electric machinebuilding, national industry has mastered production of several generalpurpose asynchronous motor series (AI, 5A) with higher efficiency and power factors. For example, “RUSELPROM” JSC has developed special crane motors of 5MTK and 7MTK series for a frequencycontrolled electric drive. The improved stator winding manufacturing technologies and the magnetic core design ensure reliable operation of electric motors when powered from autonomous voltage inverters (AVI) and a possibility for rotation frequency control in a wide range.
Today’s achievements in the development of microprocessor control facilities make it possible to solve highly complex practical tasks, such as identification of parameters, assessment of state variables, adaptive and optimal control. One of important trends in variablespeed electric drive theory and practice remains that of developing electric drives that would support technological processes with minimum energy consumption.
Practically feasible domains of FCAD functioning are determined also by the variable frequency control law and the quality of its implementation in the regulating system. At the present time there exist different AM control types,
implemented in FC on the basis of scalar and vector control systems. In its turn, vector control is divided into two basic types: with direct rotor field orientation (with rotor position sensor, speed sensor, magnetic flux sensor in the air gap) and with indirect rotor field orientation (sensorless).
Accordingly, vector control with indirect rotor field orientation allows to do without speed sensor (as well as other sensor types), but this option has the following specific disadvantages:
 in the low sliding mode, i. e., when the motor is running at low speeds, speed regulation quality deteriorates [3];
 the software and hardware part of the electric drive tends to become more complex and expensive.
The use of speed sensor to a certain extent decreases FCAD reliability due to the influences of a widerange complex of physicalchemical and climatic factors, e. g. under conditions of a confined thermally loaded space with vibrating and/or heat emitting equipment. Moreover, speed sensors (encoders) are the least reliable elements in a crane electric drive, with their failure occurring ever so often [4]. Considering all this, a scalar principle of variablefrequency control is implemented in the FCAD control system.
At the same time, selection of AM for operation in a variablespeed drive is a key factor influencing the reliability of drive/working mechanism operation. This paper considers a new traction motor (RF patent No. 184734 for a utility model is granted) with the following specifications: nominal power P_{N} = 15 kW; nominal phase voltage U_{N} = 127 V; nominal phase current I_{N} = 50.38 А; supply voltage frequency f_{N} = 400 Hz; efficiency factor = 0.8651; power factor cos φ_{N}= 0.8351; number of pole pairs z_{p} = 4; relative sliding s = 0,0269; rotor rotation speed Ω_{2} = 611.42 rad/s. The motor is manufactured for heavyduty operation under conditions of exposure to various adverse factors. To increase motor reliability, its design also features a cooling circuit with cooling channels passing through rotor in the axial direction. The adopted solutions allow to improve the internal air circulation and thus enhance the heat transfer circuit.
AM power supply from FC does not improve FCAD energy efficiency figures in a direct way. On the contrary, losses of an electric motor supplied from inverter with voltage PWM is higher than those of a motor supplied from the mains. This is conditioned both by decrease of the active voltage in the nominal mode and increased electrical and magnetic losses due to the influence of the switching component of current and stator field higher harmonics [5].
Hence, FCAD operation is accompanied by a number of negative factors: occurrence of supply voltage higher harmonics, which cause pulse overvoltages in the stator winding; high losses reducing efficiency factor and AM usable power and causing increased heating; extra inertial moments increasing vibration and noise.
In this respect, for quantitative estimation, it is proposed to undertake a comparative mathematical simulation of a particular AM powered from the mains and from FC. The simulation consists in modelling of the mode of AM direct start and achieving the ideal idling speed (Ω_{0N} = 628.3 rad/s), followed by active load surge M_{c} = 24,6 N⋅m; herewith, the speed decreases to Ω_{2} = 611.4 rad/s (relative value of rotor nominal speed ω_{2} = 1  s = 0,9731).
A number of experiments were performed on the AM mathematical model with power supplied from the mains (Fig. 1), while reading off the values of shaft rotation speed and effective values of currents and electrical losses in the stator and rotor windings. The results are given in Table 1.
Fig. 1. MATLAB Simulink mathematical model of AM supplied from the mains
For presenting a quantitative estimation of the values of electrical losses in the stator and rotor windings, as well as for establishing the causes and their specific effect on power losses in the motor, mathematical simulation of the modes of start by underfrequency relay was performed, followed by load surge on the AM shaft, with power supplied from a pulse widthmodulated voltage source.
Table 1
Values of currents and electrical losses of motor supplied from the mains
Parameter 
Мс = 6.15 N⋅m 
Мс = 12.3 N⋅m 
Мс = 18.45 N⋅m 
Мс = 24.6 N⋅m (= М_{ном}) 
Shaft rotation speed, rad/s 
624.4 
620.4 
616.3 
611.4 
Stator current effective value, A 
24.0 
30.57 
39.3 
49.2 
Electrical losses in stator winding, W 
71.29 
115.6 
191.2 
300 
Rotor current effective value, A 
10.6 
21.25 
32.1 
43.13 
Electrical losses in rotor winding, W 
24.14 
97.6 
222.4 
401.8 
Table 2
Summary table of stator current effective values
Conditions 
Stator current effective value, A 

Experiment 
Mathematical model 

n = 125,66 рад/c (0,2 Ω2), Мс = 12 Нм (0,5 М_{ном}) 
46 
45 
n = 125,66 рад/c (0,2 Ω2), М_{с} = 6 Нм (0,25) 
32.6 

n = 235.62 rad/s (0.375 ад, Мс = 10 N.m (0.41 Мном) 
37 
37 
n = 611,4 рад/c (Ω2), Мс = 12 Нм (0,5 М_{ном}) 
39 
39 
Given in Table 2 are the effective values of stator current, obtained experimentally and with the use of a mathematical model of the motor, read off under certain specified conditions, with power supplied from FC.
The data given in Table 2 confirm compliance of the mathematical model of a squirrelcage asynchronous motor with the experiment results.
The mathematical simulation was performed using the values of active resistances of the AM windings for a temperature of 150 °С provided by the manufacturer, Sarapul Electric Generator Plant (“SEGZ” JSC): stator  R_{1} = 0.0412 a; rotor  R_{2} = 0.0743 a. In the FC  AM control system, the scalar variablefrequency control law is implemented (shown as the red line in Fig. 2). The blue line designates the law of proportional variablefrequency control. The minimum voltage on low frequencies U_{boost} = 29 В (= 0,16 U_{6}). PWM carrier frequency  4 kHz.
A number of experiments were performed on the AM mathematical model with power supplied from FC with scalar control system (Fig. 3), with reading off the values of shaft rotation speed and effective values of currents and electrical losses in the stator and rotor windings. The simulation results are given in Table 3.
Fig. 3. MATLAB Simulink mathematical model of AM supplied from FC with scalar control system
Table 3
Values of currents and electrical losses of motor supplied from FC
Parameter 
М_{с} = 6,15 Нм (= 0,25 М_{ном}) 
М_{с} = 12,3 Нм (= 0,5 М_{ном}) 
М_{с} = 18,45 Нм (= 0,75 М_{ном}) 
М_{с} = 24,6 Нм (= М _{ном}) 
Shaft rotation speed, rad/s 
624.3 
620.2 
615.8 
611.4 
Stator current effective value, A 
27.80 
33.8 
42.26 
523 
Electrical losses in stator winding, W 
95.33 
141.4 
221.0 
338 
Rotor current effective value, A 
17.40 
25.7 
35.70 
46.6 
Electrical losses in rotor winding, W 
65.30 
142.5 
275.0 
468 
Fig. 4. Comparison of the results of mathematical simulation of the modes of motor operation from different voltage sources (solid lines  from the mains; dashed lines  from AVI): I_{1}  stator current effective value; I_{2}  rotor current effective value (а); ΔР_{эл1}  electrical losses in stator winding, ΔР_{эл2}  electrical losses in rotor winding (b)
For convenience of comparing the results of mathematical simulation of motor operating modes when supplied from different voltage sources, Fig. 4 shows the dependencies of the currents of stator I_{1} and rotor I_{2}, windings, electrical losses in the stator ΔР_{эл1} and rotor ΔР_{эл2} windings on the moment of load on the AM shaft when supplied from the mains (solid lines) and from an autonomous voltage inverter, AVI (dashed lines).
With AM supplied from a PWM source, at nominal load, stator current would increase by 6.3 %, and rotor current by 8.1 % (see Fig. 4). An increase in the currents of motor windings leads to respective increase in the electrical losses of stator winding by 12.7 %, and rotor winding by 16.5 % as compared with the simulation results of AM modes when supplied from a sinusoidal voltage source.
An increase in AM power losses when supplied from a pulse widthmodulated voltage source can be explained by the presence of higher harmonics in the voltage supplied to the stator windings, voltage drop on the power semiconductor devices of the FC, and voltage decrease on the motor, conditioned by the PWM use. Let us consider the following factors.
Due to the presence of higher harmonics, the effective value of stator current (Ι_{Σ}) appears to be too great, which can be described by the following formula:
where I_{(1)}  effective value of the fundamental harmonic;
K_{ВГТ}  stator current harmonic distortion factor.
In particular, for nominal load and PWM carrier frequency equal to 4 kHz, stator current harmonic distortion factor K_{ВГТ}= 0.254 (calculated in the MATLAB Simulink package by the method of [1]), hence, the voltage increase factor is small, since I + K_{ВГТ}^{2} = 1,0645. This estimate is fairly correct, since at the nominal load (M_{c} = 24.6 N⋅m), the effective stator current value is I_{1}= 52.3 А (see Table 3), and when supplied from a sinusoidal voltage source, the effective stator current value I_{1}= 49.2 А (see Table 1). The actual stator current value in the nominal mode, when supplied from a pulse widthmodulated voltage source, is 1.064 times higher (by the experimental works results  1.102 times) than when supplied from a sinusoidal voltage source.
Much greater effect is produced by current higher harmonics on the value of losses in the motor. Relative to sinusoidal power supply, electrical losses in the motor increase by 15 %, and total losses  by 4.6 %.
Moreover, nonsinusoidal power supply leads to increased losses not only in copper, but also in steel. In particular, the increase of losses from eddy currents can be approximately estimated using a similar formula:
U_{Σ} = U_{(1) } (1+ K_{ВГТ}^{2}) (2)
where U(1)  effective value of the fundamental harmonic;
K_{ВГТ }= 0,701  stator voltage harmonic distortion factor at nominal load (calculated in the MATLAB Simulink package by the method of [1]).
In the detailed MATLAB Simulink mathematical model of FCAD [1], losses in the stator and rotor steel are taken into account as well. Accordingly, stator voltage harmonic distortion factor K is obtained proceeding from this model on the basis of voltage spectral analysis by means of unit Power gui  FFTAnalysis (FFT fast Fourier transform).
It follows from this that the factor of losses increase caused by eddy currents equals to 1 + K_{ВГТ}^{2} = 1+ 0,701^{2} = 1,491. Hence, unlike the electrical losses, losses in steel from eddy currents increase, approximately, 1.5fold, which will result in an increase of total losses by ca. 7.5 %.
Summing up the increase of electrical losses and losses in steel, as well as considering increased additional losses, a degree of total losses increase at nominal load on the AM shaft can be approximately estimated as 12 % relative to the losses under sinusoidal power supply, i. e. nominal losses.
To establish a quantitative estimate of the influence of voltage level in the direct current segment on the values of current in stator and rotor windings, and, as a consequence, the values of electrical losses in the motor windings, a number of experiments were performed on a mathematical model consisting of a DC voltage source, AVI, and AM.
Because of negative effects of the AVI, such as ‘dead’ time and voltage loss on the power switches in straight and reverse direction, as well as nonideal software compensation of those effects, the required voltage level often cannot be reached.
Therefore, at the nominal frequency, the levels of driving phase voltages generated by the AVI will be by 5...10 % less than the nominal value.
Voltage of the direct current segment is maintained at a constant level U_{d} = 311В from a highpower DC voltage source. Such voltage level in the direct current segment corresponds to that available on the clamps of filter capacitor bank installed downstream of a threephase rectifier, implemented according to the Larionov circuit, supplied to the input of which is effective three phase line voltage of 220 V. The frequency of the fundamental (first) harmonic of the output voltage is 400 Hz.
The results of mathematical simulation of FCAD operating modes under varying values of load on the motor shaft and different voltages in the direct current segment are summarised in the form of plots and presented in Fig. 5 and 6.
The plots of the winding currents of stator I_{1} and rotor I_{2} (see Fig. 5) are of nonlinear character, with the values of currents increasing along with the increase of load on the AM shaft. Besides, the values of currents of the stator and rotor windings increase with the decrease of voltage in the FC direct current segment.
An increase of currents leads to the increase of respective electrical losses, which is shown in the plot families of electrical losses in stator winding ∆Р_{эл1} and rotor winding ∆Р_{эл2}, given in Fig. 6.
Fig. 5. Plot families of currents in motor windings depending on load on shaft and DC segment voltage
Fig. 6. Plot families of electrical losses depending on load on shaft and DC segment voltage
The plots of electrical losses have a nonlinear parabolic character. With an increase of load on the AM shaft, the electrical losses increase accordingly. In addition to that, the values of electrical losses in the stator and rotor windings increase along with the decrease of DC segment voltage.
Voltage decrease on the motor, conditioned by the use of PWM, is associated with natural (physical) voltage limitation, which can be obtained at the output of FC supplying the motor. The maximum permissible voltage value can also be additionally restricted by the electrical strength of motor winding insulation. Moreover, with a decrease of feed voltage frequency, the values of all reactive resistances of the AM equivalent circuit decrease proportionally, while the values of active resistances remain unchanged. Therefore, in the smallfrequency domain, the share of stator winding active resistance in the motor impedance rises. Accordingly rising is the share of voltage drop on the active resistance of stator winding and its influence on the magnetising circuit voltage at a fixed stator voltage and variation of load on the AM shaft.
In this way, the analysis of electrical losses in the AM stator and rotor windings for the cases of using sources of sinusoidal voltage and pulse widthmodulated voltage as motor power supply source demonstrated advisability of motor power supply source switchover from PWM feed to a sinusoidal voltage source in the nominal operating mode.
Proceeding from this, it is proposed to modify the circuit of motor connection from frequency converter to the supply mains by using a bypass, i. e., make a provision for motor power supply switchover from pulse widthmodulated voltage source to sinusoidal voltage source. Fig. 7 shows two options of schematic diagrams of FC with asynchronous motor connection to the supply mains.
Fig. 7. Options of schematic diagrams of FC with asynchronous motor connection to the supply mains: а  without possibility of voltage source switchover; b  with possibility of voltage source switchover
A conventional connection diagram (see Fig. 7, а) does not provide for a possibility of asynchronous motor direct connection to the supply mains. Conventionally shown in the diagram are the FC and contactors K1 and K2, without respective control windings. With power supplied to the FC, the controller sends a closing command to contactor K2 for precharging of the capacitor
bank. After that, contactor K2 opens, while contactor K1 closes, thus connecting FC to the supply mains.
One of the possible options of a schematic diagram of FC with asynchronous motor connection to the supply mains, with a possibility to switch over the voltage source feeding the motor, is given in Fig. 7 (b). As compared with the conventional arrangement, this diagram of FC with asynchronous motor connection to the supply mains is supplemented by two threephase contactors, which perform switchover of voltage source feeding the motor from pulse widthmodulated voltage to sinusoidal voltage, in accordance with the control signal state (i. e., a redundant/bypass power supply circuit is added).
Also shown in Fig. 7 (a, b) is currentlimiting resistor R, which is necessary for the procedure of capacitor bank precharging.
Let us consider an algorithm of AM feeding source switchover from pulse widthmodulated voltage to sinusoidal voltage, and vice versa, when running in the modes of electric motor acceleration and slowdown.
The initial state: all contactors are open, zero voltage in the FC direct current segment. With power supplied to the FC, the controller sends a closing command to contactor K2 for precharging of the capacitor bank. After that, contactor K2 opens, while contactor K1 closes, thus connecting FC to the supply mains. In this state, the motor accelerates to the nominal speed at a specified pace. After the motor has reached the nominal speed, contactor K3 opens and contactor K4 closes. Thereon the motor is running under power supply from a sinusoidal voltage source.
Before entering the mode of electrical braking, contactor K4 opens and contactor K3 closes. The AM is slowing down at a specified pace. When reaching zero speed, controller changes over to the start waiting mode.
It is important to point out that application of the option being considered, along with an improved drive power efficiency, implements a higher reliability of the drive, since its performance is supported in the event of FC failure.
Hence, in accordance with the policy of industrial and technological development of the integrated structure of “Almaz  Antey” Air and Space Defence Corporation for the period until 2026, one of the crucial tasks is implementation of the intellectual activity results and mastering of the output of innovative industrial products, which determines the relevance of the scientific research works described above.
Based on the results of mathematical simulation in the MATLAB Simulink software package, modernisation of power controller is performed for improving power efficiency of FCAD and drive mechanism as a whole.
About the Authors
I. V. ShestakovRussian Federation
N. R. Safin
Russian Federation
Review
For citation:
Shestakov I.V., Safin N.R. Modernization of a frequencycontrolled asynchronous electric drive system. Journal of «Almaz – Antey» Air and Space Defence Corporation. 2019;(2):2533. https://doi.org/10.38013/25420542201922533