Influence of Inverter Voltage Switching on Windings of Asynchronous Motor

Focus on the stator winding insulation of asynchronous motor, the transition process of the pulse voltage output by the frequency converter was analyzed, the voltage equivalent mathematical models and simulation model of the stator winding, reactor and cable were established. For rolling mill in metallurgical industry, the different voltage slope was verified by using real high-power asynchronous motor and frequency converter for pulse voltage test. Aiming at the voltage oscillation caused by different cables length, the second order system fitting was used to analyze the voltage spectrum characteristics, and the reactor’s suppression effect for voltage oscillation was also verified.


Introduction
With the increase of power density, high-power IGBT are widely used in metallurgical, petrochemical and other variable speed industrial situations, especially for low-voltage high-power inverter more than 130kW.For metallurgical, petrochemical and other process industries, higher requirements are put forward for the reliability, stability and rapid maintenance of frequency converter, and higher requirements are put forward for electrical insulation, voltage impact, current impact, torque impact, mechanical vibration and other aspects of variable frequency motors.In the field of variable frequency drive, the electric industry accidents caused by motor stator insulation failure results in a large number of economic losses, which has become a pain point in the development of industry technology.Without technical guidance, many technical maintenances staff could not accurately locate the fault and had to send the motor back to the motor factory for repair.Some literatures think that the voltage slope affects the motor stator insulation, and propose the structure of du/dt filter to solve this problem.Reference [1] proposed the classical design method of low-pass filter, but the large current increases the power loss of the system, which is not suitable for high-power variable frequency drive in the industrial field; Reference [2] designed one 2-order filter with RCL which was sensitive to the parameters of motor, cable, reactor, and cannot be implemented in industry applications.Some scholars have carried out relevant technical research on sine wave servo driver, and Reference [3] proposed a sine wave output filter to reduce motor du/dt, which has been applied in some small power servo drive and cannot be applied to high-power variable frequency drive.Aiming at the stator voltage slope du/dt, voltage oscillation and other technical problems of variablefrequency motor, this paper establishes a mathematical model of motor stator, builds a Simulink simulation model, and conducts a number of Matlab simulation tests to study the voltage transition process between different wire turns.At the same time, this paper uses high-power inverter motor for actual testing and verification.

Struct of Frequency Converter and Asynchronous Motor
The AC-DC-AC frequency converter rectifies the external AC voltage to DC voltage to charge the internal capacitor to form a stable DC voltage source; Pulse AC voltage is output through the opening and closing of fast IGBT, and AC current is formed after passing through the windings of variable frequency motor to drive the rotation of motor.The equivalent resistance of the stator winding of the variable frequency motor as shown in Figure 2 is very small, which is mainly reflected in the equivalent inductance component.At the same time, the EMF voltage is generated when the motor rotor rotates.Diagram of Asynchronous Motor equivalent circuit When the converter works, IGBT turns on and outputs a rectangular square wave voltage to the motor stator winding.Affected by cable, reactor, equivalent resistance and reactance of stator winding, when the rectangular square wave voltage reaches the stator winding side of the motor, the voltage rise rate will be greatly reduced [9][10], as shown in Figure 3 [12] that the square wave voltage output by the frequency converter passes through the cable and the motor stator winding, propagates in the form of electric waves, and is reflected back to the frequency converter by the cable, forming an oscillation, as shown in Figure 4.When the reflection frequency of the cable is greater than 2MHz, the voltage slope at the stator side of the motor will even be greater than that at the output side of the converter.This poses a great threat to the insulation of the stator winding of the variable frequency motor, and puts forward higher requirements for the insulation of the stator winding.  .Cable Reflection Diagram When the voltage slope increases due to cable reflection, the transitional voltage between different wire turns in the motor stator winding will also increase or decrease, which is related to the design and manufacturing process of variable frequency motor.The equivalent inductance of the motor stator winding is large, which has a strong attenuation effect on the excitation voltage.Therefore, when the pulse voltage with higher voltage slope is input to the motor stator, the voltage slope will drop rapidly after passing through a few wire turns inside the motor stator.The voltage slope du/dt only affects the insulation of the input and output terminals of the inverter fed motor stator, and has no effect on the interturn insulation of the motor stator winding.

Stator voltage model
Analyzed from the manufacturing process of variable frequency motor, the motor stator winding is made of insulated wire wound in the stator slot.It can be clearly seen from Figure 5 that the coil has the typical structure of a capacitor with respect to the ground potential with the insulating material on the wire turn surface as the dielectric: the two conductive surfaces are separated from each other by a thin dielectric.During the motor production process, each wire turn has a weak equivalent stray capacitance to the ground due to the influence of the internal resistance of the cable, the equivalent inductance, and the stator slot insulating paint.Each wire turn can be equivalent to a 2-order low-pass filter, as shown in Figure 6.After passing through the wire turns at the input end of the motor stator, the input pulse voltage is transmitted to each wire turn in series, showing obvious time lag at the output end.In the production process, the stator winding uses the same batch of insulated cables.After winding, the length of each wire turn is equal, and the equivalent inductance L is equal.Since the equivalent capacitance and resistance of each wire turn to the ground are very small, the resistance R of each wire turn to the ground can be approximately regarded as equal, which can further simplify the mathematical model of the motor stator winding.When the stator winding insulation is intact, the distribution of capacitance to ground of each wire turn is basically consistent, and the capacitance to ground is simplified to equal C. According to the connection sequence of the motor stator winding, the voltage/current model of the stator winding is established as shown in Figure 7.  1 The impedance of the penultimate wire turn is the result of the reactance n z of the last wire turn parallel connection with the turn to ground capacitance C and resistance R of the current turn.
By analogy, the impedance i z of each wire turn can be calculated.
It can be seen from the above formula that the voltage current model is an n-order system for the impedance of a stator winding with an n-turn coil, which can be expressed by a standard fraction: From the above formula, for the impedance of a stator winding with an n-turn coil, the voltage current model is an n-order system, which can be expressed by a standard fraction: the equivalent model of the wire turn winding is adopted, and the motor stator winding model is established in Matlab.Each wire turn uses the same inductance, resistance and capacitance parameters, and the pulse input voltage and the voltage of each wire turn are measured, as shown in Figure 8:  9 the voltage slope at the first wire turn on the input side of the motor stator winding is the largest.With the transmission of voltage in the stator winding, the voltage slope decreases gradually with the increase of wire turns.For each wire turn, only the voltage difference at both ends of the wire turn will affect the cable insulation, which is reflected in the voltage difference between each coil.10 the voltage of each wire turn gradually enters the steady state, and the voltage difference between the two adjacent wire turns gradually decreases with time.From the input side turn to the output side turn, the peak value of the wire turn voltage difference also shows a decreasing trend.The voltage difference between the first coil at the input end of the motor stator and the last coil at the output end is the largest, and the voltage slope is also the largest, even exceeding the input pulse voltage slope.For the winding mode of individual variable-frequency motor, if the last coil of the motor stator winding returns to the physical position of the first coil after winding, the voltage difference is maximum and the voltage slope is also maximum.When the voltage difference is lower than the insulation strength of the stator winding, theoretically, any voltage slope will not damage the wire turn insulation.Only the first and last turns of stator winding bear the maximum voltage slope and wire turn voltage difference, which is also the maximum risk point of insulation failure of variable frequency motor.Therefore, the following conclusion can be inferred: if the internal wire turn insulation of the variable frequency motor fails, but the end wire turn insulation does not fail, then the reason for the failure of the variable frequency motor is not the voltage slope and the wire turn voltage difference, should be caused by other environmental factors such as wire turn damage, conductive dust, humidity, external force, etc.In actual projects, due to the influence of the equivalent resistance of the frequency conversion cable and output reactor, voltage emission will occur at both the motor end and the output end of the frequency converter, as shown in Figure 11.The square wave voltage output by the frequency converter will form a traveling wave on the cable between the frequency converter and the motor end, and the voltage oscillation process will occur.The frequency of the oscillation is related to the cable length.Because the core parameters of each stator turn, such as equivalent inductance, equivalent capacitance to ground, and equivalent resistance to ground, are basically the same, the voltage oscillation period of each stator turn is basically the same; Influenced by the equivalent inductance of wire turns, the voltage of different wire turns has a certain period lag.However, the voltage oscillation amplitude of each wire turn is different.Based on the 2-order model of variable frequency motor stator wire turns, the UV phase stator model is established in Matlab, as shown in Figure 12.Using 7-segment modulation wave technology, the converter always changes the switching state of a pair of IGBTs at any time.This paper studies the transition process after adding 1000V step voltage at the moment of UV phase opening, and adds 50μH three-phase reactor, measure the voltage change process between UV phases, as shown in Figure 13.
Figure 13.Reactor limit voltage oscillation As shown in Figure 13, the actual voltage between the UV phases of the motor stator with reactor, the amplitude of voltage oscillation is obviously smaller than that without reactor, which verifies the inhibition effect of reactor on stator winding oscillation voltage.In the metallurgical industry the power range of the bar and wire mill product line is 200kW~3000kW, and the high-power inverter is usually equipped with an output reactor to suppress voltage fluctuations and reduce the voltage slope.As the distance varies greatly between the frequency converter and the on-site variable frequency motor, the cable length is about 50m~300m, and the cable diameter is related to the driving current, which also varies, leading to voltage oscillations at the end of some motors on the site.In the actual project site, the voltage oscillation is further suppressed by adjusting the tap of the output reactor or increasing the neutral resistance.

Engineering site test
In order to further study the influence of voltage slope and wire turn voltage difference on high-power variable frequency motor in metallurgical industry, this paper conducts a detailed test and research through the high-power tractor test platform.In this paper, the frequency converter motor YTSP450L4-6 from Shanghai Nanyang Motor Factory is used in the experiment.The rated voltage is 690V, the rated current is 530A, the rated power is 500kW, the rated frequency is 50Hz, the rated speed is 980rpm, the motor pole pairs are 3 pairs, there is no output reactor, and the frequency conversion cable is about 50m.Agilent DSO6012A is used as the oscilloscope, with a sampling rate of 2GSA/s and a single channel of 100MHz.High voltage differential isolation probe is used for voltage detection, and the maximum acquisition AC voltage is 2400V.In order to study the process of voltage oscillation, the oscilloscope data is derived and standardized.
The key system equivalent parameters are extracted by using the classical control theory for incoming line analysis and identification.The Matlab 2-order system optimal fitting method is used to identify the above step response process, and the identification results are shown in Figure 18.
The Bode diagram of the motor stator winding voltage system response is shown in Figure 19, and the cut-off frequency is 200kHz.One 50 μH reactor is added between the output side of the frequency converter, and then connected to the frequency converter motor through the frequency conversion cable.The voltage oscillation disappears which indicates that the output reactor has a good suppression effect on the voltage oscillation.

Conclusion
Aiming at the stator winding insulation problem of variable frequency motor, this paper analyzes the transition process of the pulse voltage output by the inverter in the transmission path in detail, derives the voltage equivalent mathematical models of the stator winding wire turn, output reactor and variable frequency cable, and establishes the simulink simulation model for the incoming line simulation.Taking typical rolling mill load in metallurgical industry as an example, this paper uses actual high-power inverter motor and frequency converter to conduct pulse voltage test, and verifies the change of voltage slope of different wire turns.For the voltage oscillation caused by different frequency conversion cables, this paper carries out the optimal fitting of the 2-order system, analyzes the voltage spectrum characteristics, and also verifies the effect of the reactor on suppressing the voltage oscillation.

Figure 2 .
Figure 2. Diagram of Asynchronous Motor equivalent circuitWhen the converter works, IGBT turns on and outputs a rectangular square wave voltage to the motor stator winding.Affected by cable, reactor, equivalent resistance and reactance of stator winding, when the rectangular square wave voltage reaches the stator winding side of the motor, the voltage rise rate will be greatly reduced[9][10], as shown in Figure3, thus presenting a trapezoidal wave voltage, which is called voltage slope du/dt.

Figure 4
Figure 4. Cable Reflection DiagramWhen the voltage slope increases due to cable reflection, the transitional voltage between different wire turns in the motor stator winding will also increase or decrease, which is related to the design and manufacturing process of variable frequency motor.The equivalent inductance of the motor stator winding is large, which has a strong attenuation effect on the excitation voltage.Therefore, when the pulse voltage with higher voltage slope is input to the motor stator, the voltage slope will drop rapidly after passing through a few wire turns inside the motor stator.The voltage slope du/dt only affects the insulation of the input and output terminals of the inverter fed motor stator, and has no effect on the interturn insulation of the motor stator winding.3.Stator voltage modelAnalyzed from the manufacturing process of variable frequency motor, the motor stator winding is made of insulated wire wound in the stator slot.It can be clearly seen from Figure5that the coil has the typical structure of a capacitor with respect to the ground potential with the insulating material on the wire turn surface as the dielectric: the two conductive surfaces are separated from each other by a thin dielectric.

Figure 5 .
Figure 5. Equivalent Capacitance of Stator Winding DiagramDuring the motor production process, each wire turn has a weak equivalent stray capacitance to the ground due to the influence of the internal resistance of the cable, the equivalent inductance, and the stator slot insulating paint.Each wire turn can be equivalent to a 2-order low-pass filter, as shown in Figure6.After passing through the wire turns at the input end of the motor stator, the input pulse voltage is transmitted to each wire turn in series, showing obvious time lag at the output end.

Figure 6 .
Figure 6.Equivalent diagram of motor stator In the production process, the stator winding uses the same batch of insulated cables.After winding, the length of each wire turn is equal, and the equivalent inductance L is equal.Since the equivalent capacitance and resistance of each wire turn to the ground are very small, the resistance R of each wire turn to the ground can be approximately regarded as equal, which can further simplify the mathematical model of the motor stator winding.When the stator winding insulation is intact, the distribution of capacitance to ground of each wire turn is basically consistent, and the capacitance to ground is simplified to equal C. According to the connection sequence of the motor stator winding, the voltage/current model of the stator winding is established as shown in Figure7.

Figure 7 .
Figure 7. Equivalent impedance diagram of stator Starting from the impedance analysis of the last wire turn, the voltage between the wire turn and the ground is n u .The current flowing through the wire turn is n i .The impedance of wire turn is n z .

Figure 10 .
Figure 10.Equivalent impedance diagram of stator As shown in Figure10the voltage of each wire turn gradually enters the steady state, and the voltage difference between the two adjacent wire turns gradually decreases with time.From the input side turn to the output side turn, the peak value of the wire turn voltage difference also shows a decreasing trend.The voltage difference between the first coil at the input end of the motor stator and the last coil at the output end is the largest, and the voltage slope is also the largest, even exceeding the input pulse voltage slope.For the winding mode of individual variable-frequency motor, if the last coil of the motor stator winding returns to the physical position of the first coil after winding, the voltage difference is maximum and the voltage slope is also maximum.When the voltage difference is lower than the insulation strength of the stator winding, theoretically, any voltage slope will not damage the wire turn insulation.Only the first and last turns of stator winding bear the maximum voltage slope and wire turn voltage difference, which is also the maximum risk point of insulation failure of variable frequency motor.Therefore, the following conclusion can be inferred: if the internal wire turn insulation of the variable frequency motor fails, but the end wire turn insulation does not fail, then the reason for the failure of the variable frequency motor is not the voltage slope and the wire turn voltage

Figure 11 .Figure 12 .
Figure 11.Voltage oscillation process diagram The reactor can change the equivalent circuit parameters for the convertor, and destroy the voltage oscillation process, which can effectively reduce the voltage slope.With the center frequency between 0.2MHz and 3MHz, the voltage oscillation can be restrained by 10μH~50μH output reactor.Especially for the reactor with adjustable resistance connected to the neutral point to form a 2-order low-pass filter.Phase U 1 s t Coil Phase U 2 n d Coil Phase U 3 r d Coil Phase U 4 t h Coil

Figure 14 .
Figure 14.Experimental environment Frequency converter carrier frequency is 1.5kHz, dead time is 5 μs.Pulse voltage width is 1 μs ~600μs。 The frequency converter outputs 25Hz three-phase sine voltage, and the motor speed is stabilized at 490rpm. the U-V phase voltage of the motor was measured, as shown in Figure 15.

Figure 15 .
Figure 15.Stator Voltage Diagram For the transition process of voltage oscillation, the peak to peak value of oscillation voltage Upp is 1.14kV, and the oscillation period is 3.6 μs shown in Figure 16.

Figure 18 .
Figure 18.Diagram of voltage fitting Thus, the transfer function of the system is:

Figure 19 .
Figure 19.Voltage Bode Diagram of Motor Stator This cut-off frequency is related to the switching characteristics of IGBT components and is one of the important indicators of high-power inverter product design.It determines the IGBT switching loss power.Reducing the amplitude and frequency of voltage oscillation can effectively reduce the switching loss of the inverter, which is conducive to extending the work life of IGBT components.One 50 μH reactor is added between the output side of the frequency converter, and then connected to the frequency converter motor through the frequency conversion cable.The voltage oscillation disappears which indicates that the output reactor has a good suppression effect on the voltage oscillation.