inverter - effect on isolation of motors and solutions

The massive introduction of power electronics in the field of electrical drives in recent decades has essentially revolutionized the type of power the electric motors. On the one hand this has improved the quality of the speed control, but on the other hand has compounded the stresses that the insulation of machinery must endure. Ing. Ibrahim GULESIN

INVERTER - Effect on isolation of Motors and SOLUTIONS

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Ing.Ibrahim GULESIN

introduction The massive introduction of power electronics in the field of electrical drives in recent decades has essentially revolutionized the type of power the electric motors. On the one hand this has improved the quality of speed control, and the other has compounded the stresses that the insulation of machinery must endure. These devices operate a conversion from an AC / DC and DC / AC which allows, as in the modulation Pulse Width Modulation (PWM), to be able to vary the power supply frequency of the machine, generating a sequence of pulses of variable width. It is then passed from traditional feeds with sinusoidal waveforms alternate forms of pulse voltage and high frequency, that is characterized by rapid rising edges and falling edge (of the order of a few kV / ms). The impulsive nature of these waveforms has aggravated the electrical stress to which they are subjected, the materials used for the insulation of the conductors of the windings of the machines. It 'important to note that the use of electronic devices, which now lie scattered in networks of low voltage,

absorb currents with high harmonic content on the side of the withdrawal and thus have a distorting effect

that alters the sinusoidal shape of the network itself. Thus, without appropriate filters, also all the other

connected users nearby, dimensioned for sinusoidal power supplies of the traditional type, can be affected

by these harmonic distortions. For all these reasons it became necessary to review the adequacy of

traditional insulation to be able to withstand the stresses resulting from the use of electronic converters. In

particular, for the electric motors such question was raised following the occurrence of a high number of

unexpected failures (early), probably attributable to the different electrical stress applied to the materials.

The awareness of conditions that can produce reflected waves and a selection of the appropriate motor

type Class F or Class H can guarantee a service life of the engine at the lowest possible cost.

The following information was developed to help users to understand the operation of the inverter and

motor synchronous / asynchronous and understand the phenomenon of wave reflective in terms related to

the issue, the failure mechanisms known engine and the available solutions. Although, this phenomenon

can affect only a small portion of plants, it is important to recognize the situation and consider the possible

solutions before installation.

Ing. Ibrahim GULESIN

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Sommario introduction ....................................................................................................................................................... 1

INVERTER ........................................................................................................................................................... 3

The modulation P.W.M. Pulse Width Modulation ........................................................................................ 3

Waveforms of current and voltage ............................................................................................................... 4

Advantages and disadvantages of PWM inverter ......................................................................................... 5

Mechanism of generation of overvoltages.................................................................................................... 5

Common Considerations ............................................................................................................................... 7

ENGINES ............................................................................................................................................................. 8

Insulation system of the motor ......................................................................................................................... 9

Insulating resin ............................................................................................................................................ 10

Mounting techniques and effects on engines ............................................................................................. 10

Understanding why AC variable speed motors break. .................................................................................... 10

Grounded coil slot. ...................................................................................................................................... 11

Grounded coil in edge of slot. ..................................................................................................................... 11

Damage due to locked rotor. ....................................................................................................................... 12

Damage due to short circuit. ....................................................................................................................... 12

Damage caused by phase-to-phase winding. .............................................................................................. 13

Damage due to winding short circuit between windings. ........................................................................... 13

Damage due to coil shorted. ....................................................................................................................... 14

Damage due to cooking / break of a stage. ................................................................................................. 14

Damage due to unbalanced voltage. ........................................................................................................... 15

Damage due to overvoltage. ....................................................................................................................... 15

NEMA Standards .............................................................................................................................................. 16

Conclusion and solutions ................................................................................................................................. 20

RIFERIMENTI .................................................................................................................................................... 26

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INVERTER The inverters are static converters used to generate waveforms of voltage or alternating current, controlled in amplitude and frequency, using a voltage source or current. The principle diagram of the operation of a synchronous motor / asynchronous powered by inverter is illustrated in Figure 1. It may be noted a first stage consisting of a rectifier (with which it makes continuous the mains voltage) and a second stage consisting of an inductor, which has the task of reducing the harmonics fed into the grid by the inverter. There is then a capacitor C, which has the function to reduce the peak voltage at the output of the rectifier and to maintain as constant as possible the voltage on the DC bus. It is then present a branch braking in case you want to brake the motor in electric mode. Finally, as the third stage, there are three branches of the inverter driven by the PWM modulation.

The modulation P.W.M. Pulse Width Modulation A control method for controlling the opening and closing of the static switches is the Pulse Width Modulation (PWM). The PWM is applied to electronic components and forced commutated. such prerogative is made necessary by the presence of openings and closures of the components to very precise instants and with rather high frequencies from 0.7 kHz up to 20 kHz, this means that IGBT turns on and off with a voltage from 7,000 up to 20,000 times per second. So it means that the rise time of the voltage is short, usually less than a microsecond. The rise times short determine long with electric lines between the drive controller and the voltage reflections, also said reflected waves that have high peak voltages. If the voltages are large enough to generate potentially destructive stresses in the insulation of the motor.

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In fact, the waveform of the output voltage is the better the higher is the switching frequency. The

switching instants of the components in the PWM technique arise from comparison between two

functions: a triangular-shaped constant-frequency carrier call Vpor, and a modulating Vmod of shape and

frequency equal to the desired voltage at the inverter output V (t). The amplitude of the modulating wave

is instead a part of V (t) for obvious reasons of convenience (Figure 2).

Figure 2 - Graph of the function triangular carrier and modulator.

• The voltages generated by the inverter have a harmonic content often intolerable for engines. • Piloting each branch of inverters in a complementary manner at a high frequency as if it were a chopper whose duty cycle varies with sinusoidal law is obtained by an alternating voltage with a "harmonic content" high frequency which is filtered from the engine and then tolerated. • The law of control is obtained by a comparison of two functions sine and triangular. • The three branches of inverters will have functions of piloting phase difference between them of 120 ° to obtain a three-phase system

Waveforms of current and voltage

Figure 3 - Vag waveform of current on each phase individually and Vab form of waves of voltage between the phases

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Advantages and disadvantages of PWM inverter

advantages:

• The current is more similar to a sinusoid.

• The drive is single-stage to vary the width because I do not need the chopper input.

• The motors run very regularly.

disadvantages:

• The impulse control is more complicated.

• The semiconductor switch to a higher frequency and therefore heat up more.

Mechanism of generation of overvoltages

The inverter rectifies a commercial power supply voltage and in more regulates the whole into a DC

voltage. It is the magnitude of the DC voltage that becomes approximately 1.4 times greater than that of

the source voltage (about 630V in the case of an input voltage of 440V AC). The peak value of the output

voltage is generally close to this value of DC voltage bus; but, since there exists inductance (L) and stray

capacitance (C) in the wiring between the inverter and the motor, the variation in voltage due to switching

of the elements of the inverter causes a voltage peak originating from the LC resonance and involves the

addition of a high voltage to the motor terminals (refer to Fig 2). This voltage reaches sometimes up to

about twice the voltage inverter DC (630V x 2 = about 1240 V or 620x3 = 1860V) according to a switching

speed of the elements inverter and a condition of wiring.

In addition, in the elevator industry as an output of the inverter using 2 contactors of the line if they are not

well synchronized with the inverter can create additional spikes. To avoid the peak output voltage of the

inverter, ensure that both the inverter and the motor are completely stopped before you enable or disable

the line contactor.

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As the wave PWM provides the advantages and engine performance, it can also create tensions that can put stress on the insulation system of the engine and cause damage. The chart above shows the typical waveform of PWM output from the driver to the output terminals of the frequency converter. The waveform on the graph below shows the same waveform of the motor terminals. Note that the reflected wave phenomenon has raised the peak level of the waveform of voltage to the motor. Most of the drives currently use the IGBTs of last generation that offer advantages for the design of the inverter. These drives have also the possibility that the waveform output to the motor may have peak amplitudes two or three times higher than the voltage of the DC bus. As a result, these tensions can potentially stress and damage the insulation system in some engines today. Similarly to power AC sine wave at 50 ÷ 60 Hz, when the square wave voltage urges the isolation of motors fed by a PWM converter, exceeds a predetermined threshold value; for which it has the triggering of the discharges and is defined a parameter called PDIV (Partial discharge Inception Voltage), or trigger threshold of partial discharges. Each pulse free discharge a very small amount of energy. This involves the gradual erosion and chemical decomposition of the material urged until the moment in which a branched channel of microscopic dimensions penetrates below the surface of the 'insulation. The channel is developed, simultaneously increase its size and intensity of partial discharge involving itself. Once reached the conductor that acts as a second electrode, it forms a conductor path that determines the total discharge of the insulator.

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Common Considerations

Inverter with the use of IGBT create voltage spikes 2-3 times greater than the DC-bus and this represents a

problem for the engine. It is important to remember that traditional motors with variable speed can

ultimately lost in isolation and damaged. The insulation of the motor depends on the heating temperature

of the engine to the environment, from the range of load and speed of application. Excessive vibration and

shock loads continue to be a problem as well as environmental problems such as moisture, dirt and harsh

chemicals that can affect the life of the engine.

The IGBT drive technology of today can cause excessive stress isolation of the motor which must be treated

as another problem of application. Because of the high switching speed and short rise times of IGBT

devices, the peak voltages on the motor can damage the insulation.

The rise time that determines the dv / dt, or voltage variation, for the change in time of the waveform, can

also influence the motor failure. Not only determines the distance from the converter in which the peak

voltages occur, but also affect the non-uniform distribution of this voltage inside the motor and, therefore,

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the effect on the insulation system. Since the units today with a PWM IGBT technology have higher carrier

frequencies, with the carrier frequency can increase the number of pulses that have high voltage spikes. All

these factors, therefore, determine the severity of the impact on the engine.

ENGINES Electrical machines mainly consist of three types of materials: • the copper, as regards the electrical part; • the iron, as regards the magnetic circuits; • the insulator, to separate two or more active parts in different potential. The isolation proves to be the most critical point in the realization of an electric machine, because among the three materials listed above, the insulator is that it degrades more readily in the presence of various kinds of stresses (electrical, thermal, mechanical and chemical) , of which, first of all, the thermal. An empirical evaluation allows to estimate, in fact, the halving of the life of an insulator for every increase of the operating temperature of 7 ° C. The insulators are chosen, for this, according to their thermal class, as a function of the temperatures reached in thermal regime by the machine. There is talk of life of a system to indicate the duration of its operation, that is, the time up to failure. E 'need to know the behavior of these characteristics when the insulation is subjected to stresses that will cause aging, ie it is necessary to know the degradation of properties such as dielectric strength, which is essential for the correct operation of the electrical systems. The stresses that cause degradation of insulation systems are essentially three: • thermal stress; • mechanical stress; • electrical stress. The joint action of more stress results in a reduction of the life of the insulation system in question, up to unacceptable values with respect to the design conditions, due to the synergism between the various types of stress. The study of the resistance to electric stress of an insulating material consists in evaluating the effects of a voltage applied to it, kept constant. The life of the material is much shorter the higher is the applied electric field. The failure is caused by the failure (ie, the total discharge), due to a reduction in the time of its electrical properties. It occurs after the formation of a branched channel of prior discharging (treeing, or arborescence electrical) that originates from the partial discharges, which take place in the vacuoles or in microscopic crevices that are present inside the insulating material.

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Insulation system of the motor

Insulation systems of the engine are designed to protect the electric motor. The isolation is required in the

event of a difference of electrical potential between two conductors. The integrity of the insulation system

of the engine is not determined by a single component but rather by a combination of components and

production techniques. A typical insulation system includes the wire (Wire), insulation goal stick phase-to-

phase (mid stick), head of the coil insulation between phase-to-ground (Top stick), insulating resin between

the wires and insulation from the total stator phase-land (Slot Liners).

wire

The first component is the wire insulation. The magnet wire is typically a film coated with enamel. The

thickness of the material and the concentricity of the coating can affect the quality of the insulation system.

isolation slot

The slot insulation of the motor physically separates the element of the motor. Coatings of groove and

splints increase the isolation of phase-to-ground. In between there are cues that increases the phase-to-

phase insulation. Finally, it adds a dielectric strength of the insulation system through one last cue that

plays a roll.

Coil insulation

At the head of the coil is added a splint so as to determine an additional insulation which enhances the

performance of the engine. The paper of step (splint) can be placed between the windings of different

phases and isolation can be added to the conductors which emerge from these coils. The entire assembly

can be taped and connected to increase the strength and prevent the movement of the component. This

can greatly increase the dielectric insulation between phase to phase and between groups of coil.

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Insulating resin

The insulating resin is so called because after winding is coated with an insulating resin. This resin has to fill

as much space as possible between the cables and the slots of the stator to create a solid unit. This adds to

the dielectric strength of the insulation system increases the voltage required to determine the crown. The

type of resin used may vary from BOME solvent to a reactive type 100%. The solvent resin BOME requires

that the solvents are removed during the process of hardening (Bake) leaving the final insulating material.

For the 100% reactive resin, the material solution becomes part of the final insulating material.

Mounting techniques and effects on engines

How does the system isolation? The assembled and processed can greatly affect the final quality? An

insulation system is the sum of its parts. The design of winding can improve a system of conductors with

insulation while maintaining high electric potential difference as physically distant between them.

Furthermore, the process by which the resin is applied can greatly affect the performance of insulation.

Some resins are applied by a dipping process and then the windings are cooked to assist the flow of

material during whole winding; in other cases they slide the resin above the stator first and then be cooked.

Another method is a VPI or vacuum impregnation. This method forces the resin in the windings so as to

increase the thickness of the coating by eliminating the voids. Many of these processes can be performed

repeatedly to improve the system. As well as the use of the inverter is increasing in the field of drives,

systems of motor insulation are adapting the functioning of this. In particular the first models of PWM

modulation since cause excessive heating of the motor has been added for greater thermal resistance to

the alternating-current motor, with the use of major classes of insulating materials. Efforts have thus

focused on a better insulation of phases, resins, enamelled wire and production techniques to eliminate the

possibility of faults in the engine.

Understanding why AC variable speed motors break.

To understand why variable speed motors break easily and know how you are trying to avoid this, it is

important to understand the failure mechanisms of the motors.

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Grounded coil slot. This type of insulation failure is typically caused by contaminants, abrasion, vibration or voltage spikes.

Grounded coil in edge of slot. This type of insulation failure is typically caused by contaminants, abrasion, vibration or voltage spikes.

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Damage due to locked rotor. Severe thermal deterioration of the insulation in all phases of the motor is normally caused by very high currents in the stator winding due to a condition of locked rotor. It can also occur due to excessive stops and reversals.

Damage due to short circuit. This type of insulation failure is typically caused by contaminants, abrasion, vibration or voltage spikes.

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Damage caused by phase-to-phase winding. This type of insulation failure is typically caused by contaminants, abrasion, vibration or voltage spikes.

Damage due to winding short circuit between windings. This type of insulation failure is typically caused by contaminants, abrasion, vibration or voltage spikes.

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Damage due to coil shorted. This type of insulation failure is typically caused by contaminants, abrasion, vibration or voltage spikes.

Damage due to cooking / break of a stage. Insulation faults as this is usually caused by power surges. Voltage surges are often the result of the power circuits of switching, lightning, discharges of capacitors, and solid state power devices (IGBT).

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Damage due to unbalanced voltage. Deterioration of thermal insulation in a stator phase may result from an uneven tension between the phases. Not equal voltages are usually caused by unbalanced loads on the power source, a bad connection to the terminal, or a high contact resistance. A 1% voltage unbalance may cause 6% -10% of the imbalance of the current!

Damage due to overvoltage. This type of insulation failure is typically caused by contaminants,

abrasion, vibration or voltage spikes.

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In addition to using insulated wire, most of the motors receives additional insulation immersing the windings in a tank of insulating material to form a coating on the windings. This process adds insulation value to the magnet wire, compensating nicks and variations in the thickness of the insulating material as the original. Some windings can get a second dive (with a process of treatment of wetting or another process to "drop" in which you increase the insulation in the final laps). This solves the problem of insulation faults of the engine which occur on the end turns of the winding, but not in slots. Measurements of insulation can ensure that additional windings with different potentials are electrically isolated from each other. The process of isolation, however, leaves usually microscopic air pockets in the lining. These holes can be points of insulation fault when voltage peaks were recorded on the stator by a reflected wave. Frequently, 60 to 80% of the voltage can be distributed between the first coil of the motor winding. Right now with the final data is not available to determine the exact cause of the fault isolation. In fact, the engine manufacturers are differing opinions on the results of the reflected wave phenomenon: • The voltage electrical stress exceeds the breakdown voltage of the empty air, causing a partial discharge. Subsequent partial discharge slowly deteriorate the insulation. • The voltage ionization of the surrounding air, leading to arc between the windings (known as the crown), cause immediate failure of the engine. • The peak voltage is higher than the degree of insulation of the magnet wire, causing a dielectric stress and failure existing insulation. There is no definitive research on the fact of the effects of reflected waves in 'isolation of the cable. Most cables however are immune to this phenomenon because of their substantial isolation.

NEMA Standards

In an attempt to deal with the effects of fast switching IGBT from the point of view of the manufacturer of

the engines, a commission (National Electrical Manufacturers Association) NEMA has set a standard for the

insulation capability of the system. In the standard "NEMA MG1 Part 31.40.4.2" in attempting to define Dv

/ dt or voltage variation in time and maximum peak voltage. It is described as an increase 0.1 micro seconds

from 10% to 90% of the steady state voltage has a peak maximum of 1600 volts. Which is why the variable

speed motors and AC must be in accordance with NEMA MG1 Part 31.40.4.2 standard. In addition to the

NEMA standard, these engines should also have a minimum rating (Corona Inception Voltage) CIV of 1600

volts at operating temperature rated for 460 VAC, and 1800 volts for 575 VAC. In relation to the values of

insulation (Corona Inception Voltage) CIV at nominal operating temperature provides a standard that can

be tested with controls and quality.

The cable installed between the output terminals of the drive and the motor terminals are AC impedance

for the cause of voltage pulses of the PWM converter. These cables contain relevant values of inductance

and capacitance which are directly proportional to the length of the cable. Each time that this impedance of

the cable does not match the impedance of the motor, a reflected wave occurs regardless of PWM

semiconductor technology (IGBTs, BJTs, GTO, etc..).

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The PWM waveform with peaks of the reflected wave, if it has a sufficient voltage level higher than the

(Corona Inception Voltage) level CIV, will begin the process of partial discharge and / or corona (ionization).

When the engine meets the PWM pulses with high voltage spikes, a gradient of high voltage between

conductors causes air adjacent to a conductor, or air at the end turns of the engine, to ionize that produces

ozone. The formation of this corona can cause physical damage (ion bombardment) and chemicals to the

windings and the possibility of localized spots hot high temperature. In some cases, when the resin is

applied to the motor, small bubbles remain in the material. These bubbles, known as empty, can provide

the air required to begin the process of corona.

One of the keys to solving the problems of the reflected wave is the choice of an engine with an

adequate rating (Corona Inception Voltage) CIV. As previously discussed, the voltage starts crown

can be influenced by a number of factors. These factors include:

• Concentricity and film thickness of magnet wire

• Method of assembly / quality winding including the model

• The application of resin, and the lack of mechanical damage

• The winding process including the type of resin used, the number of times in the resin which is

applied as well as the process in which it is applied.

• Environmental conditions such as humidity, temperature and contaminants.

• Expanding the area of exhibitions voltage reflected wave vibrating even a single pulse can have a

waveform of vibration which can cause a voltage higher than the value of the crown more than

once. This vibration can be influenced by the length of the cable, the type and location of the cable

used as the carrier frequency of the inverter. The area under each of these peaks is a power that

generates partial discharges.

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If the voltage exceeds the level CIV of the engine, each level exceeded CIV will cause a partial discharge.

Repeated partial discharge / corona cause a deterioration in the insulation of the motor. The combination

of power under the curve and the number of transient peaks above the level CIV determine the rate of

deterioration. For example, the length of the cable and the type of cable used can affect the frequency of

oscillation of the cable that determines the number of pulses that appear above the nominal CIV.

Furthermore, the motor cables longer able to reduce the frequency of vibration, which extends into these

pulses to cause a wider area under each pulse that creates more power for the generation of the crown.

The intensity of the partial discharge / crown is influenced by a number of factors:

• Cable length between inverter and motor, which determines the amplitude of the reflected wave.

• The time dV / dT of the device which determines the extent to any length of the cable and the motor.

• The level of voltage imposed on the motor can be also influenced by other factors, such as the input

voltage of the network. For example, the 'input 380Vac an inverter will have a DC bus 532Vdc and 616Vdc

440Vac will have a DC bus, inverter output is then creates a voltage transients min. 1064Vac and max.

1848Vac.

• The inverter switching or carrier frequency determines the number of times for PWM waveform that

exceeds the voltage level of the motor CIV. In the final process, the action of the partial discharge or corona

possibly erodes or degrades the insulation between the coils inside the motor. When insulation also fails

the current can flow phase-phase, phase-to-earth, winding-winding, wire to wire, and in these cases the

inverter stops.

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• In many cases, these failures are not severe enough to actually cause the total failure of the engine.

Engines with damaged insulation can sometimes be used throughout the line without any influence.

However, the inverters of today with sensitive electronic circuits can often detect small problems quickly

and determine malfunction of an engine having damaged insulation.

An example measured in Fig 3 illustrates a ratio of peak value of the voltage, the motor end between the

inverter and motor.

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Conclusion and solutions

Over the past 10 years, the market investigation on the damage the motor insulation due to surge from the

inverter show that the incidence of the damage is 0.013% under the condition of voltage peaks over 1,100

V and most of the damage occurs several months after the commissioning of the 'inverter. For rooting out

and driving even this small percentage were taken several measures of 13 permanent magnet synchronous

motors and asynchronous motors with gearbox. The measurements were carried out with and without

reactor reactor to see the improvement. It follows:

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• Measure dV / dt phase to phase without additional reactor V = 1060V, dV / dt = 1060/274 ns =

3868 V / us. Measurement made fully operational, the engine = 10 A.

• Measure dV / dt phase to phase without the additional reactor Inspection-(start / stop pulse), dV

/ dt, irrelevant graph only track the checks performed.

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• Measure dV / dt phase-to-ground without the additional inspection-(start / stop pulse).

dV = 2.35kV, dt = 27us. dV / dt, dV / dt is not significant.

• Measure dV / dt phase-to-ground without the additional nominal velocity to inspection.

dV = 1.01kV, dt = 3.1 us, dV / dt is not significant.

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• Measure phase-phase scheme with L from 2.7mH in series to the motor. dV = 850V, dt = 7.6 us.

dV / dt = 112 V / us, less than 3% of 3868 V / us without reactor.

• Measure phase-phase nominal velocity with L from 2.7mH in series to the motor. dV = 820V, dt

= 9.2 us. The result is similar to that of the old installation, the dV / dt is even lower because the

current measured on the motor = 13/14 A, in fact, the plant is a 480 Kg capacity instead of a 320

Kg, for the same motor .

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Asynchronous 18kW, 400V, 39 A, 930 rpm, 50 Hz, system presumably 10-12 people to 1.75 m / s,

down-load current = 30 A - Measure dV / dt phase-to-phase nominal velocity without inductors.

The dV / dt is similar to that found on the system with synchronous motor with permanent

magnets, in fact dV = 1040V, dt = 280ns, dV / dt = 3714 V / us.

• Measure dV / dt phase-phase nominal velocity with inductance from 2.7 mH, made in

the inspection because the plant is still in the process of mounting. Even in this case, the

dV / dt is negligible for about inductance added dV = 800V, dt = 8.04us, dv / dt <100V/us.

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When the distance between the drive and the motor exceeds the length recommended by the

manufacturer, additional protection is required. This may be in the form of a reactor (reactor) or in the

form of a special filter to reduce the peak voltages on the motor. So you try to suppress surges. There are

two methods for suppressing overvoltages, one is to reduce the voltage increasing and another is to reduce

the value of the peak voltage. If the cable length is relatively short, the surge can be suppressed by

reducing the voltage (dv / dt) with the installation of a CA reactor on the output side of the inverter. (See

fig. 4 (1)) filter Dv / Dt

If the length of the cable becomes long, for suppressing the peak voltage of overvoltage must install a

reactor and a filter on the output side of the inverter so as to enable the reduction of voltage peaks at the

motor terminals. (See fig. 4 (2))

Three-phase dV / dT filters (3 lines) By the use of a filter REO dV / dT voltage peaks are reduced by 70-90%.

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RIFERIMENTI

1. Persson, E., “Transient Effects in Application of PWM Inverters to Induction Motors,” IEEE Transaction on Industry Applications, Vol.

28,1992, No. 5, pp. 1095 – 1101.

2. Deisenroth, H., Trabert, Ch., “Vermeidung von Überspannungen bei Pulsumrichterantrieben,“ Antriebstechnik, etz. 114, pp. 1060 –

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inverter - effect on isolation of motors and solutions

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