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EFFECTS OF LONG MOTOR CABLES WITH VFDs

The drive converter supplies at its output, a voltage which consist of voltage blocks with the amplitude of the DC link voltage. The rate of rise dv/dt of these voltage blocks is extremely steep and is determined by the switching speed of the IGBT used. Minimum switching times range from 0.08 to 0.10 microseconds. The amplitude of the DC link voltage is 135% of the line supply voltage. dv /dt > 600 *1.35 / 0.1microsecond ~ 8kV/.usec for a 600 volt supply. This high dv/dt can, especially for long motor feeder cables, results in additional stressing at the drive (current spikes) and at the motor (insulation stressing). CHARGING CURRENT EFFECTS WITH LONG POWER CABLES Motor cables have a specific capacitance. The longer the cable length, the higher the resulting cable capacitance. These capacitances are re-charged at each commutation. This involves a charging current of the cable capacitances which is superimposed on the actual motor current, see figure 1.

Instantaneous values of the drive converter output voltage and current The drive converter must, in addition to the motor current, also supply these capacitive re-charging currents, which decay after approximately 1 microsecond. The amplitudes of these capacitive re-charging currents are higher, the higher the cable capacitance (longer the cable). Output smoothing reactors or filters must be provided in the motor feeder circuit to limit the instantaneous peak currents from tripping the drive. Alternately, the VFD can be oversized to handle the charging current and motor load. This problem is quite common on the smaller inverters where the charging current of the cable approaches the current rating of the inverter. The following table list the typical distances that the 6SE70 series drives can be used at nameplate current rating without any filtering.

HP DISTANCE FOR DISTANCE FOR

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UNSCREENED CABLES SCREENED CABLES 5HP 50 35 7.5 70 50 10 100 67 15 110 75 20 125 85 25 135 90 30 150 100

225 150 100 2000 150 100

If parallel cables are used, or multiple motor runs, then the total sum of each of the cable runs must be added together. If a motor is fed from the output of an AFD via a long cable, several potentially troublesome effects may become apparent The lumped element equivalent circuit of a cable is shown in figure A. indicating the presence of series inductance and shunt capacitive effects. when the steeply rising (and falling) wave fronts of the AFD are applied to the cable, the resulting capacitor charging current effects result in spikes of current that can cause the inverter to trip. While the size of the shunt capacitors are small, the effect is noticeable, particularly in low power inverters. The magnitude of this current is predictable and can be reduced using a series inductor. The size of the inductor is proportional to the length of the cable and the degree of current reduction required. The object of the application of series inductors is to reduce the charging current to a value low enough so as not to reduce the reliable current rating of the drive. Drives above the rating of 50 or 100 horsepower are probably not susceptible to these effects. As the power of the drive is reduced, more intervention is required. Drive manufacturers generally provide a table of inductors in a matrix relating specific inductors to frame sizes as a function of length of cable. As the frame size diminishes and the length of cable increases, the required series inductance increases accordingly. The particular selection procedure is a function of "head room" or transient current margin of the specific drive and other factors. The general effects are as stated above.

Figure A. Equivalent Circuit of Cable Figure B. Filter for Long Cable

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The high rate of rise of the voltage at each commutation (i.e. 5000 time per second in each phase for a 2.5kHz pulse frequency) can be considered as a traveling wave. It is propagated along the motor feeder cable at a speed of approximately 150 m/sec (~1/2 speed of light). The characteristic impedance suddenly changes at the motor: Rwmotor

~ 10 … 20 * Rw cable The moving voltage wave received from the drive inverter is reflected back to the drive and again back to the motor again. The initiated stabilization sequence decays to the steady state value after approximately 1 microsecond. This results in the motor voltage characteristic as shown in the right hand side of the above figure. The maximum motor voltage under load, taking into account for full voltage reflection, is determined by Vmotor ~ 1.9 * VDC Link = 2.6 * Vsupply = 1540 Volts for a 600 volt supply. During brief transients operations, the DC Link voltage can be increased. At the end of an acceleration ramp, or during an input line transient or during a regenerative condition (braking), the bus voltage can be increased to approximately 120% of the normal DC link value without tripping of the VFD, thus causing increased overshoots. The following table shows the maximum peak values at the motor terminals without any smoothing reactors or filters.

SUPPLY VOLTAGES

PEAK MOTOR VOLTAGE UNDER STANDARD

CONDITIONS

SPORADIC PEAK MOTOR VOLTAGES

DURING EQUALIZATION

PERIODS 230 600V 735V 400 1040V 1470V 460 1200V 1470V 500 1300V 1840V 575 1500V 1840V 690 1800V 2200V

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The prerequisite for maximum voltage reflection is that the voltage rise at the drive converter output is shorter than the propagation time of the traveling wave from the drive to the motor. Full Reflection will occur, if: dt < tprop = lcable / v, therefore lcable = 150 meters/microsecond * 0.1 microseconds = 15 meters, This means that full voltage reflection can occur at very low cable lengths of 10 to 15 meters. The following factors influence the maximum cable length: • ratio between drive current rating and motor current rating • line supply voltage • cable type and cross section • maximum load current • output reactor Generally, longer cable lengths are possible, if: • motor rating is less than the drive rating (lower rated motors have higher leakage

inductance) {multiple lcable by VFD Current Rating/Motor Current Rating} • input line supply voltage is lower than drive nominal input voltage {multiple lcable by VFD Voltage Rating/Supply Voltage } • using cables with a lower cross-section. (they have lower capacitance values and

higher damping due to increased resistance) {multiple lcable by (0.5 + 0.5 (Cross-section referenced /Cross-section used)} • drive overload current is not used by setting torque limiting to the drive rating or less. {multiple lcable by VFD Rating/VFD Limited} • output reactor is used {multiple lcable by ( 5.76 - 3.5 * VFD Current Used / VFD Current Rating)} The equivalent circuit of figure A. can be used to study the transient effects with cables as discussed in the previous section. One effect involves the development of high voltages at the motor terminals due to reflections in a power cable as shown in figure C. Note that the motor voltage tends to double as the reflected waves meet, cancel and add. Such excess voltages are believed to cause motor insulation failures. The use of series inductors as discussed above reduces the rise time of motor voltage but does not effectively reduce the peak voltage. It can be shown that for a given length of cable, reducing the rise time below a critical time will eliminate over voltages due to reflections as shown in figure D. This implies that a filter placed on the output of a fast switching AFD could do the same thing. One such filter is shown in figure B. Figure E. demonstrates the effects of a typical IGBT inverter applied to a motor fed by a 150 meter cable. Note the reflections before applying of the filter. Note further, the successful suppression of over voltage at the motor by using the filter (figure F.)

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Such filters can and have been designed for cables over 300 meters in length. It should be emphasized that such filters involve a significant cost and collateral power loss proportional to cable length depending on the optimization procedures used in the design. Alternative filters can be used, each having specific advantages and disadvantages. The LC or LRC circuit of figure (B.) has proven to be simple, reliable, and produces effective suppression at relatively low loss as applied by Siemens. The LRC filter discussed above provides two distinct functions. It reduces the over-voltages at the motor and reduces the rate of rise of voltage.

Figure C. 50 meter cable Figure D. 50 meter cable 0.1 microsecond rise time 0.667 microsecond rise time

Figure E. 150 meter cable. Figure F. 150 meter cable 0.1 microsecond rise time 2 microsecond rise time Other means of reducing the voltage stressing is output reactors, dv/dt filters or sinewave filters.

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Output Reactors: Output reactor limit the capacitive output currents and additionally reduce the voltage rate of rise after the reactor. The following two types of reactors can be used: Iron Reactors: For output frequencies of less than 120 hertz and a low pulse frequency of around 3 kHz. Ferrite Reactors: For output frequencies above 120 hertz and a pulse frequency between 3 and 6kHz. dv / dt Filters: The dv/dt filter reduce the voltage rate of rise to values of dv/dt < 500 Volts/microsecond. This means that the capacitive re-charging currents are reduced significantly. Also the voltage spikes at the motor are reduced to approximately 130% of the DC Link voltage or 176% of the line supply voltage. The following is a block diagram of a dv/dt filter comprising of a output filter reactor, filter capacitors, decoupling capacitors, diode bridge and DC Link Bus capacitors. The decoupling capacitor network limits the motor voltage to less than 130% of the DC Link bus voltage. The dv/dt filter is tuned for a maximum of a 3 kHz pulse frequency rate.

Sinusoidal Filter: The Sinusoidal Filter represents the most complex and expensive filter technique allowing for cable runs from 250 to over 1000 meters. The use of the Sinusoidal filter produces a waveform which is practically a sinusoidal voltage at the output. These filters are tuned for 6kHz at 460 volts and 3kHz at 600 volts. This filter only works on VFDs with a space vector or sinusoidal modulation scheme. The voltage drop associated with this scheme limits the output voltage to the motor to approximately 90% of rated voltage. This voltage limitation must be considered when selecting the motor. Sinusoidal filters when used on motors in hazardous areas do not create any extra heating in the motor over that produced by the Utility sinewave.

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HIGH FREQUENCY EFFECTS WITH MOTORS

Assuming that a motor is connected directly to an AFD with short cables to eliminate the effects discussed above, the effect of applying a high rate of voltage rise to a motor winding is to concentrate that voltage on the winding of the first coil as shown in figure (a). This effect acts to stress that portion of the winding leading to premature failure. This effect can be mitigated by applying a filter as discussed above. The amount of filtering is dependent on the magnitude of the applied voltage. It must be stressed that no agency has produced definitive limits to these prospects. However1 NEMA and other agencies have. proposed that the following criteria is acceptable:

Voltage Voltage Rise Limit

500V 0.1 Micro Seconds 1000V 2.0 Micro Seconds

It has been suggested that a straight line be drawn between these two points and applied at intermediate voltages below 1000 volts peak. The author feels that any motor supplied for a inverter should survive its normal design life if the applied voltage is within the envelope suggested above. Thus, a filter designed to satisfy the particular parameters of the envelope should certify the motor/drive combination. The use of a simple series reactor is also believed to be effective in some cases. The determination of the specific size of the reactor is now under investigation as present methods are empirical or subjective.

NEMA MG1 - 1993 PART 31, REVISION 1 MOTOR SPECIFICATIONS

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The above NEMA standards addresses all the issues of properly applying squirrel cage motors for application that do not operate across the line on utility power. A motor built to the Part 31 standards, will provide long life and performance satisfaction when applied to adjustable frequency controllers. It implies that the motor manufacturer, must know sufficient details about the variable speed controller and the load characteristics that it is to operate. There is no such thing as a Part 31 motor that is stocked on the shelf ready to ship to site as some motor manufacturers will tend to state. A true Part 31 motor can only be provided after the operating speed and load conditions of the application are confirmed and the VFD manufacturer selected. Part 31 address the following issues for the motors: • Minimum Speed • Base Speed • Maximum Speed • Starting Torque requirements • Intermittent Torque requirements • Pulsating Torque • Insulation Ratings

• Temperature Rise • Insulation Class • Service Factor • Insulation Voltage rating for high frequency pulses

• Horsepower requirements over operating range • Cooling requirements for designated speed range and torque requirements • Distance between controller and motor For centrifugal loads such as fans and pumps, a Part 31 motor can be pre-built with design exceptions or limits to torque, speed range and overload capability. These limitation will generally allow for proper motor and VFD operation for a 10 - 60 Hertz speed range. Some motor vendors even require additional filtering (reactors) in the VFD output circuit to protect their motor for them to meet the remaining Part 31 requirements. The actual cost difference to upgrade to Part 31 requirements is very little to most manufacturers, since they build the motors already to satisfy most requirements listed above and have test data providing Speed and Torque ranges that satisfy the NEMA requirements for PWM Drives. SERVICE FACTOR, TEMPERATURE RISE AND MOTOR LIFE

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The specification of induction motors with a service factor rating greater than 1.0 is quite common today. However, there are various reasons for such specifications. Most customers order motors with a conventional 1.0 service factor rating and expect to operate the motor at or below its rated horsepower. Other customers expect to operate the same way most of the time, but, like to have some extra margin built-in just in case some overload capability is needed on occasions. others have the same loads, but, take what they consider a 'safe' approach. They will ask for a 1.15 service factor rating then specify the temperature rise at the service factor for the next lower insulation class. Then there are those customers that take the opposite extreme by ordering a motor with 1.15 service factor and then operating right at the top of this rating. Assuming each of these customers were purchasing the same speed, horsepower and enclosure type, they will be receiving completely different motors. The motors will all do the job intended, but, they will be operating at different temperatures. Their expected insulation lives will differ even more. Information on insulation and insulation life expectancy can be beneficial in making the decisions described above. Several classes of insulation are commonly used and each of these has temperature capabilities assigned to them by standards prepared by The Institute of Electrical and Electronic Engineers (IEEE). Manufacturers have developed systems for these temperature classes using various materials such as mica, glass, epoxy, polyester, etc. and tests have been made on these systems to determine the operating life at the standardized temperatures. These tests are made at elevated temperatures and the results are extrapolated down in temperature and out in time. This method allows a test to be performed in a couple of years rather than a couple of decades. This same concept of reduction and expansion of insulation life with temperature will be used in the following to obtain relative life numbers.

TABLE 1. STANDARD TEMPERATURE RISE (IN DEGREES C) METHOD OF RATED RATED SERVICE TEMPERATURE RISE FOR

FOR MEASUREMENT HP VOLTAGE FACTOR INSULATION CLASS B F H

RESISTANCE ANY ANY 1.00 80 105 125 1.15 90 115

1500 OR ANY 1.00 90 115 140 LESS 1.15 100 125

EMBEDDED 7000 OR 1.00 85 110 135 DETECTOR OVER LESS 1.15 95 120

1500 OVER 1.00 80 105 125 7000 1.15 90 115

The standard temperature rises permitted for the different insulation classes, service factors and voltage ratings are shown in Table 1. The 1.0 service factor values are those appearing in NEMA MG1 and other standards. The service factor rise values are those recognized in the AISI C50.41 (Standard on Motors For Power Plants) and are those used for Siemens motors.

TABLE 2. RELATIVE INSULATION LIFE AT DIFFERENT TEMPERATURES DESIGN RELATIVE LIFE AT ___ o C THIS DESIGN

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INSULATION TEMP. RISE AT THIS LOAD CLASS @ S F 70 80 90 105 125 1 1.15

B 80 @ 1.0 2.0 1.0 0.5 0.2 - 1.0 0.2

F 80 @ 1.0 10+ 5.7 2.8 1.0 0.5 10+ 1.0

F 90 @ 1.15 10+ 5.7 2.8 0.5 0.25 10+ 2.8

F 105 @ 1.0 10+ 5.7 2.8 1.0 0.5 0.25 1.0 H 125 @ 1.0 10+ 10+ 4.0 2.0 1.0 1.0 0.2

Table 2 illustrates the relative life of motor insulation when a motor with that insulation system is operated at different temperatures. The Table shows that the relative life of a system is 1.0 (this is 1.0 per unit or 100%) when operating at its rated temperature. Operating the motor at higher than rated temperature results in a lower life and this is reflected by the lower relative lives (or multipliers) in the Table. In the same manner, the Table shows larger numbers (longer life expectancy) as a result of lower operating temperatures. The rule that fits most insulation Systems is that the life will be doubled for each 10oC decrease in total temperature and likewise, the life is cut in half for each 100C increase in total temperature. This "10 degree half-life rule" was used to construct Table 2. Using Table 2, the relative lives of the four machines previously mentioned can be determined. The first machine, with 1.0 SF and no overload, has the standard 1.0 life rating whether built with Class B insulation with an 80o rise or Class F with a 105o rise. The life of the motor in the second example will be approximately two times that of the first. For example, the motor could have Class B insulation and be designed for a 70o rise at rated load. The Table shows a relative life of 2.0 for this condition. With all other factors remaining the same, operation as in the third example above will result in even longer insulation life. The most common situation would be to specify a Class F system, designed for a rise of 90oC at 1.15 SF and then operate at 1.0 SF. The rise, as in the second example, is around 70o and Table 2 shows a life rating of over 10 times that of a standard motor. Expecting to actually achieve 10 times the normal life is perhaps being a little too optimistic, however, increased insulation life can be expected. The fourth example goes to the other extreme. This motor will be operating at temperatures that will result in an insulation life of only one-half (0.5 per unit) of the standard life. There is nothing wrong with any of these approaches. The user must balance life and reliability against cost and make the appropriate decision. The above discussion is concerned with the relative life of the stator insulation at different temperatures. Other factors also influence insulation life just as there are other

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factors which influence overall motor life. Don't expect the overall motor life to follow the numbers in Table 2 exactly, however, with good application and maintenance practices, it is certainly reasonable to expect and achieve longer machine life at lower insulation temperatures. It would be difficult to find a motor failure that was caused by thermal degradation of the insulation unless there has been overloading or other abuse to the motor. MOTORS FOR RECIPROCATING COMPRESSORS AND PUMPS Siemens Motor Division Offers Pulsating Current Analysis Reciprocating type compressor and pump applications are unique in that, during each shaft revolution, the amount of torque needed to be developed by the motor driver to drive the load, varies depending upon the exact position of the pistons within the cylinders of the compressor or pump. This is in contrast to the constant load characteristics of most other applications during a single turn of the shaft. Induction motors respond to the cyclic nature of reciprocating loads with corresponding changes in torque output and incremental changes in speed during each shaft revolution. Since the amount of current drawn by the motor is proportional to torque output, variations in instantaneous torque requirements will be reflected as current pulsations in the power line supplying the motor. Pulsating currents, if not limited, can cause voltage variations in the distribution system which, in turn can cause light flicker or improper operation of voltage sensitive equipment. These applications are addressed in NEMA MG 1-20.82 with the following statement: "When the driven load, such as that of reciprocating type pumps, compressors, etc. requires a variable torque during each revolution, it is recommended that the combined installation have sufficient inertia in its rotating parts to limit the variations in motor stator current to a value not exceeding 66 percent of full-load current" As suggested in NEMA MG 1-20.82, the severity of the current pulsation in these applications is typically controlled with the addition of pure inertia to the combined driver-driven load rotating system. This inertia load acts as a flywheel during the period of peak torque demand. A determination of the inertia vs. pulsating current relationship is termed a pulsating current analysis. This analysis involves the use of a computer program in which values from the driven equipment torque effort diagram and necessary motor data are entered into the program to determine the required inertia for any value of current pulsation. The standard NEMA pulsation current limit of 66% is based on the premise that most industrial distribution systems have sufficient capacity to absorb such current pulsations without experiencing objectionable variations in voltage. This is not always the case and it may be necessary to limit the amount of current pulsation to a value lower than 66%. This is accomplished with additional system inertia to limit the pulsations to values of

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50%, 40% or 30%. Typically, a pulsating current analysis may include the amount of system inertia required to limit pulsations to a specific value at more than one load point; such as, full load, 3/4 load or 1/2 load. Because of the damping effect of the electrically developed motor torque, higher system inertia values are frequently required at the lower load points to limit the pulsations to a specific value. Since inertia energy contained within the motor rotor is used to supplement electrically developed motor torque during periods of peak load torque demand, this inertial derived torque is in addition to the torque which would be transmitted through the motor shaft in constant load applications. The result can be abnormally high transient torque values. A torsional evaluation of the application could well indicate the need for a larger than standard shaft extension. Because of these special considerations, motor drivers to be used in reciprocating load applications should be clearly identified. The cyclic torque and torsional characteristics of a reciprocating type driver-driven load system depend upon the specific nature of the driven load; consequently, responsibility for their evaluation rests with the purchaser of the motor driver. Siemens Motor Division offers the capability to conduct a pulsating current analysis. Torque effort diagrams and inertia values of the driven equipment must be provided by the motor purchaser. Substantial engineering effort is required in the performance of a pulsating current analysis