VFD and Motor Insulation Stress
Contents
1. Now consider a VFD that is connected to a 240 V power line The DC bus voltage of this VFD is 340 V Even if the motor receives PWM voltage pulses that are twice the DC bus voltage the motor will only experience a peak of 680 V This is well within the rating of the motor s insulation so there is no need for concern Unfortunately I sometimes see specifications that call for a filter to limit the peak motor voltage for a 208 V AC applications There is no reason to do this 480 V AC When a VFD is connected to a 480 V power line its DC bus voltage will be around 680 V In a worst case scenario the peak voltage at the motor s terminals could be as high as 1360 V This could damage a general purpose motor s insulation However as shown above you don t necessarily need to confront the worst case scenario If the motor cable lengths are kept short and if the VFD used generates PWM pulses with a relatively long rise time the peak voltage at the motor can be kept to less than 1000 V 600 V AC In Canada and in a small number of locations in the United States the power line voltage may be between 575 V and 600 V AC This represents a voltage that is 25 higher than a 480 V AC power line In these cases additional care must be taken to avoid motor insulation problems This doesn t necessarily mean that the motor must conform to the peak voltage standard of NEMA MG 1 Part 31 However careful consideration may be needed to avoid pr2. ABB Inc Ph 800 752 0696 16250 W Glendale Fax 262 780 5120 New Berlin WI 53151 Variable Frequency Drives and Motor Insulation Stress by Ken Fonstad HVAC Application Engineering Manager August 3 2011 Introduction Variable frequency drives VFDs are commonly used to control the speed of standard AC induction motors VFDs have become popular in a wide range of systems because they improve the control of the system increase its overall efficiency and extend its operating life by reducing mechanical and thermal stresses Unfortunately VFDs can sometimes contribute to the degradation of the coils that produce the magnetic field in the motor s stator This paper will investigate the causes of such degradations and offer some general suggestions to help avoid such problems What damage does the motor have The vast majority of motor failures are not related to a VFD Motors failed for decades before VFDs were even invented This section gives some ideas on how to determine if a motor failure was caused by a VFD damaging the motor s stator windings When investigating a problem that occurred with a motor that was controlled by a VFD it is important to know what went wrong with the motor before looking for a solution Too often the report from the field is that the motor burned out This shouldn t be taken too literally Most people use this to describe any motor that simply stopped working Some additional investigation is
3. answer is nothing Power line harmonics deal with how the VFD draws power from the AC power line while dV dt filters deal with how energy is supplied to the motor I am always a bit surprised when I read a specification that instructs us to add output dV dt filters if an analysis of the power line harmonics shows that the system doesn t pass the requirements of IEEE 519 This just doesn t make sense The ACH550 User s Manual gives some very long maximum motor cable lengths some as long as 980 feet What is the correct maximum motor cable length This paper has looked at motor insulation stress The table in the Motor Connection Specifications table in the User s Guide looks at the limitations of the VFD and not the motor It assumes that the motor has sufficient insulation to handle any peak voltages that may occur The limitations on motor cable length from the VFD s point of view arise because of the PWM pulses that the VFD produces These pulses have high frequency components that couple electrical energy between the motor leads and from the motor cable to ground via the surrounding conduit Under normal circumstances the amount of energy that is lost in this way is insignificant However when the motor cable is extremely long a significant amount of energy may be lost through the cable Among other problems this may reduce the energy available to drive the motor at full load cause the VFD to produc
4. motor s speed To put numbers to this when the motor is running at 50 speed its cooling fan is producing 50 of full cooling air flow but the motor s current and its heating effect is around 25 of that at full speed The motor in a variable torque application should run cooler as the motor s speed is decreased So what could make the motor s stator windings overheat this much Some possible causes include Blocked cooling to the motor Running at full speed in bypass mode or from a constant speed motor starter and encountering an overload While the motor overload should take care of this sometimes things go wrong Improper wiring of the motor Running a 208 V AC motor from 460 V AC will generate a lot of heat quickly In a similar way entering incorrect motor data into some VFDs can also cause a motor to run hot For a constant torque application such as a screw compressor reciprocating compressor or some industrial application like a conveyor or hoist running the motor for extended times at low speed can cause thermal problems The problem here is that the motor must provide full torque even at low speeds So at half speed the motor s fan is providing half cooling while the current through the motor is producing full heating Not all VFDs effectively protect against this The motor just stopped running and the VFD indicated that it has an output short circuit or earth fault The motor won
5. t smell burned and a quick inspection of the motor s stator windings may not reveal any major damage A variety of situations could cause this Some possible causes are Loose wire strands in the motor s conduit box Nicked insulation in the motor cables This can occur if the conduit wasn t de burred sufficiently before the motor cables were pulled through it Moisture in the motor or in the conduit between the VFD and the motor Insulation break down in the motor s windings An insulation tester such as a Megger can apply a high voltage to the motor s stator windings to confirm if insulation break down is a problem This could be caused by the interaction between the VFD and the motor s stator windings That is the topic of this paper Figure 2 A motor stator with pinhole insulation damage page 3 of 11 What causes VFD induced motor insulation break down VFD induced motor insulation break down is the result of an interaction between the voltage pulses that the VFD applies to the motor and the stator coils of the motor The motor s coils have an electrical property called inductive reactance This causes these coils to react to a change in the current through the coils by producing a voltage that opposes this change in current flow This is sometimes called a back voltage If the change in current flow is gradual such as when a sine wave AC line voltage is applied to the mo
6. 2 s division Figure 10 A motor cable length over 200 feet causes a large voltage overshoot page 7 of 11 It is useful to note that the ACH550 produces pulses that have a relatively long rise time As a result it causes less motor insulation stress than most competitive VFDs Other factors that have an impact on motor insulation stress The rise time of the PWM pulses produced by the VFD the motor cable length and the design of the motor s insulation system are the main factors that determine the amount of stress that using a VFD will have on the motor However there are some other factors that are worth mentioning Power Line Voltage While this has a major impact on the stress on the motor s insulation I won t really spend much time on it While you can choose the peak voltage rating of the motor s insulation the design of the VFD and to some extent the length of the motor cables you seldom are able to choose the voltage that is applied to the VFD you generally use what is available Even so it is still important to understand the role that the AC line voltage plays in the stress on the motor s insulation 208 240 V AC 208 240 V motors generally have the same insulation system as 480 V motors Most of the time the motor can be converted between these voltages by simply re wiring it Therefore a general purpose 208 or 240 V motor should have an insulation system that can withstand 1000 V peak voltages
7. 600 400 200 0 200 400 600 800 0 180 360 540 720 Voltage V rms Voltage V Figure 5 The relationship between instantaneous and rms AC voltage voltage V Figure 6 Voltage levels in a PWM pulse DC Bus Voltage Max Voltage Overshoot Safety Factor VFD s DC bus voltage also approx the peak AC line voltage Up to twice the DC bus voltage page 5 of 11 For a 480 V AC power line this is 1490 V There isn t much of a difference between the two versions of this standard Note The motor s peak voltage rating doesn t correlate to the Insulation Class of the motor The motor insulation class rating describes the temperature rating of the insulation and not its ability to withstand voltage peaks For example a motor with Class B insulation is rated for a winding temperature of 130 C while a motor with Class F insulation is designed for a maximum winding temperature of 180 C The higher insulation class can be useful when operating the motor in a location with a high ambinet temperature when the motor may experience overloads or a poor supply voltage or when it must drive a constant torque load at low speed for a long amount of time While a motor with Class F insulation might be able to withstand higher peak voltages its isn t necessarily designed to To get a motor that can handle peak voltages up to 1600 V it is important to specify a motor that meets the peak voltage req
8. arantee of an installation with no electrical noise problems In this same section of the manual are instructions for wiring the system It is equally important to follow these instructions carefully when electrical noise is a concern So far I ve avoided directly answering the question about the maximum length of the motor cable The problem is that there is no definite cut off where every motor length shorter than it is fine and every motor cable length longer that it is a problem There are also variables such as the AC line voltage the type of motor being used and so on To attempt to simplify this here is a compilation of rules of thumb for motor cable lengths While a rule of thumb is never perfectly accurate it at least can provide some degree of guidance Rules of Thumb for Maximum Motor Cable Lengths Power Line Voltage Motor Cable Length1 up to 75 ft up to 100 ft up to the max length in the Motor Connection Spec longer 208 240 V AC General Purpose Motor NEMA MG 1 Part 30 3 480 V AC General Purpose Motor NEMA MG 1 Part 30 Inverter Duty Motor NEMA MG 1 Part 31 2 3 575 600 V AC General Purpose Motor NEMA MG 1 Part 30 Inverter Duty Motor NEMA MG 1 Part 31 2 3 1 These maximum motor cable lengths are rules of thumb based on the motor s stator insulation and the VFD s maximum motor cable length capability RFI EMC concerns are not taken into account All lengt
9. e nuisance Earth Fault Overload or Short Circuit trips Generally the motor cable length limitations given in the Motor Connections Specifications table are far longer than are seen in HVAC applications If it is necessary to use a longer motor cable length than the maximum shown in this table it may be necessary to connect a sine wave filter to the output of the VFD Such a filter essentially eliminates the PWM voltage pulses on the output of the VFD and smoothes them into a voltage waveform that is nearly sinusoidal This reduces the coupling of energy from the motor cables to each other or to ground If you have a need for an extremely long motor cable length contact the HVAC Application Engineers at the factory for advice PowerHelp is an excellent tool to use when you have such questions page 11 of 11 Shortly after the table mentioned above the User s Manual provides two more tables listing other maximum motor cable lengths These deal with ensuring that the installation of the VFD will meet various standards for conducted and or radiated electrical noise Some of the standards are quite stringent so some of the motor cable lengths are quite short These standards are based on European requirements While they are not commonly applied in North America it is useful to know about these tables when electrical noise is a major concern I should point out that simply complying with these maximum motor cable lengths is no gu
10. hs are based on the ACH550 VFD Competitive VFDs may need a significantly shorter motor cable length 2 Follow the maximum cable length recommendations of the motor manufacturer if they are more restrictive 3 For motor cable lengths longer than the VFD s recommendation a sine wave filter and or other considerations may be required Contact ABB
11. igned for use at high switching frequencies In commercial buildings a high switching frequency can generate excessive levels of electrical noise Radio Frequency Interference and Electro Magnetic Interference RFI and EMI Such interference is reduced by using a lower 0 V 1000 V 0 5 s division Figure 7 A PWM Pulse with a high dV dt value can cause a large voltage overshoot page 6 of 11 PWM switching frequency When a VFD is designed to operate at a lower switching frequency it can produce PWM pulses with a longer rise time and so a lower value of dV dt This results in a lower peak voltage at the motor Figure 8 shows the advantage of using a PWM pulse with a lower dV dt value Both Figure 7 and Figure 8 were made using the same motor the same motor cable length and the same load on the motor The difference is only the VFD that was used For Figure 7 a VFD that produces very fast rising pulses caused the motor to experience a peak voltage well in excess of the ratings of a general purpose motor For Figure 8 the PWM pulse has a longer rise time The peak voltage of about 820 V is well within the ratings of a general purpose motor There would be no need to replace this motor with a definite purpose motor that is specially made for use with VFDs The length of the power cables between the VFD and the motor also has a major impact on the peak voltage at the motor It is always best to keep this cable length as sh
12. needed to understand what really happened to the motor The motor stopped running This may be what is meant by the statement that the motor burned out Unfortunately it s not very descriptive Some additional questions may be needed These could include If the motor was being controlled by a VFD what alarm did the VFD display If the motor was being run in bypass is the motor overload tripped Are the system s safety interlocks closed Is the voltage at the motor s conduit box correct Does the motor s shaft turn freely Can the motor run if it is disconnected from the load The motor got really hot and it now smells kind of burned This is the one case where you could literally say that the motor burned out Something caused the motor to draw so much current that its cooling system couldn t keep up This isn t likely to happen in variable torque applications when a motor is being controlled by the ACH550 Although some people think that the motor will over heat at low speeds due to reduced air flow from the motor s cooling fan this isn t Figure 1 A motor stator with over heated insulation page 2 of 11 the case for variable torque loads While the cooling air flow across the motor does drop off proportionally to the motor s speed the motor s current drops off much faster since the torque required by the motor is roughly proportional to the square of the
13. oblems page 8 of 11 Load on the Motor While the load on the motor has a relatively minor impact on the peak voltage at the motor terminals there is a slight tendency for the peak voltage at the motor terminals to drop as more current is drawn through the motor The amount of reduction is seldom significant PWM Switching Frequency The rate of production of PWM pulses the Switching Frequency seldom has any impact on the peak voltage at the motor s terminals In most cases each pulse continues to act independently As far as motor insulation stress is concerned the main reason for reducing the switching frequency is to reduce the number of voltage pulses that are received by the motor each second This will reduce how quickly the impact of these pulses builds up in the motor Here s what is happening Each voltage pulse generates a large electromagnetic field around the motor s windings particularly at the ends of the winding where the bend in the windings tends to concentrate the field This can cause a corona effect which may generate some ozone at that point Ozone is chemically reactive and can attack the organic varnish that is used in some general purpose motors If this process eventually forms a pin hole in the varnish the motor will short out and the VFD will immediately stop Reducing the switching frequency of the VFD can significantly slow the process of generating the ozone When a very high switching f
14. ort as practical Much of the energy that causes the voltage overshoot and oscillation of the PWM pulses is from energy that is stored in electromagnetic fields in the motor cable The longer the motor cable the greater the stored energy Here is a pair of examples In the first case the motor cable was 10 feet long Because of the small amount of cable not much energy was stored in it with each PWM pulse As a result the peak voltage of the PWM pulse was about 800 V well within the limits of NEMA MG 1 Part 30 For the second test the only change was that the motor cable length was now 210 feet This caused a peak voltage at the motor cables that was about 1220 V While it is clear that keeping the motor cable length as short as is practical is a good idea don t read too much into these examples It is a combination of the dV dt or rise time of the PWM pulse and the motor cable length that determines the peak voltage at the motor For example I made a measurement of the peak voltage at a motor s terminals for a motor cable length of only 16 feet Because the VFD that I was testing produced pulses with a very short rise time the peak voltage at the motor s terminals was over 1500 V 0 V 1000 V 0 5 s division Figure 8 A PWM pulse with a lower dV dt value creates a lower voltage overshoot 0 V 1000 V 0 2 s division Figure 9 A short motor cable length causes a low peak voltage 0 V 1000 V 0
15. ot The ACH550 was designed to reduce the stress in the motor s insulation by generating pulses with rise time that is longer than most other competitive VFDs 2 Keep the motor cable length as short as practical As the motor cable length gets longer the electrical energy that is stored in the motor cables becomes greater This additional energy increases the voltage overshoot of the pulses 3 Use a motor that conforms to NEMA MG 1 Part 31 s peak voltage specification Normally the peak voltage produced by the PWM VFD won t be a problem for such a motor Do I need to buy a new motor when I convert a constant speed application to variable speed 1 It isn t always necessary to replace the motor If the VFD produces pulses with a moderately long rise time and if the motor cable length can be kept short the peak voltages at the motor can be kept to less than 1000 V the standard for general purpose motors For 480 V power lines if an ACH550 is used keeping the motor cable lengths less than 100 feet is generally sufficient 2 If the motor needs to be mounted a significant distance away from the motor a filter at the output of the VFD can reduce the motor s insulation stress to a reasonable level Two types of filters are commonly used One is a set of output reactors coils Because coils oppose fast changes in the current passing through them they will increase the rise time of the pulses at the motor Unfortunately
16. requency is used with a very long motor cable the PWM pulses can interact and build on each other Here the peak voltage at the motor can be more than twice the DC bus voltage However such cases aren t common in HVAC applications The size of the motor From experience it appears that 15 HP and smaller 480 V motors experience more stator insulation problems than larger motors There are a few factors that may be at work here First the smaller motors have less space for insulating the stator windings and the wire bends at the ends of each section of coil must be sharper These combine to make the motor more susceptible to insulation damage In addition smaller motors have a higher impedance resistance to AC current flow than large motors This can create an impedance mismatch between the motor and the lower impedance of the motor cables and the VFD s inverter section This mismatch can increase the oscillation of the voltage pulses at the motor page 9 of 11 Common questions about PWM VFDs and motor stator insulation How can problems with motor insulation damage be avoided There are a number of answers to this question The most appropriate answer can depend on the requirements of the application Here are a few thoughts 1 Use a VFD that generates pulses with a relatively long pulse rise time It is the fast increase in the voltage that reacts with the inductance of the motor s coils to cause the voltage oversho
17. such reactors occasionally cause problems particularly for relatively long motor cable lengths The reactors act like additional wire between the VFD and the motor The energy stored in the reactors can in some cases increase the peak voltage at the motor A dV dt filter reduces concerns about the possible voltage overshoot at the motor The three capacitors on the motor side of the reactors absorb energy from any high voltage pulses The resulting PWM waveform is generally suitable for use with general purpose motors as defined by NEMA MG 1 Part 30 Figure 11 A set of output reactors between the VFD and the motor Figure 12 A dV dt filter reduces the rate of rise of the PWM pulse and controls voltage overshoot page 10 of 11 3 While the methods above can allow existing general purpose motors that are in good shape to be used during a retrofit of constant speed motor starters it can be difficult to know if the motors are in good shape Unfortunately no tests can definitively project the life remaining in an old motor If the application is critical it may simply make sense to replace the existing motors with new motors that meet the NEMA MG 1 Part 31 standard However for less critical applications many end users have decided to keep their existing motors when an energy retrofit was performed Most have been happy with their decision What do dV dt filters have to do with power line harmonics The simple
18. tor the motor s reaction is also gradual The inductive reactance of the motor simply delays the change of current through the motor making it lag behind the applied voltage When a fast rising voltage pulse is applied to the stator coils of an AC induction motor the back voltage generated by the motor s coils can cause the voltage pulse to overshoot the voltage that was applied by the VFD This overshoot interacts with the inductance of motor the inductance of the motor cables and the capacitance of the motor cables and the motor to cause the voltage to oscillate The peak voltage caused by this oscillation is the major point of concern If this voltage gets to be too high it can break through the motor s insulation and cause the motor s windings to short A number of measurements are commonly used describe the shape of the PWM pulse at the motor The peak voltage Vpeak determines the amount of stress that is imposed on the motor s stator insulation Another is the rise time of the pulse While the exact definition of this varies throughout the world in North America this is generally considered to be the amount of time for the pulse to rise from 10 to 90 of the peak voltage of the pulse This is measured in microseconds s The shorter the rise time the greater the stress that the pulse will apply to the motor s insulation A pulse with a rise time of 0 1 s or shorter is generally considered to be a ver
19. uirements of NEMA MG 1 Part 31 What determines the amount of voltage overshoot There are a number of factors that impact the amount of voltage overshoot This is of interest because if the peak voltage at the motor can be kept below the 1000 V value it may be possible to use a gerneral purpose motor rather than a definite purpose motor with the VFD The rise time of the VFD s PWM pulses has a major impact on the peak voltage at the motor When a VFD is designed for industrial applications fast response is often a prime concern Some industrial VFDs use high PWM pulse switching frequencies to get precise control of the motor To achieve this the inverter section of the VFD must be designed to produce PWM pulses that quickly transition between fully OFF and fully ON These pulses may have a very short rise time often less than 0 1 s While the short rise time allows these industrial VFDs to operate at a high switching frequency there is a price to pay The very fast transition causes a very high rate of change of the voltage that is applied to the motor a high dV dt value Since the motor s stator coils produce a voltage that opposes this fast rising voltage pulse a high voltage overshoot can result In this example the peak voltage at the motor is about 1250 V A general purpose motor isn t appropriate here A motor that conforms to the peak voltage standard of NEMA MG 1 Part 31 should be used Not all VFDs are des
20. voltage that a VFD uses for making the PWM pulses that it supplies to the motor In case you were wondering rms stands for root mean square which describes the calculation that is used to determine this voltage General Purpose Motors When the National Electrical Manufacturers Association NEMA wanted a standard for how high of a voltage the insulation in a motor should be able to withstand they knew that it would need to be above the 650 V that the AC power line already provides The value that was selected for general purpose motors with a base voltage rating of up to 600 V was 1000 V This is published in NEMA Standard MG 1 Part 30 It is about 150 of the voltage that would be experienced from a normal AC power line That seems to be safe enough Definite Purpose Inverter Fed Polyphase Motors The voltage pulse that a PWM VFD applies to a motor overshoots the DC bus voltage that generated the pulse When the use of PWM VFDs became more popular NEMA needed a new standard to ensure that motors had sufficient stator insulation to ensure reliable operation when used with VFDs The peak voltage of the pulse can reach is as much as two times the DC bus voltage of the VFD Therefore the 1993 version of NEMA MG 1 Part 31 stated that a definite purpose motor for use with a VFD should have stator windings that can withstand 1600 V In 2006 NEMA MG 1 Part 31 provided the following calculation 1 1 2 2 Vrated 800
21. y fast rising pulse The other is dV dt the rate of rise of the pulse s voltage This is calculated by considering the yellow rectangle above and dividing the change in voltage dV by the change in time dt For a given motor voltage the insulation stress increases as the rise time becomes shorter or the dV dt value becomes larger Figure 3 In an AC induction motor current lags behind voltage voltage V Figure 4 When a voltage pulse reaches a motor it overshoots and oscillates 50 A 40 A 30 A 20 A 10 A 0 A 10 A 20 A 30 A 40 A 50 A 800 V 600 V 400 V 200 V 0 V 200 V 400 V 600 V 800 V 0 180 360 540 720 Voltage V Current A Vpeak Rise Time 0 9 Vpeak 0 1 Vpeak page 4 of 11 It isn t possible to talk about only VFD s dV dt or peak voltage values The shape of the pulse that the motor receives depends on the interaction between the VFD the motor power cables and the motor How high of a peak voltage should a motor be able to withstand First let s review normal AC voltage When an AC voltage is called 460 V AC this is the effective or rms voltage of the AC wave This is the DC voltage that would provide the same amount of power to a resistor as the AC voltage The AC voltage reaches a peak voltage that is 2 times the rms voltage In this case that would be 650 V This peak voltage is very close to the DC bus
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