Commutation of DC motors

IEEE INDUSTRY APPLICATIONS MAGAZINE  NOV j DEC 2010  WWW.IEEE.ORG/IAS Effects of field weakening on dc machine performance and maintenance F OR MANY YEARS, DC MOTORS Control of DC Motor Speed have been the workhorse of variable speed Many industrial processes, including those used in the drives in the paper industry Many mills paper industry, require variable speed, variable torque have inquired about operating motors machines to drive them This function has been reliably beyond the original motor rating With armature voltage limited by the drive, motor speed may be increased by provided by dc motors for over a century To maximize productivity of large capital equipment, it is reducing the shunt field current but not without consideration of vibration, © PHOTODISC machine adjustment, often possible to run the equipment at BY RICHARD D HALL & WALTER J KONSTANTY higher speeds than those anticipated when the equipment was originally brush performance, and possible designed and installed When increas- commutation issues, because the ing machine speed, there are a number machine was not designed and tested at those conditions of factors that must be taken into account to be sure that the This article examines and presents test/field data display- equipment can run safely and efficiently without a significant ing the effects of field weakening on dc machine perform- increase in maintenance requirements or even failures ance and maintenance If the driven machinery is found to be capable of running at higher speeds, consideration must be given to the 56 Digital Object Identifier 10.1109/MIAS.2010.938392 driving machinery, i.e., the dc motors If higher speeds are 1077-2618/10/$26.00©2010 IEEE can be rotated and secured to set the brush position or neutral There a number of techniques that can be used to set neutral at the OEM factory or in the field, including motor repair shops These tests may be done with the motor standing still without power or running with no load Adjusting the flux from the interpoles, or commutating fields, requires the machine to be operated under load and with controllable loads Some of the techniques used in the field, such as brush potential, may have safety issues as they require working closely with rotating machinery under load This method is discussed in [2] The brush potential method is also not practical for many of the smaller frame size motors used in the paper industry Tuning DC Machines for Commutation DC Motor Speed and Torque The speed of dc motors is controlled by adjusting the supplied armature voltage (VA ) and the main field flux (U) The dc motor speed equation is RPM $ KV (VA À IR À BD) , (U À AR) (1) where RPM, motor speed (r/min); KV , motor voltage constant—a function of the motor design; VA , voltage applied to the armature circuit; IR, armature current times the resistance of the armature circuit (V); BD, brush drop (V); U, main field flux—a function of the ampere turns in the main field; and AR, armature reaction—the flux from the ampere turns in the armature The variables that are controllable to change motor speed are the armature voltage (VA ) and the main field flux (U) The armature voltage is supplied by the drive, and increased voltage will cause the motor to run at a higher speed The main field flux is varied by changing the main field current The relationship between flux and field current is not linear and is determined by the motor saturation curve (see [1]) At lower flux levels, there is a direct proportion between flux and field current However, at higher flux levels, the curve flattens out, and changes in field current produce less change in the flux levels Since the flux is in the denominator of the earlier equation, decreasing the flux level will increase the motor speed The dc motors also produce torque to their work The dc motor torque equation is Torque $ KT IA U, (2) where KT , motor torque constant—a function of the motor design; IA , armature current (A); and U, main field flux The motor will try to produce the torque that is required to drive the load If the main field flux is reduced to increase the motor speed, the armature current will automatically increase, and so the motor supplies the required torque The armature current, on a root-mean-squared basis, must be kept at, or below, the motor-rated current, or the motor will overheat and shorten insulation life Fortunately, the motors used in many paper mill applications operate well below the motor-rated current, so there is margin to increase the motor loading without overheating This should be reviewed on a case-by-case basis For good motor performance and commutation, it is best to use the armature voltage to control speed as much IEEE INDUSTRY APPLICATIONS MAGAZINE  NOV j DEC 2010  WWW.IEEE.ORG/IAS needed to drive the process equipment, there are several approaches that can be adopted 1) If a gearbox is used in the drive, a different gear ratio can be used to obtain a higher output speed for the same input speed 2) The motors can be replaced with higher speed motors 3) The motors can be replaced with a low base speed motor to direct drive the machine without a gearbox 4) The existing motors can be operated at higher speed The first three options may be very expensive in terms of equipment that must be purchased, modification of the mounting bases, and downtime for the equipment changes The fourth option requires little capital or downtime, so in many cases this would be the preferred option By varying the supplied armature voltage or excitation (shunt field) current, dc motor speed can be easily controlled If the armature voltage is increased, the dc motor will run faster There is a limitation on the voltage that can be supplied by dc drives, so it is possible to run out of voltage control without reaching the desired motor top speed By reducing the excitation (shunt field) current, the motor speed can be further increased Many motors are purchased as speed range motors to run at times with reduced excitation (field weakening) to obtain higher speeds Sometimes, however, even the prescribed reduced field will not provide the desired speed In these cases, the machinery owner may wish to reduce the excitation current even further to obtain a higher speed There are a number of factors to be taken into account when considering the weakening of motor main fields to lower levels It is imperative to stay within the safe mechanical and electrical speeds of the motor The maximum safe mechanical speed is higher than the maximum nameplate-rated speed of the motor but may not be published and might be obtained from the motor original equipment manufacturer (OEM) There may be increased vibration or possibly mechanical resonant frequencies excited at the higher speed To produce the same torque with reduced main field excitation, the armature current will increase, but the armature current must be kept below the rated current of the motor to prevent overheating The motor may become unstable as a result of a further reduction of flux in the main field air gap because of the field weakening effect of ampere turns in the armature windings, and the motor may tend to run away and over speed Commutation may be affected, which can increase brush and commutator maintenance The conditions for which the motors are expected to operate are tested and adjusted by the motor OEM The commutation limitations are particularly important and not always well understood by many users The major adjustments that can be made to dc motors to tune them up for good commutation are: 1) brush position adjustment on the commutator surface or setting electrical neutral 2) the flux adjustment from the commutating fields (interpoles) to the correct level to assist in the commutation process Commutation, or current reversal in the armature windings, occurs as the armature coils pass from under one main pole to the next This is the area in the motor where the commutating poles or interpoles are located The commutation process is described in [1] along with basic dc motor theory The brush holders are mounted on arms that are secured to a large ring called a brush yoke or rocker ring This ring 57 as possible If the armature voltage control does not produce the required speed, only then it is necessary to weaken the field for obtaining higher speeds IEEE INDUSTRY APPLICATIONS MAGAZINE  NOV j DEC 2010  WWW.IEEE.ORG/IAS Motor Commutation Adjustment 58 If commutation is not occurring properly in the dc motor, there will be increased sparking at the brushes, which causes faster brush wear and electrical erosion of the commutator copper This will result in more motor maintenance or repair The motors must be properly assembled (see [3]) with control of the following items: n air gaps between the main and interpoles and the armature n brush box spacing above the commutator n symmetry of the circumferential spacing of the brushes around the commutator n axial alignment of the brush arms (skew) n circumferential spacing of the poles in the frame n choice of brush grade and brush construction Even with these parameters controlled, there is enough variation between individual motors that each motor must be fine tuned for good commutation The technique used by a number of motor manufacturers for tuning motors for commutation is called the black-band method of commutation adjustment It is described in [4] The black-band method has the great advantage of being able to adjust both neutral (brush position) and commutating field (interpole) strength under a variety of load and speed conditions Unfortunately, it requires specialized equipment and the ability to hold steady load on the motors Although it is an excellent technique for motor manufacturers for adjusting new machines before shipment, it is generally not applicable for field work Black commutation is a condition where there is no visible sparking at the brushes If a motor can be adjusted for black commutation, brush and commutator maintenance will be minimized The important adjustments for dc machines are setting the proper brush position on the commutator, or electrical neutral, and the correct interpole strength The black-band method uses a setup as shown in Figure The dc machine under test will have excitation provided by dc current in the main field The armature circuit includes the armature itself and the commutating fields (interpoles) in series or series/parallel, and in larger machines, a pole face winding (compensating winding) is also connected in series Power for the dc motor is provided by a drive or, more commonly for black-band testing, a dc generator A dc generator is used because it provides pure dc with no ripple that is associated with static drives that can contribute to sparking and adversely affect the results The commutating fields provide a magnetic field that generates a voltage in the armature coils undergoing commutation, which helps the current in the coils to reverse direction (see [1]) For this test, a low-voltage generator called a buck-boost generator is placed in parallel with the commutating field This allows some adjustment of current in the commutating field independent of the armature current By purposely misadjusting the strength of the commutating field gradually, visible sparking will occur at the brushes and the limits of spark-free (black) commutation can be found Knowing these commutation limits under various load conditions allows the tester to determine permanent adjustments that must be made in brush position (neutral) and the commutation field strength The commutating fields and compensating windings (if used) are in series with the armature, so there is no adjustment in the electrical circuit to vary the magnetic field strength Any adjustment made in the magnetic circuit is by interchanging magnetic (steel) and nonmagnetic (brass or aluminum) shims between the back of the commuting pole and the motor frame Setting Electrical Neutral Black-Band Setup Buck-Boost Generator Commutating Field Field Armature Drive Black-band setup Light-Load Boost i IA ≈ Field Buck-Boost Generator i Commutating Field ARM Drive Operating with boost current, but low armature current In a properly adjusted dc motor, the brushes will be contacting commutator bars connected to coils in the armature that are undergoing commutation or have the current in them reversing Finding the correct position of the brushes on the commutator is called setting electrical neutral The approximate brush position is set before operating the motor The first adjustment may use a static test of applying 120-V ac current to the shunt fields and shifting the brush yoke to get the minimum ac millivolts between brushes on adjacent brush arms Next, speed reversibility may be checked at near no load (see [5]) Finally, to set neutral by the blackband method, the machine is operated at rated armature voltage with little current in the armature circuit There should be no sparking at the brushes at this point Next, the buck-boost generator is operated to produce a current through the commutating fields in the same direction as armature current would normally flow as seen in Figure This is called boost current The boost current is slowly increased while observing the interface of the brushes and commutator Eventually, the magnetic flux from the commutating fields will miscompensate the motor enough that slight sparking will appear at the brushes The boost current that it takes to first cause sparking is recorded as the boost amperes The buck-boost generator will then be operated to produce current in the opposite direction of normal current Load Testing and Adjusting the Commutating Field Strength To adjust the machine for loaded conditions, the motor must be operated with steady and controllable armature current The motor is set to run under a variety of loads, such as 50, 100, and 150% rated armature current Under each load condition, boost current and buck current is circulated in the loop, including the buck-boost generator and Boost Amperes Buck-Boost Curve Band Center on Boost Side (Weak) No Sparking in Black Area; Sparking Outside Black Area Band Center Buck Amperes 50 Boost Amperes Buck Amperes 50 100 150 No Load Band Corrective Action: Shift Center on Buck Brush Rigging with Side (Strong) Rotation (Motor) 100 150 Corrective Action: Remove Nonmagnetic Shims and Add Magnetic Shims Load Amperes (%) Black band going weak (boost) with load Buck Amperes Buck-Boost Curve Load Amperes (%) Buck-boost curve at no load Loaded Boost Buck-Boost Generator i Field IA + i Armature Buck Amperes IA Near Perfect Black Band Centered at All Loads Boost Amperes Commutating Field Drive IA 50 100 150 Load Amperes (%) Boost current with the motor under load Black band wide and centered at all loads IEEE INDUSTRY APPLICATIONS MAGAZINE  NOV j DEC 2010  WWW.IEEE.ORG/IAS Buck-Boost Curve the commutating field coil In Figure 4, current is shown in the boost direction where the boost current (i) adds to the load current (IA ) In the buck direction, some of the armature current passes through the buck-boost generator, and the buck-boost current subtracts from the load current in the commutating field coil Under each load condition, the boost and buck current is adjusted to initiate sparking, and this value is recorded These data are then plotted on the buck-boost curve The solid area in Figure is between the sparking limits where commutation is black (no sparking) Beyond these limits, there is sparking This black band goes off toward the boost side as load increases (weak) At the higher loads, there is not as much commutation margin on the buck side To correct this and have the black band go straightout at the center would require removing some nonmagnetic shims behind the commutation pole and replacing them with magnetic shims If the black band were sloped downward, the opposite action would be required The ideal black band seen in Figure is wide (good commutation margin) and centered at all loads indicating a well-adjusted motor For a motor that is expected to run only in one direction at base speed, it is not difficult to adjust the machine for good commutation at all loads If the machine Boost Amperes flow in the armature This current will be gradually increased until there is again slight visual sparking at the brushes, and this will be recorded as the buck amperes These data will be plotted on a graph that has buck and boost amperes on the vertical axis and load amperes expressed as a percentage of rated load on the horizontal axis as seen in Figure This curve indicates that it takes less boost current to cause sparking than buck current The center between buck and boost is in the buck or strong side There should be equal margins on the boost and buck side for a welladjusted machine To correct this, the brush yoke would be loosened and the brush rigging shifted slightly in the direction the motor was rotating The test would be repeated and adjustments made on a trial and error basis until the center between buck and boost was near zero on the vertical axis, and the machine was set on electrical neutral Any time a motor is disassembled or the brush rigging is moved, neutral should be reset There are a variety of static and running tests that can be used in the field or motor shop to accomplish this (see [5]) 59 might run in either direction or the direction of rotation is not known to the motor builder, there must be compromises in brush position (neutral) The entire black band will shift slightly to the boost side when the motor is running clockwise (CW) and to the buck side when running counterclockwise (CCW) If the motor must run at strong and weak main fields, the black band will slope upward to the boost side at higher loads, similar to Figure 5, with strong main field (base speed) and downward to the buck side with weak main fields (high speed) Again, a compromise in adjustment is needed With the requirement to be able to run in both the directions and at base and high speeds, the common areas of the black band may look like Figure 7, where the band is narrower overall because of the reversibility requirement and narrows at the higher loads because of the speed-range requirement There is not very much commutation margin at the higher loads If the main fields are to be further weakened to increase motor speed beyond the speeds for which the motor was originally adjusted, the black band with the weaker main field will slope further to the buck side with increasing load, and there may be no common area of the black band where there is no sparking In addition, the armature current (load) will increase to produce the same torque, so the machine may be operating in an area where there is 60 Boost Amperes Combined Band Strong and Weak Main Field and Reversible Rotation Buck Amperes 50 100 150 Test Data on 500-hp DC Motor Black-band commutation tests were run on a 500-hp motor (500 V, 1,150/1,500 r/min, model 5CD604KA002A019, and serial number WK-2-10 WK) under a variety of voltage, speed, and main field current conditions to show the effect of field weakening on commutation The setup is shown in Figure For simplification, the data will be shown for one direction of rotation only The upper and lower lines in Figure indicate the limits of the black band Between these limits, there is no sparking (black commutation) Outside these limits, there is some sparking The motor can operate to approximately 140% load with no sparking The dark line indicates the center on the black band, indicating that the commutating field flux is slightly strong (high) It is common to adjust the machine this way at the factory, as the black band will tend to shift weaker (toward the boost side) after some time during service This is because as the commutator film builds, commutators become slightly rougher, and brush springs age and lose some force over time At the top speed, the black band becomes narrower, indicating there is less commutation margin as seen in Figure 10 This effect is typical for higher speed operation The black band still tends toward the strong side, and with the narrower limits, sparking would begin at about 115% of the armaturerated current As load increases, sparking levels would increase Load Amperes (%) Black band for reversible speed range motor Buck-Boost Curve Boost Amperes VA = 500 RPM = 1,150 lf = 15.3 Rotation: CCW Buck Amperes IEEE INDUSTRY APPLICATIONS MAGAZINE  NOV j DEC 2010  WWW.IEEE.ORG/IAS Buck-Boost Curve continuous increased sparking, which can result in reduced brush life and more commutator maintenance When dc motors are operated on rectified power supplies, there is ripple current produced from a combination of silicon-controlled rectifier firing, drive tuning, actual motor operating volts and amps, and total inductance in the circuit High ripple current can affect motor commutation, and typically, larger motors (>1,500 hp) may have line reactors installed to increase inductance in the circuit to minimize current ripple If the amount of ripple current exceeds half the black-band width, sparking may result Ripple current also affects motor heating and insulation life, as the ripple current squared times the field (or conductor) resistance is direct heat added to the windings Motors setup for load testing (Photo courtesy of GE.) 50 100 Band Center Load Amperes (%) Black band at base speed 1,150 r/min 150 The shunt fields were further weakened to obtain speeds above where the motor was originally tested as seen in Figure 11 The black band (commutation margin) is narrower still At 150% load, it was impossible to extinguish the sparking by the addition of buck or boost current Next, the speed was increased by increasing the armature voltage to 550 V, as seen in Figure 12, rather than with field weakening alone The motor specifications allow up to 10% overrated voltage In addition to increasing armature voltage, field current had to be reduced slightly to obtain 1,650 r/min The black-band commutation limits were similar to those obtained by weakening the fields alone (Figure 11) The commutating pole (interpole) shimming was adjusted by removing a 0.015-in steel shim and adding a 0.016-in aluminum shim in an attempt to eliminate the slope of the black band toward the buck side The result at the 1,150 r/min base speed is seen in Figure 13 The black band no longer sloped to the buck (strong) side with increasing load When operated at the top speed of 1,500 r/min, after adjusting commutation fields by removing a 0.015-in steel shim and adding a 0.016-in aluminum shim behind the commutating poles, the black band still does not slope to the strong (buck) side However, there is once again less commutation margin indicated by the narrower black band at the higher speed as seen in Figure 14 If the shunt field is further weakened to obtain 1,650 r/min after adjusting commutation fields by removing a 0.015-in steel shim and adding a 0.016-in aluminum shim behind the commutation poles, the black band still does not slope to the strong buck side It was impossible to extinguish the sparking with buck or boost current at 150% load as seen in Figure 15 With the slope of the black bands modified by the commutating pole shim change, one further adjustment could be made to optimize the commutation if the motors were only going to operate in one rotation That adjustment would be a slight shift in neutral away from the present position where neutral was set for a machine that could operate in either rotation That adjustment would be to make a slight brush shift opposite to the direction of motor rotation (CW shift) Buck-Boost Curve Buck-Boost Curve Band Center 100 150 Band Center Buck Amperes 50 Load Amperes (%) 10 Black band at top speed 1,500 r/min 12 Black band above top speed (1,650 r/min) by both increased armature voltage and field weakening Buck-Boost Curve Buck-Boost Curve After –0.015-in Steel + 0.016-in Aluminum Boost Amperes VA = 500 RPM = 1,650 lf = 8.4 Rotation: CCW Boost Amperes 150 Band Center Band Center 100 Load Amperes (%) Black band above top speed (1,650 r/min) with field weakening alone 150 Buck Amperes 50 11 50 100 150 VA = 500 RPM = 1,150 lf = 14.7 Rotation: CCW Load Amperes (%) Black band at base speed 1,150 r/min after adjusting commutation fields 13 IEEE INDUSTRY APPLICATIONS MAGAZINE  NOV j DEC 2010  WWW.IEEE.ORG/IAS 50 Load Amperes (%) Buck Amperes VA = 550 RPM = 1,650 lf = 9.6 Rotation: CCW 100 Buck Amperes Increased Voltage to Increase RPM Boost Amperes Boost Amperes VA = 500 RPM = 1,500 lf = 9.5 Rotation: CCW 61 100 Buck Amperes Buck Amperes 50 150 Load Amperes (%) Boost Amperes Buck Amperes 100 Load Amperes (%) 50 16 Expected black band above top speed (1,650 r/min) Buck-Boost Curve After –0.015-in Steel + 0.016-in Aluminum VA = 500 RPM = 1,650 lf = 8.2 Rotation: CCW Band Center 50 150 100 Load Amperes (%) 14 Black band at top speed 1,500 r/min after adjusting commutation fields IEEE INDUSTRY APPLICATIONS MAGAZINE  NOV j DEC 2010  WWW.IEEE.ORG/IAS Band Center 0 62 Buck-Boost Curve After –0.015-in Steel + 0.016-in Aluminum and Shift Brushes Against VA = 500 Rotation (CW) RPM = 1,650 lf = 8.2 Rotation: CCW Boost Amperes Boost Amperes Buck-Boost Curve After –0.015-in Steel + 0.016-in Aluminum VA = 500 RPM = 1,500 lf = 9.2 Rotation: CCW Band Center 150 15 Black band above top speed (1,650 r/min) after adjusting commutation fields Although that test was not run, the predictable results would be as shown in Figure 16 With these adjustments, the motor would have better commutation at high speed when operating in the CCW rotation than it would have with the original adjustment from the factory In other words, the commutation could be optimized for the speed that is higher than that was expected at the time the motor was built This particular motor was not as sensitive to varying the slope of the black bands with changes in main field current as some motors might be With the reduction of commutation margin (black-band width) at the higher speeds, this fine tuning would improve commutation This would minimize adverse effects to the motor and reduce the amount of required motor maintenance caused by operating speeds increased above the speed where the motor was originally tested and adjusted Conclusions The information presented here is generally known to dc motor manufacturers but is not common knowledge to dc motor users As there is a necessity for the equipment to perform beyond its original specifications and adjustment, commutation issues may arise that could increase maintenance requirements or downtime Understanding the commutation implications of reducing the motor field may direct a user on a course of action, should commutation deteriorate and maintenance costs increase unacceptably Some applications will tolerate this speed increase and others may not In some cases, it may be possible to shift neutral slightly in the direction of motor rotation to improve commutation at the higher loads in weak field, with a possible sacrifice in performance at lower loads or with full field The motor may be readjusted by the blackband method to improve commutation at the weak field settings at all loads, with some sacrifice of performance at base speed The motor could be returned to the OEM for black-band adjustment with the reduced main field excitation or sent to a motor shop with this capability Otherwise, it may be necessary to consider the other options of obtaining a higher speed motor, using a different gearbox or using a lower base speed motor and eliminating the gearbox References [1] R D Hall (1985, Apr 23–26) Unraveling the commutation mystery Proc IEEE Pulp and Paper Conf., Greenville, SC, Morgan Advanced Materials and Technology [Online] Available: www.morganamt.com [2] G H Gunnoe, Jr., “Analyzing D-C commutation problems by the brush potential method,” Plant Eng., Apr 1980 [3] W J Konstanty, “DC motor and generator troubleshooting and maintenance,” in Conf Rec 1991 Annu Pulp and Paper Industry Tech Conf., June 3–7, 1991, pp 262–272 [4] G H Gunnoe, Jr., “Fine-tuning D-C commutation by the black-band method,” Plant Eng., Jan 1980 [5] R D Hall (2007, June) NECP—Tuning DC motors and generators presented at the Western Mining Electrical Association and Mining Electrical Maintenance and Safety Association [Online] Available: www.wmea.net Richard D Hall (rich.hall@morganplc.com) is with National Electrical Carbon Products in Greenville, South Carolina Walter J Konstanty is with General Electric Company in Erie, Pennsylvania This article first appeared as “Commutation of DC Motors Operated at Reduced Field Current” at the 2009 Pulp and Paper Industry Conference  ... also not practical for many of the smaller frame size motors used in the paper industry Tuning DC Machines for Commutation DC Motor Speed and Torque The speed of dc motors is controlled by adjusting... individual motors that each motor must be fine tuned for good commutation The technique used by a number of motor manufacturers for tuning motors for commutation is called the black-band method of commutation. .. symmetry of the circumferential spacing of the brushes around the commutator n axial alignment of the brush arms (skew) n circumferential spacing of the poles in the frame n choice of brush grade