Commutation of DC motors

With armature voltage limited by the drive, motor speed may be increased by reducing the shunt field current but not without consideration of brush performance, and possible commutation

Effects of field weakening on

dc machine performance and maintenance

drives in the paper industry Many mills have inquired about operating motors beyond the original motor rating With armature voltage

limited by the drive, motor speed may be increased by

reducing the shunt field current

but not without consideration of

brush performance, and possible

commutation issues, because the

machine was not designed and tested at those conditions

This article examines and presents test/field data

display-ing the effects of field weakendisplay-ing on dc machine

perform-ance and maintenperform-ance

Control of DC Motor Speed Many industrial processes, including those used in the paper industry, require variable speed, variable torque machines to drive them This function has been reliably provided by dc motors for over a century

To maximize productivity of large capital equipment, it is

often possible to run the equipment at higher speeds than those anticipated when the equipment was originally designed and installed When increas-ing machine speed, there are a number

of factors that must be taken into account to be sure that the equipment can run safely and efficiently without a significant increase in maintenance requirements or even failures

If the driven machinery is found to be capable of run-ning at higher speeds, consideration must be given to the driving machinery, i.e., the dc motors If higher speeds are

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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

weaken-ing) 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

lev-els 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

res-onant 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

sur-face or setting electrical neutral

2) the flux adjustment from the commutating fields

(interpoles) to the correct level to assist in the

commu-tation process

Commutation, or current reversal in the armature

wind-ings, occurs as the armature coils pass from under one main

pole to the next This is the area in the motor where the

com-mutating 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

can be rotated and secured to set the brush position or neu-tral 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

The dc motor speed equation is

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

armature voltage is supplied by the drive, and increased volt-age 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 equa-tion, decreasing the flux level will increase the motor speed

The dc motors also produce torque to do their work The dc motor torque equation is

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 cur-rent, 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

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

Motor Commutation Adjustment

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:

armature

brushes around the commutator

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

com-mutating 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

ship-ment, 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 1 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 gen-erates 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 misad-justing 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 commuta-tion limits under various load condicommuta-tions 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 cir-cuit is by interchanging magnetic (steel) and nonmag-netic (brass or aluminum) shims between the back of the commuting pole and the motor frame

Setting Electrical Neutral

In a properly adjusted dc motor, the brushes will be con-tacting 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 black-band method, the machine is operated at rated armature volt-age 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 cur-rent through the commutating fields in the same direction as armature current would normally flow as seen in Figure 2 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

Black-Band Setup Buck-Boost Generator

Commutating Field

Field

1

Black-band setup.

Light-Load Boost Buck-Boost Generator

Commutating Field

Field

IA ≈ 0

i i

2

Operating with boost current, but low armature current.

58

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 3

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

well-adjusted machine To correct this, the brush yoke would be

loosened and the brush rigging shifted slightly in the

direc-tion 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])

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

circu-lated in the loop, including the buck-boost generator and

the commutating field coil In Figure 4, current is shown

in the boost direction where the boost current (i) adds to

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 5 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 straight-out at the center would require removing some nonmag-netic 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 6 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

Buck-Boost Curve

Boost Amperes

Buck Amperes Load Amperes (%)

0

No Load Band

Center on Buck

Side (Strong)

Corrective Action: Shift Brush Rigging with Rotation (Motor)

3

Buck-boost curve at no load.

Loaded Boost Buck-Boost Generator

Commutating Field Drive Field

IA

IA + i

i

4

IA

Armature

Boost current with the motor under load.

Buck-Boost Curve Band Center on Boost Side (Weak)

Band Center

No Sparking in Black Area;

Sparking Outside Black Area

Load Amperes (%)

0

Corrective Action: Remove Nonmagnetic Shims and Add Magnetic Shims

5

Black band going weak (boost) with load.

Buck-Boost Curve

Load Amperes (%)

0

Near Perfect Black Band Centered at All Loads

150

6

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

clock-wise (CW) and to the buck side when running

counterclock-wise (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

direc-tions 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

cur-rent (load) will increase to produce the same torque, so the

machine may be operating in an area where there is

continuous increased sparking, which can result in reduced brush life and more commutator maintenance

When dc motors are operated on rectified power sup-plies, 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 commuta-tion, 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 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 volt-age, speed, and main field current conditions to show the effect of field weakening on commutation The setup is shown in Figure 8

For simplification, the data will be shown for one direc-tion of rotadirec-tion only The upper and lower lines in Figure 9 indicate the limits of the black band Between these limits, there is no sparking (black commutation) Outside these lim-its, there is some sparking The motor can operate to approxi-mately 140% load with no sparking The dark line indicates the center on the black band, indicating that the commutat-ing 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, indi-cating 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 armature-rated current As load increases, sparking levels would increase Buck-Boost Curve

Load Amperes (%)

0

Combined Band Strong and Weak Main Field and Reversible Rotation

150

7

Black band for reversible speed range motor.

8

Motors setup for load testing (Photo courtesy of GE.)

Band Center

0

150 Buck-Boost Curve

Load Amperes (%)

VA = 500 RPM = 1,150 Rotation: CCW

l f = 15.3

9

Black band at base speed 1,150 r/min.

60

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

nar-rower 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

volt-age, 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

alu-minum 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 com-mutating 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)

50

100 Band Center

0

150 Buck-Boost Curve

Load Amperes (%)

VA = 500 RPM = 1,500 Rotation: CCW

l f = 9.5

50

100

VA V

V = 500

RPM = 1,500 Rotation: CCW

l f

ll = 9.5 f

10

Black band at top speed 1,500 r/min.

Band Center

0

150 Buck-Boost Curve

Load Amperes (%)

VA = 500 RPM = 1,650 Rotation: CCW

l f = 8.4

Band Center

150

VA V

V = 500

RPM = 1,650 Rotation: CCW

l f

ll = 8.4 f

11

Black band above top speed (1,650 r/min) with field

weakening alone.

50

100 Band Center

Increased Voltage to Increase RPM

0

150 Buck-Boost Curve

Load Amperes (%)

VA = 550 RPM = 1,650 Rotation: CCW

l f = 9.6

50

100 Band Center

Increased Voltage to Increase RPM

150

VA V

V = 550

RPM = 1,650 Rotation: CCW

l f

ll = 9.6 f

12

Black band above top speed (1,650 r/min) by both increased armature voltage and field weakening.

Band Center After –0.015-in Steel + 0.016-in Aluminum

0

150 Buck-Boost Curve

Load Amperes (%)

VA = 500 RPM = 1,150 Rotation: CCW

l f = 14.7

Band Center After –0.015-in Steel + 0.016-in Aluminum

150

VA V

V = 500

RPM = 1,150 Rotation: CCW

l f

ll = 14.7 f

13

Black band at base speed 1,150 r/min after adjusting

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

operat-ing in the CCW rotation than it would have with the original

adjustment from the factory In other words, the

commuta-tion 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

commuta-tion 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

operat-ing 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 mainte-nance 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 black-band 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 excita-tion or sent to a motor shop with this capability Otherwise,

it may be necessary to consider the other options of obtain-ing a higher speed motor, usobtain-ing a different gearbox or usobtain-ing

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 main-tenance,” 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 Electri-cal 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

Band Center After –0.015-in Steel + 0.016-in Aluminum

0

150 Buck-Boost Curve

Load Amperes (%)

VA = 500 RPM = 1,500 Rotation: CCW

l f = 9.2

Band Center After –0.015-in Steel + 0.016-in Aluminum

150

VA V

V = 500

RPM = 1,500 Rotation: CCW

l f

ll = 9.2 f

14

Black band at top speed 1,500 r/min after adjusting

commutation fields.

Band Center After –0.015-in Steel + 0.016-in Aluminum

0

150 Buck-Boost Curve

Load Amperes (%)

VA = 500 RPM = 1,650 Rotation: CCW

l f = 8.2

Band Center After –0.015-in Steel + 0.016-in Aluminum

150

VA V

V = 500

RPM = 1,650 Rotation: CCW

l f

ll = 8.2 f

15

Black band above top speed (1,650 r/min) after adjusting

commutation fields.

50 100 Band Center

After –0.015-in Steel + 0.016-in Aluminum and Shift Brushes Against

Rotation (CW)

0

150 Buck-Boost Curve

Load Amperes (%)

VA = 500 RPM = 1,650 Rotation: CCW

l f = 8.2

50 100 Band Center

After –0.015-in Steel + 0.016-in Aluminum and Shift Brushes Against

Rotation (CW)

150

VA V

V = 500

RPM = 1,650 Rotation: CCW

l f

ll = 8.2 f

16

Expected black band above top speed (1,650 r/min).

62