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 do their work.
The dc motor torque equation is
Torque $ KT 3 IA 3 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.

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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 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 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

1
Black-band setup.

Light-Load Boost
i
IA ≈ 0
Field

Buck-Boost Generator
i

Commutating
Field
ARM

Drive

2
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 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


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

0
Buck Amperes

50
Boost Amperes

0
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 (%)

5

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

0
IA + i

Armature

Buck Amperes

IA

Near Perfect Black Band Centered at
All Loads

Boost Amperes

3

Commutating
Field
Drive
IA

50

100

150

Load Amperes (%)

4
Boost current with the motor under load.

6
Black band wide and centered at all loads.

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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 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 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 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

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 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 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

0

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 8.
For simplification, the data will be shown for one direction of rotation 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 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 (%)

7
Black band for reversible speed range motor.

Buck-Boost Curve
Boost Amperes

VA = 500
RPM = 1,150
lf = 15.3
Rotation: CCW

0
Buck Amperes

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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.

8
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

9


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

0
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
0

Band Center
0

100
0

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

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50

Load Amperes (%)

Buck Amperes

VA = 550
RPM = 1,650
lf = 9.6
Rotation: CCW
100

0
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

0
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.

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Band Center
0

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 () 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.