Micromotors have become popular and their performance is remarkable,
as areas of application are wide and include home appliances, audio-visual
equipment, precision equipment, office automation, industrial applications,
As the use of electronic technology increases, the application of
the small PM motor will spread to many fields and will become part
of our everyday functions. The power that is obtained from the small
motors, with very low voltage level, is incredible when combined with
regulated power supplies, pulse width modulation, and other high-tech
In order to better evaluate the micromotors, it is necessary to look
at six areas: manufacturing the armature assembly, manufacturing of
the PM motor, brush system design and assembly, design and application
of the complete motor, power supplies and control circuits, and ways
to improve the micromotor’s performance. Due to the motor’s
increase in popularity and use in appliance applications, it is important
to review the components and assembly of a PMDC motor.
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Manufacturing the Armature Assembly
The armature core is a laminated assembly that is stamped from an
electrical steel material, but not annealed. The laminations are then
pressed onto the shaft, and the entire assembly is insulated by a fluidized
epoxy method. Next, the coils are wound into the armature’s slots
(see Figure 1). The majority of the small diameter motors (30-mm diam
and under) are made with three poles or three commutator bars; however,
there are a few motors that are made with five poles or five commutator
bars in order to improve the efficiency and reduce electrical noise
levels. Large diameter motors (30-mm diam and over) are designed with
five poles or five commutator bars, although in some cases only three
bars are included to save costs. After the coil is wound, a heavy insulation
magnet wire is wound around the armature core (armature slots) and
then automatically connected to the commutator tanks. Next, the commutator
tanks are thermo-welded to provide a monolithic connection. Finally,
the commutator surface is polished with a high-speed diamond tool and
the armature is tested. In some cases, the armature gets balanced for
reduced vibration and longer brush and bearing life.
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Figure 3. CLICK for
Manufacturing of the Stator Assembly
During this process, the front bearing housing is made along with
other various formed fingers to support the permanent magnet segments
when pressed into the housing (see Figure 1). A small iolite bearing,
either straight or self-aligning, is secured to the front of the metal
housing prior to the magnet segments installation. A hairpin type of
spring is inserted to support the two magnets in place in addition
to the magnetic field.
The permanent magnets are made of a ceramic material (barium ferrite).
When the materials are of a low energy product, or a non-orientated
grade, it is called isotropic. When the materials are of a higher energy
product, or high-orientated grade, it is called anisotropic. The magnets
are charged at a very low level in order to be handled during assembly,
but most companies have dropped this process since the introduction
of automation. Also, for low voltages and low torque, rubber type magnets—flexible
material impregnated with barium ferrite powders—are used. Late
designs of the materials can generate high-energy products.
Brush Systems Design and Assembly
Two carbon brushes are mounted to a plastic end bell, which also
supports the rear bearing. Most of the time, carbon brushes are pressed
on a copper spring arm that applies pressure as the commutator rotates
around the two brushes. Larger motors of a higher torque and current
have a brass enclosure, and the brush pressure is generated by a spring.
Under high currents or near stall, the heat can fatigue the springs,
causing a loss of pressure, which creates the problems of arcing and
commutator (bar) failure.
An improved design would include a pig tail (copper braided, short
wire) on the side of the back end of the brush to carry the current,
making the spring independent (see Figure 2). The brush grades also
play an important role in the life of the motor, as most of the brushes
have a percentage of copper to help the ability of the brush (cross-section
area) to carry the current.
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H = Coercive force measured in oersteds (equivalent to
B = Residual induction measured in gauss (equivalent to
the motor’s voltage/speed)
= Operating point of the magnet on the Hc scale
= operating point of the magent on the Br scale
operating point is picked as the highest point from the
product HdBd (maximum energy product); thus, a magnet of
a given area (Am) and a given length (Lm) will produce
the maximum flux density.
Design and Application of the Complete Motor
After the micromotor is manufactured, the armature is inserted into
the stator housing and properly spaced. The brushes are positioned
on opposite sides of the commutator, the rear end bell is secured on
the back of the motor, the permanent magnets are fully charged (magnetized),
and the motor is ready for testing.
Several areas that should be taken into consideration regarding the
design of the motor include ceramic PM materials, carbon brushes, over
hang of the magnets over the armature, and explanation of the motor
Ceramic PM Materials
Isotropic ceramic materials were first developed between 1953 and
1954. The production of anisotropic material started between 1957 and
1958. The high resistance to demagnetization as well as the resistance
to cold temperature in a closed magnetic circuit make the material
a good candidate for PM motors and in the early 1960s, it was used
as a stator for a PM motor. The formulas and Figures 3 and 4 refer
to the magnet’s length area, steel return path, and other related
Basic formulas for permanent magnet materials:
Lm = rf Bg Lg/Hd
Lm = length of magnet
Rf = reluctance factor (voltage drop in lead wires)
Bg = flux density in gap
Lg = length of gap
Hd = coercive force @ open pointAm = Ó Bg Ag/Bd
Am = area of magnet
Ó = leakage factor
Bg = flux density in gap
Ag = area of gap
Bd = residual induction @ operating point
The corrosion-resistant steel area can be estimated as:
As = Ø/18,000
As = area of steel (thickness x length)
Ø = flux
18,000 = steel’s safe density before saturation
As previously mentioned, the brush material is very important because
it is a function of the motor’s life associated with the commutator.
If the brush material fails, it causes the commutator to fail, shortening
the motor’s life. Most of the brushes found on the micromotors
contain anywhere from 10 to 70 percent of copper based on current and
load. The higher the copper percentage, the higher the current per
square inch. Each carbon material has different current value per inch.
The longer the brush, the longer the life; therefore, when a brush
is operating at 40 to 50 percent below the maximum current capacity
(ampacity) per inch and has an extra long length, the motor can have
a very long life.
The cooling effect is given by:
S = ? D L/I cm/amp
S = specific cylindrical cooling surface/amp
D = diameter of commutator
L = length of commutator
I = full load current of motor
Also, each material is given pressure per inch, then pressure applied
to each brush is:
Bp = W x T x P in lbs
Bp = pressure per brush
W = width of each brush
T = thickness of each brush
P = specific pressure of brushes
The peripheral speed is given by:
V = ? D/12 x R ft/min
V = peripheral speed (velocity)
D = diameter of commutator
R = rotational speed
The following formula is the theoretical explanation of magnet overhang
(see Figure 5):
Jm = Lm/D + Ja
Ja = armature length
D = armature diameter
Lm = magnet thickness
Jm = magnet length
There is also another practical method or explanation (see Figure
LF = length of front crown (coils)
LR = length of rear crown (coils)
LI = length of iron
Adding 60 percent of LF and LR to LI should give the total magnet
length. This is done for two reasons. First, if the magnet has equal
length to the armature core, it would be difficult to center it, and,
thus, it would lose armature flux. Second, as the armature coils enter
and exit the armature slots, they have a longer active length than
the one through the iron; therefore, a longer magnet can generate high
fields, increasing the motor’s performance. Also, one magnet
length can be used for various armature lengths and for more economical
Explanation of the Motor Curve
Load Speed—T=0; I=0.2 A approx. This point may be used for
design if the load is low or indeterminate, and the required gear motor
output speed is known.
Stall Torque—The projection of the speed line, N, down to zero
rpm: T=400 g-cm or 5.5 oz.
Maximum Efficiency Point—This is the most useful design point.
Maximum brush life is available at this point. In addition, it is the
basis for matching power required, establishing gear ratio, and developing
gear motor torque. The maximum efficiency point is approximately 25
percent below no load speed. This is the target speed for normal operations
and long life.
Maximum Power Point—This point gives the basis for “punch
out” power, an incidental point for continuous use. Brush life
can be affected by operating frequently at this point. The maximum
power point is approximately 50 percent of the no load speed. Limit
to very intermittent operations only.
Maximum Power Current—This may be the level of current at which
current protection is set. This protects mechanisms from overload.
PTC current limiters are commonly set here or somewhat below this point.
Required Current—At the maximum efficiency point, this is the
current required of the supply to operate continuously.
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Figure 6. CLICK for
Power Supplies and Control Circuits
The most important parameter for a well-operating d.c. motor is the
power supply that provides the main source of power. A well-filtered
supply capable of providing the required current to the d.c. motor
is a must. Failing these expectations, the motor may starve of power,
torque, and speed. When selecting a step down transformer to drive
power supply, the transformer’s current capacity should be equal
to the motor’s stall current.
typical formula for the transformer’s secondary current
Ep/Es = Is/Ip or Is = Ep x Ip/Es
Ep = primary voltage
Es = secondary voltage
Ip = primary current
Is = secondary current
The ripple factor is referred to when filtering is discussed. The
ripple factor is defined as:
rf = Vout (a-c)/Vout (d-c)
rf = ripple factor
Vout (a-c) = effective value of the a-c components in the output
Vout (d-c) = average value at the output
The electrolytic capacitor used as a filter is often called a “shunt
capacitor filter.” This filter not only improves the ripple but
also provides a constant voltage across the motor, improving the motor’s
A well-performing motor for production can be developed at a low
cost by manipulating the following areas: supply voltage, flux ring,
varistor, filtering (for EMI reduction), and PTC for motor protection.
Supply Voltage—As stated earlier, a full wave bridge and a
large electrolytic capacitor are used to test various values and select
the best part for the job.
Flux Ring—Most of the mircomotors have an over-saturated steel
housing. For economical reasons, rotating a paper clip around the motor’s
housing can check the saturation. One point (the middle of the magnet’s
arc) will attract the strongest. This can also be achieved with the
probe (Hall effect) of a flux-meter. By pressing on a heavier corrosion-resistant
steel ring, the attraction will be reduced. Then, the leakage flux
and the magnet’s operating slope is reduced by increasing the
thickness until the attraction is at a minimum. The motor’s operating
slope without the flux ring is at (Br and Hr). When the flux ring is
placed on the motor case, the operating slope drops to (Bd and Hd),
torque increases by the relationship of Hd/Hr (up to 25 percent), and
the no load decreases by Bd/Br or approximately 25 percent (see Figure
Varistor—When a voltage is applied across a varistor’s
terminals, its resistive valve decreases sharply. Compared to an ordinary
resistor, the varistor has a non-linear characteristic, which increases
current at a ratio of a third to a fifth power against voltage. The
disk varistor is positioned between the armature’s windings and
the commutator parallel (across) with the armature’s windings,
making each armature winding lower with effective resistance. Since
the current increases in a non-linear function, the voltage drop (1xR)
decreases, reducing the “spikes” across the commutator
bars that generate the EMI (see Figure 8). The introduction of the
varistor into the armature assembly (circuit) does not reduce or interfere
in any way with the overall motor performance, but does help the motor
to operate with reduced noise effects.
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Figure 8. CLICK for
PTC for Motor Protection
It is very important to protect the d.c. motor from frequent stalls.
A stall condition generates high currents that generate heat, which
can destroy the armature windings. Therefore, the best way to protect
the micromotor is to use a fuse type of a device called PTC (Positive
Temperature Coefficient). The PTC is a thermally sensitive resistor
made out of polycrystalline ceramic materials.
The benefits of a PTC include the
- Better Protection, Maintenance Free—PTC resets after
an overcurrent situation. Protection levels may be set lower than
possible with fuses, without worrying about nuisance trips (see Figure
Non Cycling—Functioning as a manual reset
device, PTC overcurrent protectors remain latched in the tripped
stated and automatically reset only after voltage has been removed.
continuous cycling and protects against reclosing into a fault condition.
No Hysteresis—After resetting, ceramic
PTCs return to the initial resistance value, providing repeatable,
consistent protection levels. Unlike polymer-type PTCs, Cera-Mite
devices exhibit no resistance hysteresis application problems.
The best way to select the value of a PTC is to take the motor’s
stall current at the operating voltage. Then, take 85 percent of this
value and the number should become the max holding/tripping current
in order to protect the system.
A micromotor has the following specs:
E = 24 V d.c., Is = 2 A
2 x .85 = 1.7 A, the PTCs tripping
current should be 1.7 A @ 24 V d.c.
By reviewing the components and assembly associated with PMCD motors,
they can be successfully implemented into appliance designs to satisfy
direct customer needs.