Part 2: Motors and Air-Moving Devices
Constructing a Small PMDC Motor

by A.N. (Tom) Tsergas, vice president of Engineering and Research, Molon Motor and Coil Corporation

The permanent magnet d.c. (PMDC) motor is considered one of the most economical and efficient motors because the stator is generating its own magnetic field without any applied power.

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, and automobiles.

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 electronics.

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.

Figure 1. CLICK for large graphic.

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.

Figure 2. CLICK for large graphic.

Figure 3. CLICK for large graphic.

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.

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 curve.

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 information.

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 density
18,000 = steel’s safe density before saturation

Carbon Brushes

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

Magnet Overhang

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 6):

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 reasons.

Explanation of the Motor Curve

No 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.

Figure 5. CLICK for large graphic.

Figure 6. CLICK for large graphic.

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.

A typical formula for the transformer’s secondary current is:

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 torque/speed performance.

Improving Performance

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 7).

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.

Figure 7. CLICK for large graphic.

Figure 8. CLICK for large graphic.

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 following:

• 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 8).
• Resetting, 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. This prevents continuous cycling and protects against reclosing into a fault condition.
• Repeatable, 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.

Example:

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.