POWER SUPPLIES FOR DC MOTORS BASIC INFORMATION AND TUTORIALS

Power supplies to dc motors may be batteries, a dc generator, or rectifiers. The permanent-magnet and miniature motors use battery power supplies. Large integral-horsepower dc motors such as rolling-mill motors use dc generators as the power supply. Most fractional-horsepower and integral-horsepower dc motors operate with rectifier power supplies. Some of the types of rectifier power supplies are as follows:

1. Single-phase, half-wave

2. Single-phase, half-wave, back rectifier

3. Single-phase, half-wave, alternating-current voltage controlled

4. Single-phase, full-wave, firing angle controlled

5. Single-phase, full-wave, firing angle controlled, back rectifier

6. Three-phase, half-wave, voltage controlled

7. Three-phase, half-wave, firing angle controlled

The NEMA standard letter designations of dc motor test power supplies are as follows:

Power supply A—dc generator

Power supply C—3-phase 6-pulse controlled rectifier (230 V L-L, 60 Hz)

Power supply D—3-phase 6-pulse controlled rectifier (with three thyristors and three diodes)

with free-wheeling diode (230/460 V L-L, 60 Hz)

Power supply E—3-phase 3-pulse controlled rectifier (460 V L-L, 60 Hz)

Power supply K—1-phase full-wave controlled rectifier with free-wheeling diode (230/115 V, 60 Hz)

When a direct-current integral-horsepower motor is operated from a rectified alternating-current supply, its performance may differ materially from that of the same motor when operated from a low ripple direct-current source of supply, such as a generator or a battery. The pulsating voltage and current waveforms may increase temperature rise and noise and adversely affect commutation and efficiency.

Because of these effects, direct-current motors must be designed or specially selected to operate on the particular type of rectified supply to be used. Armature-current form factor and ripple are two important parameters to be specified for motors which are required to operate with rectifier power supplies.

The form factor is defined as the ratio of the rms value to the average value of the armature currents. Recommended rated form factors vary from 2.0 for 1-phase half-wave rectifier supplies to 1.1 for 3 phase full-wave rectifier supplies (see NEMA MG1-14.60).

Because the letters used to identify the power supplies in common use have been chosen in alphabetical order of increasing magnitude of ripple current, a motor rated on the basis of one of these power supplies may be used on any power supply designed by a lower letter of the alphabet. For example, a motor rated on the basis of an E power supply may be used on a C or D power supply.   

PERMANENT MAGNET DC MOTORS BASIC INFORMATION

Permanent-magnet (PM) motors are available in fractional and low integral-horsepower sizes. They have several advantages over field-wound types.

Excitation power supplies and associated wiring are not needed. Reliability is improved, since there are no exciting field coils to fail, and there is no likelihood of overspeed due to loss of field.

Efficiency and cooling are improved by elimination of power loss in an exciting field. And the torque versus-current characteristic is more nearly linear. Finally a PM motor may be used where a totally enclosed motor is required for a continuous-excitation duty cycle.

Temperature effects depend on the kind of magnet material used. Integral-horsepower motors with Alnico-type magnets are affected less by temperature than those with ceramic magnets because flux is constant.

Ceramic magnets ordinarily used in fractional-horsepower motors have characteristics that vary about as much with temperature as do the shunt fields of excited machines.

Disadvantages are the absence of field control and special speed-torque characteristics. Overloads may cause partial demagnetization that changes motor speed and torque characteristics until magnetization is fully restored.

Generally, an integral-horsepower PM motor is somewhat larger and more expensive than an equivalent shunt-wound motor, but total system cost may be less.

A PM motor is a compromise between compound-wound and series-wound motors. It has better starting torque, but approximately half the no-load speed of a series motor.

In applications where compound motors are traditionally used, the PM motor could be considered where slightly higher efficiency and greater overload capacity are needed. In series-motor applications, cost consideration may influence the decision to switch.

For example, in frame sizes under 5-in diameter the series motor is more economical. But in sizes larger than 5 in, the series motor costs more in high volumes. And the PM motor in these larger sizes challenges the series motor with its high torques and low no-load speed.

ROTOR DESIGN OF DIRECT CURRENT GENERATORS BASIC INFORMATION

Rotor Speeds. Standards list dc generator speeds as high as are reasonable to reduce their size and cost. The speeds may be limited by commutation, maximum volts per bar, or the peripheral speeds of the rotor or commutator.

Generator commutators seldom exceed 5000 ft/min, although motor commutators may exceed 7500 ft/min at high speeds. Generator rotors seldom exceed 9500 ft/min. If the prime mover requires lower speeds than these, generators can be designed for them but larger machines result.

Rotor Diameters.

Difficult commutating generators benefit from the use of large rotor diameters, but diameters are limited by the same factors as rotor speeds. The resultant armature length should be not less than 60% of the pole pitch, because such a small portion of the armature coil would be used to generate voltage.

Direct-current motor speeds must suit the application, and often the rotor diameter is selected to meet the inertia requirements of the application. Core lengths

may be as long as the diameter. Such motors are usually

force-ventilated.

Number of Poles and Other Rotor Design Factors.

The rotor diameter usually fixes the number of main poles. Typical pole pitches range from 17.5 to 20.5 in on medium and large machines. When a choice is possible, high-voltage generators use fewer poles to allow more voltage space on the commutator between the brush arms.

However, high-current generators need many poles to permit more current carrying brush arms and shorter commutators. Commutators for 1000 to 1250 A/(brush arm) (polarity) are costly, and lower values should be used where existing dies will permit.

The main-pole air-gap flux density Bg is limited by the density at the bottom of the rotor teeth. The reduced taper in the teeth of large rotors permits the higher gap densities.

Ampere conductors per inch of rotor circumference (q) is limited by rotor heating, commutation, and, at times, saturation of commutating poles. The commutator diameter is usually about 55% to 85% of the rotor diameter, depending on the sizes available to the designer, the peripheral speed, and the resulting single clearances.

Heating may also limit the choice. Brushes and brush holders are chosen from designs available to limit the brush current density to 60 to 70 A/in2 at full load, to obtain the needed single clearance, and to obtain acceptable commutator heating.

TEMPERATURE RISE CAUSES OF DIRECT CURRENT GENERATORS BASIC INFORMATION

What are the causes of heating of DC generators?

The losses in a dc machine cause the temperature of the parts to rise until the difference in temperature between their surfaces and the cooling air is great enough to dissipate the heat generated.

Permissible measured temperature rises of the parts are limited by the maximum “hot-spot” temperature that the insulation can withstand and still have reasonable life. The maximum surface temperatures are fixed by the temperature gradient through the insulation from the hot spot to the surface.

The IEEE Insulation Standards have established the limiting hot-spot temperatures for systems of insulation. The American National Standards Institute Standard C50.4 for dc machines gives typical gradients for those systems, listing acceptable surface and average copper temperature rises above specified ambient-air temperatures for various machine enclosures and duty cycles.

Typical values are 40°C for Class A systems, 60°C for Class B, and 80°C rise for Class F systems on armature coils.

Class H systems usually contain silicones and are seldom used on medium and large dc machines. Silicone vapors can cause greatly accelerated brush wear at the commutator and severe sparking, particularly on enclosed machines.

Heating of End Connections of Armature Windings. Small machines often have “solid” end windings banded down on insulated “shelf”-type coil supports. Larger machines are more heavily loaded per unit volume and usually have narrow coil supports, air spaces between the end turns, and ventilating air scouring both the top and bottom surfaces of the coil extensions.

With this construction, the approximate allowable product of ampere conductors per inch of outer circumference times the amperes per square inch for various rotor velocities is shown in Fig. 8-51 for a 40°C rise on the end turns.

CAUSES OF SPARKING AND POOR COMMUTATION OF DC GENERATORS BASIC INFORMATION

Sparking and bar burning are usually due to one or more of the following causes:

1. Brushes not in the proper position.

2. Incorrect spacing of brushes. This may be checked by marking an adding-machine tape around the commutator.

3. Projecting-bar-edge mica. Mica between bars should be undercut about 0.063 in below the commutating surface, but occasionally slivers of mica are left inadvertently along the bar.

4. Rough or burned commutator. The commutator should be ground according to the manufacturer’s instruction book.

5. Grooved commutator. This may be prevented by properly staggering the brush sets so that the spaces between the brushes of an arm are covered by brushes of the same polarity of other arms.

6. Poor brush contact. This is due to improper fitting of the brushes to the commutator surface. To seat the brushes, sandpaper should be moved between the commutator and the brush face. Emery cloth should not be used because its abrasive is conducting.

7. Worn brushes replaced by others of wrong size or grade.

8. Sticking brushes. These brushes do not move freely in their holders so that they can follow the irregularities of the commutator.

9. Chattering of the brushes. This is usually due to operation at current densities below 35 A/in2 and must be corrected by lifting brushes to raise the density or by using a special grade of brush.

10. Vibration. This may be due to poor line up, inadequate foundations, or poor balance of the rotor.

11. Short-circuited turns on the commutating or compensating fields. These may be obvious on inspection but usually must be found by passing ac current through them for voltage-drop comparisons.

12. Open or very high resistance joints between the commutator neck and the coil leads. In this case, the bar at the bad joint will usually be burned.

13. An open armature coil. A broken coil conductor produces an effect similar to that produced by the poor joints described in the previous item. For emergency operation, the open coil may be opened at both ends, insulated from the circuit, and a jumper placed across the two affected necks. Since some sparking will probably result, operation should be limited.

14. Short-circuited main-field coils. With the resulting unbalanced air-gap fluxes under the poles, large circulating currents must be expected even with good armature cross connections. The offending coil may be found by comparing voltage drops across the individual coils.

15. Reversed main-field coil. This is an extreme case of the one described in the previous item.

16. Overloading.

DIRECT CURRENT GENERATORS PARALLEL OPERATIONS BASIC INFORMATION

Shunt-Wound Generators in Parallel. A and B of Fig. 8-59 are two similar generators feeding the same bus bars C and D. If A tends to take more than its share of the total load, its voltage falls and more load is automatically thrown on B.

Also, if the driver of one of the generators slows down to stop, the emf of the machine falls until the other generator starts to drive it as a motor. This continues until its driver takes over again.

The external characteristics of the two machines are shown in Fig. 8-60. At voltage E, the currents in the generators are Ia and Ib, and the line current is Ia # Ib. To make machine A take more of the load, its excitation must be increased to raise its characteristic curve.

If a 1000-kW generator and a 500-kW machine have the same regulation curves, the machines will divide the load according to their respective capacities, as shown in Fig. 8-61.

Compound-Wound Generators in Parallel. A and B of Fig. 8-62 are two compound-wound machines. If A tends to take more than its share of the load, the series excitation of A increases, its voltage rises, and it takes still more of the load.

Thus, the operation is unstable. If this continues until A takes all the load and the voltage of B drops to the point that A reverses the current in B, B will be driven as a motor. With the reversed current in the series field of B it becomes a differentially compounded motor, and the series weakens the flux to speed up the motor.

This may progress to a point at which the unit may be damaged mechanically and electrically. To prevent this, a bus bar of large section and of negligible resistance, called an equalizer bus, is connected from e to f (Fig. 8-62).

Points e and f are then practically at the same potential. Therefore, the current in each series coil is independent of the current in its particular generator, is inversely proportional to the resistance of the coils, and is always in the same direction.

When a single compound generator has too much compounding, a shunt in parallel with the series field coils will reduce the current in these coils and so reduce the compounding.

When compounded generators are operating in parallel using an equalizer bus, the current in the series field coils depends only on the resistance of the coils and a shunt connected across one of them is actually across all of them, reducing the compounding of all but not disturbing the relative compounding between the machines.

To reduce the compounding of a single machine, it is necessary to place a resistance in series with the coils. This may require a large resistor to handle the large load current it must carry.

ARMATURE SLOTS AND COIL DESIGN BASIC INFORMATION

The depth of an armature slot is limited by several factors, including the tooth density, eddy losses in the armature conductors, available core depths, and commutation. For reasonable frequencies (up to 50 Hz on medium and large dc machines), slots about 2 in deep can ordinarily be used.

Acceptable slot pitches range from 0.75 to 1.5 in. Small machines have shallower slots and a lower range of slot pitches. For medium and large machines, a reasonable tooth density usually results if the ratio of slot width to slot pitch is about 0.4.

Eddy losses in the conductors can be large compared with their load I 2R losses. Sometimes these must be reduced by making each armature conductor from several strands of insulated copper wire.

The number of strands and their size depend on the frequency and the total depth of the conductor. An approximate formula for reasonable eddy losses is

No. of strands = (0.168) (f ^0.83)(de^ 0.4)

where f is the frequency in hertz, (r/min # poles)/120, and dc is the total depth of a conductor.

The insulation space required depends on the type used. Typical conductor strands have about 0.018 in of glass strands and varnish total. Mica wrappers, binding tapes, and varnish and slot finish allowance (0.010 in) total about 0.085 in on the coil width.

If the space for the wedge and its retainer is included, the two coils depth wise total about 0.315 in.

Approximate Slot Design

Width [see text preceding and 0.563 in

following Eq. (8-17)] 0.4 # 1.407

Depth 2.0 in

Approx. total cond. depth 0.875 in

Frequency 42.8 Hz

No. of strands/conductor [Eq. (8-17)] 3

Slot width, in Depth, in

Approx.

Size 0.563 in 2.000 in

Insulation 0.139 (0.085 # 0.054) 0.423 (0.315 # 0.108)

Bare copper 0.424 in 1.577 in

Strand size 0.141 in 0.263 in

Use 3 (0.144 0.289) in strands/conductor

Use available slot 0.570 in 2.250 in

TWO CIRCUIT OR WAVE WINDINGS OF DC GENERATORS BASIC INFORMATION

Figure 8-16 shows a wave type of coil. Figure 8-17 gives a 6-pole wave winding. Study reveals that it has only two parallel paths between the positive and negative terminals. Thus, only two sets of brushes are needed

Each brush shorts p/2 coils in series. Because points a, b, and c are at the same potential (and, also, points d, e, and f ), brushes can be placed at each of these points to allow a commutator one-third as long.

The winding must progress or retrogress by one commutator bar each time it passes around the armature for it to be singly reentrant. Thus, the number of bars must equal (kp/2) # 1, where k is a whole number and p is the number of poles. The winding needs no equalizers because all conductors pass under all poles.

Although most wave windings are 2-circuit, they can be multicircuit, as 4 or 16 circuits on a 4-pole machine or 6, 12, or 24 circuits on a 12-pole machine. Multicircuit wave windings with the same number of circuits as poles can be made by using the same slot and bar combinations as on a lap winding.

For example, with an 8-pole machine with 100 slots and 200 commutator bars, the bar throw for a simplex lap winding would be from bar 1 to bar 2 and then from bar 2 to bar 3, etc. For an 8-circuit wave winding, the winding must fail to close by circuits/2 bars, or 4. Thus, the throw would be bar 1 to 50, to bar 99, to bar 148, etc.

The throw is (bars # circuits/2)(p/2), in this case, (200 # 4)/4 # 49. Theoretically such windings require no equalizers, but better results are obtained if they are used. Since both lap and multiple wave windings can be wound in the same slot and bar combination simultaneously, this is done by making each winding of half-size conductors.

This combination resembles a frog’s leg and is called by that name. It needs no equalizers but requires more insulation space in the slots and is seldom used.

Some wave windings require dead coils. For instance, a large 10-pole machine may have a circle of rotor punchings made of five segments to avoid variation in reluctance as the rotor passes under the five pairs of poles.

To avoid dissimilar slot arrangements in the segments, the total number of slots must be divisible by the number of segments, or 5 in this case. This requires the number of commutator bars to be also a multiple k of 5.

However, the bar throw for a simplex wave winding must be an integer and equal to (bars # 1)(p/2). Obviously (5k # 1)/5 cannot meet this requirement.

Consequently one coil, called a dead coil, will not be connected into the winding, and its ends will be taped up to insulate it completely. No bar will be provided for it, and thus the bar throw will be an integer. Dead coils should be avoided because they impair commutation.

MULTIPLE OR LAP WINDINGS OF DC GENERATORS BASIC INFORMATION

Figure 8-12 shows a lap-winding coil. The conductors shown on the left side lie in the top side of the rotor slot. Those on the right side lie in the bottom half of another slot approximately one pole pitch away. At any instant the sides are under adjacent poles, and voltages induced in the two sides are additive.

Other coil sides fill the remaining portions of the slots. The coil leads are connected to the commutator segments, and this also connects the coils to form the armature winding. This is shown in Fig. 8-13.

The pole faces are slightly shorter than the rotor core.

Almost all medium and large dc machines use simplex lap windings in which the number of parallel paths in the armature winding equals the number of main poles. This permits the current per path to be low enough to allow reasonable-sized conductors in the coils.

Windings. Representations of dc windings are necessarily complicated. Figure 8-14 shows the lap winding corresponding to the Gramme-ring winding. Unfortunately, the nonproductive end portions are emphasized in such diagrams, and the long, useful portions of the coils in the core slots are shown as radial lines.

Conductors in the upper layers are shown as full lines, and those in the lower layers as dotted lines. The inside end connections are those connected to the commutator bars. For convenience, the brushes are shown inside the commutator.

Note that both windings have the same number of useful conductors but that the Gramme-ring winding requires twice the number of actual conductors and twice the number of commutator bars. Figure 8-15 shows a 6-pole simplex lap winding. Study of this reveals the six parallel paths between the positive and negative terminals. The three positive brushes are connected outside the machine by a copper ring T# and the negative brushes by T#.

The two sides of a lap coil may be full pitch (exactly a pole pitch apart), but most machines use a short pitch (less than a pole pitch apart), with the coil throw one-half slot pitch less than a pole pitch. This is done to improve commutation.

Equalizers. As shown in Fig. 8-15, the parallel paths of the armature circuit lie under different poles, and any differences in flux from the poles cause different voltages to be generated in the various paths. Flux differences can be caused by unequal air gaps, by a different number of turns on the main-pole field coils, or by different reluctances in the iron circuits.

With different voltages in the paths paralleled by the brushes, currents will flow to equalize the  voltages. These currents must pass through the brushes and may cause sparking, additional losses, and heating. The variation in pole flux is minimized by careful manufacture but cannot be entirely avoided.

To reduce such currents to a minimum, copper connections are used to short-circuit points on the paralleled paths that are supposed to be at the same voltage. Such points would be exactly two pole pitches apart in a lap winding.

Thus in a 6-pole simplex lap winding, each point in the armature circuit will have two other points that should be at its exact potential. For these points to be accessible, the number of commutator bars and the number of slots must be a multiple of the number of poles divided by 2.

These short-circuited rings are called “equalizers.” Alternating currents flow through them instead of the brushes. The direction of flow is such that the weak poles are magnetized and the strong poles are weakened. Usually, one coil in about 30% of the slots is equalized. The crosssectional area of an equalizer is 20% to 40% that of the armature conductor.

Involute necks, or connections, to each commutator bar from conductors two pole pitches apart give 100% equalization but are troublesome because of inertia and creepage insulation problems. Figure 8-15 shows the equalizing connections behind the commutator connections. Normally they are located at the rear coil extensions, and so they are more accessible and less subject to carbon-brush dust problems.

CORE LOSSES OF DC GENERATORS BASIC INFORMATION AND TUTORIALS

The flux in any portion of the armature passes through p/2 c/r (cycles per revolution) or through (p/2 [(r/min)/60] Hz. The iron losses consist of the hysteresis loss, which equals Kb1.6fw watts, and the eddy current loss, which equals Ke(Bft)^2w watts.

K is the hysteresis constant of the iron used, Ke is a constant inversely proportional to the electrical resistance of the iron, B is the maximum flux density in lines per square inch, f is the frequency in hertz, w is the weight in pounds, and t is the thickness of the core laminations in inches.

The eddy loss is reduced by using iron with as high an electrical resistance as is feasible. Very high resistance iron has a tendency to have low flux permeability and to be mechanically brittle and expensive. It is seldom justified in dc machines. The loss is kept to an acceptable value by the use of thin core laminations, 0.017 to 0.025 in thickness.

Another significant loss is the pole-face loss. These losses are included in the core loss.

Unfortunately, there are other losses in the core that may differ widely even on duplicate machines and that do not lend themselves to calculation. These include:

1. Loss due to filing of slots. When the laminations have been assembled, it will be found in some cases that the slots are rough and must be filed to avoid cutting the coil insulation. This burrs the laminations and tends to short circuit the interlaminar resistance.

2. Losses in the solid spider, core end plates, and coil supports from leakage fluxes may be appreciable.

3. Losses due to nonuniform distribution of flux in the rotor core are difficult to anticipate. In calculating core density, it is customary to assume uniform distribution over the core section.

However, flux takes the path of least resistance and crowds behind the teeth until saturation forces it into the less used, longer paths below. As a result of the concentration, the core loss, which is about proportional to the square of the density, is greater than calculated.

Thus, it is not possible to predetermine the total core loss by the use of fundamental formulas. Consequently, core-loss calculations for new designs are usually based on the results from tests on similar machines built under the same conditions.

Such test results are plotted in Fig. 8-54 for machines using ordinary laminations 0.017 in thick and a limited amount of filing. They do not include the pole-face losses, which would increase the values about 30%.

DIRECT CURRENT (DC) GENERATORS BASIC AND TUTORIALS

Applications. The most important role played by the dc generator is the power supply for the important dc motor. It supplies essentially ripple-free power and precisely held voltage at any desired value from zero to rated.

This is truly dc power, and it permits the best possible commutation on the motor because it is free of the severe waveshapes of dc power from rectifiers. It has excellent response and is particularly suitable for precise output control by feedback control regulators.

It is also well suited for supplying accurately controlled and responsive excitation power for both ac and dc machines.

The dc motor plays an ever-increasing vital part in modern industry, because it can operate at and maintain accurately any speed from zero to its top rating. For example, high-speed multistand steel mills for thin steel would not be possible without dc motors.

Each stand must be held precisely at an exact speed which is higher than that of the preceding stand to suit the reduction in thickness of the steel in that stand and to maintain the proper tension in the steel between stands.

General Construction Of DC Generator.

Figure 8-1 shows the parts of a medium or large dc generator. All sizes differ

from ac machines in having a commutator and the armature on the rotor. They also have salient poles

on the stator, and, except for a few small ones, they have commutating poles between the main poles.

Construction and Size.

Small dc machines have large surface-to-volume ratios and short paths for heat to reach dissipating surfaces. Cooling requires little more than means to blow air over the rotor and between the poles. Rotor punchings are mounted solidly on the shaft, with no air passages through them.

Larger units, with longer, deeper cores, use the same construction, but with longitudinal holes through the core punchings for cooling air. Medium and large machines must have large heat-dissipation surfaces and effectively placed cooling air, or “hot spots” will develop.

Their core punchings are mounted on arms to permit large volumes of cool air to reach the many core ventilation ducts and also the ventilation spaces between the coil end extensions.

Design Components.

Armature-core punchings are usually of high-permeability electrical sheet steel, 0.017 to 0.025 in thick, and have an insulating film between them. Small and medium units use “doughnut” circular punchings, but large units, above about 45 inches in diameter, use segmental punchings shaped as shown in Fig. 8-2, which also shows the fingers used to form the ventilating ducts.

Main- and commutating-pole punchings are usually thicker than rotor punchings because only the pole faces are subjected to highfrequency flux changes. These range from 0.062 to 0.125 in thick, and they are normally riveted.

The frame yoke is usually made from rolled mild steel plate, but, on high-demand large generators for rapidly changing loads, laminations may be used. The solid frame has a magnetic time constant of 1/2 s or more, depending on the frame thickness. The laminated frame ranges from 0.05 to 0.005 s.

The commutator is truly the heart of the dc machine. It must operate with temperature variations of at least 55 DEG C and with peripheral speeds that may reach 7000 ft/min. Yet it must remain smooth concentrically within 0.002 to 0.003 in and true, bar to bar, within about 0.0001 in.

The commutator is made up of hard copper bars drawn accurately in a wedge shape. These are separated from each other by mica plate segments, whose thicknesses must be held accurately for nearly perfect indexing of the bars and for no skew.

This thickness is 0.020 to 0.050 in, depending on the size of the generator and on the maximum voltage that can be expected between bars during operation. The mica segments and bars are clamped between two metal V-rings and insulated from them by cones of mica.

On very high speed commutators of about 10,000 ft/min, shrink rings of steel are used to hold the bars. Mica is used under the rings.

Carbon brushes ride on the commutator bars and carry the load current from the rotor coils to the external circuit. The brush holders hold the brushes against the commutator surface by springs to maintain a fairly constant pressure and smooth riding.

STATIC CHARGE - BASIC ELECTRICAL PARAMETER INFORMATION AND TUTORIALS

What Is Static Charge?


A current can only flow as long as a potential difference is sustained; in other words, the flowing charge must be replenished. Therefore, some currents have a very short duration.

For example, a lightning bolt lasts only a fraction of a second, until the charge imbalance between the clouds and the ground is neutralized.When charge accumulates in one place, it is called static charge, because it is not moving.

The reason charge remains static is that it lacks a conducting pathway that enables it to flow toward its opposite charge. When we receive a shock from static electricity—for example, by touching a doorknob—our body is providing just such a pathway.

In this example, our body is charged through friction, often on a synthetic carpet, and this charge returns to the ground via the doorknob (the carpet only gives off electrons by rubbing, but does not allow them to flow back).

As our fingers approach the doorknob, the air in between is actually ionized momentarily, producing a tiny arc that causes the painful sensation.14 Static electricity occurs mostly in dry weather, since moisture on the surface of objects makes them sufficiently conductive to prevent accumulations of charge.

However startling and uncomfortable, static electricity encountered in everyday situations is harmless because the amount of charge available is so small,15 and it is not being replenished.

This is true despite the fact that very high voltages can be involved (recall that voltage is energy per charge), but these voltages drop instantaneously as soon as the contact is made.