A mechanical seal is a sealing device which forms a running seal between rotating and stationary parts. They were developed to overcome the disadvantages of compression packing. Leakage can be reduced to a level meeting environmental standards of government regulating agencies and maintenance costs can be lower. Advantages of mechanical seals over conventional packing are as follows:

1. Zero or limited leakage of product (meet emission regulations.)

2. Reduced friction and power loss.

3. Elimination of shaft or sleeve wear.

4. Reduced maintenance costs.

5. Ability to seal higher pressures and more corrosive environments.

6. The wide variety of designs allows use of mechanical seals in almost all pump applications.

All mechanical seals are constructed of three basic sets of parts .

The primary seal is achieved by two very flat, lapped faces which create a difficult leakage path perpendicular to the shaft. Rubbing contact between these two flat mating surfaces minimizes leakage. As in all seals, one face is held stationary in a housing and the other face is fixed to, and rotates with, the shaft. One of the faces is usually a non-galling material such as carbon-graphite. The other is usually a relatively hard material like silicon-carbide. Dissimilar materials are usually used for the stationary insert and the rotating seal ring face in order to prevent adhesion of the two faces. The softer face usually has the smaller mating surface and is commonly called the wear nose.

There are four main sealing points within an end face mechanical seal  The primary seal is at the seal face, Point A. The leakage path at Point B is blocked by either an 0-ring, a V-ring or a wedge. Leakage paths at Points C and D are blocked by gaskets or 0-rings.

The faces in a typical mechanical seal are lubricated with a boundary layer of gas or liquid between the faces. In designing seals for the desired leakage, seal life, and energy consumption, the designer must consider how the faces are to be lubricated and select from a number of modes of seal face lubrication.

To select the best seal design, it's necessary to know as much as possible about the operating conditions and the product to be sealed. Complete information about the product and environment will allow selection of the best seal for the application.

If your band, school or church's sound system has evolved along a familiar path, what started out as a pretty simple, small group, “sound-on-a-stick” has gradually become more and more sophisticated. Many have replaced that powered mixer with separate components and added a snake to allow for a mix position in the listening area. Words like “direct box”, “balanced”, “low impedance”, “crossover”, etc. have become part of the sound team's vocabulary as they strive to provide today's expected level of sound quality and production - for both the listeners and the performers. In this article we just might add some additional terms to your audio vocabulary as we discuss microphone splitters.

As your sound system expands, it will eventually be necessary to provide additional mixes from locations other than the main mix position. Although it's possible to provide a separate monitor mix from the main console, a person located nearer to the performance area can hear what the performers hear, see their cues more easily and just generally be able to provide a better monitor mix.

Or you may be called upon to provide a separate mix for recording or broadcasting your performances. That mix will be at its best if the person providing it is isolated from the confusion of hearing the live sound. In any event, you'll most likely need to split your mic signals and feed more than one mixing console.

IMPEDANCE

Proper design of signal flow in an audio system dictates that low impedance outputs (mics) feed high impedance inputs (mixers). When a signal is split to be sent to more than one mixing console, the input impedances of those consoles provide additional paths for the electrical current. This actually increases the overall load presented to the mic signal and limits how many times it can be split without degrading tone or introducing distortion. (See tech article “High and Low Impedance” for a more detailed explanation.)

Microphones can usually be split to up to three, and in some cases even four, destinations without the use of electronics. The number of splits that can be accomplished depends on the application, impedances present in the system, length of the cables and the quality of the components used in the splitter. This is called passive splitting - no power required.

Active electronic splitters will most likely be required when splitting microphones to four or more consoles.

There are two types of passive splitters: parallel and transformer isolated .

PARALLEL SPLITS

The simplest form of splitter is the parallel type split. This involves taking a mic cable and simply “Y” connecting the plus, minus and ground wires to two other cables.

This method successfully connects the mic to multiple mixing consoles but connects the consoles directly to each other as well. Most modern consoles behave well when connected to each other but keep in mind that there is no DC isolation between them. Also, differences in the impedances of the legs in active balanced inputs of multiple consoles can make the system more susceptible to hums and buzzes caused by outside interference. However, if a system works well with a parallel split, this type is popular because they are simpler to construct and do not require employing isolation transformers - an added expense. If a splitter is to be used in a noisy environment or is to be connected to many different systems (such as a mobile recording setup) the possibility of encountering problems can be lessened by using a microphone splitting transformer as described below.

TRANSFORMER ISOLATED SPLITS

In a transformer splitter, the microphone is wired straight through to a “Direct Out” and also to the input of a splitting transformer. (See figure below.) This transformer has a 1:1 turns ratio and its output side is connected to the second or “Isolated” split output. (Transformers with two or more secondaries are used for achieving more than one iso split.)

The transformer will pass the microphone's AC audio signal but will block DC voltage in either direction. Impedances are still reflected from the destination to the source across the transformer just as in a parallel split. Therefore, transformer isolation doesNOT change the impedance loading of the circuit and does not allow for an increased number of splits over parallel splitting for impedance reasons alone.

One of the outputs is usually wired as a direct connection because the transformer will also block phantom power (DC). Remember to plan on connecting this direct leg of the split to the console that will be providing the phantom power.

A major benefit of using a transformer split is that it increases each leg's ability to reject interference by improving the “balanced” characteristic of the line (called “Common Mode Rejection” or CMR).

(A much more detailed explanation of the theory behind this is discussed in this whitepaper from Jensen transformers, Theory and Construction of Mic “Splitters”.)

A disadvantage of this type of split is the added expense of the transformers. High quality transformers are essential for providing proper shielding and for preserving the frequency response of the mic signal - don't cut corners here! 

Schematic diagram for a 2-way isolated split with Whirlwind TRSP-1F transformer with dual Faraday shields.

GROUND LIFTS

 In fact any time two pieces of audio gear are plugged in, their actual resistance to earth ground can vary quite a bit - even when the outlets are on the same circuit. This can be due to the designs of the power supplies, the length of the cable from the outlet to the service box, poor or oxidized connections within the outlet boxes and service panels - anything that can affect the resistance of the path to ground. Even when using a transformer split, a problem can arise when the consoles' grounds are connected directly to each other through a splitter.

If console A “sees” a lower resistance to ground through its connection through the splitter to console B, then part of its AC ground return current will take that path of least resistance. AC current flows in the shields of the cable, through the splitter, and over to console B. This is called a ground loop . Now, instead of the shields providing a defense against unwanted interference, they are carrying 60Hz AC and radiating it directly into the signal conductors that they are supposed to be protecting!

Although it might solve the hum problem, you should NEVER use a three-prong ground lifter on the AC power cable of either console! This is not safe and can present an electrical shock danger to the people using the system.

A better solution to this problem is to break the ground connection of one or more channels between the consoles. This is accomplished by disconnecting each offending ground connection at one end (usually the splitter) and leaving it connected at the opposite end. The shield for that channel will continue to work because it is still grounded at one end.

Some technicians will clip all of the split grounds, leaving them permanently disconnected but it's better to install ground lift switches for each channel or use lift adapters when necessary.

This way, the ground can normally be left connected but lifted if there's a problem. Since phantom power requires ground connections to work, if the main console is unplugged or disconnected, the grounds can be left connected to the split console so it can provide phantom power.

Remember that a direct out of a parallel split will not pass phantom power with the ground lifted at either end and a transformer isolated split will not pass phantom power even with the ground connected at both ends.

SOME DIFFERENT TYPES OF HARDWARE

Single channel, transformer isolated split boxes are available in SP1X2 or SP1X3versions. Both feature ground lift switches.

Screw terminal strips or punchdown blocks are popular for parallel splits in permanent installations as the connections are kept neat and orderly without the need for soldering. They are best used where all the mic lines can be gathered in one fixed location.

Pictured at the left is the Whirlwind/ADC MASS Punch . It is available in 58, 28 or 16 channel versions and the punch connections provide quick, solid, gas-tight connections.

For transformer splits in permanent installs, commercial units like Whirlwind's SPCSeries Splitters provide a pre-made, rack mountable transformer split. Phoenix connectors employ screw terminals for easy on site termination. Phantom power will pass through the direct out or can be injected at the splitter with an external power supply.

Stage boxes using XLR connectors for all ins and outs can be used with standard microphone cables. These are used more often when a splitter is only occasionally needed. However, for permanent use, all those cables make for a lot of “spaghetti” lying around and the mic cables are an added expense.

Standard Medusa® snakes can be built with a second parallel output, each output having the proper length cable for a particular situation. This is one of the least expensive alternatives for portable systems although the cabling can be a bit difficult to coil and store.

Multipin disconnects are a popular option with portable splitting systems. This unit pictured at the left is part of a splitting system employed in a mobile broadcast truck by the Rhema Bible Church of Broken Arrow Oklahoma. It features a 40-channel 4-way transformer isolated split with 16 return lines, ground lift switches and all outputs via MASS Connector multipin disconnects

Quartz crystals can be an excellent choice for the frequency-determining component in wireless communication systems. Designers appreciate the high Q value (quality factor), reasonable cost, and temperature performance of quartz crystals relative to other options, such as inductor-capacitor, ceramic, microstrip and surface acoustic wave resonators. When quartz crystal oscillators are used in wireless designs, multiplication and phase lock loop circuits are often used to extend their practical frequency range across the wireless spectrum.

When specifying crystals for use in oscillators, informed consideration of basic parameters early in the design process is important. These parameters become critical when developing voltage controlled crystal oscillators (VCXOs) and temperature compensated crystal oscillators (TCXOs), as such fundamental choices set the stage for these devices and affect the success of the resulting designs.

Making the Cut
Quartz is uniquely suited, in terms of mechanical, electrical and chemical properties, for the manufacture of frequency control devices. Quartz crystals are cut from a quartz bar, which is grown in an autoclave. The angle at which the saw blade cuts through the quartz determines many of the electrical properties of the crystal.

The most popular angle of cut for crystals used in wireless applications is the AT cut (see Figure 1). Crystals fabricated in this manner are available at relatively high frequencies, exhibit excellent frequency vs. temperature stability, and come at a moderate cost. Fundamental resonant frequencies from 1MHz to over 1 GHz are possible, but most AT-cut crystals are manufactured to have fundamental frequencies between 1.8 and 40MHz, due to price and other constraints. The resonant modes (overtones) of AT-cut crystals are at roughly odd multiples of the fundamental mode (see Figure 2). Crystals specified for these overtones typically fall in the 24 to 200MHz frequency range.

Important Specification Parameters
When specifying crystals for use in wireless designs, some parameters are of particular importance, including tolerance, stability, temperature range and load capacitance. These parameters require special consideration because they are essential to a sound design and can also impact the manufacturability and cost of the specified crystal.

Tolerance of a crystal refers to the maximum allowable frequency deviation from the target frequency at room temperature, expressed in parts per million (ppm). For example, a 10MHz crystal having a 10ppm frequency tolerance could have an actual resonant frequency response that is higher or lower than 10MHz by as much as 100 Hz (10ppm X 10MHz = 100 Hz). Tolerance default values range from 30 to 50ppm; values below 10ppm are available depending on frequency and holder, at an increased price.

Crystal stability is the allowable frequency error over the operating temperature range referenced to a baseline 25�C reading. The default values are 50ppm to 100ppm, although crystals having stabilities as low as 10ppm are available, depending again on the frequency and package desired.

For a given operating mode, the frequency of a crystal is inversely proportional to its thickness. Higher frequency crystals are thin and more sensitive to the kinds of shock and vibration typically encountered in portable and mobile wireless applications.

Several crystal holders are available to the wireless design engineer. The lowest cost holder with the greatest amount of design flexibility is the HC49U. This resistance welded package is a direct replacement for the older solder-sealed HC-18/U. The HC80U and the FD are also popular holder models.

In addition to the mechanical demands of wireless application environments, designers need to consider the thermal requirements. Most wireless applications require crystals that can operate over the industrial temperature range of -40�C to +85�C. Crystals can also be specified for use over the commercial temperature range (0�C to 70�C) and, with some products, over the military temperature range (-55�C to +105�C).

Load Capacitance
Load capacitance is one of the most overlooked parameters when specifying crystals. In the popular Pierce oscillator circuit, which has a capacitor to ground on either side of the crystal, the load capacitance is equal to the series combination of the two capacitors plus Cstray. (Cstray is the sum of capacitances that are contributed to the circuit by the layout, board material, and the input and output impedance of the active device.)

A good rule of thumb for Cstray is 5pF. If one of the capacitors is replaced with a varactor, the frequency can be "pulled" by applying a tuning voltage on the varactor. This configuration can be the basis for a simple VCXO. If the tuning voltage is derived from a thermistor network, the frequency can be adjusted to null out the effects of temperature. This configuration is an approach for a TCXO. A combination of both features is known as a TCVCXO.

A pullability specification for an HC49U crystal used in a fundamental mode VCXO might have the following form:

CL = 20 to 45pF, pullability = -100ppm max, CL = 20 to 10pF, pullability = +100ppm min.

Smaller crystals have about half the pullability of the HC49U. The pullability of overtone crystals is reduced by 1/n2, where n is the overtone mode (i.e. 1, 3, 5, etc.).

Crystal Modeling
Crystals are often modeled as series R-L-C networks (see Figure 3), which are known as the motional arms of the model. Generally, only the arm representing the frequency of operation is specified and often even this is left to the manufacturer's defaults. In the model, R is called equivalent series resistance (ESR). Motional capacitance, C, represents the elasticity of the quartz; and motional inductance, L, represents the vibrating mass of the crystal unit. The CO, or shunt capacitance, is capacitance formed by the metal electrodes deposited on either side of the quartz blank, plus the strays associated with the holder and leads. CO is generally specified as 7pF maximum.

If an electrode is especially large, it will cause a large motional capacitance in addition to causing a large CO. The larger the motional capacitance, the greater the pullability of the crystal. The converse is true as well. Small motional capacitance, especially on overtones, makes a crystal less sensitive to pulling effects. This can be useful in TCXO and VCXO designs. There are trade-offs, however, between maximizing the motional capacitance and adequately suppressing the spurious responses. A good design must balance these trade-offs in a way that best meets the needs of a particular application.

A common error in specifying crystals is to assume that the motional parameters and oscillator performance will remain the same when board designs are modified to use smaller crystal packages. Whenever a design goes through a "shrink," it is best to re-evaluate the impact on the oscillator design.

Additional Design Considerations
Controlling spurious responses -- unwanted responses that can cause problems such as temperature anomalies, VCXO linearity bumps, etc. -- often is a significant part of the design process. Typically, these unwanted responses should be 6dB down relative to the fundamental and 3dB down relative to the overtone responses. Alternatively, these unwanted responses can be specified in terms of a spur to main mode resistance ratio. For example, 6dB is equivalent to a spur resistance ratio of 2:1 (20 log10 Rspur/ESR). Development cycles for square and rectangular crystals typically take more iterations to achieve adequate spurious rejection.

The drive level is another important element to take into consideration when modeling VCXOs and TCXOs. This is the amount of power the crystal will have to dissipate in a given oscillator circuit. Overdriven crystals tend to age faster and have more and stronger temperature anomalies. The smaller the crystal, the more pronounced these problems become. Most crystal manufacturers recommend a maximum drive level of 100mW.

An undesirable characteristic, but one to which designers must attend, is the aging factor. The aging of a crystal refers to the amount of frequency drift per year at 25�C. Default aging specifications are in the 3 to 5ppm/year range. Specials can be produced at 1ppm/year depending on the frequency and holder. Generally, crystals age the most in their first year of operation.

Design to Test Standards
Environmental and reliability specifications, for the most part, should reference the test procedures and methods outlined in MIL-STD-883 and MIL-STD-202. Some of the procedures include tests for shock, vibration, solvent resistance, terminal strength, gross leaks, fine leaks, low-temperature exposure, high-temperature exposure, thermal cycling, solder heat resistance, and others.

Closing the Loop
This tutorial provides the basis for specifying crystals for VCXO and TCXO designs used in wireless applications. It is possible to over-specify a crystal or specify crystals that cannot be manufactured cost-effectively. Because this is the case, Fox Electronics' Engineering Department routinely offers advice on customer drawings and designs. 

James B. Northcutt is an Engineer with Fox Electronics, Fort Myers, Florida, a leading supplier of frequency control products.

Because it offers frequency and cost benefits, the AT cut crystal is preferred for use in wireless application
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The overtones of AT-cut crystals fall at roughly odd multiples of the fundamental mode. (Re-published by permission of Intel Corporation; Copyright 1983 Intel Corporation)
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A common modeling series for crystals is the R-L-C network, where R is equivalent series resistance (ESR); L is motional inductance, and C is motional capacitance.

Yoke of dc generator serves two purposes,
(i) It holds the magnetic pole cores of the generator and acts as cover of the generator.
(ii) It carries the magnetic field flux.

In small generator, yoke are made of cast iron. Cast iron is cheaper in cost but heavier than steel. But for large construction of DC generator, where weight of the machine is concerned, lighter cast steel or rolled steel is preferable for constructing yoke of dc generator. Normally larger yokes are formed by rounding a rectangular steel slab and the edges are welded together at the bottom. Then feet, terminal box and hangers are welded to the outer periphery of the yoke frame.

Let's first discuss about pole core of dc generator. There are mainly two types of construction available.
One: Solid pole care, where it made of a solid single piece of cast iron or cast steel.
Two: Laminated pole core, where it made of numbers of thin, limitations of annealed steel which are riveted together. The thickness of the lamination is in the range of 0.04" to 0.01". The pole core is fixed to the inner periphery of the yoke by means of bolts through the yoke and into the pole body.

The pole shoes are so typically shaped, that, they spread out the magnetic flux in the air gap and reduce the reluctance of the magnetic path.

Due to their larger cross - section they hold the pole coil at its position.

Pole Coils: The field coils or pole coils are wound around the pole core. These are a simple coil of insulated copper wire or strip, which placed on the pole which placed between yoke and pole shoes as shown.

Armature Core of DC Generator

The purpose of armature core is to hold the armature winding and provide low reluctance path for the flux through the armature from N pole to S pole. Although a dc generator provides direct current but induced current in the armature is alternating in nature. That is why, cylindrical or drum shaped armature core is build up of circular laminated sheet. In every circular lamination, slots are either die - cut or punched on the outer periphery and the key way is located on the inner periphery as shown. Air ducts are also punched of cut on each lamination for circulation of air through the core for providing better cooling. Up to diameter of 40", the circular stampings are cut out in one piece of lamination sheet. But above 40", diameter, number of suitable sections of a circle is cut. A complete circle of lamination is formed by four or six or even eight such segment.

Armature winding are generally formed wound. These are first wound in the form of flat rectangular coils and are then pulled into their proper shape in a coil puller. Various conductors of the coils are insulated from each other. The conductors are placed in the armature slots, which are lined with tough insulating material. This slot insulation is folded over above the armature conductors placed in it and secured in place by special hard wooden or fiber wedges.

The commutator plays a vital role in dc generator. It collects current from armature and sends it to the load as direct current. It actually takes alternating current from armature and converts it to direct current and then send it to external load. It is cylindrical structured and is build up of wedge - shaped segments of high conductivity, hard drawn or drop forged copper. Each segment is insulated from the shaft by means of insulated commutator segment shown below. Each commutator segment is connected with corresponding armature conductor through segment riser or lug.

The brushes are made of carbon. These are rectangular block shaped. The only function of these carbon brushes of dc generator is to collect current from commutator segments. The brushes are housed in the rectangular box shaped brush holder. As shown in figure, the brush face is placed on the commutator segment with attached to the brush holder.

For small machine, ball bearing is used and for heavy duty dc generator, roller bearing is used. The bearing must always be lubricated properly for smooth operation and long life of generator.

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