Alkaline Batteries

           

The most popular battery composition is alkaline. Alkaline batteries have a sloped discharge curve. Their voltage gradually drops off over the life of the battery until eventually, the battery is no longer usable. Although the discharge curve is sloped, it is still flatter than zinc-chloride and carbon batteries. Alkaline batteries can deliver up to 80% of their original capacity after being stored for four years. This makes alkaline batteries ideal for emergency devices such as flashlights, radios, and TVs.

Alkaline batteries are 8 to 10 times more powerful than the zinc-chloride or zinc-carbon batteries. They’re very good for use in high-current applications, such as motorized toys and portable TVs. Since they are able to emit large amounts of current for short periods of time without significant voltage loss, alkaline batteries are also ideal for photo flash units.

Lithium Batteries

           

Lithium batteries are great for long-term use. They last 2-3 times longer than alkaline batteries. Lithium batteries also perform well n extreme temperatures and are an excellent choice for devices such as smoke detectors and wireless alarm systems. These batteries are able to ahndle excessive current applications, like CD players and portable computer equipment. The discharge curve of a lithium battery is similar to that of an alkaline battery; however, some lithium batteries have shelf lives of up to 10 years!

Nickel-cadimum

           

Nickel-cadmium (Ni-Cd) batteries introduce a different spectrum of battery use. Once these batteries have reached the end of their effective lifespan, they may be recharged over and over again, up to 300 times. Ni-Cd batteries feature a flat discharge curve, which means they will deliver their full voltage for a length of time, then suddenly discharge. This works well in motor-driven devices, such as RC cars and CD players.

Hi-capacity Ni-Cd

           

Hi-capacity Ni-CD batteries are a great step-up from standard Ni-Cd batteries because of greater current capacity. The voltage rating on hi-capacity Ni-Cd batteries are the same as standard Ni-Cd batteires. One major disadvantage of Ni0Cd batteries is the cell capacity.  The cells in Ni-Cd batteries produce only 1.25 volts each. Batteries with multiple cells are usually rated in multiples of 1.2 volts. For example, the 9 volt equivalent Ni-Cd battery only provides 7.2 volts (6 cells x 1.2 volts). As a result, Ni-Cd batteries are not recommended for voltage-dependent devices such as clocks.

Rechargeable Alkaline

           

Rechargeable Alkaline batteries are yet another option. They have the advantage of having the exact same voltage as standard alkalines, so they are interchangeable in every situation. Unlike any of the other rechargeables, they come charged and ready to use (because they do not lose their charge as quickly as the others). Rechargeable alkaline batteries cannot be recharged as many times as other rechargeable batteries, however. An absolute maximum is about 100 times, but the maximum voltage is reduced as the number of cycles increases.

Current Capacity

           

Batteries are rated according to voltage and current capacity (ampere-hours). Each battery is composed of one or more cells. In our traffic analogy, these cells are representative of parking garages for our cards, or electrons. When the battery is hooked into a circuit, the cars have a path on which to travel. They then begin to flow and the circuit has current. When all of the cars have left the garage and run out of gas, the battery is dead and the current flow ceases. Current capacity is the approximate gas mileage of our cars. The voltage of a battery is determined by the chemical composition of the material within the cells. Just as diesel fuel and gasoline both have the capacity to do work within our cars, the different chemical compositions have capabilities of doing work similar to one another.

mA x hours = mAH

(Current capacity)

           

The current capacity measures how quickly a battery will discharge under certain circumstances. This figure is determined by multiplying the flow of the current from the battery into the circuit by the amount of time the battery is able to provide that current.

So,

Time = mAH/mA

           

If you are given the current capacity of a battery (in mAH), dividing this number by the current requirement of the circuit will give you the time of the battery will last under a constant load.

Let’s take, for example, a 9.6 volt battery rated at 1500 mAH (milliampere-hours) at 250 milliamps. By dividing 1500 mAh by 250 mA, we find the battery will discharge in about six hours.

1500 mAh ÷ 250 mA = 6 hours

In contrast, the battery might be rated at 1250 mAh if used in a circuit requiring 500 mA. In this case, the battery would discharge in about 2.5 hours.

1250 mAh ÷ 500 mA = 2.5 hours

It is this concept that makes predicting the usable time of camcorder batteries difficult. Camcorders require a great deal of current, especially when using certain features, such as auto-focusing, fast forward, or rewind. Each of these features requires a different amount of current. This factor alone makes the usable time difficult to predict. However, other factors must be taken into consideration, such as temperature. Therefore, a 2000 mAh battery will not necessarily last twice as long as a 1000 mAh battery -- although it will typically be close.

Alkaline cells, regardless of their size, produce 1.5 volts. So, the parking garages may only hold a certain number of cars (remember, the TOTAL number of cars is voltage). Therefore, a “D” battery is composed of one alkaline cell. Conversely, a 9-volt battery is composed of six alkaline cells wire together in series. To explain this, we will deviate slightly from our traffic scene to a nearby railroad. Hooking batteries or cells in series is just like using multiple locomotives to pull cargo on a railroad -- the train is able to utilize the cumulative power of all of the engines to pull the railroad cars.

Voltage Discharge Curve

           

The voltage discharge curve is a vital tool to help you determine the appropriate type of battery to sell. This factor is determined by the chemical properties of the battery. Certain batteries exhibit what is known as a sloped curve. In this type of battery, the voltage gradually degrades as the battery gets older. Batteries exhibiting flat voltage curves maintain their voltage at a somewhat constant level until the end of their life, where the voltage suddenly drops off. In the graph at left, the top curve represents a flat discharge curve, the bottom curve represents a sloped discharge curve.
For battery basics part-II click http://bit.ly/10lZdub

Alternators or synchronous generators can be classified in may ways depending upon their application and design.
According to application these machines are classified as
1) Automotive type – used in modern automobile
2) Diesel electric locomotive type – used in Diesel Electric Multiple Unit
3) Marine type – used in marine
4) Brush less type – used in electrical power generation plant as main source of power
5) Radio alternators – used for low brand radio frequency transmission

These ac generators can be divided in many ways but we will discuss now two main types of alternator categorized according to their design. These are1) Salient pole type 
It is used as low and medium speed alternator. It has a large number of projecting poles having their cores bolted or dovetailed onto a heavy magnetic wheel of cast iron or steel of good magnetic quality. Such generators are characterized by their large diameters and short axial lengths. These generator are look like big wheel. These are mainly used for low speed turbine such as in hydral power plant.
2)   Smooth Cylindrical type
It is used for steam turbine driven alternator. The rotor of this generator rotates in very high speed. The rotor consists of a smooth solid forged steel cylinder having a number of slots milled out at intervals along the outer periphery for accommodation of field coils. These rotors are designed mostly for 2 pole or 4 pole turbo generator running at 36000 rpm or 1800 rpm respectively.

A brushless DC motors speed curve is reduced by the effects of motor inductance.  Below is a torque speed curve showing curves that both include and do not include the effects of inductance.

The red curves are the motor performance curves with the effects of inductance included.   If your design requires your motor to operate above the speed curve that includes inductance you will be disappointed.   The curve that includes inductance has a steeper slope caused by the inductance. Since the equation for N (no inductance effects):

 N=(V-IR)/Ke

shows that the slope is due to motor resistance, people have calculated the resistance that fits the actual performance curves and termed this Reff, the effective resistance.  However this is misleading on multiple counts.  The calculated effective resistance is supply voltage (speed) dependent and the motor I^2R losses will be overestimated.

As a three-phase brushless DC motor rotates, voltage is applied to the leads in a particular pattern, termed a 6 Step Drive.  The frequency that the motor is commutated as is termed the commutation frequency.  At each commutation, one motor lead that is attached to a supply rail is open-circuited and another lead that was previously open circuited is attached to a supply rail.  The lead that was previously attached to a supply rail has current flowing in it and this current continues to produce useful torque.  The lead that previously was open circuited has no current flowing in it and voltage must be applied across it to overcome the winding inductive impedance.

When the lead with current flowing is open circuited, the phase voltage flies from the supply rail it was attached to by the drive, to the other supply rail, where it is clamped by the drive diodes.  Since the back EMF curve is almost at the same level as when the lead was energized, and the torque constant is proportional to the back emf constant, the torque produced by this freewheeling current is almost the same magnitude as produced prior to switching.

In effect the motor inductance acts as a switch inductance resistor, consuming voltage and reducing peak motor speed on one hand, while producing torque and reducing the measured motor current on the other.  Since the freewheeling current is not measured, a torque constant calculated from an actual motor performance curves will overestimate the torque constant.

Actual measurements of brushless DC motor stators has shown that there is very little inductance coupled between motor phases, therefore the inductance switched at each commutation point can be approximated by the motor inductance divided by two.  Since the effects of inductance is already a secondary effect, the effects of coupled inductance and of losses in laminations due motor lead commutation can be safely ignored.  In other words, we will assume no coupled inductance and that the inductor is lossless.

Many drive related effects reduce the motor speed, such as EMI filter resistance, attachment wire resistance and switching transistor resistance.  To predict the actual expected motor performance all must included in your motor model.  The effects of motor inductance has a significant effect and also must be included in your motor model.  Unfortunately, the speed torque curves supplied by manufacturers are regularly incorrect, predicting results that are unattainable in practise.  Care is required in any design where minimum speeds are a critical specification.

Understanding Volt AMP (VA) rating:

The VA rating of your unit represents the volume of electricity that the unit is designed to protect. Exceeding the VA rating of your ups is a common cause of UPS problems, even when the power is not out.

How does it work?

The VA rating of the unit is similar to the circuit breakers in your home that protect you from overloads – most breakers are 15amp breakers, meaning that you should not ever plug in enough devices to use more then 15 amps of power, or the breaker will trip, and disconnect the load. Without this safety, you risk a fire by causing the wires in the wall to heat up to the point that they ignite and burn. The same basic idea works with your UPS VA rating – the components inside your UPS are designed to never have a load higher then the VA rating, so overloading the UPS can damage the internal components, creating a risk of fire and damage to equipment. Think of your VA rating as the redline on an engine- you can rev it up to the max rating, but exceeding it, or even just running it for long periods near the redline will shorten the life of the engine, or even cause it to fail.

How do I make sure I am not overloading my UPS?

If your load indicator reads more then 60%, you should upgrade your unit to a higher VA unit. When choosing a unit, keep in mind that you want your baseline load to be about 50% of the UPS capacity, to allow your unit to perform best. This means that if I have a 500 VA load from my pc, monitor, and internet connection, I would look for a UPS rated for at least 1000 VA.

UPS Sizing Calculator

Welcome to the RefurbUPS.com free UPS sizing calculator. To simplify UPS purchasing for our customers, we have built this free tool designed to easily determine the electrical load of your equipment, and the minimum recommended UPS VA rating for your needs based on your desired run time. It's easy to use - simply select the type of equipment you are looking to protect, either from our list of generic equipment, or by HP, Cisco, or Dell part numbers. Add a line for each device that you are looking to protect, and then press the 'calculate your requirements' button to see what size UPS you should consider. if you would like one of our power protection experts to prepare a quote for a suitable solution, please provide us with your name, company name, email address, and phone number, and someone will get back to you within a few hours.

Maximum Power transfer theorem:

• In electrical engineering, the maximum power (transfer) theorem states that, to obtain maximum external power from a source with a finite internal resistance, the resistance of the load must be made the same as that of the source. It is claimed that Russian engineer Moritz von Jacobi was first to discover the maximum power (transfer) theorem around 1840, which is also referred to as "Jacobi's law".
• The theorem results in maximum power transfer, and not maximum efficiency. If the resistance of the load is made larger than the resistance of the source, then efficiency is higher, since a higher percentage of the source power is transferred to the load, but the magnitude of the load power is lower since the total circuit resistance goes up.
  
  
  
• If the load resistance is smaller than the source resistance, then most of the power ends up being dissipated in the source, and although the total power dissipated is higher, due to a lower total resistance, it turns out that the amount dissipated in the load is reduced.
• The theorem can be extended to AC circuits which include reactance, and states that maximum power transfer occurs when the load impedance is equal to the complex conjugate of the source impedance.