What is an AC generator

 

Good to know generators

A generator is required to convert the mechanical power of a rotating shaft into electrical power. In the car it is also known as an alternator or, earlier, a generator.

Whether electrical energy is to be generated with a small wind generator or a nuclear power plant, the principle is always the same:

An electrical conductor (usually made of copper) is pushed through a magnetic field. This magnetic field generates (induces) a voltage in the moving copper wire. If you now connect the ends of this conductor via an electrical resistor, a current flows. The electrical power is then obtained from the product of current and voltage.

The faster the conductor moves in the magnetic field (or the stronger the magnetic field is), the greater the induced voltage. It follows that the higher the speed of the generator, the more power can be drawn with the same design.

This also explains why the generators with low speed are always so big and heavy. In order to achieve the same performance, the rotor must have the largest possible diameter.

 

 

 

By the way, the reverse is also possible: If you feed a current through a conductor that is located in a magnetic field, the conductor wants to move in the magnetic field. This is then the principle of the engine.

Many engines can also be used as generators if they are operated correctly. Particular attention must be paid to starting up the motor operated as a generator. If no permanent magnets are used, an excitation current has to be generated somehow. If it is an asynchronous motor, the reactive power must be made available.

 

The physical quantity that makes the generator turn is called torque, which is measured in Newton meters [Nm]. In a Pelton turbine, for example, a jet of water sprays onto the turbine blade and thus causes a torque (measured in Newtons) on the blade. The torque is now generated via the lever arm of the turbine wheel radius (measured in meters). If the torque generated is greater than the opposite torque, the shaft begins to turn. The speed of the generator is so high that the generated and the opposite torque are equal. The opposite torque depends on the friction in the bearings or seals from the magnetic fields and on the power that the generator delivers.

In the case of wind turbines, the so-called cogging torque is of decisive importance. This counter-torque is primarily generated by the permanent magnets and the iron sheet in the stator of the generator. Of course, there is also the static friction of the bearings and seals here. The lower this moment, the sooner the wind turbine begins to turn.

 

 

Important parameters for the generator are also the maximum load current, the maximum output voltage and the maximum speed (in 1 / min).

 

The load current through the generator winding generates the heat in the housing. So that the temperature in the generator does not rise arbitrarily high and destroy the winding or the bearings, attention should also be paid to the cooling of the generator. Seen in this way, the maximum load current also depends on the cooling and the ambient temperature. The better the cooling, the higher the permissible load current.

 

The level of the voltage at the output of the generator depends on the one hand on the winding of the generator, on the strength of the magnetic field, but also on the speed. The higher the speed, the greater the output voltage. Generators with a high nominal speed are smaller than generators with a low speed because fewer copper windings are required.

You don't want to go so high with the speed, because otherwise the bearings have to be replaced more quickly. Especially with a water turbine, which should run 24 hours a day, the speed should not be higher than 1500 1 / min.

Since one would like to do without a gearbox with small wind turbines, a generator has to be found which works with a speed of up to approx. 500 1 / min. Generators with low speed and high output voltage are now also available. However, these are then relatively heavy and large.

 

 

Generator power

The power on the generator shaft is essentially dependent on the speed of the generator and the torque that is applied to the shaft.

 

The formula is. P = M x 2 x Pi x f

 

In which

P: power in watts [W]

M: torque in Newton meters [Nm]

Pi: natural constant 3.14159

f: speed in heart or revolutions per second [HZ or 1 / s]

 

The power delivered by the generator is the product of current and voltage at the output terminals.

 

The formula is for the generator with 2 phases (2 wires):

 

P = U x I

 

In which

P: power in watts [W]

U: voltage in volts [V]

I: current in amperes [A]

 

However, the generators are very often designed with a three-phase winding, i.e. 3 active ones are brought out.

The formula for the power then looks like the following:

 

P = U x I x √3

 

In which:

P: power in watts [W]

U: voltage between two conductors in volts [V]

I: current in the conductor in amperes [A]

√3: constant 1.732

 

The higher the speed that can be selected for the generator, the smaller the design with the same power conversion. Therefore, a generator for a wind turbine is usually much larger and heavier than a generator for a Pelton turbine, although the power output is the same. This is because a wind turbine usually has a lower speed than a free jet turbine. Incidentally, this is the same for the electric motor.

 

A basic distinction is made between direct current and alternating current generators

 

Torque

The torque is understood to be the 'turning force' required to bring the generator's rotor to the desired speed. The torque is measured in Newton meters.

You can differentiate between two types of torque on the generator.

 

The starting torque

states what torque is required to move the rotor from its resting state. This statement is important for wind generators insofar as a generator with a low starting torque starts rotating at lower wind speeds. This torque is essentially dependent on the construction of the generator. The simple arrangement of the stator core and the rotor magnets can lead to an enormous cogging torque and thus a high starting torque due to the magnetic forces. Generators that do not require iron in the stator have an extremely low starting torque.

 

The 'working torque'

is mainly determined by the current speed and the power (current flow in the stator), which is drawn from the generator.

Many a 'perpetuum mobile' developer has already disregarded this counter-torque in their considerations.

 

The formula for calculating the torque is. M = P / (2 x Pi x f)

 

In which

P: power in watts [W]

M: torque in Newton meters [Nm]

Pi: natural constant 3.14159

f: speed in heart or revolutions per second [HZ or 1 / s]

 

The power delivered by the generator is the product of current and voltage at the output terminals.

 

 

 

Direct current generator (DC generator)

Actually, the DC generator would also generate an alternating current. Using carbon brushes, however, the current is rectified during the transition from the rotating to the fixed part.

The rotating part is called the rotor, the fixed part is called the stator. So that no energy is wasted in generating the magnetic field, the direct current generators are usually equipped with permanent magnets.

The direct current generators for pico power plants (P> 5kW) are often optimized to charge a battery directly via a charge controller.

Maurer Elektromaschinen also tried to transfer power from the DC generator to the public grid via an inverter. However, since the generators were built for purely capacitive loads (battery), the commutator fire was too large. This means that the carbon brushes produced sparks that were too strong. Operating an inverter directly from the DC generator without a battery is therefore not recommended.

 

Alternator (AC generator)

The advantage of alternators is that they do not have carbon brushes. Because these have to be replaced from time to time. If direct current is required, e.g. to charge a battery or to power an inverter, the alternating current can be converted into direct current by a rectifier. This rectifier is a straightforward device made up of diodes and heat sinks.

An alternator should definitely be used for water turbines or water wheels that are in use 24 hours a day, as the wear and tear on the carbon brushes of the DC generator is too high. An AC generator is also recommended for wind generators. A DC generator (direct current generator) can certainly be used for demo objects that are only in use for a short time.

 

In the case of AC generators, too, the direct current values ​​as they occur after the rectifier are often specified.

 

In the case of generators for 24-hour operation, it is essential to ensure that high-quality shaft bearings are used. However, these should be replaced preventively after 5 years, before a defective bearing destroys the generator.

 

Hub generators also called external rotors

In most generators, the shaft rotates with the magnets. The winding is mounted around the outside. This remains.

With the hub generator it is exactly the opposite. The shaft remains stationary and the outer part rotates. In this way, the gap size of the generator can be increased. This means that a higher voltage can be achieved at the same speed. The disadvantage is that the attachment of these generators is more complex and unconventional.

 

 

Alternator, also called alternator (Lima)

The alternator is mainly installed in vehicles such as cars, trucks, buses and boats to charge the starter batteries. The magnetic field is generated by an excitation winding on the rotor. With the standard alternator, this is controlled by a regulator so that the battery is charged with a speed-dependent current. As soon as the end-of-charge voltage specified in the regulator is reached, this voltage is maintained.

The alternators need a relatively high speed to generate electricity. Depending on the alternator, 1500 to 2000 rpm is required for power generation to begin. The maximum current is delivered at approx. 6000 rpm.

In mobile homes and on boats, a second battery (consumer battery) is usually charged by the alternator via an isolating relay or battery splitter. The big disadvantage of this inexpensive charging option is that the consumer batteries (mostly gel or AGM) have a different charging characteristic than the starter batteries.

As an alternative to the isolating relays and battery splitters, charging converters are often used, which are basically multi-stage chargers that work with a nominal input voltage of 12V.

Another option is to replace the alternator regulator with an intelligent alternator regulator. This then controls the excitation of the alternator so that the alternator charges with the characteristics of a multi-stage charger.

 

 

engine

In principle, almost any electric motor can be used as a generator.

Squirrel cage rotors (asynchronous motors) first need energy for the excitation magnetic field.

Brushless DC motors are often used. It should be noted that these usually have a cogging torque.

 

Applications

Generators are used wherever mechanical power is to be converted into electrical power.

 

Wind turbines

Since the rotor of the wind turbine only turns slowly, generators with a low speed are used here.

It is also extremely important that the starting torque is as low as possible so that the rotor begins to turn even at low wind speeds. I.e. the generator may no detent torque have, which is not easy with permanently excited synchronous machines.

Often, external rotors are also used in wind turbines.

 

The nominal voltage of the generator is determined depending on whether the wind turbine is to be used to charge batteries or to feed into the public grid using an inverter.

 

Water turbines

The starting torque plays a subordinate role in water turbines.

The generators are often used at a speed of 1500 rpm or higher. It is important to ensure that the bearings are of good quality, as water turbines are mostly in use around the clock.

 

Water wheels

Water wheels have a very low speed. The lower the speed of the generator can be selected, the less the gearbox has to translate. This has a great influence on the efficiency.

 

You can find more information about batteries at: http://www.maurelma.ch/batterien.htm

 

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