What is the difference between an AC generator and an Alternator?

AC generators can be broadly referred to any type of electricity generator that generates electrical energy in Alternating Current (AC) form. So, in a way, yes, an Alternator is a type of AC generator. Just so, an Induction generator is a type of AC generator too.

A more appropriate term for an alternator is - AC synchronous generators.

More specifically, in an alternator, the excitation winding (which produces magnetic field) is mounted on the Rotor shaft and the armature winding (which generates electrical current) is mounted on the Stator (stationary part of machine which surrounds the rotor). While in most of the other types of electrical generators, including DC generators, the excitation winding is stationary and the armature winding is mounted on the rotor shaft.

So basically, you may distinguish it like - the magnetic field is stationary in generators, whereas it is rotating in alternators. Or the other way - the armature winding is rotating in generators, whereas it is stationary in case of alternators.

So why the armature winding is stationary in an alternator?

• At high voltages, it is easier to insulate the stationary armature winding, which may be as high as 11 kV or even more in some cases.
• The generated high voltage output can be directly taken out from the stationary armature. Whereas for a rotary armature, there will be large brush contact drop at higher voltages, also the sparking at the brush surface will be a problem to look after
• If the field exciter winding is placed in the rotor, low voltage DC can be transferred safely to the exciter winding via slip-rings.
• The armature winding can be braced well, to prevent deformation caused by high centrifugal force if it was in the rotor.

Take it like this, all alternators are AC generators but NOT all AC generators can be alternators.

In general terms you can read more about Difference between Generators and Alternators here.

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Basic construction and working of a DC Generator.

DC Generator

A dc generator is an electrical machine which converts mechanical energy into direct current electricity. This energy conversion is based on the principle of production of dynamically induced emf. This article outlines basic construction and working of a DC generator.

Construction of a DC machine:

Note:Theoretically, a DC generator can be used as a DC motor without any constructional changes and vice versa is also possible. Therefore, a DC generator or a DC motor can be broadly termed as a DC machine. These basic constructional details are also valid for both DC Generator and DC motor. Hence, let's call this point as construction of a DC machine instead of just 'construction of a dc generator'.

The above figure shows constructional details of a simple 4-pole DC machine. A DC machine consists of two basic parts; stator and rotor. Basic constructional parts of a DC machine are described below.

1. Yoke: The outer frame of a dc machine is called as yoke. It is made up of cast iron or steel. It not only provides mechanical strength to the whole assembly but also carries the magnetic flux produced by the field winding.
2. Poles and pole shoes: Poles are joined to the yoke with the help of bolts or welding. They carry field winding and pole shoes are fastened to them. Pole shoes serve two purposes; (i) they support field coils and (ii) spread out the flux in air gap uniformly.
3. Field winding: They are usually made of copper. Field coils are former wound and placed on each pole and are connected in series. They are wound in such a way that, when energized, they form alternate North and South poles.



Armature core (rotor)

4. Armature core: Armature core is the rotor of a dc machine. It is cylindrical in shape with slots to carry armature winding. The armature is built up of thin laminated circular steel disks for reducing eddy current losses. It may be provided with air ducts for the axial air flow for cooling purposes. Armature is keyed (fixed) to the shaft.
5. Armature winding: It is usually a former wound copper coil which rests in armature slots. The armature conductors are insulated from each other and also from the armature core. Armature winding can be wound by one of the two methods; lap winding or wave winding. Double layer lap or wave windings are generally used. A double layer winding means that each armature slot will carry two different coils.
6. Commutator and brushes: Physical connection to the armature winding is made through a commutator-brush arrangement. The function of a commutator, in a dc generator, is to collect the current generated in armature conductors. Whereas, in case of a dc motor, commutator helps in providing current to the armature conductors. A commutator consists of a set of copper segments which are insulated from each other. The number of segments is equal to the number of armature coils. Each segment is connected to an armature coil and the commutator is keyed (or fixed) to the shaft. Brushes are usually made from carbon or graphite. They rest on commutator segments and slide on the segments when the commutator rotates keeping the physical contact to collect or supply the current.

Commutator

Working principle of a DC generator:

According to Faraday’s laws of electromagnetic induction, whenever a conductor is placed in a varying magnetic field (OR a conductor is moved in a magnetic field), an emf (electromotive force) gets induced in the conductor. The magnitude of induced emf can be calculated from the emf equation of dc generator. If the conductor is provided with a closed path, the induced current will circulate within the path. In a DC generator, field coils produce an electromagnetic field and the armature conductors are rotated into the field. Thus, an electromagnetically induced emf is generated in the armature conductors. The direction of induced current is given by Fleming’s right hand rule.

Need of a Split ring commutator:

According to Fleming’s right hand rule, the direction of induced current changes whenever the direction of motion of the conductor changes. Let’s consider an armature rotating clockwise and a conductor at the left is moving upward. When the armature completes a half rotation, the direction of motion of that particular conductor will be reversed to downward. Hence, the direction of current in every armature conductor will be alternating. If you look at the above figure, you will know how the direction of the induced current is alternating in an armature conductor. But with a split ring commutator, connections of the armature conductors also gets reversed when the current reversal occurs. And therefore, we get unidirectional current at the terminals.

Types of a DC generator:

DC generators can be classified in two main categories, viz; (i) Separately excited and (ii) Self-excited.
(i) Separately excited: In this type, field coils are energized from an independent external DC source.
(ii) Self-excited: In this type, field coils are energized from the current produced by the generator itself. Initial emf generation is due to residual magnetism in field poles. The generated emf causes a part of current to flow in the field coils, thus strengthening the field flux and thereby increasing emf generation. Self excited dc generators can further be divided into three types -
    (a) Series wound - field winding in series with armature winding
    (b) Shunt wound - field winding in parallel with armature winding
    (c) Compound wound - combination of series and shunt winding

You can learn more about types of a DC generator/machine here.

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Why inverter generators are quiet compared to normal generators

From many of the generator types in the market, most household owners prefer to use either conventional generators or portable inverter generators. There are many benefits of choosing inverter generators over others, but in between them, the major difference is the noise.

Inverter generators are quieter than conventional generators, but why? even though they have the same fossil fuel engine to generate the power. The answer to that question is the difference in their design parameters as well as in their working mechanisms.

Keep reading to know the reasons for the noise from the generators in the first place and how the design and the working of inverter generators are advantageous to reduce the noise level of the generator.

Why do generators produce noise in the first place?

The answer to that question is because it involves the rotating and moving mechanical parts, which are the main reason for the noise.

The main part of any generator is the engine. The engine runs by burning the fuel, which transfers the power to the pistons inside the engines, which will be responsible for a rotational motion inside the alternator, and the electricity is generated.

The burning of fuel inside the engine are small explosions that make the most of the noise. Companies take some measures to reduce it to some extent, but they can’t totally diminish it, and the matter gets worse when the generator is an open frame generator.

Moreover, a generator will need to maintain a constant speed of 3600 RPM to keep the supply frequency to 60 Hz. And for that, the engine constantly runs and burns the same amount of fuel to maintain that speed to maintain the frequency.

Besides the internal combustion engine, the friction between the moving parts also produces a significant amount of noise.

Why are Inverter generators so quiet?

There are many differences between the conventional generator and the inverter generator in terms of their design and working. It makes the inverter generator quieter.

The first thing is the engine; the engine used in the inverter generators is smaller and a bit quieter than those used in conventional generators. Besides, they don’t need to run continuously to maintain the frequency of the output voltage.

The working of the inverter generator ensures the constant frequency. The engine runs the alternator to generate the AC electricity, which is converted to DC and then again AC. The whole process is controlled with the help of a microprocessor.

When the load is less, the microcontroller will reduce the speed of the engine, and so the noise is also reduced, while the frequency remains the same.

The reduced noise is one of the biggest points of inverter generators. All the inverter generators are equipped with advanced noise-reducing technologies and very good mufflers.

Other than that, the casing made out for the generator is from the noise absorbing and quality material.

Can conventional generators be as quiet as inverter generators?

No, you can’t make the conventional generators as quiet as the inverter generators. There are major differences in their working mechanisms, as we discussed.

Still, you can reduce the noise of the conventional generator to quite an extent with the use of some external means, but it will be more than the inverter generators.

Even if you enclose the whole generator in the casing, then you will need the air vents and the exhaust vents. And simply stating, exhaust pipes are the second most noisy part of the generator.

Conclusion

Inverter generators are the best option in the market in terms of noise level and portability. They run very smoothly and offer fuel economy. The engines designed for the inverter generators are inherently quieter than for the conventional generators. Moreover, they are equipped with other accessories to reduce the noise even more.

They are best for home use and for camping use as well. Their initial prices are high, but you get the power with the silence so you can enjoy nature.

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Difference between Generators and Alternators

The terms alternator and generator will sound the same to you if you are not a technical person. But, let me tell you, they are not. They are different machines despite the fact that both of them have the same function, to generate useful electricity.

If you want to know what alternators and generators are, how they work, and their differences then we have all of that in this article.

Before we begin, just understand that electricity and magnetism are both related. You can produce one with the help of the other one.

You can produce electricity either by rotating the conductor inside the static magnetic field or by rotating the magnetic field around the static conductor.

Both alternator and generator use these principles to generate electricity.

How an Alternator Works?

The alternator is also known as a synchronous generator. Whether it is a generator or an alternator, the fundamental principle to generate electricity remains the same. They both work on the principle of faraday’s law of electromagnetic induction.

The law states that a relative motion between the magnetic field and the conductor will generate the electricity in the conductor.

The same goes for the alternator as well, but unlike generators, it will only give out the AC electricity. The construction of an alternator is simple; the rotor contains the exciting winding, whereas the stator will have the armature winding.

The rotor is the one that rotates, and the exciting winding generates the magnetic field. So, a magnetic field in the alternator rotates while the armature winding stays stationary. An armature winding is a term used to denote the winding in which the electricity is induced.

The reason for the armature winding to be stationary is few. First, the voltage induced in alternators will be higher, so the insulation will be much easier compared to the rotating winding.

Also, If the armature winding will be on the rotor, then there will be a high voltage drop at the brushes, and also, there will be a problem of sparking.

The excitation winding is powered by the DC, and it will be easier to connect the current through the brushes if the winding is on the rotor.

The alternator is a device that generates AC electricity from mechanical energy, and they only provide this energy when it is needed so the operation will be efficient.

How a Generator Works?

The generator also works on the same principle of Faraday's law of electromagnetic induction. The alternator only supplies the AC electricity, but the generator provides the DC or AC electricity.

For a generator, the placement of the armature and the excitation winding is opposite to the alternator. The armature winding is situated on the rotor, and the excitation winding is on the stator.

The electricity gets induced in the armature winding on the rotor. On the stator, for magnetic flux, it will either be the permanent magnets or windings are used. The armature winding will be rectangular, and both of its ends will terminate to the brushes.

The rotor inside the generator will rotate with the help of some external mechanical force, and the electricity will get induced in the windings. It will be taken out from the brushes.

The generators produce electricity continuously. It can also be used to recharge the completely drained batteries when they are completely drained, without any problems. Their ability to generate constant power makes them a strong choice as home backup solutions or to power the power tools at the job site.

DC and AC electricity: What is it?

We know that the generator can supply the AC or DC electricity, whereas the alternator can only supply the AC electricity. In this section, let’s discuss what AC and DC electricity are.

AC electricity

You may be knowing that the flow of electrons inside the conductor is referred to as the electric current.

For AC electricity, the flow of electrons will be back and forth, and this means the current is alternating in nature. This is the type of current most of the appliances in the house works on. But the voltage and the frequency of the electricity should be as per the standards.

This periodic change in the current direction is demonstrated in the form of the sine waveform, which is also known as the AC waveform.

The reason for this kind of power generator is because of the magnetic poles inside the generator and alternator. When one side of winding passes from under one of the poles, the electric current will be in one direction.

When the same side comes under another pole, the direction of the current reverses, this keeps going on as the winding keeps rotating and the AC is generated.

DC electricity

The DC means the Direct Current. The DC is the one in which the flow of electrons will be only in one direction. And the magnitude of the current stays the same; it doesn’t change with time.

As there is no oscillation, we can say that the frequency of the DC is zero. The use of DC is mainly to power electronics due to its constant flow of electrons.

The generated current inside the alternator or the generator will always be AC, and you can’t generate DC from them. This AC for the generator is converted to DC with the use of either the brushes or the rectifier.

The process of conversion of AC to DC is known as rectification.

Key Differences between the generator and an alternator

Please note that the alternators used in cars are different from the alternators (also known as a synchronous generator), which are used in a power station to produce electricity in bulk. The alternators used in the vehicles are to charge the batteries. Another thing you should note is to never charge the fully drained batteries with the alternator, and it is hazardous.

Both the generator and alternator have their own advantages and disadvantages.

The alternator has the stationary armature winding because -

• Stationary winding can easily be insulated for high voltages and doesn't deal with any centrifugal forces
• Hence Armature winding can be constructed more rigidly to avoid any mechanical stress
• Elimination of brushes to collect the output current from the rotor.
• The 3-phase output of armature winding can directly load without slip rings and brushes.
• Since the DC current to supply field winding is lower, only two slip rings are required, which are of very light construction.

Note that any alternators rated above 5 kVA employ the stationary armature and revolving field winding (rotating magnetic field).

Alternators are capable of generating electricity at the specified frequency.

When prime mover speed is lower, the salient rotor is used in an alternator. So as the number of poles increases, the RPM will decrease.This same concept is used in water turbines where the prime mover speed is very low. But all commercial generators or portable generators use a cylindrical design with a two-pole arrangement to cope with high rotation speeds.

Output Current

Alternators can only supply the AC electricity while the generators can provide the AC as well as DC currents.

Construction

Another major difference between them is the constructional one. For an alternator, the magnetic field rotates with the rotor as the field winding will be placed on the rotor. While for the generator the field stays stationary and the armature rotates on the rotor, the field winding will be situated on the stator.

Size

The alternators used in power stations are usually large and they accommodate large areas. On the other hand, the portable generators rated below 10Kw are not that large and won’t require that much space.

The power output

In alternators you can change the voltage output by adjusting field current which is achieved through an automatic voltage regulator. The latest inverter generator can regulate their speed to produce less power without sacrificing the desired frequency.

Polarisation

For an alternator, there is no need for a polarization. But, the generator will need to be polarized after it is installed.

Use of them

Alternators and generators, due to their differences, used for different purposes. Alternators are used in power stations and the generators are used where electricity needs are small.

Conclusion

That’s it. Most of the important things related to alternator and generator we have covered in this article. They look the same and have the same function to generate electricity. But they differ from each other in many things.

Their construction is different, one of them can supply the DC power and one of them can’t. These differences led them to be useful and suitable for different applications. You can’t use one instead of another.

You can read about difference between an Alternator and AC generator specifically here.

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7 Motor Parts and Their Common Faults

Understanding common motor problems is an important aspect when it comes to understanding engineered products or machinery. This comes in handy in many unfavorable situations, so one can define the appropriate solution and protective measure for such faults and mishaps. This also makes it important to understand various important terms related to assessing the parts and the working structure of your motors even during onsite testing of motors.

Cut Away View of an Electric Motor (Credit: Wikimedia commons)

Here we have noted down

Some common motor problems caused by certain parts

Shaft

This mechanical component in your motor is basically used to transmit torque and rotation. Shaft misalignment is the most common problem when it comes to this particular part. This will destroy your bearings before their complete working life. Analyze your motors through a vibration measurement procedure and check the force and the load that your motor is put through when they are running to avoid wear and tear.

Rotor

Rotor elements can likewise help decide the reason for untimely gear disappointment. A fan's exhibition can be influenced by typical wear. On the off chance that appropriate protection support isn't executed, rotors may turn out to be slim and ultimately break.

Stator

On the off chance that you sense that your motor comes up short on the force it once had, or it's increasingly hard to begin, test your source coils. It is doubtlessly going to bite the dust. While you test it, you may need to likewise test your pick-up coil, as it could be failing as well. The actual stator may be dead and isn't sending any current to the battery. Never forget to replace your stators connectors. A bad connection generates heat, and heat fries the connector, making a short circuit that may burn your stator and other electrical parts.

Bearing Issues

In the three-phase machine, two sets of bearings are installed within the engine lodging, for supporting the machine shaft. They comprise an external and inward ring which is called races and a bunch of moving components which are called bearings. The bearing balls, both external and internal ring are damaged because of which the motor completely jams or gets struck. Bearing issues can also be checked by vibration measurement.

Insulation Brakdown

The most regular disappointment in electrical hardware is the degradation and breakdown (flashover) of the insulation. Natural protection material comprises enamels, resins, varnishes, or polymers that are applied to the steel surface to give high inter-laminar (between windings) resistance as found on most air-cooled apparatus and some oil-submerged transformers.

Hipot test

Another form of Electric Motor Testing is A hipot (high potential) test, additionally called a dielectric strength test, which checks for shortcomings in the link or wire protection. To play out this test, you apply flow between the electrical circuits and the edge. Note that particular overvoltage levels applied are subject to the motor and its predetermined voltage.

During this type of electric motor testing, you measure the overflow of current and figure the comparing mega-ohms even during onsite testing of motors. Zones with lower or higher than recommended mega-ohm readings have damaged protection.

Motor winding resistance

The objective of an machine winding test is a disconnected test used to find winding issues. You ought to play out this test at whatever point you see breaks or burns, or if you've noticed a burning smell coming from the machine. You will begin by clearing off the windings with shop air and investigating them. Then, set the multi-meter to midrange and design it to gauge resistance in ohms, at that point contact the leads together to confirm that the reading is 0 ohms.

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Power System Stability

Power System Stability is the ability of a power system network to regain its equilibrium state even after being subjected to a disturbance. The main objective one must understand in power transmission is that the maximum amount of real power is aimed to be transferred to the load. Achieving this is practically not possible due to frequent load variation (either increase or decrease), but with various methods and analysis operating the system in the most stable region can be implemented and that is where this area of study comes into the picture.

Understanding the type of instability introduced into the system, the network is brought back to its equilibrium operating condition to achieve maximum power transfer. Let us first understand the main parameters that need to be taken into account to ensure a stable network.

We know the equation to determine the real power transferred in a transmission line:

Pe =  

EV sin𝛿

---

X

Where,
Pe = Magnitude of real power transferred
E = Excitation voltage
V = Terminal Voltage
𝛿 = Load angle/ Power angle (the angle between the Excitation voltage phasor and the terminal voltage phasor)
X = The total reactance

Therefore the relation can be plotted as follows:

This is defined as the power angle curve.

If we notice, as the load angle increases the power transferred also increases and reaches the maximum at 90°. But, when it further increases the power transferred decreases significantly.

Thus, the load angle should be maintained in such a way that the power should not be decreased.

A change in load results in a number of issues:

1. Change in frequency
2. Change in load angle
3. Change in rotational characteristics (especially speed of the machine)

Thus, a variation in load makes the system lose its synchronism and that is brought back with the help of Load Frequency Control.

The system stability can be classified depending on the type of disturbances it incurs in the system. They may be classified into:

1. Steady State Stability
2. Transient Stability
3. Dynamic Stability

Steady State Stability

The ability of the machine (synchronous machine) to deliver maximum real power to the loads by maintaining equilibrium even when it experiences a small and gradual variation of load. Small load variations may occur when the frequency of oscillations made by the rotor is less than the natural frequency of the system (basically the change in rotational characteristics of the synchronous machines may fall under this category).

Transient Stability

Transient Stability is the ability of the machine to deliver maximum real power to loads when it experiences a sudden and large variation of load. This type of variation of the load is due to the occurrence of three-phase fault that lasts for a few cycles. The types of three-phase faults may be:

CASE 1: WHEN IT OCCURS NEAR TO THE BUSBAR
CASE 2: WHEN IT OCCURS MIDWAY BETWEEN THE SENDING AND RECEIVING END
CASE 3: WHEN IT OCCURS ON THE BUSBAR

Dynamic Stability

The ability of the system to remain stable against smaller disturbances that do not last more than a few seconds. These smaller disturbances may occur due to random small variations in load or power generation levels. If these disturbances are cleared in few seconds, the system is said to be dyanamically stable.

Assumptions

There are a few assumptions taken and need to be kept in mind while analyzing the stability of a power system network.

1. Effect of the shunt capacitance is neglected
2. The resistance is neglected
3. Mechanical input to the alternator is assumed to be constant (angular velocity is constant, running at synchronous speed)
4. Effect of damper winding is neglected

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Author: Vaishnav Chathayil is pursuing his B.Tech. in Electrical and Electronics engineering at National Institute of Technology, Calicut.

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Power System Security

Author: Vaishnav Chathayil is pursuing his B.Tech. in Electrical and Electronics engineering at National Institute of Technology, Calicut.

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The sole objective of any power system network is to transmit power within its acceptable limit. The network should also work or stay under its limits event under any contingency. Contingencies may be of any type:

1. Generator outage
2. Line outage

Thus, irrespective of the issue evolved in a network, the power transfer between generating units and its loads must be “secure”.
The following article deals with what power system security means and how a power system network is classified into various states of operation.

What does Power System Security mean?

Power System Security may be defined as: The ability of a power system network to withstand contingencies(changes) and remain in its secure state or operate within its acceptable limits.

Various parameters may be taken into consideration and each have their own constraints. Violation of any such constraints may deviate the network’s secure operation. The constraints to be met for a secure operation are:

Inequality Constraints:
Pgi_min ≤ Pgi ≤ Pgi_max
Vi_min ≤ Vi ≤ Vi_max
Qgi_min ≤ Qgi ≤ Qgi_max
f_min ≤ f ≤ f_max
Where,
Pgi - real power generated from i^th unit
Vi - voltage magnitude
Qgi - reactive power generated from i^th unit
f - frequency

Equality Constraints:

Total Generated Power from all the units = Power demand + Power loss

If these constraints are met irrespective of any disturbance or change in the system, then the network is in secure operation.

Functions of Power System Security

The two functions that are taken care of under power system security are:

1. Security Control: Make sure all the parameters are within their limits.
2. Security Assessment: Detects the change in the parameters and identifies in which state the system is operating in.

System State Classification

Dyliacco’s classification:

In 1968, Dyliacco was the first one to introduce the classification of states in power system security. The operating states were classified into:

1. Preventive State: this state basically highlights the secure operation of the system. It states that the system is working under its parameter limits and is also capable of withstanding the contingencies that occurs. Thus the operator on analyzing the situation should take preventive measures in advance and let the system not deviate from its state even during the contingency.
2. Emergency State: This state indicates that the system constraints has been violated, I.e., it is not operating in its limits.
3. Restorative State: In this state power transfer does not take in some parts due to outage I.e., contingency occurs in some parts of the system. Thus necessary action is to be taken to deviate it back to the normal state.

L.H. Fink and K Carlsen’s classification:

In 1978, the next type of state classification was suggested by Fink and Carlsen. In their classification the operating states are of 5 types:

1. Preventive
2. Alert
3. Emergency
4. Extreme
5. Restorative

The following state transition diagram can be used to explain the flow that take place.

STATE	EC	IC	REMARKS
Normal	0	0	The system is secure and all constraints are met
Alert	0	0	When the constraints are on the verge of violation,I.e., the operator is notified to foresee contingency
Emergency	0	1	When inequality limits are violated
Extreme	1	1	When both equality and inequality limits are violated
Restorative	1	0	The inequality limit violation has been sorted but the load demand is yet to be satisfied.

*EC= 0 (equality constraint is met)
IC=0 (when inequality constraint is met)
Each becomes 1 when both are not met

In the extreme state it is the situation where blackouts occur and restoring it back to normal state may take hours or even days. Thus maximum the operator tries to deviate it from that state.

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