The idea behind electricity generation is that a form of rotational (kinetic) energy is converted into electricity. Think of a steam or wind turbine, where the blades are attached to a rotating shaft. AC machines (mostly synchronous machines) convert this rotation into 3-phase electricity.

There are two types of AC machines:

  • Synchronous Machines &
  • Induction Machines (Asynchronous)

Synchronous vs Induction Machines

Let’s see what the biggest difference is.

First of all, what do we mean by ‘machines’? AC machines are based on the principle of electromagnetic induction. Their purpose is to change rotational energy to electrical energy and vice-versa. So, both synchronous and induction machines can be used as generators or motors.

What does ‘synchronous’ mean?

In case of a simple synchronous generator, the rotation of the prime mover is in synch with the synchronous speed (magnetic field). If the turbine turns 50 times per second, the output voltage will have a frequency of 50Hz! (50 cycles per second)

Induction machines, also called ‘asynchronous machines‘ are slightly lagging behind the synchronous speed. They operate at a more simplistic principle and they’re also cheaper to manufacture.

So, for example, if our turbine turns 50 times per second, the induction machine will generate only 45Hz. We will see why in a bit.

Induction Machines

Induction machines are very commonly used. We have them in the kitchen, fans or even air conditioning systems. They are also widely used as 3-phase generators.

The basic idea for both induction machines and synchronous is very similar. They consist of 2 main parts:

  • Stator – a hollow steel cylinder, that contains the 3-phase coils around
  • Rotor – which is inside the stator

Construction of a 3-Phase Induction Machine

In addition, there are 3 important parts:

  • A Rotating Shaft
  • Squirrel Cage
  • Stator Windings

In a case of a generator, the rotating shaft (prime mover in other names) is attached to a turbine. For example water moves the turbine and it makes the prime mover spin. It is then attached to a squirrel cage, that also rotates.

The squirrel cage is essentially 2 circular plates that are joined by bars. This cage is next placed inside the stator.

The stator is the stationary part of the machine. It is a hollow steel alloy that contains the 3-phase conductors rolled up in coils.

Diagram: Induction Machine Construction

Figure 1: Construction of a 3-phase induction motor. The rotating magnetic field induces a current flow in the squirrel cage bars based on the law of electromagnetic induction. The cage will start to rotate due to the Lorentz Force.

Induction Machines Working Principle

So, these machines can either be generators or act as motors. Let’s take the example of a motor:

  • 3 conductors come in the motor’s stator, each representing a phase.
  • The 3 phases are placed inside the stator 120º apart. Check this topic for details about 3-phase.
  • The voltage & current in each phase are 50Hz.
  • If we have a current (electron) moving in a conductor, the moving electron will induce a magnetic field around the wire. This is happening in all 3 phases. This field will have a direction. The magnetic field can be easily drawn using the Right Hand Rule where your fingers will indicate the direction.

The Right Hand Rule

The Right Hand Law indicating the direction of magnetic field that is induced by a current moving in a wire.

Figure 2: Electrons, as they move in a wire, create a magnetic field. Current, which is the flow of ‘holes’ or in other words the absence of electrons, flow in the opposite direction. Hold a wire in your hand. Your our thumb will point to the direction of current flow. Your other four fingers will indicate the direction of the magnetic field. This is the Right Hand Rule.

The direction of the Magnetic Field in a single phase in the Stator

Let’s isolate one phase in the stator and see how the magnetic field is generated. This conductor is rolled up in a coil. Note, we have AC in the conductor meaning the current, thus, the magnetic field too will change directions 50 times a second:

Diagram: Showing both direction of current and induced magnetic field in a coil (1 phase) in the Stator of an Induction Machine..

Figure 3: The image shows a single phase isolated in the stator of a 3-phase motor. Alternating Current is fed into the winding. The current will induce a magnetic field around the wire based on the Right Hand Rule mentioned above. What we have is a coil. The direction of the magnetic field always goes from North to South, so referring to the magnetic flux lines outside the loop, we can draw in the N & S poles. As the current alternates, so does the direction of the magnetic field and poles. The image shows the two states of the current and how it changes the magnetic poles accordingly.

  • Using the Right Hand Rule, we can determine the direction of the magnetic field for both positive and negative currents.
  • A coil with current in it will have a North and a South magnetic pole. The magnetic field always points from North to South (the field outside the coil).
  • In the above diagram, the North and South poles will change positions 50 times a second. If we add back the other two conductors, each being 120 degrees apart physically, the overall effect would be a rotating magnetic field.

Current & Magnetic Field in the Squirrel Cage of Induction Motor

Now we have a rotating magnetic field in the stator, which is around the squirrel cage. This rotating magnetic field will induce a current in the squirrel cage bars based on electromagnetic induction.

Diagram: Rotor bars of an Induction Machine showing the direction of current and induced magnetic field as well as the rotating magnetic field in the Stator.

Figure 4: Squirrel Cage (part of the rotor) in an Induction Motor. The cage has a rotating field around (which is from the 3-phase windings in the stator). Note, it is heading in a different direction inside the cage than outside (blue arrows). We define the North and the South poles based on the magnetic field’s direction outside the cage. Thus, we can draw the N & S poles. The field’s direction always points from North to South. This field induces a current in the bars. As the N & S poles rotate, so will the current change direction in each bar. But the induced current itself in each bar will have its own magnetic field that will repel the stator’s field, thus, creating a force. This force will make the cage rotate.

  • The direction of the current in the cage bars will vary back and forth with the rotating magnetic field. Why? Because initially the cage is static and all a particular bar sees is a constantly changing North & South poles (from the rotor).
Fleming’s Right Hand Rule

It is a handy tool to determine the direction of the current in a particular bar at a given point. Let’s take the example that we have in the picture.

Fleming's Right Hand Rule

Figure 5: Fleming’s Right Hand Rule. The Law of Electromagnetic Induction states that if a conductor is placed in a varying magnetic field an Electromotive Force will be induced. If the conductor is part of a closed circuit, this force will move electrons and current will flow. The hand help us determine the flow of current based on the moving magnetic field’s direction (where the arrows point) and motion (which way does it move).

Lorentz Force

So, now we have an alternating current in the squirrel cage bars. Although, when there is a current in a wire (or bar in our case), the current will induce a magnetic field itself. This magnetic field will interact with the 3-phase rotating field and the result is a force that starts to rotate the squirrel cage. This is called Lorentz Force. This is because both magnetic fields point at the same direction and will repel each other.

Lorentz Force Right Hand Rule

Figure 6: Lorentz Force Right Hand Rule. Hold out your hand. Thumb shows the direction of the current and the fingers indicate the direction of the magnetic field. The Lorentz Force will be symbolised by an arrow that come out from your palm.

Why Do Induction Machines Lag?

It is very simple. We said that the current in the bars are due to the constantly rotating magnetic field around. Now, if the squirrel cage (rotor) starts to rotate, eventually it catches up with the synchronous speed (rotor’s magnetic field).

But in this case there’s no more rotating magnetic field around the bars, as they move with the same speed. So, no current, no force. This leads to the rotor to slow down. But hey, once the rotor slows down, the magnetic field seems to rotate again and we start the whole process from the beginning.

This happens so fast that what we witness in real life, is that the machine rotates a bit slower than the synchronous speed.

Starting an Induction Motor

When an Induction Motor is connected to 3-phase power, the magnetic field starts to rotate instantaneously. The rotor is still. This means the rotating magnetic field will be super fast compared to the stationary cage and will generate a huge initial current.

Since there is a sort of transformer action between the cage and the stator windings, this rush of current will also reflect in drawing a massive current from the 3-phase supply. And could cause a drop in the grid, so taking current away from other devices.

To come across this issue, we use the Star-Delta Starter method. Initially the motor is wired in a star configuration, where the phase voltage is smaller than the line voltage, next, when the motor is running it is switched to a Delta config.

To know more about the Star & Delta config, check out this earlier post.

Also, here is a good video that explains more about a Star & Delta starter.

Figure 7: Star and Delta connection of a 3-phase motor.

Induction Generators Working Principle

So, far we covered how Induction Motors work. We also know that the rotating shaft’s speed will slightly lag behind the synchronous speed. This is called slip.

Now, let’s attach this shaft to a turbine that will generate a rotation faster than the synchronous speed. The slip will be negative and the current will be generated in the opposite direction, meaning, electrons will flow back in the supply.

Synchronous Machines

Synchronous machine are similar to Induction machines. The idea is that this time the rotating shaft (prime mover) is attached to a DC electro-magnet.

The magnet needs power. There are so called ‘slip rings‘ attached to the pole. These are connected to an external power source via brushes. This is because the rotor, thus, these slip rings spin, and the only way to feed electricity in them is via brushes. The brushes only touch the rings.

Diagram: Synchronous Machines construction consisting of a DC electromagnet, 3-phase coils, prime mover and a squirrel cage.

Figure 8: Construction of a Synchronous Motor. Current comes in via the 3 phases and since it is alternating current it creates a rotating magnetic field in the stator. The 2-pole DC electromagnet in the diagram has a fixed N and S poles and they lock with the rotating N & S poles of the 3-phase magnetic field. Since it is hard to lock for the DC electromagnet onto the fast spinning stator field, a squirrel cage is used to give an initial rotation for the rotor. This works the same principle as in induction motors.

Synchronous Machines (Motor) Operation Principle

We already know that in the stator there is a rotating magnetic field. So, in the case of synchronous motors, instead of generating a Lorentz Force, we lock the poles of the stator’s DC electro-magnet with the rotating 3-phase’s field. This means for example the North pole of the electromagnet will attract the South pole of the rotating magnetic field.

This sounds easy, but when the motor is initially started, the magnetic field changes very fast around the DC electro-magnet, so it attracts and next repels. We need something that initially gives a kick for the rotor to start rotating so it can lock onto the 3-phase field.

We already have a great tool for this: the squirrel cage!

The trick is to disconnect the slip rings initially while the cage is in action, next, once the rotor is spinning, we connect them back and the fields will lock.

Salient Pole Rotor for Synchronous Machines (Generators)

A Synchronous Motor operated backwards work as a generator. Based on the number of rotor poles we can distinguish between:

  • Cylindrical Pole Rotor – 2 poles
  • Salient Pole Rotor – 4+ poles

Why do we even want multiple poles?

In the above example we had an electromagnet as a rotor that had a North and a South pole (2 poles). It is essentially a coil where you start rolling up a wire at one end and finish it up the other.

What if we used the same wire and wrap it around 4 poles instead? We will end up with something like this:

Diagram: 4-pole Rotor of an Induction Machine indicating the direction of the current.

Figure 9: 4-pole rotor of a 3-phase synchronous generator. This is a DC electromagnet. The electricity is supplied from brushes to the slip rings. Normally, for a solenoid (coil) one end is N and the other S with 180° between them. In a 4 pole scenario the N and S poles change every 90°.

Now, instead of a North and a South pole, we have two of each. If we look at the rotor from one particular angle, let’s say from the top, by one full 360º rotation this way we have a change between N&S happening 4x. North – South – North -South.

So, in a 2-pole scenario, the rotor rotates 50 times per second to generate 50Hz electricity. In the case of a 4-pole setup, the rotor only has to turns 25 times to generate the same 50Hz. Yet the change of magnetic field (synchronous speed) would still be 50 times/s.

Think about it, this way the rotor doesn’t need to rotate crazy fast. We can even have more poles. The following relationship is used to calculate the synchronous speed (magnetic field in the stator) in RPM in the cases when a salient pole rotor is used:

Formula: for Salient Pole Rotor Synchronous Machines calculating the synchronous speed based on the prime mover's frequency and number of poles.

Where:

  • Ns: the synchronous speed (magnetic field)
  • P: number of poles
  • f: frequency

2-pole vs 4-pole rotor in a 3-Phase Synchronous Machine (Generator)

Diagram: Comparison between a 2-pole and a 4-pole rotoro synchronous machine.

Figure 10: Difference between a 2-pole rotor ,3-pole stator machine and a 4-pole rotor, 6-pole stator machine.

On the left we see the most simplistic scenario: 2-pole rotor and a 3-pole stator. We have a 3-phase AC in the stator. The colours indicate each phase, The direction of the current is usually denoted by an arrow, so the X means it is going in the page, whereas the dot means coming out. Essentially, we have a coil that is wrapped around the rotor. Although, in real life it looks more like in Figure 6.

On the right, we have a 4-pole rotor with a 6-pole stator. As mentioned before, in the case of a generator, the rotor needs to be turned with half the speed to generate the same frequency as in the 2-pole rotor setup.

Why have a 6-pole stator?

In the first picture, one side of the rotor is N and the other is S. In the 4-pole case, the opposite side of the North pole is another North pole. Left picture: N points to an X, S points to a •. Right picture: Both N points to an X and both S to a •. Don’t forget, this is one wire. We can’t have the two N poles send the electrons against each other.

Diagram: Comparison between a 3-pole and a 6-pole stator synchronous machine.

Figure 11: Differences between a 3-pole and a 6-pole stator winding. When we have the same type of pole at the opposite side, the two sides are wired out of phase. So, the wire goes from outside-in, and on the opposite side inside-out. The turn is also in the opposite direction. This makes the currents flow in different directions. Use the Right Hand Rule to see the magnetic field. For North poles. the field points from outside in. Don’t forget, the magnetic field goes in a loop, but we consider the arrows that are outside the pole.