Electric Machines (3)

Question 1

What is the basic equation for angular speed, ω?

Answer 1

ω = 2πf = 2π(rpm/60)

Question 2

What is the equation for frequency, f, using number of poles, p, and angular speed, ω?

Answer 2

f = (p/2) x (ω/2π)

where:
* p = number of poles
* ω = angular speed

Note:
ω/2π = rpm/60,

so we can write:
f = (p/2) x (rpm/60)

Question 3

What is the equation for rpm using frequency, f and the number of poles, p?

Answer 3

rpm = 120f/p

Question 4

How can we work out the peak voltage, Em, in an equation like the one below without knowing ϕm?

Answer 4

Instead of using E = Em sin(2πft), where Em = ω(ϕm)N.

We can use the other version of Faraday’s Law:

Em = NBLu

where, N is the number of turns in the phase coil, B is the flux density, L is the length of the coil being cut by the rotor, u is the linear velocity.

So we can use this equation but note that there are two sides of the coil being cut by the rotors magnetic field, therefore we must have 2Lrotor for each coil being cut.

so final equation is, Em = 2NBLrotor u

Question 5

What are the two main types of rotors?

Answer 5

• Non-salient pole cylindrical rotor: conductors are wound round the rotor in a specific way carrying DC current to generatre N and S poles
• Salient Pole rotor: conductors are just wound round these big jutting points and DC current is passed through them generating the N and S poles.

Question 6

What are the benefits and drawbacks each type of rotor?

Answer 6

Non-salient pole cylindrical rotor:
-Pros: Very uniform magnetic field comparted to salient rotor
-cons: Difficult to manufacture and tell where and which poles are N or S.

Salient pole rotor:
-pros: Easier to manufacture and see where and which pole is N or S.
-cons: Doesn’t create as uniform magnetic fields compared to non-salient rotor.

Question 7

How are stators created?

Answer 7

The stator (or sometimes called armature laminations) are just layers of thin (often steel) that means that it reduces eddy currents and magnetic reluctance, helping guid the flux flow through the conductor coils.

The conductors themselfs are layed in the divits shown below, which are made of Roebel bars, which are many copper ‘wires’ that are hollow as most AC current flows to the outside because of the skin effect. Also these copper wires are wound and twisted to reduce the effects of induced noise current.

Question 8

If coils rotating in a magnetic field also generates AC currents, why don’t we often use it for large scale power generation?

Answer 8

Coils rotating in a magnetic field is useful for small scale energy generation for small currents but it’s bad for larger currents because the brushes will need to pass high voltages which can cause sparking across the brushes and the brushes will slowly ware out and require changing.