ELECTRICAL MACHINES : PART 2 MCQ & NOTES

In this blog i will mention few points from the topic DC Generator and DC Motor. I hope you like it. The information provided here are true to the best of my knowledge. Still if you find any mistake please inform me in comment section i will rectify it.

How the DC Generator works to produce Voltage can be found in any standard textbook, it’s the first thing the authors start with so I will skip the entire theory part.

The basic principle is that when a coil is placed in a uniform magnetic field and the coil is rotated then an emf is induced in it. This induced emf is taken out from the coil with the help of slip rings and contact brushes.

Now that we know how voltage is generated let’s move on with the blog.

  1. We can increase the voltage generated (output voltage) and also make it smoother by increasing the number of coils. These coils are put in the armature.
  2. Basically when the coil rotates in the Magnetic field the voltage generated is AC. We convert that AC voltage to the corresponding DC voltage with the help of “Commutators”.
  3. The uniform magnetic field that we are speaking about is produced by the field coils. When DC current is passed through the coils (made of copper wire), it electro magnetizes the filed poles and hence produce the strong magnetic field around the armature.
  4. The generated voltage mainly depends upon three(3) factors:

(i) Armature speed

(ii) Magnetic fields strength

(iii) Number of armature conductors

The formula:

 

Where, Eg = Generated emf in the armatures

P = Number of poles

Ø = Magnetic field in Flux per pole (Webers)

Z = Total number of armature conductors

N = the rotation of armature (revolution per minute)

A = No. of parallel paths in armature.

Z, which is the Total number of armature conductors, can also be written as

Z = (Conductor per slot) . (Number of slot)

The value of A depends upon the type of armature winding.

For Lap winding , A = P i.e. No. of parallel paths in armature is equal to Number of poles

Whereas For Wave winding, A = 2 i.e. No. of parallel paths in armature is equal to a constant value which is 2.

Note: See the Picture for lap winding and Wave winding in any image search engine of your choice.

  • 5. The armature wingdings are connected either in lap winding form or in wave winding form.
    (i) In lap winding the coil ends are connected to adjacent Commutators bars. Armature conductors are divided into as many parallel paths as the number of poles in the generator.
    (ii) In wave winding, the coil ends are connected to the Commutators bars under the alternate poles. Armature conductors are divided into 2 parallel paths independent of the number of poles in the generator.
  • 6. Eg is inversely proportional to no. of parallel paths (A).As A increases Eg decreases.
    Wave winding has the lowest value of A=2, so for such winding Eg is more. That’s why wave winding is used when the voltage requirement is more.
    Whereas, for Lap winding A=P which is greater than or equal to 2, so Eg is less for it. But as it has as many parallel paths as the number of poles so its current capacity is more.
  • 7. Small machines use wave windings as the current carrying capacity is NOT RELATIVELY important than the voltage. But, voltage is easily build up in Large machines so current carrying capacity is important out there so we need Lap winding in large machines.
  • 8. Lets now see few Electrical Machine Tests and what quantities they measure in the test:
    (a) Sumpner’s test measures Copper Loss and Iron Loss.
    (b) Short Circuit test measures Copper Loss.
    (c) Open Circuit test measures Iron Loss.
    (d) Load test is used to find Total loss.

9. If the load is Fan then Torque-speed characteristics for it will be:
Torque directly proportional to (Speed)2
Where as for Hoist it will be:
Torque inversely proportional to (Speed).

10. Synchronous condensers are overexcited synchronous motor operating at No load. They are used to improve the power factor of Load.

 

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