Segmented commutator

 

Each paid of segments connects to each set of windings, only the desired windings are connected to the circuit

 

In any position, the armature will have windings in the optimum location.

 

Types of DC Motors

3 types series

Shunt (parallel)

Compound (mix of both)

 

Instead of magnets, we use coils to create magnetic field.

 

 

I series (universal motor - it works on D.c or A.c

 

Reversing. change field or armature direction but not both. 

 

Dc generators have few applications. 

 

Power tools

 

AC motors - induction motors

How do they work?

Field windings - AC current and magnetic field

No brushes

Current gets to the rotor by induction.

This current produces its own magnetic field. 

 

2 magnetic fields --> force

3∅ induction motor. 

3 sets of windings - one for each phase. 

The sets of windings take turns in producing the maximum magnetic field.

 

 a 3∅ induction motor creates a rotating magneticfield, The rotor has no electrical or mechanical connection with the stator. Currents are induced in the rotor by the rotating stator field. These currents produce there own magnetic field, the two fields (internal and external) interact producing a force.  

Unlike 3∅ motors that are self starting, 1∅ motors require a 'kick' to start the rotation. 

Once started the rotor will turn attracted to each pole in urn the stator field changes polarity. 

There are a number of ways of starting a 1∅ induction motor. 

 

1. split-phase motor

this has two sets of windings

run windings (main)

and start windings. (offset to one side)

For this to work we need a slight delay between start and run windings. 

 

But, these windings are all inductors and we know from CIVIL that inductors give us a lag.

Run windings - use heavy gauge wire R->low

Start windings - use fine wire R->high

 

 

2. capacitor start

Capacitor connected to start windings. From CIVIL we get a lead --> delay between start and run. 

Centrifugal switch again. Switches off start windings when motot is up to speed. 

 

3. shaded pole motor. 

- simple type of motor.

The shading ring is a simple inductor (coil with 1 turn)

CIVIL --> Lag 

 

4. universal motor

A.C but not an induction motor (same as series D.C motor)

 

When using ac, as the supply changes polarity

the magnetic field also changes

But so does the current in the armature, These cancel out and the motor turns in the same direction throughout the cycle. 

 

Rotors

There are two types, Cage rotor

and wound rotor

Which is more like a DC armature)

Thin wire used but very high number of turns so altough current is low due to high resistance of windings the magnetic field is strong because of the number of turns. 

 

Applications of 1∅ motors

Split phase:

Poor starting torque 

Maintains constant with carying loads

Belt drives - Production lines

Maintains constant

Speed with varying loads

 

Motor starting (speed control) 

2 problems, Starting high torque

 

 

 

 

 

Rotor resistance starter. 

Current in the rotor is restricted by adding resistors

This only works for wound rotors connecting to the resistance via sliprings

The resistance is removed from the circuit at a pre-determined speed. 

 

electronic starters (soft start)

Control Either voltage or current

_variable frequency control (used on larger motors) 

Variabe frequency control (larger motors)

Changing f Varies the reactance of the coils (windings)

 

 

Slip 

The speed of rotation for the ext al (stator) rotating magnetic field is called synchronous speed (Ns)

         f

Ns = p x60 

d - cycles per second

Ns - Revolutions per minute.

f - Supply frequency

p - number of pairs of poles 

 

Nr is the rotor speed

Nr is always less than Ns

Slip is the difference between Ns and Nr 

 

Slip = Ns-Nr

can be a decimal or a difference

0.04 - 4%