Presentation 2

• classified according to the manner in which their field flux is produced:

• Separately excited generator• Shunt generator: • Series generator• compounded generator

A simplified equivalent circuit of a DC generator, with RF combining the resistances of the field coils and the variable control

resistor

Separately Excited Generator

AL II

A separately excited DC generator is a generator whose field current is supplied by a separately external DC voltage source

VT = Actual voltage measured at the terminals of the generatorIL = current flowing in the lines connected to the terminals.EA = Internal generated voltage.IA = Armature current.

The terminal characteristic of a separately excited dc generator (a) with and (b) without compensating windings (EA = K)

• For DC generator, the output quantities are its terminal voltage and line current. The terminal voltage is VT = EA – IARA (IA = IL)• Since the internal generated voltage EA is independent of IA, the terminal characteristic of the separately excited generator is a straight line.

•When the load is supplied by the generator is increased, IL (and therefore IA) increase. As the armature current increase, the IARA drop increase, so the terminal voltage of the generator falls. (Figure (a) PREVIOUS SLIDE)

• This terminal characteristic is not always entirely accurate. In the generators without compensating windings, an increase in IA causes an increase in the armature reaction, and armature reaction causes flux weakening. This flux weakening causes a decrease in EA = Kω which further decreases the terminal voltage of the generator. The resulting terminal characteristic is shown in Figure (b) PREVIOUS SLIDE)

Major types of dc motors

• Self excited dc motor Series dc motor

Shunt dc motor Compound dc motor

• Separately excited dc motor • Permanent magnet dc motor

Series motors •Series motors connect the field windings in series with the armature. •well-suited for high-torque loads like power tools and automobile starters because of their high torque production and compact size. •Poor speed regulation,

Ea

Rf

M VT (dc supply)

Raia

)( faaaT RRiEV

La iinote :

aa IKKE 21

Shunt motors • Shunt motors use high-resistance field windings

connected in parallel with the armature. • Varying the field resistance changes the motor

speed. • Shunt motors are prone to armature reaction, a

distortion and weakening of the flux generated by the poles that results in commutation problems evidenced by sparking at the brushes.

• Installing additional poles, called interpoles, on the stator between the main poles wired in series with the armature reduces armature reaction.

Shunt motors

Ea VT (dc supply)

Raia

if

RfM

iL

)( aaaT RiEV

ffT

faL

RiV

iiinote

:

Compound motors • the concept of the series

and shunt designs are combined.

)( 2faaaT RRiEV

1

:

ffT

faL

RiV

iiinote

Ea VT (dc

supply)

Raia

if

Rf1M

iLRf2

Separately Excited Motor

• There is no direct connection between the armature and field winding resistance

• DC field current is supplied by an independent source

• (such as battery or another generator or prime mover called an exciter)

Separately Excited Motor (Cont)

• Circuit analysis:

nKniKC

pnZE fffa 60

2

Where p= no of pole pair n= speed (rpm) Z=no of conductor =Flux per pole (Wb) C= no of current/parallel path

=2p (lap winding) =2 (wave winding)

Ea

Ra Laia

M

Rf

VTVfLf

If

fff RiV

aaaT RiEV

KVL:

La iinote :

Permanent Magnet motors

• PMDC is a dc motor whose poles are made of permanent magnets.

• Do not require external field circuit, no copper losses

• No field winding, size smaller than other types dc motors

• Disadvantage: cannot produce high flux density, lower induce voltage

Speed Control for shunt motor and separately excited dc motor

• There are three variables that can influence the speed of the motor,

VIfRa

• Thus, there are three methods of controlling the speed of the shunt and separately excited dc motor,

• Armature terminal – voltage speed control• Field speed control• Armature resistance speed control

Speed Control for shunt motor and separately excited dc motor

• Armature resistance speed control- Speed may be controlled by changing Ra

- The total resistance of armature may be varied by means of a rheostat in series with the armature

- The armature speed control rheostat also serves as a starting resistor.

• Torque –speed characteristic

Ra1

nNLnn1

m

Ra2

Ra3

Ra1 < Ra2 < Ra3

n2n3

• Advantages armature resistance speed control:i. Starting and speed control functions may be combined in one

rheostatii. The speed range begins at zero speediii. The cost is much less than other system that permit control down to

zero speediv. Simple method

• Disadvantages armature resistance speed control :i. Introduce more power loss in rheostatii. Speed regulation is poor (S.R difference nLoaded & nno loaded)

iii. Low efficiency due to rheostat

• Field Speed Control• Rheostat in series with field winding (shunt or

separately ect.)• If field current, If is varied, hence flux is also

varied• Not suitable for series field

• Torque –speed characteristic

nNL3nn1

m

n2 n3 nNL2

nNL1

If1 < If2 < If3

1 < 2 < 3

Advantages field speed control:• Allows for controlling at or above the base speed• The cost of the rheostat is cheaper because If is small value

Disadvantages field speed control :• Speed regulation is poor (S.R difference nLoaded & nno

loaded)• At high speed, flux is small, thus causes the speed of the

machines becomes unstable• At high speed also, the machines is unstable mechanically,

thus there is an upper speed limit

Speed Control for shunt motor and separately excited dc motor

iii. Armature terminal – voltage speed control- Use power electronics controller

- AC supply rectifier- DC supply chopper

- Supply voltage to the armature is controlled- Constant speed regulation

nNL1n1

V3 < V2 < V1

n2n3 nNL2nNL3

Torque –speed characteristic

• Advantages armature terminal voltage speed control:• Does not change the speed regulation• Speed is easily controlled from zero to maximum safe

speed

• Disadvantages armature terminal voltage speed control :

• Cost is higher because of using power electronic controller