DC Machines

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Basics of a Electric Motor

DC Machines

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A Two Pole DC Motor

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A Four Pole DC Motor

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Armature of a DC Motor

DC Machines

Significant Features of DC Machines • Conventional DC generators are being replaced by the

solid state rectifiers where ac supply is available.

• The same is not true for dc motors because of– Constant mechanical power output or constant torque

– Rapid acceleration or deceleration

– Responsiveness to feedback signals

• 1W to 10,000 hp

• Applications – in electric vehicles to extend their range and reduce vehicle weight, in steel and aluminum rolling mills, traction motors, electric trains, overhead cranes, control devices, etc.

Introduction

Electromagnetic Energy Conversion:

1. When armature conductors move in a magnetic field produced by the current in stator field winding, voltage is induced in the armature conductors.

2. When current carrying armature conductors are placed in a magnetic field produced by the current in stator field winding, the armature conductors experience a mechanical force.

These two effects occur simultaneously in a DC machine whenever energy conversion takes place from electrical to mechanical or vice versa.

Constructional Features of DC Machines

• Commutator along with the armature on the rotor

• Salient-pole on the stator and, except for a few smaller machines, commutating poles between the main poles.

• Field windings (as many as 4):– Two fields that act in a corrective

capacity to combact the detrimental effects of armature reaction, called the commutating (compole or interpole) and compensating windings, which are connected in series with the armature.

– Two normal exciting field windings, the shunt and series windings

Schematic Connection Diagram of a DC Machine

Armature Reaction

If a load is connected to the terminals of the dc machine, a current will flow in its armature windings. This current flow will produce a magnetic field of its own, which will distort the original magnetic field from the machine’s field poles. This distortion of the magnetic flux in a machine as the load is increased is called the armature reaction.

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Armature Reaction(AR)

• AR is the magnetic field produced by the armature current

•AR aids the main flux in one half of the pole and opposes the main flux in the other half of the pole

The net effect is the reduction of the field current

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Minimizing Armature Reaction

•Since AR reduces main flux, voltage in generators and torque in motors reduces with it. This is particularly objectionable in steel rolling mills that require sudden torque increase.

•Compensating windings put on polefaces can effectively negate the effectof AR. These windings are connectedin series with armature winding.

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Minimizing commutation problems•Smooth transfer of current during commutation is hampered by a) coil inductance and b) voltage due to AR flux in the interpolar axis. This voltage is called reactance voltage.

•Can be minimized using interpoles. Theyproduce an opposing field that cancels out the AR in the interpolar region. Thus this winding is also connected in series with the armature winding.

Note: The UVic lab motors have interpoles in them. This should be connected in series with the armature winding for experiments.

Equivalent Circuit of a DC Machine

aaat

fff

RIEV

RIV

Ia_gen

If

Vf VtRf

+

- Ea

+

-

Ia_mot

Ra

+

Ia

If

VtRf

Ea

-

IL

Ra

+

+

-

Generated emf and Electromagnetic Torque

aaat

fff

RIEV

RIV

mdaa KE

adae IKT

meaaem TIEP

Voltage generated in the armature circuit due the flux of the stator field current

Electromagnetic torque

Ka: design constant

Motor: Vt > Ea

Generator: Vt > Ea

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Magnetization Curve

maa KE

•Flux is a non-linear function of field current and

hence Ea is a non-linear function of field current

•For a given value of flux Ea

is directly proportional to m

Types of DC Machines

Both the armature and field circuits carry direct current in the case of a DC machine.

Types:

Self-excited DC machine: when a machine supplies its own excitation of the field windings. In this machine, residual magnetism must be present in the ferromagnetic circuit of the machine in order to start the self-excitation process.

Separately-excited DC machine: The field windings may be separately excited from an eternal DC source.

Shunt Machine: armature and field circuits are connected in parallel. Shunt generator can be separately-excited or self-excited.

Series Machine: armature and field circuits are connected in series.

Separately-Excited and Self-Excited DC Generators

If

IL

If

DC Supply VtRf

+

- Ea

+

-

Ra

+

Ia

VtRf

Ea

-

IL

Ra

+

Separately-Excited Self-Excited

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Shunt Field Coil Armature

RA

Shunt Excited DC Machine

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Series Field Coil

Armature

RA

Series Excited DC Machine

26


RA

Compound Excited DC MachineSeries Field Coil

•If the shunt and series field aid each other it is called a cumulativelyexcited machine•If the shunt and series field oppose each other it is called a differentiallyexcited machine

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Field CoilArmature

Ra

Vf

Separately Excited DC Generator

+

-

Rf

Vt

+

-

Field equation: Vf=RfIf

If

Ia

+

-

Ea

Armature equation: Vt=Ea-IaRa

Vt=IaRL, Ea=Kam

RL

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Shunt Generators

Field equation: Vt=Rf If

Rf=Rfw+Rfc

Armature equation: Vt=Ea-Ia Ra

Vt=(Ia – If) RL, Ea=Kam


Ra

RL

If IaIa – If

Vt

+

-

Rfc

Ea

+

-Field coil has Rfw : Implicit field resistance

Example 1

A 100-kW, 250-V DC shunt generator has an armature resistance of 0.05 and field circuit resistance of 60 . With the generator operating at rated voltage, determine the induced voltage at (a) full load, and (b) half-full load.

Solution to Example 1(a) At full load,Vt=Ea-IaRa

If=250/60=4.17 AIL_FL=100,000/250=400 AIa=IL_FL+If=400+4.17=404.17 AEa=Vt+IaRa=250+404.17*0.05=270.

2 V

(b) At half load, If=250/60=4.17 A IL_HL=50,000/250=200

A

Ia=IL_HL+If=200+4.17=204.17 A

Ea=Vt+IaRa=250+204.17*0.05=260.2 V

Armature

Ra

RL

If IaIa – If

+

-

Rfc

Ea

+

-

Example 2

Example 2-solution

Field CoilArmature

Ra

Vf+

-

Rf

Vt

+

-

If

Ia

+

-

Ea

RL

DC Generator Characteristics

In general, three characteristics specify the steady-state performance of a DC generators:

1. Open-circuit characteristics: generated voltage versus field current at constant speed.

2. External characteristic: terminal voltage versus load current at constant speed.

3. Load characteristic: terminal voltage versus field current at constant armature current and speed.


Open-circuit and load characteristics

The terminal voltage of a dc generator is given by

aa

mf

aaat

RI

dropreactionArmatureIf

RIEV

,


It can be seen from the external characteristics that the terminal voltage falls slightly as the load current increases. Voltage regulation is defined as the percentage change in terminal voltage when full load is removed, so that from the external characteristics,

External characteristics100

V

VEregulationVoltage

t

ta

Self-Excited DC Shunt Generator

Schematic diagram of connection

Open-circuit characteristic

Maximum permissible value of the field resistance if the terminal voltage has to build up.

Comparison between the Shunt and Series Connected DC Motor

Power Division in DC Machines

Input from

prime-mover

Elec-magnetic

Power =EaIa

Arm. terminal

power = Vta Ia

Output power

= Vt IL

No-load rotational loss (friction+windage+core)+stray load loss

Arm. copper loss Ia

2Ra+brush contact loss

Series field loss IL2Rs

+shunt field loss If2Rf

Input power from

mains =Vt IL

Elec-magnetic

Power =EaIa

Arm. terminal

power = Vta Ia

Output available

at the shaft

No-load rotational loss (friction+windage+core)+stray load loss

Arm. copper loss Ia

2Ra+brush contact loss

Series field loss IL2Rs

+shunt field loss If2Rf

DC Motor

DC Generator

Efficiency

InputPowerLosses

InputPowerLossesInputPower

InputPowerOutputPower

1

The losses are made up of rotational losses (3-15%), armature circuit copper losses (3-6%), and shunt field copper loss (1-5%). The voltage drop between the brush and commutator is 2V and the brush contact loss is therefore calculated as 2Ia.

DC Machines Formulas

Example 3

Example 3-solution

Problem 9-1 to 9-5

Solution to Problem 9-1 (Page 621)

Problem 9-13 (Page 623)

The End

DC Machines

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