dc motor basics - abb · pdf filehelp dc motor highlights dc motors are well known for full...

DC Motor BasicsABB Motors

E-Learning, DC drives

© ABB Group March 15, 2010 | Slide 1

DC_MOTOR_BASICS_R0201

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Objectives

This training module covers:

DC motor construction

Magnetic force and flux

Further windings

The circuit diagram

Typical DC motor characteristics



http://search.abb.com/library/ABBLibrary.asp?DocumentID=9AKK101130D4959&LanguageCode=en&DocumentPartId=&Action=Launch

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DC Motor highlights

DC motors are well known for

Full torque from zero speed

Wide field weakening range

Excellent control behavior

Correlation for motor control

Torque: Field current and Armature current

Power: Armature voltage and current

DC motors have half size comparedto Standard AC motors




Help

Torque and power compared to motor size

Power is equal

Torque is equal

P = 11 kWn = 1140 min-1

M = 76 Nm

P = 11 kWn = 960 min-1

M = 110 Nm

P = 11 kWn = 730 min-1

M = 150 Nm

P = 22 kWn = 1440 min-1

M = 150 Nm

P = 15 kWn = 960 min-1

M = 150 Nm

P = 11 kWn = 730 min-1

M = 150 Nm© ABB Group March 15, 2010 | Slide 4



Help

Stator of a DC machine

Stator is the stationary part

Main poles as field winding

Further windings

Interpole

Compensation

eliminate unwanted effects




Help

Rotor of a DC machine

Shaft as center axis

Armaturewinding

Commutator connected with windings




Help

Commutator of a DC machine

Commutator is used to transfer energy

Fins are connected with windings

Brushes provide electrical contact

Neutral zone is perpendicular to main field




Help

Compact DC machine

Typical ABB DMI machine

Motor

Generator

Shaft

Terminal board




Help

Drawing of a DMI machine

Terminal box includes connectors

Back side of the machine with commutator, analog tacho or encoder

Middle part with windings

Front side with shaft output




Help

Typical ABB DMI motors

Air cooled variant



Water cooled variant


Help

Magnetic force

Permanent magnet generates magnetic field

Current flowing conductoris affected by force

Right-Hand-Rule




Help

The Right-Hand-Rule

Right hand index along the direction of current (I)

Middle finger in the direction of magnetic flux (B)

Direction of force is along the thumb (F)




Help

Magnetic field in a DC machine

Stator of a 2 pole machine

Pole windings

Transfer magnetic principle to a DC machine

Field winding generates an electro-magnetic field

FI

FI




Help

Expand the model

Replace single wire with a wire loop

Many such loops result in a rotor

Electric current causes rotor to turn




Help

Basic construction of a DC motor

Magnitude and direction of force remains constant

Every half turn the current will be reversed

Process of switching current direction is called commutation

Brushes are attached to two external wires




http://hades.mech.northwestern.edu/wiki/index.php/Image:Motor_Commutators.jpg

Help

Rotation motion and torque

Conductors have to be implemented

Current in conductors is required

Force transferred to torque

Brushes

FI

FI




Help

Interpole windings

Inductance in armature circuit affect the electro-magnetic-field

Interpole windings generate an opposite field

Smoother commutation

n

IA

nN

-1/2 Ia

1/2 Ia

Time

Ia




Help

Reaction inside the poles

Interpole windings neutralize flux in rotor

Second unwanted flux in the poles

Uncompensated behavior




Help

Effect of compensation windings

Neutralizes effect of unwanted flux

Windings carry rotor current

Operation at higher loads




Help

Magnetic flux

Without compensating winding



Compensating winding


Help

Compensation winding

0

20

40

60

80

100

120

Speed

Pow

er (%

)

Uncomp

Comp

1:5

1:3




Help

Sum up windings

Field winding

Create electro-magnetic field

Used for flux

Interpole winding

Prevent uneven field


Prevents magnetic saturation

Increases field weakening range




Help

Circuit diagram

Field circuit

Armature circuit

Equations:

DCMotor

IA

If

UA UEMF

Φ

Ri

( )( )φ

φ

φ

××−

=

=

××=

kIRUn

If

IkT

AiA

f

Aoutput

K

K

K

.3

.2

.1




Help

Characteristics of a DC machine

Commutation limit

Field weakening factor:

Field weakening

nb nmax

UN

IN

IN

TN

PN

UA

IA

If

T

Pn

max

1nn

fbase=

max::1 nnf base=



Armature current

Armature voltage

Field current

Output [kW]

Torque


Help

Summary

Key points of this module are:

Construction of a DC motor

Magnetic force and flux inside the machine

Further windings to optimize the machine performance

The circuit diagram of a DC machine

The typical DC motor characteristics




Help

Additional information

Armature windingWindings around the rotor which are connected with the commutator

Field windingWindings around the stator which generate the main field

Interpole windingWindings vertically to the main field

Compensation windingWindings inside the pole to prevent magnetic saturation and increase field weakening range

BrushesCoal conductors to transfer energy to the rotor

Neutral zoneIs an axis perpendicular to the main field

CommutatorFins which are connected with armature windings

Field weakeningSpeed greater base speed with reduced field current




1

DC Motor BasicsABB Motors

E-Learning, DC drives



Welcome to the DC motor basics training module for ABB DC Drives.

If you need help navigating this module, please click the Help button in the top right-hand corner. To view the presenter notes as text, please click the Notes button in the bottom right corner.

2

Help

Objectives

This training module covers:

DC motor construction

Magnetic force and flux

Further windings

The circuit diagram

Typical DC motor characteristics



•After completing this module, you will know about

• DC motor construction

• Magnetic force and flux

• Further windings

• The circuit diagram and

• Typical DC motor characteristics

3

Help

DC Motor highlights

DC motors are well known for

Full torque from zero speed

Wide field weakening range

Excellent control behavior

Correlation for motor control

Torque: Field current and Armature current

Power: Armature voltage and current

DC motors have half size comparedto Standard AC motors



DC motors are used in combination with DC drives. They have the following features:

• DC motors are well known for full torque from zero speed, the wide field weakening range and excellent control behavior.

• Correlation for motor control: Field current and armature current are responsible for motor torque. The armature voltage and current are indicators of motor power.

• DC motors are half the size of standard AC motors.

4

Help

Torque and power compared to motor size

Power is equal

Torque is equal

P = 11 kWn = 1140 min-1

M = 76 Nm

P = 11 kWn = 960 min-1

M = 110 Nm

P = 11 kWn = 730 min-1

M = 150 Nm

P = 22 kWn = 1440 min-1

M = 150 Nm

P = 15 kWn = 960 min-1

M = 150 Nm

P = 11 kWn = 730 min-1

M = 150 Nm© ABB Group March 15, 2010 | Slide 4


The image shows the torque and power compared to the motor size.

• In case of constant motor power, the motor size could be different. The first line of the image shows three motor types with the same motor power meanwhile other motor data are different, e.g. torque and speed.

• Motor types with equal torque have the same size even though the motor power is different.

• The conclusion from this small experiment is: The motor size is dependent on the motor torque and is independent of motor power.

5

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Stator of a DC machine

Stator is the stationary part

Main poles as field winding

Further windings

Interpole

Compensation

eliminate unwanted effects



• Let’s start this module with an explanation of the stator of a DC machine.

• The stator is the stationary part of a DC machine. It includes the "main poles" which are represented by field winding and metal to create the magnetic flux. Further windings in a DC machine are the interpole and the compensation winding. These additional windings are used to eliminate unwanted physical effects in the stator.

• The stator, which is shown in the image, includes 4 main poles. A main pole can be identified by its thick winding. The small windings through the main poles are the interpole and compensation windings which are in series with the armature winding.

6

Help

Rotor of a DC machine

Shaft as center axis

Armaturewinding

Commutator connected with windings



The next part is about the rotor of a DC machine:

The mobile part of a DC machine is called a rotor. A typical rotor for a DC machine is shown in the picture.

The shaft of the rotor is a center axis which is mounted in the stator. The rotor winding, which is connected with the commutator, is placed around the shaft. The commutator is used to transfer the electrical energy from stator to rotor. It is a very delicate part of the rotor which has to be serviced regularly.

The load will be connected with the shaft on the "A-side". A speed measurement instrument can be connected with the "B-side" of the shaft. In some cases both sides of the shaft can be used to connect it with the load or for tandem motor configuration.

7

Help

Commutator of a DC machine

Commutator is used to transfer energy

Fins are connected with windings

Brushes provide electrical contact

Neutral zone is perpendicular to main field



On this slide, the commutator of a DC machine has to be examined more closely.

The commutator is used to transfer energy from the stationary part to the rotating part. Therefore, the fins are connected with the armature windings. It is produced in copper to reduce the transient resistor between the rotating and stationary part.

Brushes provide electrical contact between the rotating and the stationary parts. Coal brushes are very good for transferring energy. Each pole in a machine needs a separate brush. To distribute the current from one pole, multiple brushes are mounted in a line.

The commutator requires a certain current load to get an optimum operation temperature. If the load current is too small, then the temperature is too low and parallel connected brushes have to be removed.

The neutral zone is vertically to the main field. This zone has to be adjusted during commissioning to achieve optimal control.

8

Help

Compact DC machine

Typical ABB DMI machine

Motor

Generator

Shaft

Terminal board



The individual parts from the previous slides form to create the compact DC machine.

This machine can be used as a motor which drives the load or as a generator which generates energy for the supply network.

The picture shows the mechanical construction of a DC machine. The shaft is mounted between bearings which fix the rotor in the correct position. Between the poles and the rotor is a small air gap to allow a freewheeling of the rotor.

The terminal board, used for making electrical connections to the DC converter, is mounted on the top or on one side of the machine. Often DC machines have a motor-fan which is used to cool the rotor.

9

Help

Drawing of a DMI machine

Terminal box includes connectors

Back side of the machine with commutator, analog tacho or encoder

Middle part with windings

Front side with shaft output



The picture shows a drawing of an ABB DMI machine.

The terminal box includes connectors for the armature, field and in some machines a clixon which is used for temperature protection.

The back side of the machine includes the commutator on the inside and has the option of connecting an analog tacho or an encoder on the outside.

The middle part of the machine includes the windings of the stator. This is the field, interpole and compensation winding.

The front side of the machine is used for shaft output.

10

Help

Typical ABB DMI motors

Air cooled variant



Water cooled variant

• The picture shows typical ABB DMI motors.

• On the right hand side is the air cooled DC machine with the motor fan mounted on the back side. This is the most used variant for DC machines.

• Another option is the water cooled variant. More investment is needed for the water cooled variant.

• A third option is to have an air-air heat exchanger. The method of cooling is determined by the environment and the location of the motor.

11

Help

Magnetic force

Permanent magnet generates magnetic field

Current flowing conductoris affected by force

Right-Hand-Rule



In the next slides, the principle functionality of a DC machine will be explained.

Let’s keep it easy with a physical experiment about magnetic force. The starting point is a magnetic field generated from a permanent magnet. The picture shows the north pole on the top and the south pole on the bottom of the picture. Between these poles are streamlines of the field.

Inside the field is a conductor which will be affected by a force. The conductor is affected by a force depending on the direction in which the current in the conductor is flowing.

The right-hand-rule is essential for the direction of magnetic force!

12

Help

The Right-Hand-Rule

Right hand index along the direction of current (I)

Middle finger in the direction of magnetic flux (B)

Direction of force is along the thumb (F)



Fundamental is the "Right-Hand-Rule". The "Right-hand-Rule" states that if you point your right index finger along the direction of current (marked with I) and your middle finger in the direction of magnetic flux (marked with B), the direction of force is along the thumb (marked with F).

13

Help

Magnetic field in a DC machine

Stator of a 2 pole machine

Pole windings

Transfer magnetic principle to a DC machine

Field winding generates an electro-magnetic field

FI

FI



The next step is to transfer this physical experiment to a DC machine. A stationary magnetic field is required to generate mechanical force and rotation.

The picture shows the principle construction of the stator of a two pole DC machine. Around the poles are windings which generate a north and a south pole. These windings are called "field windings" which generate the electro-magnetic-field in a DC machine. Normally, this electro-magnetic-field is constant and supplied by the field current. The field which is created from field winding is called the main field.

14

Help

Expand the model

Replace single wire with a wire loop

Many such loops result in a rotor

Electric current causes rotor to turn



The next step is to expand the model.

• Replace the single wire with a wire loop construction.

• Many of these loops result in a rotor which is a free moving part.

• Electric current around the wire causes the rotor to turn.

• Conclusion: When electric current (marked with I) passes through a coil in a magnetic field (marked with B), the magnetic force (marked with F) produces torque which turns the DC motor.

15

Help

Basic construction of a DC motor

Magnitude and direction of force remains constant

Every half turn the current will be reversed

Process of switching current direction is called commutation

Brushes are attached to two external wires



The Result is the basic construction of a DC motor.

• The magnitude and direction of the forces on the wires remain approximately constant. However, the resultant torque varies with the angle.

• To maintain nearly constant torque on the rotor, we can reverse the current through the coil every half turn.

• The process of switching the current direction is called commutation. To switch the direction of the current, DC motors use brushes and commutators.

• The brushes are attached to the motor's two external wires and the commutator segments slide over the brushes, so that current through the coils switches at appropriate angles.

16

Help

Rotation motion and torque

Conductors have to be implemented

Current in conductors is required

Force transferred to torque

Brushes

FI

FI



The aim of the construction of a DC machine is a torque which can be used to drive a load.

To get this torque, conductors have to be implemented into the electro-magnetic-field. Without any current flowing through these conductors there is no effect visible.

The next step is to supply the conductors with current which is called "armature current". The volume of this current is responsible for the generated magnetic force. If we transfer this force to a rotation motion we can call it "torque". The armature windings are mounted on a rotating shaft which ends at the commutator.

A problem with a DC machine is the transfer of current to the rotor. Brushes supply the rotor winding with current in the correct direction.

17

Help

Interpole windings

Inductance in armature circuit affect the electro-magnetic-field

Interpole windings generate an opposite field

Smoother commutation

n

IA

nN

-1/2 Ia

1/2 Ia

Time

Ia



Interpole windings have the following effect:

The inductance in the armature circuit tries to counteract a change of the current with an electro-magnetic-field. This is marked in the picture with the dashed red line.

The interpole windings create an electro-magnetic-field in the opposite direction. This is marked in the picture with the dashed blue line.

The result of these electro-magnetic-fields in the opposite direction is an intermediate characteristic curve. This causes a smoother commutation and avoids electrical brush sparking.

18

Help

Reaction inside the poles

Interpole windings neutralize flux in rotor

Second unwanted flux in the poles

Uncompensated behavior



There is another reaction inside the poles which is explained in the following:

The interpole windings have only neutralized the unwanted flux inside the rotor. This has been explained in the previous slides.

But there is still unwanted flux in the poles. This is marked in the picture with red arrows.

At high loads, this flux will lead to magnetic saturation of the metal at the poles. The reason for this is that the flux is not distributed uniformly in the poles. This will be shown in the picture at narrower flux arrows.

This phenomenon is called uncompensated behavior.

19

Help

Effect of compensation windings

Neutralizes effect of unwanted flux

Windings carry rotor current

Operation at higher loads



The effect of compensation windings can be seen in the picture. They are located around the poles.

The compensation winding neutralizes the effect of the unwanted flux in the poles.

These windings also carry the same current as the rotor windings.

As a result, the DC machine can be operated with higher loads without reaching magnetic saturation.

20

Help

Magnetic flux

Without compensating winding



Compensating winding

The magnetic flux of a DC machine with and without compensation winding is shown in these pictures.

Without compensation winding there is a visible field shifting in the poles. This effect leads to an uneven disposition of the field.

This can be compared with the picture on the right of the magnetic flux of a compensated machine. In this picture, the field in the pole is arranged because of compensation windings.

21

Help


0

20

40

60

80

100

120

Speed

Pow

er (%

)

Uncomp

Comp

1:5

1:3



The compensation winding directly influences the field weakening range.

An uncompensated motor is limited in the working range. The maximum field weakening factor is 1:3. A higher field weakening factor reduces the motor power.

A machine with implemented compensation winding can be used up through a field weakening factor of 1:5.

22

Help

Sum up windings

Field winding

Create electro-magnetic field

Used for flux

Interpole winding

Prevent uneven field


Prevents magnetic saturation

Increases field weakening range



Let’s sum up the windings in a DC machine.

• The field winding is located in the stator and used to create the electro-magnetic field in the DC machine.

• The interpole winding prevents an uneven field in the rotor.

• The compensating winding creates a uniform electro-magnetic field in the stator and prevents magnetic saturation in the main pole.

• A motor equipped with compensation winding has a larger field weakening range compared to an uncompensated motor. A higher torque is also needed.

23

Help

Circuit diagram

Field circuit

Armature circuit

Equations:

DCMotor

IA

If

UA UEMF

Φ

Ri

( )( )φ

φ

φ

××−

=

=

××=

kIRUn

If

IkT

AiA

f

Aoutput

K

K

K

.3

.2

.1



The circuit diagram of a DC machine is shown in the picture. Basically, the dc machine has got two separate circuits which represent the armature and the field circuit.

• The field circuit characterizes a high inductance. Thus it is sufficient to draw the field circuit only as an inductance. If a field current flows through the windings, the magnetic flux is generated.

• The armature circuit characterizes an inductance and a resistor in series. Armature current influences the behavior of the DC machine. The voltage in the DC motor is called EMF voltage.

• Three equations apply to the DC machine.

• The motor torque will be calculated by the machine constant, multiplied by the flux and the armature current. The flux depends on the field current and the machine construction. So the motor speed is proportional to the armature voltage minus the voltage which drops at the resistor, divided by the machine constant and the flux.

24

Help

Characteristics of a DC machine

Commutation limit

Field weakening factor:

Base speed Field weakening

nb nmax

UN

IN

IN

TN

PN

UA

IA

If

T

Pn

max

1nn

fbase=

max::1 nnf base=



Armature current

Armature voltage

Field current

Output [kW]

Torque

The characteristics of a DC machine are shown in the picture. On the x-axis the motor speed is plotted.

First, the normal speed range up to base speed should be discussed. In this area the armature voltage and the motor power is proportional to motor speed if the field current is constant. Regarding the armature current, it is proportional to motor torque.

If the motor speed is greater than base speed, the motor will work in field weakening mode. In field weakening, the armature voltage will be constant but the field current has to be decreased to increase the motor speed. Note that the torque is decreasing in this area. If the motor is very deep in field weakening range it is necessary to decrease the armature current, otherwise there can be problems with the commutation. In this case electrical brush sparking could appear.

The ratio of field weakening will be calculated by dividing the base speed by the maximum speed.

The DCS800 has parameters which can be used to set the speed depending on current limitation.

25

Help

Summary

Key points of this module are:

Construction of a DC motor

Magnetic force and flux inside the machine

Further windings to optimize the machine performance

The circuit diagram of a DC machine

The typical DC motor characteristics



The key points of this module are:

• Construction of a DC motor

• Magnetic force and flux inside the machine

• Further windings to optimize the machine performance

• The circuit diagram of a DC machine and

• The typical DC motor characteristics

26

Help

Additional information

Armature windingWindings around the rotor which are connected with the commutator

Field windingWindings around the stator which generate the main field

Interpole windingWindings vertically to the main field

Compensation windingWindings inside the pole to prevent magnetic saturation and increase field weakening range

BrushesCoal conductors to transfer energy to the rotor

Neutral zoneIs an axis perpendicular to the main field

CommutatorFins which are connected with armature windings

Field weakeningSpeed greater base speed with reduced field current



dc motor basics - abb · pdf filehelp dc motor highlights dc motors are well known for full...

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