DC MOTORLovely Professional University

Magnetism

Opposites Attract / Likes Repel

Magnetic FieldsDirection of Field: North South

A Simple Electromagnet• A Nail with a Coil of Wire

• Q – How do we set up a magnet?

• A – The battery feeds current through the coil of wire. Current in the coil of wire produces a magnetic field (as long as the battery is connected).

Right Hand Rule # 1

Electric Currents Magnetism!

Right Hand Rule # 2

Electric Currents Magnetism!

Electromagnetism

Right Hand Rule # 3

• The Right Hand Rule is used to determine the direction of the force when the direction of the current and the direction of the magnetic field are known.

Forc

e

Current

Magnetic

Field

Right Hand Rule # 4Electric Currents & Magnetism Force

Force in a Conductor

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Force in a Conductor

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Simple DC Motor

Motor Basics

Motors convert electric energy to mechanical motion.

Either an AC or DC electrical energy source serves as the input to the motor.

Motors are powered by electricity, but rely on principles of magnetism to produce mechanical motion.

The result is mechanical motion of the output shaft, that is a rotation about or a translation along the shaft, provided the load carried by the shaft does not exceed the maximum load the motor is designed to carry.

Choosing a Motor

There are numerous ways to design a motor, thus there are many different types of motors.

The type of motor chosen for an application depends on the characteristics needed in that application.

These include:◦How fast you want the object to move, ◦The weight, size of the object to be moved,◦The cost and size of the motor,◦The accuracy of position or speed control needed.

Motor Parameters

The level of performance a motor can provide is described by its parameters. These include:

Rated Speed ◦Speed measured in shaft revolutions per minute

(RPM),  a way to specify how fast the motor turns.

Torque◦Rotational force produced around a given point, due

to a force applied at a radius from that point

Torque-Speed performance of a motor

Types of Motors

The different types of motors possess different operating characteristics.◦ Heavy Industrial applications: AC motors ◦ Mobile robotics : dc motor, dc servo motor, and stepper

motors

Brief overview of the operation characteristics of:◦ AC motors ◦ DC motors ◦ DC servo motors ◦ Stepper motors

DC Electric Motors

Electric Motors or Motors convert electrical energy to mechanical motion

Motors are powered by a source of electricity – either AC or DC.

DC Electric Motors use Direct Current (DC) sources of electricity:◦Batteries◦DC Power supply

Principle of How Motors Work:

Electrical current flowing in a loop of wire will produce a magnetic field across the loop.

When this loop is surrounded by the field of another magnet, the loop will turn, producing a force (called torque) that results in mechanical motion.

Practical DC Motors

Every DC motor has six basic parts –

axle,

rotor (a.k.a., armature),

stator,

commutator,

field magnet(s),

and brushes. For a small motor the magnets is made from permanent magnet

DC Machine Construction

20

DC motor stator with poles visible.

DC Motors – Components

• Field pole• North pole and south pole• Receive electricity to form

magnetic field

• Armature• Cylinder between the poles• Consists of coils of insulated wires wound around

an iron and so arranged that electric current are induced in these wires, when armature is rotating.

• Electromagnet when current goes through• Linked to drive shaft to drive the load

• Commutator• Form of rotating switch placed between armature and

external circuit.

• Reverse the current direction between the rotor and external circuit.

• Overturns current direction in armature.

• Brushes• To collect current from the commutator and supply it to

external load circuit.

How the Commutator Works

As the rotor turns, the commutator terminals also turn and continuously reverse polarity of the current it gets from the stationary brushes attached to the battery.

Controlling Motor Direction• To change the direction of rotation:

• Simply switch the polarity of the battery leads going to the motor (that is, switch the + and – battery leads)

+-

-+

Direction of Rotation

Brushed DC Motor Components

Diagram of a Simple DC Motor

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How Does an Electric Motor Work?

DC Motor Operation

• In a dc motor, the stator poles are supplied by dc excitation current, which produces a dc magnetic field.

• The rotor is supplied by dc current through the brushes, commutator and coils.

• The interaction of the magnetic field and rotor current generates a force that drives the motor

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Shaft

Brush

Coppersegment

Insulation

RotorWinding

N S

Ir_dcIr_dc/2

Rotation

Ir_dc/2

Ir_dc

12

3

4

5

6

7

8

Polewinding

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DC Motor Operation

• The magnetic field lines enter into the rotor from the north pole (N) and exit toward the south pole (S).

• The poles generate a magnetic field that is perpendicular to the current carrying conductors.

• The interaction between the field and the current produces a Lorentz force,

• The force is perpendicular to both the magnetic field and conductor

30NS Vdc

a

b

1

2

B

v v

Ir_dc

(a) Rotor current flow from segment 1 to 2 (slot a to b)

Vdc30

NS

Bv

v

a

b

1

2

Ir_dc

(b) Rotor current flow from segment 2 to 1 (slot b to a)

DC Motor Operation

• The generated force turns the rotor until the coil reaches the neutral point between the poles.

• At this point, the magnetic field becomes practically zero together with the force.

• However, inertia drives the motor beyond the neutral zone where the direction of the magnetic field reverses.

30NS Vdc

a

b

1

2

B

v v

Ir_dc

(a) Rotor current flow from segment 1 to 2 (slot a to b)

Vdc30

NS

Bv

v

a

b

1

2

Ir_dc

(b) Rotor current flow from segment 2 to 1 (slot b to a)

• To avoid the reversal of the force direction, the commutator changes the current direction, which maintains the counterclockwise rotation.

DC Motor Operation

• Before reaching the neutral zone, the current enters in segment 1 and exits from segment 2,

• Therefore, current enters the coil end at slot a and exits from slot b during this stage.

• After passing the neutral zone, the current enters segment 2 and exits from segment 1,

30NS Vdc

a

b

1

2

B

v v

Ir_dc

(a) Rotor current flow from segment 1 to 2 (slot a to b)

Vdc30

NS

Bv

v

a

b

1

2

Ir_dc

(b) Rotor current flow from segment 2 to 1 (slot b to a)

DC Motor

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DC Motor

The brush

DC Motor

2 pole motor

Animate

2 pole motor

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2 pole motor

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2 pole motor

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2 pole motor

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2 pole motor

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2 pole motor

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2 pole motor

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2 pole motor

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continue

3 pole DC motors

+ −

The coil for each poles are connected serially.

The commutator consist of 3 sector, consequently one coil will be fully energized and the others will be partially energized.

1

3 2

3 pole DC motors

animate

next

The commutator and the coil is arranged in such a way that the polarity of each pole is as shown

3 pole DC motors

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next

The commutator and the coil is arranged in such a way that the polarity of each pole is as shown

3 pole DC motors

animate

next

The commutator and the coil is arranged in such a way that the polarity of each pole is as shown

3 pole DC motors

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next

The commutator and the coil is arranged in such a way that the polarity of each pole is as shown

DC motors

• As the rotor is rotating, back emf (Ea) will be produced, the faster the rotor turn the higher Ea and the smaller Ia.

• The starting current of motors will be much higher then the rating current.

motorV Ea

Ia

DC motors

For big motors the magnet is made from coil and core. The current flowing in the coil is called If and the current flowing in the armature is called Ia.

The armature winding and the field winding are connected to a common power supply

Field winding Armature winding

• The armature winding and the field winding are often connected in series, parallel, or compound. The torque characteristic will be different for each connection.

SERIES DC MOTOR

Field and armature winding are series connected, this type of motor is called series DC motor

Shunt DC motors

Field and armature winding are parallel connected, this type of motor is called shunt DC motor

Compound DC MOTOR

Compound DC motor is DC motor having 2 field winding the first one is connected parallel to the armature winding and the other is connected series

DC MOTOR

Torque: T = KΦIa

•K is a constant•Φ magnetic flux • Ia is armature current

• Magnetic flux is constant if it is from permanent magnet

Starting a DC Motor

• If we apply full voltage to a stationary motor, the starting current in the armature will be very high (20-30 times the nominal load current) and we run the risk of:• Burning out the armature

• Damaging the commutator and brushes, due to heavy sparking

• Overloading the supply

• Breaking the shaft due to mechanical shock

• Damaging the driven equipment due to the sudden torque surge

• All DC motors must therefore be provided with a means to limit the

• starting current to reasonable values

Stopping a DC Motor

• There are three ways to brake a DC motor:

Mechanical (friction) braking

Dynamic braking

Plugging

Merits

• High starting torque

• Speed control over a wide range,both above and below normal speed

• Accurate seedless speed control

• Quick starting and stopping

Demerits

• High initial cost

• Increased operating and maintenance costs because of commutator and brush gear

Applications

•  Lathes, Drills, Boring Mills, Shapers, Spinning and Weaving Machines.

•  Elevators, air compressors, vacuum cleaners, hair drier

• Sewing machines, Presses