DC Machines

Question 1

Types of Generators

Answer 1

1. DC Generator (DYNAMO)
2. AC Generator (ALTERNATOR)

Question 2

Theory of Operation (1)

Basis for Operation of the Generator

Answer 2

Electric current flowing through a conductor creates a MAGNETIC FIELD around the CONDUCTOR

Michael Faraday demonstrated that this could be accomplished.

This is the basis for the OPERATION of the GENERATOR.

Question 3

Theory of Operation (2)

Electric Current created by Magnetic Field through Cardboard Tube

Answer 3

To show how Electric Current is created by Magnetic Field:
Several turns of wire are wrapped around a CARDBOARD TUBE and ends of CONDUCTOR connected to GALVANOMETER.

Bar Magnet is then moved through the tube; as magnet’s lines of flux are cut by turns of wire, galvanometer deflects from its zero position.

Question 4

Theory of Operation (3)

Magnet and Galvanometer Interactions

Answer 4

If magnet is at rest, the galvanometer shows NO CURRENT FLOW.

When the magnet is moved through the tube in the OPPOSITE DIRECTION, GALVANOMETER indicates a deflection in the OPPOSITE DIRECTION.

Question 5

What is a Simple DC GENERATOR consist of?

Answer 5

An armature coil with a single turn of wire.

Question 6

Theory of Operation (4)

Armature Coil and Direction of Current

Answer 6

ARMATURE COIL cuts across magnetic field to produce voltage.

If complete path is PRESENT, current will move through circuit in the CLOCKWISE direction

Question 7

Theory of Operation (5)

Armature Rotation with Commutator and its Brushes

Answer 7

As ARMATURE ROTATES in DC GENERATOR, commutator allows one brush to remain in contact with portion of loop that moves DOWNWARD through the flux lines.

The OTHER BRUSH to remain in contact with the portion of loop that moves UPWARD.

COMMUTATOR action produces pulsating DC VOLTAGE varying from ZERO to MAXIMUM and does it TWICE per revolution.

Question 8

DC Generator Construction

YOKE / FIELD FRAME

Answer 8

Outer frame of DC Generator is called the YOKE/FIELD FRAME

This provides MECHANICAL STRENGTH to the whole assembly; also CARRIES MAGNETIC FLUX produced by FIELD WINDING.

Yoke/Field Frame is made up of CAST IRON or STEEL.

Question 9

DC Generator Construction (2)

Magnetizing Force is produced by?

Answer 9

Magnetizing force inside a generator is produced by ELECTROMAGNET consisting of:
1. WIRE COIL (called as Field Winding / Field Coil); and
2. CORE (called as POLE / POLE SHOE FIELD POLE).

Question 10

DC Generator Construction (3)

Field Winding / Field Coil

Answer 10

Electromagnet that produces FLUX cut by the ARMATURE.

Composed of MANY TURNS OF WIRE that are securely fastened over IRON CORE of pole shoes.

Question 11

DC Generator Construction (3)

Field Poles

Answer 11

Joined to the yoke with help of bolts or welding.

Serve two purposes:
1. SUPPORT FIELD COILS
2. SPREAD OUT FLUX in AIR GAP UNIFORMLY

Question 12

DC Generator Construction (4)

Armature Assembly

Answer 12

Consists of ARMATURE CORE, ARMATURE COILS / WINDINGS, COMMUTATOR, and other associated mechanical parts.

Question 13

DC Generator Construction (4) - Armature Assembly

Armature Core

Answer 13

Considered as the ROTOR of the GENERATOR

Mounted on shaft rotating in bearings located in generator’s end frames.

Acts as a CONDUCTOR when rotated in the magnetic.

May be provided with air ducts for axial air flow for cooling purposes.

Question 14

DC Generator Construction (4) - Armature Assembly

Armature Coil / Armature Winding

Answer 14

Former wound copper coil rests in ARMATURE SLOTS.

Question 14

DC Generator Construction (4) - Armature Assembly

Armature Winding

Answer 14

Two methods:
1. Lap Winding
2. Wave Winding

Question 15

DC Generator Construction (4) - Armature Assembly

Commutator

Answer 15

Used to convert alternating current flowing in its armature into direct current at terminals.

Located at one end of armature and consists of wedge-shaped segments of HARD-DRAWN COPPER w/ one pair of segments for each armature coil.

Question 16

DC Generator Assembly (4.1) - Commutator

Brushes

Answer 16

Ride on surface of commutator; acts as the ELECTRICAL CONTACT between ARMATURE COILS and an EXTERNAL CIRCUIT

Made of high-grade carbon and held in place by spring-loaded brush holders insulated from the frame.

Question 17

Field Excitation

Answer 17

DC Generators have TWO MAIN CATEGORIES of FIELD EXCITATION USED:

1. SEPARATELY EXCITED. Field coils energized or excited from INDEPENDENT external DC source, such as a BATTERY.
2. SELF-EXCITED. GENERATOR supplies OWN EXCITATION; field coils energized from CURRENT produced by generator itself.

Question 18

Field Excitation (2)

Field Windings

Answer 18

1. Series-wound. Field winding IN SERIES with ARMATURE WINDING.
2. Shunt-wound. Field winding IN PARALLEL with ARMATURE WINDING.
3. Compound-wound. Combination of SERIES and SHUNT winding. Used more extensively as it can be designed w/ WIDE VARIETY of CHARACTERISTICS.

Question 19

Field Excitation (2.1)

Compound Winding Shunts

Answer 19

1. Short-shunt. Field IN PARALLEL only with armature.
2. Long-shunt Field IN PARALLEL wiith BOTH ARMATURE and SERIES FIELD.

Question 20

DC Motors

Answer 20

Mechanical construction VERY SIMILAR w/ Generators, function is DIFFERENT.

Function: DEVELOP a turning effort or TORQUE to produce mechanical rotation.

Question 21

Function of DC Generator

Answer 21

Generate a voltage when CONDUCTORS move through a FIELD.

Question 22

Parts of a DC Motor

Answer 22

1. Stator
2. Rotor
3. Armature Winding
4. Field Winding
5. Commutator
6. Brushes
7. Poles
8. Slot/Teeth
9. Motor Housing

Question 23

DC Motor Parts (2)

Stator, Rotor, and Armature Winding

Answer 23

STATOR: Carries FIELD WINDING and POLES; is a HOLLOW cylinder which constitues magnetic circuit or core of the machine with the ROTOR.

ROTOR: Carries ARMATURE WINDING; cylindrical in shape, armature is LOAD CARRYING MEMBER.

ARMATURE WINDING: LOAD CARRYING MEMBER mounted on the ROTOR, which rotates in the MAGNETIC FIELD set up at stationary winding (Field Winding). Composed of a NUMBER OF COILS in a series.

Armature Winding is a CONTINUOUS WINDING, has no beginning or end.

Question 24

# DC Motor Parts (2) Field Winding, Commutator, and Brushes

Answer 24

***FIELD WINDING*:** Exciting system which may be an **ELECTRICAL WINDING or PERMANENT MAGNET** located on the **STATOR.** ***COMMUTATOR*:** Rectifies the **INDUCED VOLTAGE and CURRENT** in the armature, which are both AC. ***BRUSHES*:** **Conducting carbon graphite spring** loaded to **ride** on commutator, acting as **INTERFACE** between **EXTERNAL CIRCUIT** and **ARMATURE WINDING**.

Question 25

# DC Motor Parts (2) Poles, Slot/Teeth, and Motor Housing

Answer 25

***POLES:*** Provided to place **FIELD WINDING**; number determined by **VOLTAGE** and **CURRENT ratings** of the machine. ***SLOT/TEETH:*** **Magnetic material** used for **MECHANICAL SUPPORT**, **protection** from **ABRASION**, and further **ELECTRICAL INSULATION**. ***MOTOR HOUSING:*** Provided to support **IRON CORE**, **BRUSHES**, and **BEARINGS**.

Question 26

# DC Motors (3) **TORQUE**

Answer 26

Starting **TWIST or FORCE** producing **ROTATION** in a motor. Rotation necessary to get **MOTORS STARTED.** Has four types: 1. **FULL-LOAD TORQUE** 2. **PULL-UP TORQUE** 3. **BREAKDOWN TORQUE** 4. **LOCKED-ROTOR TORQUE**

Question 27

# DC Motors (3.1) - Types of Torque Full-Load and Pull-Up

Answer 27

**FULL-LOAD TORQUE:** Necessary to produce **RATED HORSEPOWER** at **FULL-LOAD SPEED**. **PULL-UP TORQUE:** Required to **BRING MOTOR** to **RATED SPEED**.

Question 28

# DC Motors (3.1) - Types of Torque Breakdown and Locked-Rotor Torque

Answer 28

**BREAKDOWN TORQUE:** **Maximum torque** the motor will **develop w/ rated voltage**, applied at **rated frequency** with **NO ABRUPT DROP in SPEED.** **LOCKED-ROTOR TORQUE:** Developed when **full power** is **APPLIED** when **rotor is stationary**; the **starting torque or twist** that gets **rotor running**. ## Footnote **LOCKED-ROTOR TORQUE** is the **most common type of torque** used when **wiring** electric motors.

Question 29

**Types of DC Motors**

Answer 29

1. **Shunt Motor** 2. **Series Motor** 3. **Compound Motor**

Question 30

# DC Motors (4) - Types of Motors **Shunt Motor**

Answer 30

**Most common type** of DC Motor **Connected in the same way** as the **shunt generator.**

Question 31

# DC Motors (4) - Types of Motors **Series Motor**

Answer 31

Field of this motor is **connected in series** with the **armature**. **Speed varies** from very **high speed at light loads** to **lower speed at full loads**. ## Footnote Suitable for **starting with heavy connected loads**, as it develops a **high torque** and **operates at low speed**.

Question 32

# DC Motors (4) - Types of Motors **Compound Motor**

Answer 32

**Combines operating characteristics** of the **shunt** and **series** motors. May be **operated safely at no load**; as **load** is **added**, **speed decreases**, and **torque** is **greater** than **shunt motor**.