Dc motor

What is motor

Principe of working

The action is based on the principle that whenever an current carrying conductor is paced In a magnetic field it experiences a mechanical force.

F=BIL

Actual working

Basic constuction

Significance of back e.m.f

• Concept of Back emf

• Factors on which back emf depends

Torque equation

• Let Ta be the torque developed by the armature of a motor running at N rps if Ta is in N/m then ,

Power developed =Ta 2 л N w….(1)

We also know that electrical power converted into mech.

Power in the armature =Eb Ia w……(2)

Equating 1&2,

We get Ta 2 л N= Eb Ia w

Eb =Φ л Z (P/A) Ia OR Ta = Φ л Z (P/A) I0 (1/2 л)=0.159 N-m

Ta =0.159 Φ л Z (P/A) Ia

Types of dc motors

• For series motor

•

Characteristics of DC motor

Current Ia Current Ia Torque

Shunt Motor Characteristics

Compound motor characteristics

Starting of dc motors

• Necessity of starter

• Different types of starter

1. 3point starter

2. 4point starter

• Ia= V-Eb/Ra

Need Of Starter

3 Point Starter

OLC

NVC

4 Point Starter

Speed control of dc series and shunt motor

Flux control method

Armature control method

Voltage control method

Braking methods

Braking

Plugging

Regenerative Braking

Dynamic Braking

Electric Braking

• Braking torque can be applied Mechanically or Electrodynamically to bring motor to rest quickly.

ComparisonMechanical BreakingExcessive wear and tear occurson break blocks.Dust is produced.Large maintenance is required.Large amount of heat isproduced.Effective in holding torque.Regenerative breaking is notpossible.Noisy operation.It is effective and accurate.

Electrical BreakingNo wear and tear.

No dust is produced.Requires less maintenance.Less amount of heat is produced.Effective in holding torquerequirement.Regenerative breaking ispossible.Noiseless operation.For effective and accuratebreaking only electric is notsufficient.

Types of Braking:

1. Plugging.

2. Rheostatic braking.

3. Regenerative braking.

Plugging

• The reversal of direction of rotation of motor is the main principle of plugging in motor.

In case of plugging any two terminals of supply of either armature or fieldwindings are interchanged, so that the direction of rotation hence directionof torque changes.

This produces a torque in reverse direction and the motor tries to rotate inopposite direction, effectively the brakes are applied to motor.

Thus during plugging motor itself acts as a break.

Rheostatic Braking.

• Rheostatic braking of dc motor is obtained by connecting theexternal resistance across the armature

Regenerative Braking.• Without disconnecting motor from supply, it is made to generate instead of motor operation

and feedback energy to supply is achieved in regenerative braking.

• Magnetic drag produced due to generation action offers the breaking torque.

• This method is most efficient method.

• The transition from motoring to generation action is smooth and without switching.

• About 60-80% of energy which would have been lost can be recovered by regenerativebraking.

loses and efficiency

• Losses :

copper losses

magnetic losses

mechanical losses

Condition of maximum efficiency

Variable losses = Constant losses

Power stages

Motor input Driving power in armature

Motor output

Cu losses

Iron and friction losses

PMDC Motors.

• Basic construction

• 2pole & 4pole

• Magnet as per wattage

• Supply & torque production

• Overall working.

Internal structure

Performances

• Speed torque

• Input current

• Power output

• Efficiency.

Factors accounted

• Speed control at constant flux

• Merits

• Demerits

• Applications.

Types of routine tests acc. to is specifications

Routine tests• Visual inspection of conformity to design

• Measurement of windings resistance

• Continuity test of auxiliary circuits

• No load test

• Visual check of commutation during overload

• Over speed test

• Test with voltage applied at environment temperature

• Measurement of dielectric rigidity

• Check of accessories

Type tests

• Visual inspection of conformity to design

• Measurement of windings resistance

• Continuity test of auxiliary circuits

• No load test

• Recording of commutation black band

• Test at nominal load and speed

• Visual check of commutation during overload

• Heating test

• Determination of efficiency (with the indirect method)

• Over speed test

• Voltage test with hot machine

• Recording of the no-load magnetization characteristic

• Recording of field weakening characteristic (for field weakened motors)

• Check of accessories

Special tests

•Measurement of noise level•Measurement of vibrations•Measurement of armature winding inductance•Measurement of field winding inductance

DC Motor Applications

• Automobiles

– Windshield Wipers

– Door locks

– Window lifts

– Antenna retractor

– Seat adjust

– Mirror adjust

– Anti-lock Braking System•Cordless hand drill

•Electric lawnmower

•Fans

•Toys

•Electric toothbrush

•Servo Motor

By

SANDESH POOJRI

MONALI SADAVARTE

KRISHNOM BHAVSAR

NILESH SHINDE

NEHA ARANKAR