By Abdul Basit 8-10-2021

1. DEPARTMENT OF ELECTRICAL ENGINEERING
FACULTY OF ENGINEERING, SCIENCE AND TECHNOLOGY (FEST)
INDUS UNIVERSITY KARACHI
BEE(POWER) (BATCH 19)
PRESENTATION TOPIC
GROUP DISCUSSION TOPIC
MOTOR AND GENERATOR
• Group Presented by: Abdul Basit
• Roll Number: 2426-2019
• Group Presented by: Fawad Hussain
• Roll Number: 2437-2019
• Group Presented by: Aziz U Rehman
• Roll Number: 2189-2019

2. MOTORS & GENERATORS

3. MOTORS AND GENERATORS
• Simple devices that use basic principles of electromagnetic
theory
• Technologically important
• Motors drive everything from hybrid cars to vibrating phones.
• Most electrical power is provided by generators
• Work on the same principles: converting between
mechanical and electrical energy using the magnetic force

4. ELECTRIC AND MAGNETIC FIELDS
• Electric and magnetic fields are vector fields
• A vector has magnitude and direction
• A vector field describes a vector for every point in space
• An electric field exerts force on electric charge.
• The force is in the direction of the field - charges get pushed in
the direction of the field.
• F = qE (q is magnitude of charge).

5. MAGNETIC FORCE
• Magnetic fields exert force on moving charges
• Force is perpendicular to field and to velocity
• Units of Gauss: 1 G = 1 N*s/C*m
• FB = qvBsin(ϴ) : ϴ is the angle between v and B.

6. ELECTRIC MOTORS
• In a motor, current passes through a coil of wire in a
magnetic field
• Magnetic field exerts force on charges moving in the coil
•Current, and thus force, is in opposite directions on
opposite ends of the coil
•Creates torque on the coil

7. ELECTRIC MOTORS
• When the coil is pulled "flat" by the magnetic force, the
direction of the current must be reversed:
• This reverses the direction of the force
• Momentum continues the rotation, and the new force
accelerates the rotation

8. ELECTRIC GENERATORS
• In a generator, we rotate the rotor from "outside"
• Wire moves in opposite directions on either side of loop
• Opposite forces on either side create voltage around loop
• As the loop makes a rotation, the direction of current
reverses
• This produces alternating current

9. ELECTRIC GENERATORS
• In my generator, coil is the "stator", magnet is the "rotor"
• Circuit demonstrates how the current alternates

10. MOTORS AND GENERATORS
• Different designs: magnet can be either rotor or stator
• Some motors use an electromagnet instead of a permanent
magnet
• All designs operate on the same principle described here
• Charges moving relative to a magnetic field are
pushed perpendicular to their motion and the field

11. BACK-EMF AND SYMMETRY
• Guarantees conservation of energy
• Current through a motor's coil causes it to rotate
• A rotating coil in a magnetic field induces voltage!
• By the Right-Hand Rule, this voltage is always in the opposite direction as
the supplied voltage
• This is called back-EMF (ElectroMotive Force)
• Likewise, current induced in a generator induces torque
• Torque opposes rotation of the generator
• These reaction forces always resist the applied forces
• This is required by the Maxwell-Faraday Equation:

12. BACK-EMF AND SYMMETRY
• Load on the motor extracts rotational energy
• Motor does not reach the same top speed, so back-EMF is
always less than supply voltage
• Heavier load => lower top speed => more current flows
• We will calculate the peak back-EMF of a square coil

13. BACK-EMF FOR SQUARE COIL
Assume B and v are perpendicular:
• Force on charge: FB = qvB (v and B are perp.: drop sin(ϴ))
• Force per unit charge: FB/q = vB
• Work per unit charge over distance L: LFB/q = LvB
• This is the Back-EMF over a distance L

14. BACK-EMF FOR SQUARE COIL
Over the top and bottom edges of square coil:
• ϴ = 90 degrees, sin(90) = 1
• Speed of edge v = 2π*f*r = 2π*f*0.019 m
• 6 turns, so length L = 6*0.038 m
• B = 0 T over top edge, 0.083 T over bottom edge
• 6*2π*f*(0.019 m)*(0.038 m)*(0.083 T) = (0.0023 m2T)*f
• (0.0023 m2T)*f = Vback

15. BACK-EMF FOR SQUARE COIL
• What about the 3 turn coil?
• 3*2π*f*(0.019 m)*(0.038 m)*(0.083 T) = (0.0012 m2T)*f =
Vback
• At maximum speed, Vback should be equal for both coils
• So 6*2πfrLB = 6*2πfrLB
• 2f1 = f2
• Top speed of the 3 turn coil should be about twice that of the
6 turn coil. Is it?

16. EFFICIENCY
• Vin = 2.7 V, but only applied half the time, so 1.35 V
• These frequencies are much lower than we'd expect
• These motors have very low efficiencies
• Efficiency is defined in terms of power, energy-per-time
• Efficiency n = Pout/Pin: ratio of input power to output power
• Ideally, mechanical power of a motor equals electrical input power
• Electrical power of a generator equals mechanical input power
• In reality, this never happens