Lectur Lecture 20: e 20: DC M DC Motor otors Exam Exam 2 - - PowerPoint PPT Presentation

Lectur Lecture 20: e 20: DC M DC Motor

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Exam Exam 2 Results 2 Results

Most M

• st Missed Questions:

issed Questions:

Most M

• st Missed Questions:

issed Questions:

Most M

• st Missed Questions:

issed Questions:

Exam Exam 2 M 2 Make- ake-up up

• Fix all your mistakes on the solution problems only.
• Receive up to 1/3 of the points missed on solution problems.
• No partial credit will be given.

Instructions:

• For each problem you want to resubmit, rework each problem

in it's entirety on NEW paper (even if you only missed one part

• f the problem).
• Clearly identify each problem, subproblem, i.e. a, b, c, etc.,

and each answer.

• Credit given by subproblem.
• Must have correct and complete answers and must CLEARLY

show work to receive credit.

• As a minimum, make sure you write down any/all equations

used in variable form prior to substituting numbers.

DC M DC Motor

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• DC Motor electromechanical energy conversion device.

– Uses DC electricity (obviously). – Converts electrical energy / power into mechanical energy / power.

• Advantages:

– Easy use with batteries (transportation/portable applications) – Simpler / more precise speed and torque control than AC motors

• Disadvantages:

– Requires “commutation” - mechanical (brushes) or power electronic based – Brushed systems require regular maintenance / replacement

Elecr Elecrom

• magnetic Fundam

agnetic Fundamentals entals

Ampere's Law – Moving charge (current) induces a magnetic field

`

Right Hand Rule ( Right Hand Rule (Conductor Conductor)

• Determines direction of

magnetic field

• Imagine grasping conductor

with right hand

• Thumb in direction of current

flow (not electron flow)

• Fingers curl in the direction of

magnetic field DO NOT USE LEFT HAND RULE IN BOOK

Right Hand Rule ( Right Hand Rule (Coil) Coil)

• Imagine grasping coil with right hand
• Fingers in direction of current flow (not

electron flow)

• Thumb points in direction of magnetic

field through coil DO NOT USE LEFT HAND RULE IN BOOK Creates electromagnet

Magnetic For agnetic Force on M ce on Moving

• ving

Char Charge ge

• A magnetic field has a force on a moving charge
• Lorentz Force Law

Two right hand rules. Choose which one is best for you. I like the one on the right.

DC M DC Motor

• tor Oper

Operation ation

• Consider simplest DC Motor – single loop coil

If No Commutation

DC M DC Motor

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Operation ation

• Consider simplest DC Motor – single loop coil

With Commutation

DC M DC Motor

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Operation ation

• Real DC Motors

– Rotor has multiple windings around ferromagnetic material core

• Increase total force on wires (torque)
• Smoother torque

Rotor – Rotating electromagnetic

• component. In most DC machines,

this is the armature. Stator – Stationary electromagnetic (or just magnetic) component. In most DC machines, these are the field magnets.

DC M DC Motor

• tor Oper

Operation ation

Electrochemically generated rotor poles repelled by like stator poles, attracted to

• pposite stator poles

If poles align, there is no force and thus no torque To have continuous torque, commutation required

Motor

• tor Com

Commutation utation

Motor

• tor Tor

Torque que

• Torque is the product of force and distance:
• Force per conductor:

Motor

• tor Tor

Torque que

• Then, total torque exerted on rotor depends on:

– Number of conductors (individual windings) – Length of conductors – Strength of main field magnetic flux density – Magnitude of the current flowing in each conductor

Motor

• tor Tor

Torque que

Example: Find the torque developed by a motor with 20 armature conductors, each 10 in. in length, and radially 3 in. from the center of the armature. The current in each conductor is 20 A, and the flux density is 60,000 lines per square inch.

Motor

• tor Tor

Torque que

Example: Find the torque developed by a motor with 20 armature conductors, each 10 in. in length, and radially 3 in. from the center of the armature. The current in each conductor is 20 A, and the flux density is 60,000 lines per square inch.

F=8.85 B I l 10

8

B=60,000lines/in

2

I=20 A l=10in. F=1.062lb

Per conductor: Total armature torque:

R=3in. =F R =0.2655lb− ft 0.2655lb− ft x 20 5.31lb− ft

Counter Counter EM EMF

• Motor windings “cut” through magnetic fields
• Equivalent of changing magnetic field
• Generates voltage in windings: Counter

electromotive force (CEMF, aka Back emf)

• Generated voltage in opposition to applied
• voltage. Effect opposes current flow.

Motor

• tor Speed

Speed

• CEMF is proportional to motor speed

– EMF proportional to derivative of change in magnetic flux, which is proportional to speed.

• Actual motor speed can differ from rated speed

depending on load.

– Rated hp and speed given for a specific load

• A motor is rated at its maximum horsepower and

speed at a constant load

• Speed regulation: Motor speed changes by load

Wednesday Wednesday

• Continue discussion of DC Motors.

– Various types of motor electrical connections – Special types of DC motors

• Distribution Project Assignment

– Due Monday 12/2