# Computer Aided Manufacturing I

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```					ENT 311 Process Control
Interface Design
Questions ?

ENT311 Learning Module 1--Circuits
Review-Inductance             1
Inductance

   Inductance is a property of a coil to resist a
change in current. Current through a coil
creates a magnetic field. This magnetic field
has a certain inertia to it, so it takes energy to
change it’s state. When a change in current
occurs, the inductor will create a voltage that
opposes the change, according to the
formula

ENT311 Learning Module 1--Circuits
Review-Inductance             2
Inductance

e = L*di/dt where:
 L = inductance

 e = generated voltage

 i = current

ENT311 Learning Module 1--Circuits
Review-Inductance             3
The polarities that normally appear on a
resistor and inductor are shown below:

ENT311 Learning Module 1--Circuits
Review-Inductance             4
Inductance

   If a switch in the circuit
is suddenly opened, a
voltage is generated
across the inductor to
keep the current
traveling in the same
direction, as shown.

ENT311 Learning Module 1--Circuits
Review-Inductance             5
   Vg is a generated voltage that helps keep the
same current. Note that if the resistance is
very high (as it is in an open circuit), Vg will
be very high also.

ENT311 Learning Module 1--Circuits
Review-Inductance             6
Time Constant

   Time Constant of an Inductor
   The time constant of an inductor is equal to:
    =L / R
   If an inductor is charging (i.e. the switch is
closed when the inductor has no current
traveling through it and no magnetic field),
the formula to calculate the voltage at an
instant in time is:
   VL = V((-t/))

ENT311 Learning Module 1--Circuits
Review-Inductance             7
Problems

  Suppose in the Figure in slide 5, R =
4.7ohms, L =5H, and V = 20V. If the switch
is closed for 3s, what is the current through
the inductor?
 How long does it take to reach steady state?

 What happens when the switch is opened?

(this circuit is similar to an auto ignition.)

ENT311 Learning Module 1--Circuits
Review-Inductance             8
Relays

   A relay contains a coil with a movable arm
inside the coil. As current is applied the arm
moves. When the arm moves, it opens or
closes a contact. A relay can have normally
open or normally closed contacts. Normally
open or normally closed refers to the de-
energized state.
    The schematic and photo of a relay follow.

ENT311 Learning Module 1--Circuits
Review-Inductance             9
Relay
http://electronics.howstuffworks.com/relay.htm

ENT311 Learning Module 1--Circuits
Review-Inductance             10

```
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