CHAPTER 10 Time Control

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CHAPTER 10Time Control

• Control With Timers
• Prepared by Adam Keeth for EE483
• 4-29-03

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When Should One Use a Timer?

• Major function is to place information about
  elapsed time into a control circuit.
• Timers are not always the best way to control a
  circuit
• Overshoot/Undershoot example
• Critical criteria (time, pressure, temperature,
  or position)

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Timers and Time-Delay Relays

• Time Delay Relays
• Devices having a timing function after a timer
  coil has been energized. Then either NO or NC
  contacts activate. The only time function is the
  preset time.
• Timers ( Much more versatile)
• The time function may start on one or many
  contacts at energizing, or at a preset time after
  energizing.

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The Timer (continued)

• The timer opens or closes electrical circuits to
  selected operations according to a timed program.
• Timing function begins when the timer receives an
  electrical signal initiated through a manual
  switch or automatic sensor.
• Preset time is user-defined.

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Types of Timers

• 2 Primary Types
• Synchronous Motor Driven
• Solid-State Type
• Other Types
• Dashpot
• Mechanical
• Electrochemical
• Thermal

• Timing Functions
• ON delay
• Timing function after energizing.
• OFF delay
• Timing function after deenergizing.

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Synchronous Motor Driven Timer

• Very Common in Industrial Applications
• Available in three types
• Reset type
• Repeat-cycle type
• Manual-set type
• Symbols

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Reset Timers

• Use a clutch and a synchronous motor. Most reset
  timers have 3 contacts to provide output to the
  electrical circuit. Each contact has three
  conditions
• Reset clutch and motor deenergized.
• Timing clutch energized.
• Timed Out motor deenergized.
• Sequence Chart Figure 10-5

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Repeat Cycle Timers

• This type of timer is used to control several
  electrical circuits in a predetermined sequence.
• Adjustable cams on the motor shaft determine the
  point of closing and opening a switch during a
  revolution. In many cases, these cams are
  screwdriver-adjustable.
• Activation time and duration time are adjustable.
  
• Most include external calibrated discs to observe
  the progress of the timer (Figure 10-9).

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Manual-Set Timers

• This type requires manual operation to activate
  the timer.
• Timer runs and stops after a selected time.
• Figure 10-10AB

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Solid State Timers

• Microprocessor-type timers are the most versatile
  timers available.
• Many incorporate keypad input and digital/LCD
  readout.
• Very accurate
• 100 accurate for count operations.
• Within 50 ms (or 0.5) accurate for timing
  operations.
• Can retain data for many years.
• Most are repeat-cycle capable.

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