Electrical Motor Controls - Key Concepts and Devices

Limit Switches

• Pivot point heads attach to the limit switch body and trigger a contact change when activated.
• Operator types: plunger, lever, cat whisker; contact actuators included.
• Contacts: normally open (NO) and normally closed (NC); switch may be SPDT (NO/NC).
• Symbols: learn both NEMA and IEC symbols.

Limit Switch Installation

• Two key factors:
  • Match the switch range of operation to the system’s mechanical rotation and direction of travel.
  • Correct mounting of the switch body is crucial for proper performance and avoiding damage; mounting location affects operator speed.

Limit Switch Operators (Heads)

• Heads are the actuating surface at the top of the switch body.
• Operator types include: plunger, lever, cat whisker, contact actuators, spring wire actuators.

Other Frequently Used Operators

• Adjustable roller lever
• Adjustable wire lever
• Flexible spring lever
• Flexible loop lever

Lever and Fork Lever Switches

• Lever: actuated in one direction (detection of moving objects); can operate in either direction.
• Fork lever: maintained or momentary contacts; requires precise mounting; suitable for motion traveling in two directions.

Push Rod and Wobble Stick Switches

• Wobble stick: actuated from any lateral direction; resilient to over-travel damage; detects movement from multiple directions.
• Push rod: least travel; suitable for end-of-travel sensing on linear actuators; roller used if force is not parallel with rod.

Two Factors for Limit Switch Installation (Summary)

• Range matching: switch characteristics must align with system characteristics.
• Mounting: affects performance and operator speed; keep within switch limits.

Optional Trip Dog Mounting and Travel

• Trip dogs and travel limits influence actuator positioning and activation sequence.

Miniature or Micro Switches (Operation)

• Spring attached to pole past the pivot; pole moves to NC and NO contacts in snap-action fashion.
• Zero over-center action; pole cannot stop between NO and NC.

Example: Roller Lever Micro Switch

• Rollers actuate the lever; terminals for field wiring; contact ratings provided on the switch.

Micro Switches Used in Limit Switches

• Micro switch travel affects limit switch lever characteristics.

Flow Switches and Symbols

• Paddle (sails) type; available for liquids (varying viscosities) and for air/gas in ducts.
• Paddle action indicates flow rate above or below set point.
• SPDT terminations with paddle or sail resistance to flow to trip contacts.

Level Switches and Symbols

• Closed tank level switches vs open tank level switches.
• Most level switches feature snap-action (quick-make/quick-break).

Level Switch Applications (Mercury Switches)

• Horizontal liquid level control via mercury bulb type switches.

Multilevel Level Switch Applications - Mercury Bulb

• Mercury bulb floats with liquid level; multiple set points achieved by installing float switches for each level.
• Cable serves as support for the bulb; calibrate by adjusting cable length.
• Normally open contacts used most often.

Multilevel Level Switch Applications - Reed Switches

• Reed switches use a magnetic field; two reeds close/open a circuit; typically low amperage (e.g.,
  $ext{Contact current} \le 3 \ \text{A}$).

Replacement of Level Switches (Factors)

• Corrosiveness: use non-corrosive materials (plastic, stainless steel, polymers) for floats.
• Temperature: high-temperature level devices for hot liquids.
• Adhesiveness: paint or buildup can prevent actuation.
• Turbulence: liquid movement may trigger false acts; use less sensitive devices if needed.

Float/Level Switches (Example Data)

• Example spec: 130 $^ ext{o}C$ max; 100 $\text{PSIG}$ max; 30 W SPST; ratings for: 240 $\text{VAC}$, 0.14 A; 120 $\text{VAC}$, 0.28 A; 120 $\text{VDC}$, 0.07 A; 24 $\text{VDC}$, 0.28 A.

Understanding Pressure

• Pressure types: absolute, gauge, and differential.
• Gauge pressure is referenced to atmospheric pressure (zero gauge equals $0$ gauge pressure).
• Absolute pressure is referenced to a perfect vacuum; zero absolute corresponds to atmospheric pressure (≈ $14.7\ \text{psi}$).
• Differential pressure is the difference between two pressure sources.

Introduction to Pressure Switches

• Open or close a contact based on process pressure; measurement range from $1\ \text{psi}$ to over $15000\ \text{psi}$.
• Categories: positive pressure, vacuum (negative pressure), differential pressure.
• Principles: gases are compressible; liquids are not.

Pressure Switches and Electrical Symbols

• Field wire terminals; set-point adjustment; general-purpose pressure port.
• NEMA vs IEC notations; NO held closed, NC held open (and differential NO/NC).

Positive Pressure Switches

• Not activated vs activated positions show direction of pressure increase across the port; plate movement activates contacts (NO/NC).

Vacuum Pressure Switches

• Not activated vs activated positions show direction of pressure decrease (increased vacuum) activating the switch differently than positive pressure.

Differential Pressure Switches

• Not activated vs activated: higher port pressure moves the plate toward the lower port pressure.

Deadband (Definition)

• Deadband: neutral zone where no action occurs; prevents hunting.
• Deadband may be embedded in sensor or controller; it reduces oscillations due to small input changes.

Introduction to Deadband (General)

• All automatic switches have some deadband; more common in pressure/temperature devices.
• Deadband changes with setpoint; smaller at low setpoints, larger at high setpoints.

Deadband in a Pressure System

• Operational pressures (e.g., $40\sim 50\ \text{psi}$) require tolerance within the deadband; larger deadband widens the acceptable range.

Tank Pressure Deadband Example

• Gas-over-liquid sealed tank: pump raises tank pressure; max gas pressure $ext{PSI} = 50$ with deadband $= 10\ \text{psi}$; energized at $40\ \text{psi}$ and de-energized at $50\ \text{psi}$.

Temperature Switches

• Function: temperature sensing device mechanically changes electrical contacts at a set temperature.
• Applications: HVAC, fire alarm systems, process control.
• Sensing devices: bimetallic strip, capillary tube, direct immersion probe.

Bimetallic Strip Operation

• Two metals with different expansion rates bend with temperature change.
• Bend drives opening/closing of electrical contacts.

Capillary Tube Temperature Switch

• Bulb and capillary tube connected to main switch body; set-point adjustment changes capillary-tube tension.
• SPDT terminals for field wiring; capillary tube and bulb control the actuator.

Capillary Tube Temperature Switch Operation

• Mechanism includes differential blade spring, capillary tube, bulb, and adjustable tension to set the trip point.

Direct Immersion Temperature Switch

• Attachment point for M12 cable; installed through tank wall or pipe side into the liquid being measured.