Motor Control Flashcards by Leah Theobald

Define a contactor

An electro mechanical device that opens and closes when the coil is energized

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What two devices are found in a contactor?

A coil, and contacts

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What three devices are found in a starter?

A coil, contacts, and OLR

Coil and contacts are ‘the contractor’

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What is LVP and UVP and it’s purpose?

Low voltage protection, under voltage protection

Prevents machinery from restarting without pushing start button. Safety purpose.

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What does N.C and N.O. Mean?

Normally closed - energized closed, denergized open
Normally open - denergized open, energized closed

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What is overcurrent protection?

Fuses, breakers, devices that trip when current above its normal path is detected

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What is overload protection?

Heater relays

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What is 3 wire control?

LVP / UVP

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What is a 2 wire control circuit?

LVR. Restarts equipment without pressing start

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What is a power circuit?

Current path to motor

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What are other names for holding contacts?

Seal in contact, maintaining contact.

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Device that prevents chattering in coils

Shading coils

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What are three common types of manual motor starters?

Toggle switch, push button start, drum switch

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Three ratings found on manual motor starters

Voltage, amperage, horsepower

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Drum switch ratings

1.5hp @ 120V, 3hp @ 240V

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Which AC motor starter does not have overload protection?

Drum switch

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What type of rating do contacts of manual motor starter have?

Horsepower

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Why does a manual motor starter for a 240V motor have two power contacts?

For each ungrounded conductor

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What problem can occur if a starter is constantly being reset?

Overloads will wear out and will cause nuisance trips

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Heater selection for motors is based on what?

Full load current

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Typical inrush current is approximately how large?

3-6 times normal running current

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What is a drum switch used for?

Manually control speed, change motor direction.

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What happens when a motor overload occurs?

Like current increases, heater elements warm, heater trips the O.L contacts and stops motor.

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In the control circuit, what function does the NC contact have?

OLR, ectronical interlock

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What does “full voltage control” mean?

Line voltage for control circuit

How do you change the direction of a motor?

Interchange any two leads of the motor

Difference between jogging and inching

Jogging is a momentary connection to full line voltage Inching is a momentary connection to reduced line voltage

Over current protection protects the _________ and overload protection protects the _________

Conductors, equipment/motor

Overloads should trip at ___% to _____% of full load current

115 - 125%

What is a magnetic motor starter?

Electromagnet is used to operate a set of contacts for making and breaking the electric circuit.

What does across the line mean?

The motor is started with full line voltage.

What is a fuse called that provide both short circuit protection and overload protection?

Dual element time delay fuse

Two types of starters

Clapper type, vertical lift type

If the motor starter has tripped because of an overload, what must you do?

1. Find out cause of overload 2. Wait for OLR to cool down 3. Reset OLR

Low voltage starters are rated by the EEMAC, what is their lowest and highest rating?

Size 00 (2hp max), size 8 (900hp max)

What elements does a power circuit usually contain?

Disconnect switch, NO contacts (go rated contacts) overload heaters, fuses (overcurrent protection)

Isolation switches are horsepower rated, T/F?

False. They are not.

What is the fundamental difference between a two wire and three wire control circuit?

The additional NO contact in parallel with the start button in the three wire switch.

When are TX’s used in a control circuit?

CT’s are used to step down the line voltage from the power circuit to operate at a safer level

What do wiring diagrams show?

Shows the physical relation of all devices and each conductor termination between each device.

What do ladder diagrams show?

Symbols and lines identifying components in the electrically connected way

What is the dangerous version of a jogging circuit called?

Double block, or blocking

Name two versions of jogging circuits that work and are safe.

Control relay, jog run selector switch

Are shading coils needed for DC coils?

What is the purpose of a motor starter?

Stop, start a circuit, and provide OLP

Four enclosure types for motor starters.

Open (not enclosed), explosion proof, waterproof, dust proof

What is a downside to using bimetallic strips for overload relays?

High ambient temperatures can cause nuisance trips

Disadvantage if manual motor starters compared to magnetic motor starter?

Manual motor starters do not provide low voltage protection

Why must starters be connected to the ungrounded conductor?

Ground condition could bypass the motor running protection

What are the two different types of contacts in a magnetic starter?

Stationary, moving

Fuses act quicker than overload relays. T/F?

False. OLR act quicker.

What type of contact is the overload relay in the control circuit?

Normally closed auxiliary contact

Magnetic type overload relays are _______ type

Solenoid

Thermal overload relays are _______ type

Heater

Why are components rated in horsepower?

So that they can safely handle in rush current and full load current

What is the purpose of a motor starter?

Stop, start, provide overload protection

Define in-rush current

Locked rotor current. Current drawn by the motor when first energized.

What are push button starters rated at?

5HP @ 230V

Why is a motor rated in HP and not just current?

Must be able to interrupt locked rotor current

Example of a two wire pilot device

Float switch, thermostat, flow switch

What is electrical interlocking?

Connection of contacts in control circuit to prevent forward and reverse from being energized at the same time

Why are both electrical and mechanical interlocks used?

Electrical interlocks act quicker than mechanical interlock, but electrical interlocks can fail. Arching can cause an uneven surface on the contacts which can prevent them from acting normal.

What are two different types of time delay relays? What are their differences?

Hydraulic and Pneumatic. Hydraulic is oil filled, pneumatic is air filled and uses bellows

Advantages of timer relays

Automatic repeat of functions, multiple trip points available, long timing functions

The overall pressure over which a pressure switch can function is called the ______

Differential

Starter size required for a motor load depends on _____

Motor duty cycle

There’s types of pressure switches

Bellows, diaphragm, bourdon tube

What type of oil is used in a dashpot?

Silicon

What are the two types of proximity switches?

Inductive: detects presence of metals Capacitive: detects presence of almost any material