motor controls

a motor starter is a combination of what two devices?

a contactor and an overload realy

why isnt a motor disconnect not designed to open the circuit while the motor is running?

a spark will jump across when opened and can burn the switch blades or fuse them to their contacts. This is due to the motor's high inductance

what is an internal overload protection?

a temperature detector device located in the motor windings

one reason reduced voltage starting is used to start a motor is to?

apply torque gradually

why are internal overload protection recommended for applications where the ambient temperature can change drastically?

because external motor over load protection devices are not sensitive to the temperature around the motor

how is a contactor able to interrupt the load current while an ordinary disconnect connot?

constructed with baffles and passages to snuff out the arc

motor starter is made up of what?

contactor and overload

function of a shading coil?

designed to prevent chatter and ac hum when ac current passes through the contactor

what is an example(s) of an overcurrent protection device

fuse, circuit breaker

how is counter emf generated?

generated by the rotor cutting through magnetic lines of force this reduces the current supplied to the motor

the counter emf of the motor does what after the motor has be started

increases as the speed of the motor increases

which type of overload is better for frequent starting and stopping applications?

internal overload protection

what are the two types of motor overload protection?

internal protection and external protection

what is a motor starter?

is a device that combines a contactor and thermal overload relays

what is motor protection?

it safeguards the motor, supply system and personnel

a contactor differs from a relay in that

its contacts can interrupt large current motors

the current that flows before a motor begins to turn is called its

locked rotor current

the operation of an electric motor depends upon the interaction of two "what"

magnetic fields

what are the four common forms of external overload protection

magnetic overload relays, thermal overload relays, electronic overload relays, fuses

a manual starter is held closed "how"

mechanically

what is motor overcurrent protection?

not designed to protect motor but rather to protect the electrical supply system if motor or circuit shorts

what is the function of a magnetic overload relay?

operate on magnetic action of load current through coil. If current becomes to high a plunger is pulled into the coil interrupting the current

what is "low-voltage protection"?

operates when supply voltage drops below a set value to provide machine operation

what type of breaking applies to stopping a motor by electronically reversing the motor while it is still running in the forward direction

plugging

in a three phase starter forward and revers are mechanically interlocked to "what"

prevent a short circuit across the lines

the purpose of the shading coil mounted on the magnetic assembly of a starter is to "what"

prevent chatter and ac hum, set up auxiliary magnetic field

overcurrent protection provided by circuit breakers and fuses is designed to provide "what"

short circuit protection

known in industry as "blank" this starting method increases the voltage gradually as the motor starts, providing smooth, stepless, torque controlled acceleration?

soft start

what starter increases the voltage gradually as the motor starts?

solid state starter

in an across the line motor starter, starting, stoping, and overload protection are accomplished by acting on the "what"

starter coil

reduced voltage starters are used often in industry to limit the large "what" of big motors

starting current

internal motor protection is usually accomplished by using "what"

temperature sensors

motor overcurrent protection is designed to protect "what"

the electric supply system

what is the locked rotor current?

the magnitude of the current that flows before a motor begins to turn (high inrush current)

the purpose of overload protection on a motor is to protect "what"

the motor from sustained overcurrents

when the reset button does not reestablish the control circuit immediately after an overload, the probable cause is?

the overload trip has not cold sufficiently

the current drawn by the motor upon being started and before the motor begins to turn is high because?

there is no counter emf

what are the four types of internal overload protection?

thermostatic devices, RTD, thermocouple, thermistor

function of a across the line starter? and what are the two types?

to apply full line voltage to the motor upon starting. magnetic or manual

what is the purpose of a disconnect

to open the supply conductors to the motor allowing safe work

what is the function of an overload protection device?

to protect the motor from mechanical overload conditions

what is the function of an external-overload protection

to simulate the heating and cooling of a motor by sensing the current flowing to it. usually mounted in the starter.

function of reduced voltage starting

used to limit the voltage to starting the motor

function of a electronic overload relay

uses a current transformer that senses the current flowing to the motor and uses electronic circuitry that senses the changes in motor current then trips when current reaches full load

function of a "thermal overload relay"

uses a heater connected in series with motor supply this increases supply current. if overload occurs the heat produced causes a set of contacts to open interrupting the current

internal motor protection is usually in bedded within "where"

windings of the motor

where should a suitable disconnect be located?

within sight of the motor