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