Motor Basics
This stationary part of an induction motor holds the windings that create the rotating magnetic field.
What is the stator?
In a conventional three-wire start/stop circuit, the START pushbutton uses this normal contact state.
What is normally open?
This device converts coil energization into the opening or closing of control contacts.
What is a control relay?
A transformer winding connected to the incoming source is called this winding.
What is the primary?
In a ladder diagram, two normally open contacts in series must both close to energize a coil. This is the equivalent logic operation.
What is AND logic?
This rotating part of a motor transfers mechanical power to the shaft.
What is the rotor?
In the same conventional circuit, the STOP pushbutton uses this normal contact state.
What is normally closed?
This control device changes contact state when a moving machine part reaches a set position.
What is a limit switch?
A transformer winding supplying the load is called this winding.
What is the secondary?
Two normally open contacts in parallel can energize the same coil when either closes. This is the equivalent logic operation.
What is OR logic?
This nameplate rating expresses a motor's mechanical output, commonly abbreviated hp.
What is horsepower?
This electrically operated power-switching device closes the motor's main power contacts when its coil is energized.
What is a contactor?
This protective device responds to sustained motor overcurrent and can open the starter control circuit.
What is an overload relay?
An ideal transformer has 480 volts on its primary and 120 volts on its secondary. This is its primary-to-secondary voltage ratio.
What is 4 to 1?
An on-delay timer is set for 5 seconds. With continuous coil power, its timed normally open contact closes after this interval.
What is 5 seconds?
On a standard three-phase induction motor, interchanging any two supply phases has this effect on rotation.
What is reversing the direction? (Make changes only after proper isolation and verification.)
This auxiliary contact is wired in parallel with START to keep the contactor energized after START is released.
What is a holding or seal-in contact?
A standard overload relay alone does not provide this high-fault-current branch-circuit protection.
What is short-circuit and ground-fault protection? (An appropriate fuse or circuit breaker is also required.)
A 100-VA control transformer supplies a 24-volt secondary. This is its rated secondary current, rounded to two decimal places.
What is 4.17 amperes? (100 / 24.)
A starter runs only while START is held. The STOP circuit and coil are good. This parallel holding path should be checked first.
What is the seal-in auxiliary contact and its wiring?
A four-pole induction motor is supplied at 60 hertz. This is its synchronous speed before accounting for slip.
What is 1,800 rpm? (120 x frequency / number of poles.)
A conventional three-wire starter loses all control power, then power returns. With START released, this is the expected motor state.
What is stopped until START is pressed again? (The seal-in contact opened when the coil dropped out.)
Forward and reverse contactors must not close together. These two types of interlocking are commonly used to prevent simultaneous closure.
What are electrical and mechanical interlocks?
An ideal 480-to-120-volt single-phase transformer delivers 8 amperes on its secondary. This is its primary current.
What is 2 amperes? (120 x 8 / 480; losses ignored.)
In a simplified control rung, STOP and overload contacts are closed, but an open guard-interlock contact is in series with the coil. Pressing START produces this coil state.
What is deenergized? (The open series interlock prevents current; do not bypass it.)