Motor Theory
What causes the rotor to turn in an induction motor?
The interaction between the stators rotating magnetic field and induced current in the rotor,
What does HP on a motor nameplate mean?
Horsepower - the rated mechanical output power of the motor.
What is th emain difference between a single phase and three phase motor?
Single phase uses one AC line; three phase uses three lines 120 degrees apart.
What is the purpose of braking a motor?
To stop oe slow the motor faster than coasting.
What NEC article covers motors and motor circuits?
Article 430
What part of the motor produces the rotating magnetic field?
The stator windings.
What does FLA stand for?
Full Load Amps - the current the motor draws under full load.
What helps a single phase motor start?
A start winding and start capacitor.
What is friction braking?
Using a physical mechanism like brake pads to stop the motor.
What NEC table gives full load currents for three phase motors?
Table 430.250
What is slip?
The difference between synchronous speed and rotor speed.
What does "SF" indicate?
Service Factor
How can you reverse the direction of a three phase motor?
Reverse any two of the three leads.
What is DC injection braking?
Applying DC voltage to the stator to create a stationary magnetic field that stops the motor.
What NEC table gives full load currents for single phase motors?
Table 430.248
What two factors determine the synchronous speed of an AC motor>
Supply frequency and the number of poles.
What value tells you how fast a motor runs under full load?
RPM(Full load speed)
What are the two common wiring configurations for a three phase motor?
Wye and Delta
What is dynamic braking?
Using the motor as a generator and dissipating energy through resistors.
What is the general rule for sizing motor circuit conductors?
They must be sized at 125% of the motors full load current.
Why do we call these types of motors induction motors?
Because rotor current is induced by the stators magnetic field.
What does frame size on a nameplate refer to?
The building characteristics of the motor.
Why does a single phase motor need a starting circuit while a three phase does not?
Single phase power doesn't produce a rotating magnetic field on its own.
Which braking methods require no physical contact?
DC injection and dynamic braking.
What is the purpose of a motor disconnecting means per NEC 430?
To safely isolate the motor for service or maintenance .