Motors & Motor Starters. Prepared By: Erik Redd & Jeremy Roberts. Motors. AC-Motors Parts of an Electric Motor A. Stator : Stationary Frame B. Rotor : Revolving Part The rotary motion in an ac-motor is caused by the fundamental law of magnetism.
Parts of an Electric Motor
A. Stator : Stationary Frame
B. Rotor : Revolving Part
The rotary motion in an ac-motor is caused by the fundamental law of magnetism.
This law states that like poles repel and unlike poles attract.
This shows a three phase, two pole stator.
Where A, B, and C are the three phases
Fig. 13-2 Pg. 244
Poles 1 and 4 are at their greatest magnetic field at time equal to one, because phase A (red line) is connected to those poles, and the same for the other poles when their corresponding phases are at maximum current magnitude.
Speed at which it takes the motor to go one cycle and one revolution.
For a three-phase, 60 Hertz, 2 pole motor:
S=[120*60]/2=3600 revolutions per minute
A 2 pole, 60 Hz motor runs at a full-load speed of 1760 rpm.
What is the slip?
k is machine constant
ø is the main pole flux
IA is the armature current
Where torque is in lb-ft
full load rpm
2) Primary Resistance
3) Wye – Delta
4) Part Winding
1) Start contactor
- Closes first and connects motor to the line
through an autotransformer
2) Run contactor
- Motor switches to this contacter which has
1) Line contactor
- First to energize connecting motor to the
line voltage through a resistor
- After preset time, contactor opens
2) Accelerating contactor
- Causes smooth acceleration to full voltage
1) Line contactor and start contactor
- Energizes first and connects motor in wye
putting about 58% of line voltage across
each motor phase
- Contacts open after preset time
2) Run contactor
- Energizes connecting motor in delta and
putting full voltage on the motor
1) Two-Step - one winding connected to
full voltage line and, after a preset time,
the other connects
2) Three-Step – one winding is connected in series
with a resistor to the voltage line; after interval, resistor
is shorted out and then second line is connected to
full voltage line