14-08-2012, 09:59 AM
Electric Motors
motors.ppt (Size: 1.05 MB / Downloads: 211)
Classification / types
DC Motors
AC Motors
Stepper Motors
Linear motors
Function
Power conversion - electrical into mechanical
Positional actuation – electrical signal to position
Series Wound DC motors
Armature and field connected in a series circuit.
Apply for high torque loads that do not require precise speed regulation. Useful for high breakaway torque loads.
locomotives, hoists, cranes, automobile starters
Starting torque
300% to as high as 800% of full load torque.
Load increase results in both armature and field current increase
Therefore torque increases by the square of a current increase.
Speed regulation
Less precise than in shunt motors
Diminished load reduces current in both armature and field resulting in a greater increase in speed than in shunt motors.
No load results in a very high speed which may destroy the motor.
Small series motors usually have enough internal friction to prevent high-speed breakdown, but larger motors require external safety apparatus.
Shunt wound DC motors
Field coil in parallel (shunt) with the armature.
Current through field coil is independant of the armature.
Result = excellent speed control.
Apply where starting loads are low
fans, blowers, centrifugal pumps, machine tools
Starting torque
125% to 200% full load torque (300 for short periods).
Permanent Magnet DC Motors
Have permanent magnets rather than field windings but with conventional armatures. Power only to armature.
Short response time
Linear Torque/Speed characteristics similar to shunt wound motors. Field magnetic flux is constant
Current varies linearly with torque.
Self-braking upon disconnection of electrical power
Need to short + to – supply, May need resistance to dissipate heat.
Magnets lose strength over time and are sensitive to heating.
Lower than rated torque.
Not suitable for continuous duty
May have windings built into field magnets to re-magnetize.
Best applications for high torque at low speed intermittent duty.
Servos, power seats, windows, and windshield wipers.