Three-phase asynchronous motors
A drive task first requires a drive motor whose characteristics with regard to speed, torque and control
options are in accord with the set task.
The three-phase asynchronous motor is the world’s most common electric motor. Its popularity is the result of a
rugged, simple construction, high degrees of protection, standardized sizes and low cost.
Three-phase motors have typical starting characteristics, with tightening torque MA,
pull-out torque MK and rated-load torque MN.
The three-phase motor contains three phase windings that are offset from one another by
120 ˚/p (p = number of pole pairs). To generate a rotating field in the motor, a voltage is applied to each
phase in turn at a time delay of 120 ˚.
The effect of induction produces the rotating field and a torque in the rotor winding. The motor speed is
determined by the number of pole pairs and the frequency of the supply voltage. The direction of
rotation can be reversed by swapping over two of the supply phases:
ns = Revolutions per minute
f = Frequency of voltage in Hz p = Number of pole pairs
Example: 4-pole motor (number of pole pairs = 2), mains frequency = 50 Hz, n = 1 500 r.p.m. (synchronous speed, speed of
rotating field)
Because of the induction effect, the asynchronous motor’s rotor can not reach the rotating field’s synchronous speed
even at idle. The difference between synchronous speed and rotor speed is termed slip.
P2 = Shaft rating in kW
M = Torque in Nm
n = Speed in r.p.m.
The motor’s electrical and mechanical rating are recorded on its nameplate.
As a rule, three-phase asynchronous motors are connected to their power supply with six terminal bolts. There are
basically two connection configurations: star and delta.
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Star connection |
Delta connection |
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ULN = 3 UW |
ILN = IW |
ULN = UW |
ILN = 3 × IW |
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NoteIn continuous operation, the mains voltage must be the same as the motor’s rated
voltage.
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