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Rotary current motor
In its simplest version, a rotary current asynchronous motor consists of a stator and its three-phase winding. The current is supplied from the rotary current mains by way of a terminal board, the necessary circuitry and fuse elements. The three-phase rotor has has bearings on both ends and no conductive connection to the stator winding; following the transformer principle the rotor induces a current which in turn generates a magnetic field. This magnetic field has the tendency to follow the rotary field in the stator winding which means that the rotor is revolving at approximately the same speed as the surrounding stator field. The difference in speed is called slip and is calculated with the following formula:
| s | ( n o - n ) | x | 100% | |
| = | ---------- | |||
| n o |
s = slip (%)
no = no-load speed (1/min)
n = load speed (1/min)
The rotary frequency of rotary current asynchronous motors cannot be freely chosen since it depends on the number of pole pairs. It can be calculated with the following formula:
| s | f x 60 | x | 100% | |
| = | ----------- | |||
| Po |
no = no load speed (RPM)
f = frequency (1/sec)
P = number of pole pairs
60 = conversion factor sec/min
The pole number is physically predetermined and can only be 2, 4, 6, 8, 10 or 12. It is obvious that only a change in frequency is capable of producing a change in rotational speed. Rotary current asynchronous motors have set speeds depending on frequency and number of pole pairs.
Rotary current asynchronous motors have different types of rotors than rotary current synchronous motors. The magnetic flux required for the inductor is generated by direct-current excited coils. The rotational speed of the rotary current synchronous motor is equivalent to the theoretically calculable speed. Synchronous motors are of limited use in technical applications.
The power input of a rotary-current motor can be determined by the following formula:
| Po | f x 60 | x | 100% | |
| = | ---------- | |||
| Po |
Ps = 1,73 x U x I
Ps = apparant power(VA)
U = operating voltage (V)
I = operating current (A)
1,73= tree-phase interlinking factor MÖ 3
The power output of a rotary-current motor can be determined by the following formula:
P = 1,73 x U x I x cos j
Ps = effective power (W)
cos j = power factor (always < 1)
The ratio between the effective power (P) and the apparent power (PS) is called power factor cos j
| cos j | P (W) | ||
| = | ----------- | ||
| Ps(VA) |
cos j = power factor
Finally, the ratio of the delivered power (P2) to the consumed power (P1) is called the efficiency ratio of the motor.
| h | P2 | x 100% | |
| = | --------- | ||
| P1 |
P1 = power input (rated output) (VA)
P2 = power output (W)
h = efficiency ratio (%)
The rotary current asynchronous motor with its simple design is the most frequently used electric motor.
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