Mechanical characteristics of an induction motor
The mechanical characteristics of the motor is called the dependence of the rotor speed on the shaft moment n = f (M2)... Since under load the idle moment is small, then M2 ≈ M and the mechanical characteristic is represented by the dependence n = f (M)... As the relationship s = (n1 — n) / n1 is considered, then the mechanical characteristic can be obtained by presenting its graphical dependence in coordinates n and M (Fig. 1).
Rice. 1. Mechanical characteristics of an induction motor
A natural mechanical characteristic of an asynchronous motor corresponds to the basic (passport) scheme of its inclusion and nominal parameters of the supply voltage. Artificial characteristics are obtained if additional elements are included: resistors, reactors, capacitors. When the motor is supplied with rated voltage, the characteristics also differ from the natural mechanical characteristics.
Mechanical characteristics are a very convenient and useful tool for analyzing static and dynamic modes of an electric drive.
An example of calculating the mechanical characteristics of an induction motor
A three-phase squirrel-cage induction motor is fed from a network with a voltage of = 380 V at = 50 Hz. Engine parameters: Pn = 14 kW, нn = 960 rpm, cosφн= 0.85, ηн= 0.88, multiple of the maximum torque km = 1.8.
Determine: rated stator winding phase current, number of pole pairs, rated slip, rated shaft torque, critical torque, critical slip and construct the mechanical characteristics of the motor.
Answer. Nominal power consumed from the network
P1n =Pn / ηn = 14 / 0.88 = 16 kW.
Nominal current consumed by the network
Number of pole pairs
p = 60 f / n1 = 60 x 50/1000 = 3,
where n1 = 1000 — synchronous speed closest to the nominal frequency нn = 960 rpm.
Nominal slip
сн = (n1 — нн) / n1 = (1000 — 960 ) / 1000 = 0.04
Rated motor shaft torque
Critical moment
Mk = km x Mn = 1.8 x 139.3 = 250.7 N • m.
We find the critical slip by substituting M = Mn, s = sn and Mk / Mn = km.
To draw the mechanical characteristics of the engine, using n = (n1 — s) determine characteristic points: idling point s = 0, n = 1000 rpm, M = 0, nominal mode point сn = 0.04, нn = 960 rpm, Mn = 139.3 N • m and the critical mode point сk = 0.132, нk = 868 rpm, Mk = 250.7 N • m.
For the actuation point with n = 1, n = 0 we find
On the basis of the obtained data, a mechanical characteristic engine is built. For a more accurate construction of the mechanical characteristics, it is necessary to increase the number of design points and to determine the moments and the frequency of rotation for the given slides.
