General information about asynchronous electric motors
The main ratios characterizing operation of the asynchronous engine are that. Phase windings of the stator are located on the circle at an angle 120/r where р — number of couples of poles of the rotating magnetic field formed by the three-phase current. The Skoyorost of rotation of the field is defined by the following ratio:
n1 = 60f1: p,
where f1 - the frequency of current of the stator (Hz),
р — number of couples of poles of the rotating magnetic field of the machine.
Stabilized speed of the rotor is less than the rotational speed of magnetic field as the rotating effort is created only when magnetic field peremeyoshchatsya concerning the rotor winding, creating in it electric current. From here возникло the name "asynchronous engine" as rotational speeds of the rotor and magnetic field do not match.
The relation of the difference between the synchronous speed of n1 and rotational speed of the rotor p2 to the synchronous speed of n1
s = (n1 - p2): n1,
is called sliding which is usually expressed as a percentage, i.e.
s = (n1 - p2): n1 100%,
At the normal load of the engine sliding is in limits of 2 - 7%. Rotational speed of the rotor is defined by the following value:
n2 = n1 (1 - s),
Current frequency is in the rotor winding on such formula:
f2 = f 1 xs ,
where f2 — current frequency in the rotor (Hz),
f 1 — current frequency in network (Hz),
s — sliding,
The current consumed by the engine from network is defined by the following expression:
I1 = P: (1.73 x Ulx cosφ )
where P - the power consumption (W);
Ul - linear stress of network (B);
cosφ - engine electrical power factor during the work in this mode.
The greatest current during the long work is equal:
Il = Rn: (1.73 x Uлxcosφ x efficiency)
where Rn-rated nameplate capacity of the engine (kW),
efficiency - engine efficiency,
The equation of capacities and losses in the engine has the following appearance,
P = Rmekh. in + Rmekh.p + Rm.R + Rm. with + Rs. with,
where P — the power consumed by the engine from network (W)
Rmekh. in — net power on the shaft of the engine (W),
Rmekh.p — the power of friction losses (W),
Рм р — the power of losses in the winding of the rotor (W),
Рм. with — the power of losses in the winding of the stator (W),
Рс. with — losses in engine steel on magnetic reversal and whirling currents (W).
Power (W) on the shaft of the engine is determined by the following formula:
Rmekh. in = 1.025 x M x n2
where M — the torque (кг*м),
p2 — engine shaft rotational speed (about. / mines),
The torque of the asynchronous engine is proportional to the square the napryazheyoniya therefore the engine is very sensitive to voltage variation in network. The Vrayoshchayushchy moment depends on sliding size, and sliding, in turn, depends on the pure resistance of the chain of the winding of the rotor.
Installation of the engine of the bigger moshchyonost, than it is necessary, increases capital expenditure and reduces efficiency (and if the asynchronous engine is installed, then, besides, reduces also electrical power factor). If the moshchyonost of the engine is insufficient on the sravyoneniye with expected loading, then temperature of separate parts preyovysit it sizes, admissible on norms, and service life of the engine sniyozitsya sharply.
For example, the overload of some engines for 25% within 1,5 months results in wear of isolation, and the peregruzyoka for 50% can cause isolation destruction even within several hours.
The engine capacity specified by the zayovodom-manufacturer on its guard or in the catalog corresponds to the ambient temperature of 35 °C. At decrease in temperature below 35 °C the engine can be a little overloaded, and at temperature povysheyoniya loading of the engine has to be lowered.
For the purpose of unification of parts of the asinkhronyony engines released by the industry the uniform series of engines of the three-phase current is created.
Electric motors of the uniform series of the general application (A, JSC, also AOL is SCARLET) predyonaznachatsya for the drive of mechanisms when special trebovayoniya concerning control characteristics, sliding and power indicators are not shown.
The uniform series of asynchronous electric motors with power from 0,6 to 125 kW soyostoit from seven dimensions (from the 3rd on the 9th) and is executed as in pig-iron (all dimensions), and in the aluminum cover (the 3rd, 4th and 5th dimensions). Electric motors in the alyumiyoniyevy cover on average 30% easier pig-iron. Irrespective of the cover sort, electric motors have identical electrotechnical characteristics and the ustanovochyony sizes.
On the way of protection these electric motors are executed protected and zakryyoty blown.
Designations of types of electric motors are given in fig. 1.Top