Report content
1. Nominal data of the researched motors and metrological characteristics of measuring devices.
2. Chart of carrying out of experiments.
3. Table of measuring and computations and used computation formulas.
4. Speed-torque and moment characteristics of motors.
5. Short conclusions on work.
Control questions
1. Write equation of electric equilibrium in the contour of generator armature and direct current motor armature. What distinctions do you see between these equations?
2. Write a formula of electromagnetic moment of direct current motor. Whether the same formula does determine an electromagnetic moment, that is created on the shaft of direct current generator?
3. Explain for what physical reason does the initial starting current of motor exceed much more nominal one.
4. How will rotor speed of direct current motor with the parallel excitation be changed, that runs without load, if break an excitation circuit?
5. What features has motor with series excitation at no-load operation?
Laboratory work 4
Research of three-phase asynchronous motor with squirrel-cage rotor Aim of work
Study of design, operation principle, properties and characteristics of asynchronous motor with the squirrel-cage rotor. Determination of the explored motor parameters by experiments results of open and short circuit; construction of speed-torque characteristic of motor.
Order of work performance
1. To acquaint itself with the experimental plant and write out nominal data of the explored motor. To measure the resistance of stator winding phase, using the chart (fig. 4.3). The current value in the stator winding is set by the R rheostat, which must not exceed the rated value.
Measuring
of current and voltage needs to be done not earlier, than through 15
min. after the motor switching, and to define phases resistance by
the formula R1
=
U/2І,
where U
− voltmeter
indication, І
−
ammeter indication.
To bring the measuring and calculations results in tabl. 4.1. As phase resistance take arithmetic mean of three - five measuring.
2. To assemble a circuit (fig. 4.4) and research open circuit mode, changing motor stator voltage by means Fig. 4.3 of choke within the limits of 0,6....1,0Unom. Measuring (at this s.c. current /0 ) power of one phase ΔP01 and phase voltage U, to read three - five indications of meters. For this purpose it is necessary to find power consumed by motor at open circuit, for every voltage value:
ΔP0 = 3ΔP01
________________________________________________Table 4.1
_________Measurings_________________Calculations_____
U, v 1 І, A R1, Ω Rav, Ω ______________________________________________________ ___
With the aim of parameters determination of magnetizing contour of equivalent circuit to make distribution on the mechanical losses ΔPmech and core losses ΔPc.
Fig. 4.4
To bring the measuring and calculations results in tabl. 4.2.
Table 4.2
Measuring______________ Calculation
U, І0 ΔP01 ΔP0 ΔPMH ΔPc Zm Rm Xm cosφ0
v A W W W W W Ω Ω
__________________________________________________________
The assumption that lies in the basis of losses distribution is that the mechanical losses do not depend on stator voltage, while the core losses depend on square of voltage.
Thus
Putting aside on the abscissas axis (fig. 4.5) voltage square in the relative units, we get linear function of losses:
The mechanical losses are determined by the OA segment, that is cut off by this straight line on y-axis.
For the determination of active resistance of magnetizing contour, it is needed to divide power of core losses, found from the graph (fig. 4.5), by th proper value of s.c. current:
Impedance of magnetizing contour
Zm = U / I0
Inductive resistance of magnetizing contour
Xm = √(Z2m – R2m)
Power-factor at short circuit
Fig. 4.5 cosφ0 = ΔP0 / 3I0U
3. To define the parameters of short circuit of asynchronous motor. The determination of parameters of short circuit is carried out by circuit (see fig. 4.4), but at the blocked rotor. The stator is powered by lowered voltage, which changes by choke from zero up to the value, at which the stator current does not exceed rated value.
At short circuit experience the same electric quantities are measured, that and at open circuit experience.
Because power is measured (see fig. 4.4) in one phase only, the triple value of wattmeter indication sets to the full power.
To bring the measuring and calculations results in tabl. 4.3.
______________________________________________Table 4.3
Measuring Calculation
Usc Іsc ΔPsc Zsc Rsc Xsc cosφsc
v ______ _А W Ω Ω Ω___________
At short circuit experience the voltage brought to the motor is very lowered, therefore it is possible to neglect by the o.c. current and consider, that power brought to the motore defrays the losses in the copper of stator and rotor winding.
Active resistance of short circuit
where ΔPsc - active power of one phase, measured at short circuit experience; Isc - current of phase at the experimental short circuit.
Impedance of short circuit
where Usc - phase voltage at the motor terminals at short circuit experience.
The impedance of short circuit in machines with the closed rotor slots depends on the value of short circuit current and only at the large values of current, when core of rotor tooth is saturated, it is possible to be considered constant.
Power-factor at the short circuit
Inductive
resistance of short circuit is determined similarly to short circuit
experience:
Definite from the short and open circuit experiments, the parameters of equivalent circuit of asynchronous motor allow to find the critical sliding ssc and the maximal moment Mmax by formulas (4.5) and (4.6).
The knowledge of these quantity allows by the Kloss formula:
;
Scr
= R’2
/(X1
+ X’2);
Mmax
=
(p∙m1∙U21)
/ (4∙π∙f1∙Xscr)
to build dependence of electromagnetic moment on the sliding M = f(s). For this purpose, set by the sequence of s find the proper values M. To bring calculations data in tabl. 4.4.
Table 4.4
S M, N·м |
4. By table 4.4 data to plot functions M = f(s) and M = (Ω2), where Ω2 is determined by formula:
where p — number of poles pairs of armature winding.