电机学考试必备（The motor science test must be）.docVIP

电机学考试必备（The motor science test must be） 1. The transformer excitation reactance is good, still small good, why? A: good, from the point of view the no-load running hope no-load current as small as possible, while increasing the excitation reactance can reduce Io, so its power loss is reduced, thereby further increase the power and power factor of the transformer. 2. When the transformer is idle, will it be used to achieve power from the grid? Answer: to get power from the power grid, reactive power is mainly used to build main flux. Active power is mainly used for iron consumption and copper loss. How does the power voltage increase affect the transformer main flux, empty load current, excitation impedance, and iron consumption? A: by the U1 ~ = E1 = 4.44 fN1 Ф m, know U1 increases, the higher Ф m, or main magnetic flux increases, the voltage is higher no-load current, by Zo = U1 / Io, Zo has shrunk, and Zo = Z1 + Zm, Z1 is constant value, the smaller Zm, namely excitation impedance decreases; And Pfe ∝ f1.3 Bm2, because Ф m increases, then the B, f the same again, the iron loss increased. In the power system, what is the use of three-winding transformers? A: when you need to connect the power systems of several different voltages, three winding transformers are used. 5. What are the main structural features of turbo generators and hydraulic turbines? Why is this characteristic? Answer: the rotor of turbo generator adopts the implicit type, thin and long, the air gap is uniform, suitable for high speed rotation; The rotor of water wheel generator adopts the salient pole type, coarse and flat, suitable for low speed rotation. 6. What is synchronous reactance? How does it affect the performance of the synchronous generator? Answer: when Xt = Xa + X6 (the electric comb reflects an anti-xa, stator winding leakage potential X6), it is called the synchronous reactance of the implicit synchronous generator. It represents a comprehensive parameter of the total effect of arm