英语学习电工学概论习题答案_第四章

How does 4-1. distinguish the cage form and the winding pattern from the structural characteristics of the three-phase asynchronous motor?

Answer: rotor winding induced electromotive force is generated, currents and electromagnetic torque, the structure type with cage and wound rotor two, squirrel cage rotor of each rotor slot insert a piece of copper conducting bar, in an extended at both ends of the core at the notch, with two short circuit copper rings respectively, the all guide bar ends are welded together. If the core is removed, the overall shape of the winding is like a cage, so it is called a cage rotor. The rotor winding and the stator winding is similar to that of the insulated conductor embedded in the rotor slot and connected into a star three-phase symmetric winding, winding three outlet terminals are respectively connected to the rotor shaft three slip ring (ring and ring, the ring and the shaft are insulated from each other), the carbon brush to draw out the current.

4-2. how to change the three-phase induction motor to change?

Answer: any of the two will be connected with three-phase power supply connected three wire switch, three-phase asynchronous motor direction change.

4-3. known a three-phase cage induction motor rated power = 3kW, rated speed = 2880r/min. Find the number of pole pairs (1); (2) when the rated slip ratio; (3) rated torque.

Solution: (1) synchronous speed, so the logarithm of the motor pole P is 1;

(2)

(3) =9.95

Technical data of 4-4. Known Y112M-4 type asynchronous motor = 4kw, delta connection method, rated voltage = 380V, = 1440r / min, rated current = 8.8 a, power factor = 0.82, efficiency = 84.5%. Find the number of pole pairs (1); (2) input power during normal operation; (3) when the rated slip ratio; (4) rated torque.

Solution: (1) synchronous speed, so the logarithm of the motor pole P is 2;

(2)

(3)

(4) =26.5

4-5. Known Y132M-4 type asynchronous motor rated power 7.5kw, rated current = 15.4A, rated speed = 1440r / min, rated voltage = 380V, rated power factor of 0.85 and rated efficiency = 0.87, starting torque and rated torque = 2.2, the starting current and rated current = 7.0, the maximum torque / rated torque = 2.2. Tries (1) rated power input; (2) rated torque; (3) rated slip; (4) the

starting current; (5) starting torque; (6) the maximum torque.

Solution: (1) rated input power

(2) =49.74

(3)

(4) =7=107.8A

(5) =2.2=109.43

(6) =2.2=109.43

4-6. in the three-phase asynchronous motor start instant (s = 1), why the rotor current is large, and the power factor of the rotor circuit is small?

A: the electric motor is connected to the power supply, the induction electromotive force and the induced current of the rotor circuit are the largest, which is called the starting current or the blocking current. In general, the starting current of the small and medium sized three phase induction motor is 5 to 7 times of the rated current. Although the motor starting current is very large, but the power factor of the rotor is very low, in fact, the starting torque of the motor is not large.

4-7. if there is a large number of asynchronous motors in the three-phase power grid at the same time, will be the impact of the grid voltage? Why?

A large number of asynchronous motors start at the same time, the power grid will have a larger voltage drop. Because asynchronous motor starting current is 5 ~ 7 times the rated current, if in the three-phase power grid has a large number of asynchronous motor start at the same time, will produce a large starting current, large starting current will in power line produced larger voltage landing, transformer power supply with other work load.

4-8. three-phase induction motor torque characteristics of the shape of the curve by what factors?

Answer: electromagnetic torque can be expressed as

When the S is very small, can be neglected, T is proportional to S; when the S is closer to 1, can be omitted, T is inversely proportional to S. So on the torque characteristic curve (T ~ S) as shown in figure.

T ~ S curve