Motor and electrical control exam and answer

I. Fill in blank questions (1 point per grid, total 20 points)
1. DC motor is a general term for DC generators and DC electric. The DC motor consists of stator and rotor. The main role of the stator is: the main role of the rotor is.

2. Start methods of DC motor include ,,.

3. There are three methods for Speed Control of DC motor.

4. When the DC motor starts directly, the starting current is too large, causing adverse consequences.

And.

5. Transformer loss △p includes sum.

6. the rated speed of the three-phase asynchronous motor with a rated frequency of 50Hz is 725r/min, the number of poles of the motor is, the synchronous speed is r/min, and the conversion rate of the rated operation is.

7. The output power of a three-phase asynchronous motor is 5.5kw, the voltage U1 = 380 V, the current I1 = 11.7a, and the power factor of the motor is 0.83. The input power is, and the efficiency is.

8. When the contactor coil is powered, the contactor is closed and disconnected.

9. The function of fuses in the circuit is
.
2. Answer questions (1 point for each question, 10 points in total. Fill in "yes" or "error ".)

1. Thermal Relay and overcurrent relay can replace each other during overload protection. ()

2. When the knife switch is installed, the handle should be mounted. When wiring, the power cord is connected to the upper end and the lower end is connected to an electrical appliance. ()

3. The running three-phase asynchronous motor suddenly experiences an open circuit and the electric opportunity stops immediately. ()

4. current and voltage transformers are special transformers. Short circuit is prohibited on the secondary side of the voltage transformer, and open circuit is prohibited on the secondary side of the current transformer. ()

5. The induction potential in the DC Generator coil is alternating. ()

6. In either a DC generator or a DC motor, the commutated pole winding is always connected in series with the armature winding. ()

The phase voltage difference between the source and secondary winding is 240 ° for the transformers in the 7. Y and Y8 connection groups. ()

8. When the three-phase asynchronous motor is started with step-down, the starting current decreases and the starting torque increases compared with the direct start. ()

9. The conversion rate s is an important parameter for analyzing the running performance of an asynchronous motor. The higher the speed of the motor, the higher the conversion rate. ()

10. Asynchronization refers to the difference between the rotor speed and the magnetic field speed. ()

Iii. Q & A (6 points for the first question, 10 points for the second question, 16 points in total)

1. Write the equation for the mechanical properties of three additional excitation DC motors?

2. How does the three-phase asynchronous motor work?

4. Graphic questions (10 points for the first question and 12 points for the second question. 22 points in total)
1. For the three-phase transformer winding wiring, draw the winding potential phase diagram and determine its connection group.

2. Draw the automatic control line of the three-phase Cage Asynchronous Motor Y-△decompression start.

5. Analysis questions (12 points in total)

Compare the differences between A and B. It also explains the concepts and functions of self-locking and mutual lock.
6. Design Questions (10 points in total)
 
 
Design a control line for automatic reciprocating cycle control and draw the main circuit and control line. The specific requirements are as follows: (1) the motor can be turned forward or reversed; (2) the workbench automatically reciprocating Between Blocks 1 and 2;

(3) necessary short-circuit and overload protection.

VII. Calculation questions (10 points in total)
 
The rated data of a DC shunt motor is UN = 220 V, Pn = 46kw, in = 231a, NN = 580r/min, Ra = 0.045 Ω. When the rated load is: if the excitation is reduced to 15%, how much is the motor speed and Rotor Current?

 

Answer and scoring criteria for the motor and electrical control questions ()

(For reference)

I. Fill in blank questions (1 point per grid, total 20 points)

1. Establish the main magnetic field, generate the electromagnetic torque and the induction potential force, and realize the energy conversion.

2. Start directly; Reduce the rotor terminal voltage; rotor loop string resistance.

3. Rotor String Resistance Speed Control; Adjustable Voltage Speed Control; weak magnetic speed control.

4. Great sparks are generated during reversing, resulting in grid voltage fluctuations.

5. Copper loss and iron loss.

750; 0.03.

7.6.39kw; 0.86.

8. electric shock.

9. short circuit protection.

2. multiple choice questions (1 point for each question, 10 points in total)

1. Error 2. Right 3. Error 4. Right 5. Right 6. 7. Wrong 8. Wrong 9. Wrong 10. Right

Iii. Q & A (6 points for the first question, 10 points for the second question, 16 points in total)

1. Answer: the mechanical properties of the rotor loop when the link resistance is rad: (2 points)

Artificial Mechanical properties when rotor voltage is changed: (2 points)

Artificial Mechanical properties when the magnetic flux is changed: (2 points)

2 answer: when a symmetric three-phase alternating current is entered into the three-phase stator winding, a rotating magnetic field rotates clockwise at the synchronous speed N1 along the stator and the inner circular space of the rotor. (3 points)

As the rotating magnetic field rotates at the speed of N1, the rotor conductor is static at the beginning. Therefore, the rotor conductor will cut the stator's rotating magnetic field and generate an induction electromotive force (the direction of the induction electromotive force is determined by the right hand ). Because the two ends of the rotor conductor are short-circuited by a short loop, the rotor conductor will generate a generate current which is basically consistent with the direction of the induction electromotive force. (4 points)

The rotor's streaming conductor is subjected to the electromagnetic force in the stator's magnetic field (the force direction is determined by the Left Hand direction ). The electromagnetic force produces an electromagnetic torque on the rotor shaft, and the rotor is driven to rotate along the direction of the rotating magnetic field. (3 points)

Or: when the three-phase stator winding of the motor (120 degree electrical angle of each difference), a rotating magnetic field will be generated after three-phase alternating current is introduced; (3 points) this rotating magnetic field is used to cut the rotor winding and generate induction current in the rotor winding (the rotor winding is closed path), (4 points) the rotor Conductor Carrying the current will generate an electromagnetic force when the stator rotates the magnetic field, so as to form an electromagnetic torque on the motor shaft to drive the motor rotation, and the motor rotation direction is the same as that of the rotating magnetic field. (3 points)

4. Graphic questions (10 points for the first question and 12 points for the second question. 22 points in total)

1. The phase diagram (8 points) is shown in Figure A. The Join group is Y and D7 (2 points ).

 

2. (12 points)

5. Analysis questions (12 points in total)

A diagram can realize independent forward and reverse conversion. Because the control circuit does not have a mechanical mutual lock, direct switch of forward and reverse cannot be realized.

Figure B adds the mechanical lock link, which can both forward and reverse, and realize direct switch of forward and reverse. (4 points)

Self-locking: that is, after the coil is released, it can still continue to power through its own open contact next to the start button in parallel. For example, km1 and km2. (4 points)

Mutual lock: that is, the electric circuits of the two coils are connected to each other to connect the other's own closed contact or the other's Starting Button's normally closed contact. For example, the common closed contact method of km1 and km2. (4 points)

6. Design Questions (10 points in total)

Answer: The design line is as follows:

 

Main Circuit (4 points) control line (6 points)

VII. Calculation questions (10 points in total)

Solution:

(3 points)

Because the speed remains unchanged before and after the speed limit

(3 points)

The rotation speed after the weak magnetic field is

(4 points)