Motor and Drive Basics --- Final Review

foreword

for final review

knowledge points

(1) Although the magnetic circuit and the circuit have a category relationship, the properties of the two are different. There are also the following differences in the analysis and calculation: <1> When there is a current I in the circuit,
there is a power loss I2R; and the DC magnetic circuit In , when a certain magnetic flux is maintained, the core has no power loss.
<2> In the circuit, it can be considered that all the current flows in the wire, and there is no current outside the wire. In the magnetic circuit, there is no absolute magnetic insulator. In addition to the magnetic flux in the core, in fact, there is always a part of the leakage flux scattered in the surrounding air.
<3> The resistivity of the conductor in the circuit is constant at a certain temperature, but the magnetic permeability of the iron core in the magnetic circuit is not a constant value, it changes with the saturation degree of the iron core.
<4> For linear circuits, the superposition principle can be applied in the calculation, but for the iron core magnetic circuit, the superposition principle cannot be applied in the calculation, because the magnetic circuit is nonlinear when the iron core is saturated.
Therefore, the magnetic circuit and the circuit are only similar in form, not in physical essence .
(2) After the external magnetic field is removed, the magnetic flux density B still retained in the ferromagnetic material is called residual magnetic flux density, or remanence for short .
(3) The phenomenon that the change of the magnetic flux density B of the ferromagnetic material lags behind the magnetic field strength H is called hysteresis . The BH closed loop showing hysteresis is called hysteresis loop.
(4) According to the shape of the hysteresis loop , ferromagnetic materials can be divided into two categories: soft magnetic materials and permanent magnetic (hard magnetic) materials .
(5)
<1>Hysteresis loss—the energy consumed by the material being repeatedly magnetized by the AC magnetic field and the magnetic domains rubbing against each other.
<2> Eddy current loss - the heat energy loss generated by the eddy current inside the core on the core resistance.
<3> Core loss - the sum of hysteresis loss and eddy current loss.
(6)
<1>Rated power: The power that the motor can provide under rated conditions. For generators, it refers to the rated electric power output between the brushes. For motors, it refers to the output mechanical power on the shaft
<2> Rated voltage: Under rated conditions, the average voltage at the outlet of the motor. For generators, it refers to the output rated voltage. For motors, it is the rated input voltage.
<3> Rated current: Under the rated voltage, the corresponding current when running at the rated power.
<4>Rated speed: Under the rated voltage and rated current, the corresponding speed when running at the rated power.
<5> Rated excitation current: corresponds to the excitation current at rated voltage, rated current, rated speed and rated power
<6> Excitation method: refers to the connection method between the excitation coil and armature coil of the DC motor
(7)
<1> Motor During operation, all physical quantities are the same as the rated value, and the motor operates in the rated state.
<2> The operating current of the motor is less than the rated current, and the motor is under-loaded.
<3> The operating current is greater than the rated current, overloaded.
<4> Long-term under-load operation will cause waste of the motor, and long-term overload operation will shorten the service life of the motor.
<5> It is better for the motor to run in the rated state or near the rated state. At this time, the operating efficiency and working performance of the motor are relatively good.
(8) The winding brushes of the DC generator do not move, but the commutator moves.
(9) The number of branches of a single stack winding is equal to the number of poles of the motor.
(10) The motor can run without load, at this time there is only excitation magnetomotive force. When the motor is loaded, there is not only the excitation magnetomotive force, but also the armature magnetomotive force.
(11) The branch logarithm of a single-wave winding has nothing to do with the magnetic pole logarithm, and is always equal to 1.
, the motor is said to be in braking operation. Separately excited motors have three braking methods: dynamic braking, reverse braking, and regenerative braking .
(13) Motor type: transformer; DC motor; AC motor (AC motor is divided into synchronous motor and asynchronous motor, synchronous motor is often used as a generator, and asynchronous motor is often used as a motor) (14) The most basic characteristics of ferromagnetic materials
:
< 1>High magnetic permeability
<2>Nonlinearity of magnetization curve
<3>Hysteresis phenomenon and hysteresis loss
<4>Eddy current phenomenon and eddy current loss
(15)The materials used in the motor are divided into four categories:
<1>Conductive materials
< 2> Magnetic material
<3> Insulating material
<4> Mechanical support material
(14)
<1> The influence of armature magnetomotive force (rotor) on main magnetic pole magnetomotive force (stator) is called armature reaction.
<2> The influence of the armature magnetic field on the air gap magnetic field is called armature reaction.
(15) The main magnetic circuit of the DC motor can be divided into five sections: the air gap (air) between the stator and the rotor, the armature teeth, the armature yoke (rotor), the main magnetic pole and the stator yoke (stator).
(16)
<1> E and Ia of the generator are in the same direction; E and Ia of the motor are in the opposite direction.
<2>The direction of the electromagnetic torque of the generator is opposite to the direction of the motor speed, and the electromagnetic torque acts as a brake;
<3>The direction of the electromagnetic torque of the motor is the same as that of the motor speed, and the electromagnetic torque acts as a drive.
(17) Starting requirements for separately excited DC motors:
<1> Sufficient starting torque


Starting method: series resistance of armature circuit, reducing armature voltage
(18) Braking method of separately excited DC motor: energy consumption braking, reverse connection braking (voltage reverse connection, reverse pull reverse connection), feedback braking .
(19) Evaluation indicators for speed regulation:
<1> Speed ​​regulation range
<2> Static difference rate (relative stability)
<3> Smoothness of speed regulation
<4> Economy of speed regulation
(20) Armature magnetomotive force The effect on the air gap magnetic field generated by the excitation magnetomotive force is called armature reaction
(21) The effect of armature reaction not only distorts the air gap magnetic field in the motor, but also has a demagnetization effect that also affects the operation of the motor (
22) In order to facilitate measurement, voltage is applied to the high-voltage side, and the low-voltage side is short-circuited
(23) The load test is a test performed when the rated current at the rated frequency passes through one winding of the transformer and the terminals of the other winding are short-circuited.
(24) The no-load test can be carried out on both the primary side and the secondary side of the transformer. Usually, for safety reasons, it is generally carried out on the low-voltage side.
(25) Copper loss caused by no-load current is negligible.
(26) Completely consumed in the excitation circuit is called excitation loss, and this part of power consumed in the armature winding resistance is called armature copper loss ( 27
) There are only two types of transformers: iron loss and copper loss.
The load loss refers to the iron loss, while the load loss refers to the copper loss.
(29) Like the DC motor, the basic iron loss of the transformer is mainly hysteresis loss and eddy current loss.
(30) The hysteresis loss is related to the properties of the silicon steel sheet material, the maximum value of the magnetic flux density and the frequency of the power supply. The eddy current loss is related to the thickness, resistivity and maximum value of the magnetic flux density of the silicon steel sheet.

topic

If it is found that the voltage of the shunt excited DC generator cannot be established during the test, how to find the reason? How to deal with it?

(1)
<1> The residual magnetism disappears.
<2> The excitation winding is reversed.
<3> The direction of rotation is wrong.
<4> The field winding is disconnected.
<5> The armature is short-circuited.
<6> The brushes are in poor contact.
<7> The resistance of the magnetic field loop is too large.
(2)
<1> It should be magnetized, and direct current is passed into the shunt winding to generate a magnetic field.
<2> Rewire after proofreading.
<3> Change the rotation direction of the prime mover according to the direction indicated by the arrow on the casing.
<4> It is necessary to check whether the connection between the field winding and the field rheostat is loose or wrong, and whether there is an open circuit in the field winding or the field rheostat.
<5> Check whether there is a short circuit on the reversing surface of the external wiring or the joint piece or check with a millivolt meter.
<6> Check whether the brush holder spring is loose, and improve the contact surface.
<7> Check the resistance of the field rheostat and the excitation winding, and check the resistance of the contact surface of the contact.

Definition of Slip

Slip refers to the proportion of the difference between the actual speed of the asynchronous motor and the synchronous speed in the synchronous speed. That is, the difference obtained by subtracting the actual speed from the synchronous speed is divided by the synchronous number. Usually represented by the letter S. See the following formula for definition:

$$s=\frac{n_1-n_2}{n_1}$$

<1>s – slip rate. Unitless.
<2>n1–synchronous speed, that is, the speed of the motor’s rotating magnetic field, unit [rev/min] (r/min or RPM or rpm)
<3>n2–actual speed of the motor, unit [rev/min] (r/min or RPM or rpm), when the actual speed is opposite to the rotation of the rotating magnetic field, it is a negative value.

What materials are often used for the magnetic circuits of motors and transformers? What are the main properties of this material?

(1) Ferromagnetic material with high magnetic permeability
(2) High magnetic permeability

Explain the causes of hysteresis loss and eddy current loss in the iron core

The flipping of magnetic domains back and forth creates friction that causes hysteresis losses.
The loss caused by the circulation is called eddy current loss.

What are the basic laws of magnetic circuits? When several magnetomotive forces act on the magnetic circuit of the iron core at the same time, can the calculation of the magnetic circuit use the superposition principle, why?

(1) Arrange the loop law, Ohm's law of the magnetic circuit, the series law and the parallel law of the magnetic circuit;
(2) No, because the magnetic circuit is nonlinear and there is a saturation phenomenon.

In the following structural parts, the term DC motor stationary part is ()

A, main magnetic pole B, armature winding C, commutator D, brushes
(1) A, D
(2) Note: The part of the motor connected to the external power supply is called the armature, and the armature of the AC motor generally specifies the stator winding.

What is overload, what is underload, and what are the hazards of overload operation and underload operation?

(1) The operating current of the motor is less than the rated current, and the motor is under-loaded.
(2) The operating current is greater than the rated current, and it is overloaded.
(3) Long-term under-load operation will cause waste of the motor, and long-term overload operation will shorten the service life of the motor.

Series wound motors cannot be run under light or no load

correct

Why are DC motors generally not allowed to start directly, and what will happen if they are started directly?

(1) Such a large current will cause sparks in the commutation
(2) Generate a large electromagnetic force and damage the winding
(3) Excessive electromagnetic torque proportional to the armature current will cause a strong impact on the transmission mechanism
(4) Cause the voltage fluctuation of the power grid and affect the normal operation of other electrical equipment.
Therefore, the DC motor is not allowed to start directly, and the starting current is limited to no more than (1.5~2) IN

When separately excited DC motor step-down or series resistance speed regulation, the larger the value of the maximum static difference rate, the larger the speed regulation range

correct

The hardest mechanical characteristics of DC motors are generally ()

A, shunt-excited motor B, separate-excited motor C, series-excited motor D, compound-excited motor
Answer: B