1. Why do residual current circuit breakers (RCCBs) trip easily when used with frequency converters?
The waveform output by a frequency converter will contain high-order harmonics, but leakage current will occur in the cables between the motor and the frequency converter. This current will be much larger than when the motor is driven at the mains frequency, thus causing a tripping phenomenon. Since the leakage current of the frequency converter output is about three times the mains frequency current , plus the leakage current of the motor, the current of the residual current device (RCD) should be about 10 times greater than the mains frequency current .
2. When conducting a variable frequency speed control experiment with an electric motor, is it necessary to use a variable frequency motor?
If you are conducting a variable frequency speed control experiment, you don't necessarily need to use a variable frequency motor; ordinary AC motors or DC motors are also acceptable.
An AC variable frequency motor is essentially a type of motor that adjusts its speed by regulating the frequency of the AC power supply, primarily using a frequency converter. Note: The motor itself does not change frequency; a regular motor needs to be coupled with a frequency converter to achieve frequency conversion.
DC motor frequency conversion is different. Taking DC motor frequency conversion air conditioner as an example: it converts the industrial frequency AC motor into DC power and then sends it to the power module. The microcomputer controls the DC power and sends it to the DC motor of the compressor to control the compressor's displacement and achieve frequency conversion speed regulation.
3. What is the function of a speed reducer ?
Reduce the motor's output speed and increase the torque.
4. Why is the motor running slowly when starting?
There are two possibilities. The first is that the speed is slow at startup but normal after startup. In this case, it may be due to a mismatch in the starting capacitor; or it may be due to the motor design (motor designs vary depending on the application); or it may be due to excessive load resistance causing the startup time to be too long.
The second scenario is that the speed is still very slow after starting. In this case, factors such as insufficient voltage, capacitor mismatch, and high rotational resistance need to be considered.
5. How to select the brushes for an asynchronous motor?
First, it's necessary to determine whether the operating conditions of the brush can meet the requirements for current density and linear velocity at the edge of the slip ring. This can be determined using formulas.
Brush current carrying capacity = Brush current density × Brush width × Brush thickness ≥ Rated current of motor rotor
Linear velocity of the slip ring edge = rated motor speed × slip ring circumference ≤ applicable range of brushes
Electric brushes generally include graphite brushes, metal brushes, and electrographite brushes. During use, it is important to regularly check the brush movement, brush pressure, and degree of wear. The brush should move freely up and down in the brush holder without any obstruction. To prevent the brush from getting stuck, simply smooth the two sides of the brush on sandpaper. The brush pressure should be adjusted appropriately according to the type and model of the brush. Currently, most brush clamping springs attached to the brush holder are tension/compression springs, and their pressure gradually decreases as the brush wears. Therefore, the brush pressure should be adjusted continuously during motor operation.
6. What precautions should be taken when using a 60Hz motor on a 50Hz power supply?
This is because the motor's current frequency is lower than the design frequency. In order to reduce the no-load back electromotive force generated during its rotation and increase the no-load current, which would damage the motor, the no-load voltage needs to be reduced.
In variable frequency speed control technology, the motor frequency and stator voltage change simultaneously. Even if the frequency decreases, the voltage must also decrease simultaneously to prevent the motor from experiencing overcurrent and to achieve the desired operating effect.
7. Why is an output reactor added to the output terminal of a frequency converter, and what is its function?
Adding an output reactor to the output terminal of the frequency converter increases the distance between the frequency converter and the motor. The output reactor can effectively suppress the instantaneous high voltage generated when the frequency converter's IGBT switches, reducing the adverse effects of this voltage on cable insulation and the motor.
The main function of a reactor is to limit the capacitive charging current of the motor connection cable and to limit the voltage rise rate on the motor winding to within 540V/μs . It is also used to passivate the steepness of the inverter output voltage (switching frequency) and reduce the disturbance and impact on power components (such as IGBTs ) in the inverter.
8. Can an AC servo motor be controlled by a frequency converter?
Because frequency converters and servos differ in performance and function, and their applications are also quite different, it is not possible to use them together.
In applications where speed and torque control requirements are not very high, frequency converters are generally used. Some frequency converters also use a closed-loop system with position feedback signals added to the host computer for position control, but the accuracy and response are not high. Some frequency converters now accept pulse sequence signals to control speed, but they don't seem to be able to directly control position.
In situations requiring strict position control, only servo motors can achieve this. Furthermore, the response speed of servo motors is much faster than that of frequency converters. Servo control is also used in some situations where high speed accuracy and response are required. In almost all motion situations where frequency converters can be used for control, servo motors can replace frequency converters.
The key differences are twofold: first, servo motors are significantly more expensive than frequency converters; second, there's the power difference: frequency converters can reach hundreds of kilowatts or even higher, while servo motors only reach tens of kilowatts at most . The fundamental concept of a servo motor is accurate, precise, and rapid positioning. Frequency conversion is an essential internal component of servo control, and servo drives also incorporate frequency conversion (for stepless speed regulation).
9. Can a variable speed motor be started frequently?
Speed-regulating motors can be started frequently. Our company uses speed-regulating motors for testing, and they frequently start like this without any problems. However, minimizing frequent starts is always best. Regardless, frequent starts will damage the motor.
10. How can I tell if a motor is connected in a star or delta configuration?
In a star connection, one end of the three-phase windings is connected, and the other end is connected to the three-phase power supply respectively, forming a shape like the letter " Y ". In a delta connection, the three-phase windings are connected end to end to form a "△" shape, and the top of the triangle is connected to the three-phase power supply.
Their phase voltages are different. Generally, the rated voltage of a star-connected motor is 220V , while the rated voltage of a delta-connected motor is 380V . The connection method is usually marked on the inside and outside of the junction box cover, and different connection methods correspond to different power supply voltages.
11. How does the number of poles of a motor affect its selection?
Motors currently come in 2/4/6/8 pole configurations, with special-purpose motors potentially having even more poles. The more pole pairs a motor has, the lower its speed, but the greater its torque.
When selecting a motor, you need to consider how much starting torque the load requires. For example, a motor that starts under load requires more torque than one that starts under no-load. If it is a high-power, high-load motor, you also need to consider reduced-voltage starting (or star-delta starting).
Regarding the matching of the motor's pole pairs with the load speed after the pole pairs have been determined, one can consider using pulleys of different diameters for transmission or using a gearbox. If the power required by the load cannot be met after belt or gear transmission based on the determined pole pairs, then the motor's operating power must be considered.
12. What is a series-wound motor, and what is its specific operating principle?
A series-wound motor is a motor in which the stator winding and the rotor winding are connected in series.
Working Principle: The principle of generating rotational torque when powered by AC power can still be explained using the operating principle of a DC motor. When current flows through a conductor, a magnetic field is generated around the conductor, and the direction of the magnetic field lines depends on the direction of the current. When a current-carrying conductor is placed in a magnetic field, the interaction between this magnetic field and the magnetic field generated by the current-carrying conductor will cause the conductor to experience a force F , thus causing it to move. The conductor will move from the direction of denser magnetic field lines to the direction of sparser magnetic field lines. When a coil consisting of two opposing conductors is placed in a magnetic field, the two sides of the coil also experience forces, and these two forces are in opposite directions, generating torque. When the coil rotates in the magnetic field, the corresponding two coil sides move from one magnetic pole to the other. At this time, because the polarity of the magnetic field changes, the direction of the force on the conductor changes, and the direction of the torque also changes, causing the coil to rotate in the opposite direction. Therefore, the coil can only oscillate back and forth around its central axis.
