1. Why do residual current circuit breakers (RCCBs) tend to trip when used with frequency converters?
This is because the output waveform of the frequency converter contains high-order harmonics, and leakage current will be generated in the motor and the cable between the frequency converter and the motor. This leakage current is much larger than that when the motor is driven by the mains frequency, so this phenomenon occurs.
The leakage current on the output side of the inverter is about three times that of the mains frequency operation. In addition, the leakage current of the motor, etc., should be selected. The operating current of the leakage current protection device should be more than 10 times that of the leakage current at the mains frequency.
2. I want to conduct a motor frequency conversion speed control experiment. Can a regular motor achieve frequency conversion speed control? Or do I have to buy a frequency conversion motor?
For the experiment of variable frequency speed control of motors , a regular AC motor will suffice.
DC motors can also achieve frequency conversion, such as in modern DC inverter air conditioners: they convert industrial frequency AC power into DC power and send it to the power module. The module is controlled by control signals sent by a microcomputer. Unlike AC inverters, the module outputs controlled DC power to the DC motor of the compressor to control the compressor's displacement, thereby achieving "frequency conversion speed regulation".
3. What kind of motor is an AC variable frequency motor?
Simply put, it means that frequency conversion technology is used in the control of AC motors. An AC variable frequency motor is actually a type of motor that adjusts its speed by regulating the frequency of the AC power supply. Adjusting the AC power frequency requires a frequency converter; the motor itself does not change frequency. In many applications where the requirements are not high, a regular motor is used as an AC variable frequency motor by adding a frequency converter for speed regulation.
4. Can a single-phase 220V frequency converter output three-phase 380V?
No, it's not possible. A frequency converter itself cannot boost voltage, let alone convert single-phase 220V to three-phase 380V. Theoretically, it's possible to use a transformer to boost single-phase 220V to 380V, and then convert that single-phase 380V to three-phase 380V.
5. High-power motors driven by belts all have a speed reducer connected to the motor. What is the function of the speed reducer here?
The uses of speed reducers can be briefly summarized as follows:
While reducing speed, the output torque is increased. The torque output ratio is calculated by multiplying the motor output by the reduction gear, but care must be taken not to exceed the rated torque of the reduction gearbox.
Deceleration reduces the load's inertia, and the reduction in inertia is equal to the square of the reduction ratio. You can see that most motors have an inertia reading.
6. Please explain why the motor starts at a slow speed.
If the starting speed is slow but normal after starting, it could be due to a mismatched starting capacitor, the motor's inherent design (depending on the environment), or excessive load resistance causing the long starting time.
If the engine speed is still slow after starting, the problem may be insufficient voltage, capacitor mismatch, or high rotational resistance.
7. How to select the brushes for the rotor slip rings of a wound-rotor asynchronous motor?
The determination is mainly based on whether the working conditions of the brush meet the requirements of current density (A/cm2) and linear velocity (m/s) at the edge of the collector ring.
Determine the formula:
Brush current carrying capacity (A) = brush current density (A/cm2) × brush width L (cm) × brush thickness b (cm) ≥ motor rotor rated current (A);
Linear velocity of the circumference edge of the slip ring (m/s) = Rated speed of motor (r/m) / 60 (s) × circumference of slip ring (m) ≤ Applicable range of brush (m/s).
There are three common types of brushes: non-graphite brushes, electrographite brushes, and metal-graphite 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, the brush clamping springs attached to the brush holder are mostly tension/compression springs, and their pressure gradually decreases as the brush wears. Therefore, the brush pressure should be adjusted continuously during motor operation.
8. 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's frequency and stator voltage change simultaneously. That is, as the frequency decreases, the voltage must also decrease simultaneously to prevent overcurrent in the motor and achieve the desired operating effect.
9. 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.
10. 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 also 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, power consumption differs: 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).
11. Can a variable speed motor be started frequently?
Speed-regulating motors can start frequently; our company uses them for testing and commissioning, and they frequently start without any problems. However, minimizing frequent starts is always best. Regardless, excessive starting will damage the motor.
12. 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, 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 or outside of the junction box cover, and different connection methods correspond to different power supply voltages.
13. How does the number of poles of a motor affect its selection?
Motors currently come in 2/4/6/8 pole number options, 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 the starting torque required by the load. For example, motors starting under load require more torque than those starting under no-load. For high-power, high-load starting, you also need to consider reduced-voltage starting (or star-delta starting). Regarding matching the motor's pole pair number with the load speed, you can consider using pulleys of different diameters for transmission or using a gearbox.
If the power required by the load cannot be achieved after determining the number of pole pairs of the motor through belt or gear transmission, then the operating power of the motor needs to be considered.
14. 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 by 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 the 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 where the magnetic field lines are dense to the direction where the magnetic field lines are sparse. When a coil consisting of two opposing conductors is placed in a magnetic field, the two sides of the coil are also subjected to a force. These two forces are in opposite directions and produce a torque.
When the coil rotates in the magnetic field, the 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. Thus, the coil can only swing back and forth around the central axis.
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