1. Motor protection level
For example, an IP23 rating for a motor means that it can prevent the intrusion of solid objects larger than 12mm, prevent human fingers from touching internal parts, and prevent the intrusion of medium-sized foreign objects (diameter greater than 12mm). It can also prevent the intrusion of sprayed water, or prevent water sprayed at an angle of less than 60 degrees from entering and causing damage.
The IP (International Protection) rating system was drafted by the IEC (International Electrotechnical Commission). It classifies motors according to their dust and moisture protection characteristics. Foreign objects, including tools and human fingers, must not come into contact with the live parts inside the motor to avoid electric shock.
IP protection rating consists of two digits. The first digit indicates the level of dust and foreign object intrusion prevention for the motor, while the second digit indicates the degree of moisture and water resistance for the motor. The larger the number, the higher the protection level.
2. When conducting a motor frequency conversion speed control experiment, can a regular motor achieve frequency conversion speed control, or is it necessary to purchase 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. What could be causing a buzzing sound when a frequency converter is added to a motor?
The buzzing sound is caused by the carrier frequency of the inverter's output waveform. Usually, if the inverter you are using has a fixed carrier frequency, the motor will emit a high-pitched scream, which is quite irritating to the human ear. You can adjust the carrier frequency (this function parameter can be found in the function table of the inverter's technical manual).
A higher carrier frequency results in a quieter sound, but it also makes the motor more prone to overheating. Therefore, you need to consider both the heating level and the noise level when choosing your carrier frequency. Generally, the optimal carrier frequency is set at the factory for the rated current, and you usually don't need to change it!
If the inverter uses a random carrier wave, the humming sound from the motor will be softer, but generally more pleasant than that from a fixed carrier wave. If you don't accept this or want quiet operation, you can increase the carrier frequency until you are satisfied.
5. Can a single-phase 220V frequency converter output three-phase 380V?
No. 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 single-phase 380V to three-phase 380V.
6. Two capacitor starting methods for motors
1) Capacitor starting (refers to the capacitor being disconnected after the motor starts)
2) Capacitor starts and operates (meaning the capacitor is responsible for both starting and operating the system).
7. High-power motors driven by belts all have a speed reducer connected to the motor. The function of the speed reducer here is...
1) While reducing speed, increase output torque. 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.
2) Deceleration reduces the load's inertia, and the reduction in inertia is the square of the reduction ratio. You can see that most motors have an inertia value.
8. Reasons for slow motor speed during startup
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.
9. How to select the brushes for the rotor slip rings of a wound-rotor asynchronous motor?
The determination is mainly based on whether the brush's operating conditions meet the requirements for current density (a/cm²) and linear velocity (m/s) at the edge of the collector ring. The formula for determination is:
① Brush current carrying capacity (a) = Brush current density (a/cm2) × Brush width l (cm) × Brush thickness b (cm) ≥ Rated rotor current of motor (a)
② 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)
The commonly used brushes include three types: 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 jamming, simply smooth both sides of the brush on sandpaper.
The pressure of the brushes needs to be adjusted appropriately according to the type and model of the brushes. Currently, most of the brush clamping springs attached to the brush holders are tension and compression springs, and their pressure gradually decreases as the brushes wear down. Therefore, the brush pressure should be adjusted at any time during motor operation.
10. In the transformer SFZ-32000/220TH, what do Z and TH represent?
"z" means on-load tap changer, and "th" indicates its use in humid tropical regions.
What precautions should be taken when placing a 11.60Hz 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.
12. For the same type of motor, are the resistances the same for a 50Hz motor and a 60Hz motor?
If the output power is the same, then the resistance is the same.
13. Can an AC servo motor be controlled by a frequency converter?
Because frequency converters and servo drives differ in performance and function, and their applications are also quite different, it is not possible to use them together.
1) 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 control 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.
2) In situations with strict position control requirements, only servo motors can be used. Also, the response speed of servo motors is much faster than that of frequency converters. Servo motors are also used in some situations where high speed accuracy and response are required. In almost all situations where frequency converters can control motion, 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 kW or even higher, while servo motors only reach tens of kW 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).
14. 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.
15. How can I tell if a motor is delta (Δ) or y-connected?
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 and outside of the junction box cover, and different connection methods correspond to different power supply voltages.
16. How does the number of poles in a motor affect its selection?
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, a motor starting under load requires more torque than one starting under no-load. If it is a high-power, high-load starting 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, one can consider using pulleys of different diameters for transmission or using a gearbox. If, after determining the number of motor pole pairs, the power required by the load cannot be met through belt or gear transmission, then the motor's operating power must be considered.
17. 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, and thus 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 generate 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.
18. A submersible pump with a rated current of 12A experiences a maximum starting current of 227A. Would this trigger the thermal-magnetic protection of the upstream switch to trip?
The instantaneous value of the starting current is independent of the load; even if the pump impeller is jammed, it should not cause a change in the maximum value of the instantaneous starting current. If the pump impeller is indeed jammed, it will only cause the starting current to last for a longer period and not decrease (which may cause the upstream switch thermal magnetic protection to trip).
If the insulation of the motor windings to ground is normal, the reason for an excessively high maximum starting current is likely due to a decrease in the insulation resistance between phases or between turns of the windings. Checking for a decrease in phase-to-phase insulation is relatively easy, but checking for a decrease in inter-turn insulation is much more difficult.
Another reason for an excessively high starting current is a partial break in one phase of the three-phase winding (if the winding uses a double-wire parallel winding). A double-arm bridge can be used to measure the DC resistance of the three-phase winding. If a large deviation is found, a partial break in one phase (the phase with the higher resistance) should be suspected.
In addition, it should be noted whether the motor has a capacitor connected in parallel to improve the power factor. If the capacitor performance deteriorates, it will also cause the starting current value to be too high.
19. How to determine if a three-phase asynchronous motor is good or bad?
The following instruments should be used to check the condition of the coils in a three-phase asynchronous motor:
1. Megohmmeter; can be used to measure the insulation resistance between phases and between phase and ground in motors, and the resistance must not be less than 0.5 megohms.
2. Multimeter; used for checking the continuity of motor coils.
3. Single-arm bridge; accurately measures coil resistance, allowing you to determine if the resistance of each phase coil is close, especially after rewinding.
Electric motor failures generally fall into two main categories: mechanical and electrical.
In terms of machinery:
1. Is the bearing lacking lubrication or damaged?
2. Check if the end cap has shifted outwards, and if the bearing has shifted outwards.
The main electrical aspects include:
1. Is the insulation resistance up to standard?
2. Are the three-phase DC resistances up to standard? Measure them using a double-arm bridge.
3. Is the rotor bar broken? The DC resistance of the motor is an important indicator for judging the motor's condition.
20. Can a circuit breaker and a fuse be added to the neutral wire?
1) When there is only a single-phase circuit, a circuit breaker can be added, meaning the neutral and live wires can go to the switch or the fuse.
2) In a three-phase circuit, the neutral wire must never go through a circuit breaker, switch, or fuse.
21. Can a motor soft starter save energy?
Soft start has limited energy-saving effect, but it can reduce the impact of starting on the power grid and achieve smooth starting, thus protecting the motor unit.
According to the law of conservation of energy, due to the addition of relatively complex control circuits, soft start not only does not save energy, but also increases energy consumption. However, it can reduce the starting current of the circuit and play a protective role.
22. What are the changes in the starting current and starting torque of the motor when operating with a frequency converter?
When operating with a frequency converter, the frequency and voltage increase accordingly as the motor accelerates, limiting the starting current to below 150% of the rated current (125%~200% depending on the model). When starting directly with a mains frequency power supply, the starting current is 6~7 times higher, thus causing mechanical and electrical shocks.
Using a frequency converter drive allows for smooth starting (though starting time is longer). The starting current is 1.2 to 1.5 times the rated current, and the starting torque is 70% to 120% of the rated torque; for frequency converters with automatic torque boosting function, the starting torque is over 100%, allowing for full-load starting.
23. What is the relationship between motor overload and short circuit?
There are two types of motor overload:
First, there is mechanical overload, which refers to an overload where the load exceeds the rated value or there is jamming in the transmission system. This is unrelated to short circuits.
Second, if the load is normal but the motor current is overloaded, this may be due to a local short circuit to ground or between turns in the motor windings.
24. In what applications is variable frequency speed control used? What are its advantages?
Before the realization of variable frequency speed control (which was theoretically possible long ago, but was actually realized after the invention of power electronic devices), traditional speed control used DC power.
The disadvantages of DC speed control are:
1) DC motors have complex structures and high maintenance costs.
2) Due to the presence of the commutator, there is not much room for improvement in the power of the DC motor.
Therefore, the advantages of variable frequency speed control are:
1) Enables AC motors to achieve speed regulation performance as excellent as DC speed regulation.
2) AC squirrel-cage asynchronous motors are simple and convenient to maintain.
3) AC motor power is not limited by the commutator.
25. Is a 100kVA transformer sufficient to power an appliance with a total power of 300kW (maximum 37kW)?
What load can a 100kVA transformer handle? The formula tells you:
p = Capacity * Power Factor * 80% = 100 * 0.9 * 80% = 72kW
Running at 20% overload for 1 hour is generally permissible, so it's sufficient.
The main thing to check is whether the total current exceeds the limit. For a 100kVA transformer, the high-voltage current is 5.8A and the low-voltage current is 150A. Even if it occasionally exceeds this limit, it's not a big deal. The main thing to watch out for is that the temperature rise doesn't exceed 55 degrees Celsius. The temperature rise equals the actual temperature minus the ambient temperature.
