Electric motors are the most frequently used rotating tools to date. With the development and popularization of frequency converters, more and more electric motors need to be used in conjunction with frequency converters. However, in the process of using frequency converters and electric motors together, many problems inevitably arise. These problems are increasingly worthy of our deep consideration and discussion.
1. 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, achieve smooth starting, and protect the motor windings.
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.
2. When the motor is running with a frequency converter, what are the starting current and starting torque of the motor?
When using 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). Direct starting with a mains frequency power supply results in a starting current 6~7 times the rated current, causing mechanical and electrical shocks. Frequency converters allow for smooth starting (though starting time is longer). The starting current is 1.2~1.5 times the rated current, and the starting torque is 70%~120% of the rated torque; for frequency converters with automatic torque boosting, the starting torque is over 100%, allowing for full-load starting.
3. What is the relationship between motor overload and short circuit?
There are two types of motor overload: 1. Mechanical overload: This occurs when the load exceeds the rated value or there is obstruction in the transmission system. This is unrelated to short circuits. 2. Normal load: This occurs when the motor current is overloaded, which may be due to a partial short circuit to ground or between turns in the motor windings.
4. In what applications is variable frequency speed control used? What are its advantages?
In what applications is variable frequency speed control used?
It can be applied to rotating machinery that requires speed regulation.
What are the advantages of variable frequency speed control?
Before the realization of variable frequency speed control (which was theoretically possible long ago, but actually realized after the invention of power electronic devices), traditional speed control used DC. 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 DC motors.
Therefore, the advantages of variable frequency speed control are:
1. It can enable AC motors to achieve speed regulation performance that is as excellent as DC speed regulation.
2. AC squirrel-cage asynchronous motors are simple and convenient to maintain.
3. There is no commutator limitation on the power of AC motors.
5. Is a 100KVA transformer sufficient to power appliances with a total power of 100kW (maximum 37kW)?
What load can a 100KVA transformer handle? The calculation formula below will tell you.
P = Capacity * Power Factor * 80% = 100 * 0.9 * 80% = 72KW. Generally, operating at 20% overload for 1 hour is permissible, so it is 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 the 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.
6. How do I measure the insulation resistance of a motor?
If it is a three-phase AC motor, measure the insulation resistance between phases and to ground of the three-phase windings of the motor.
For DC motors, measure the armature winding to ground, series winding to ground, separately excited winding to ground, and series winding to separately excited winding. Select the appropriate megohmmeter according to the voltage level of the motor being tested.
Measurement steps:
---Disconnect the power
---Discharge to ground
---If it's a three-phase AC motor, open the center point (if possible).
---If it is a DC motor, lift the brushes.
---Use a megohmmeter to test the phase-to-phase and phase-to-ground insulation resistance respectively.
---Discharge to ground
---Restored Line
---Record the insulation resistance and ambient temperature.
7. What is a brushless, ringless starter?
Brushless and ringless starters are starting devices that overcome the disadvantages of wound-rotor asynchronous motors, which have slip rings, carbon brushes, and complex starting devices, while retaining the advantages of wound-rotor motors, such as low starting current and high starting torque. All JR, JZR, YR, and YZR three-phase wound-rotor AC asynchronous motors (excluding variable-speed and those equipped with phase shifters) that were originally started using resistance starters, reactors, frequency-sensitive rheostats, liquid rheostats, or soft starters can be replaced by brushless and ringless starters.
8. What are the different types of capacitor-start motors?
There are two ways to start:
1. Capacitor starting (meaning the capacitor disconnects after the motor starts);
2. The capacitor starts and operates (the capacitor participates in the operation after starting).
9. Can a transformer be used as a load for a frequency converter?
In principle, it should be possible, but in practice, it's impractical. Even without a transformer for voltage boosting, there should be inverter models suitable for circuits above 380V. For even higher voltages, there are circuits that directly convert 220V or 380V and then use a voltage multiplier to obtain the high voltage. Inverters are primarily used for load driving (such as motors), rarely for power supply frequency conversion. Furthermore, the function of an inverter extends far beyond frequency conversion itself, including many additional functions such as various protections. Using an inverter to obtain a variable frequency power supply is not economically feasible; alternative frequency conversion circuits are recommended.
10. Can the frequency converter be adjusted to 1Hz? What is the highest possible frequency (Hz) for use?
If a frequency converter is used on a general AC asynchronous motor, the frequency converter is already close to DC when it is set to 1Hz, which is absolutely not allowed. The motor will operate at the maximum current limit of the frequency converter, which will cause the motor to overheat and may burn out.
Running at frequencies above 50Hz will increase the iron losses of the motor, which is also detrimental to the motor. It is generally best not to exceed 60Hz (exceeding it for a short period of time is permissible), otherwise it will also affect the service life of the motor.
11. What is the working principle of the frequency adjustment resistor in a frequency converter? Why can adjusting the resistor change the frequency?
The frequency adjustment resistor in a frequency converter is used to proportionally divide the 10V reference voltage of the frequency converter and then send the voltage back to the main control board of the frequency converter. The main control board then performs analog-to-digital conversion on the voltage returned by the resistor, reads the data, and then converts it into a proportional value of the rated frequency to output the current frequency. Therefore, adjusting the resistor value can adjust the frequency of the frequency converter.
12. Can a frequency converter decouple the motor current?
Can a frequency converter decouple the motor? No! But as long as the output frequency f and synchronous speed n1 keep the slip rate in the stable region or the rated slip rate Se, it is equivalent to decoupling the motor current, because the rotor power factor is 1 at this time, and the rotor current is the torque current that you want to decouple and control! A frequency converter is a speed control device for asynchronous motors, and it cannot exceed the mechanical characteristics of asynchronous motors to perform any so-called control!
13. Why does an induction motor draw a large current when starting, and then the current decreases after starting?
When an induction motor is stationary, from an electromagnetic perspective, it resembles a transformer. The stator winding connected to the power source is equivalent to the primary coil of the transformer, while the closed-circuit rotor winding is equivalent to the short-circuited secondary coil. There is no electrical connection between the stator and rotor windings, only a magnetic connection; the magnetic flux forms a closed circuit through the stator, air gap, and rotor core. At the instant the circuit is closed, before the rotor starts rotating due to inertia, the rotating magnetic field cuts the rotor windings at its maximum cutting speed—synchronous speed—causing the rotor windings to induce the highest possible electromotive force. Consequently, a large current flows through the rotor conductors. This current generates magnetic energy that cancels out the stator magnetic field, just as the secondary magnetic flux in a transformer cancels out the primary magnetic flux.
In order to maintain the original magnetic flux compatible with the power supply voltage, the stator automatically increases the current. Because the rotor current is very large at this time, the stator current also increases significantly, even reaching 4 to 7 times the rated current. This is the reason for the large starting current.
Why is the current small after startup? As the motor speed increases, the speed at which the stator magnetic field cuts the rotor conductor decreases, the induced electromotive force in the rotor conductor decreases, and the current in the rotor conductor also decreases. As a result, the part of the stator current used to counteract the magnetic flux generated by the rotor current also decreases, so the stator current decreases from large to small until it returns to normal.
14. What impact does the carrier frequency have on the inverter and the motor?
The carrier frequency affects the inverter's output current:
(1) The higher the operating frequency, the larger the duty cycle of the voltage wave and the smaller the higher harmonic components of the current. That is, the higher the carrier frequency, the better the smoothness of the current waveform.
(2) The higher the carrier frequency, the smaller the current that the inverter can output;
(3) The higher the carrier frequency, the smaller the capacitive reactance of the wiring capacitor (because Xc=1/2πfC), and the greater the leakage current caused by the high-frequency pulse.
The effect of carrier frequency on motors:
The higher the carrier frequency, the less vibration, the lower the operating noise, and the less heat the motor generates. However, the higher the carrier frequency, the higher the frequency of harmonic currents, the more severe the skin effect of the motor stator, the greater the motor losses, and the lower the output power.
15. Why can't a frequency converter be used as a variable frequency power supply?
The entire circuit of a frequency converter power supply consists of AC-DC, AC, and filtering components. Therefore, its output voltage and current waveforms are pure sine waves, very close to an ideal AC power supply. It can output the grid voltage and frequency of any country in the world.
A frequency converter is composed of circuits such as AC-DC-AC (modulated wave). The standard name for a frequency converter should be variable frequency speed controller. Its output voltage waveform is a pulsed square wave with many harmonic components. The voltage and frequency change proportionally simultaneously and cannot be adjusted separately, which does not meet the requirements of AC power supply. In principle, it cannot be used as a power supply and is generally only used for speed control of three-phase asynchronous motors.
16. Why is the motor temperature rise higher when using a frequency converter than when using the mains frequency?
Because the output waveform of the frequency converter is not a sine wave but a distorted wave, the motor current at rated torque is about 10% higher than that at the power frequency, so the temperature rise is slightly higher than that at the power frequency.
Another point is that when the motor speed decreases, the motor cooling fan speed is insufficient, and the motor temperature rises higher.
17. What does the protection rating of an electric motor mean?
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. The IP protection rating consists of two digits: the first digit indicates the level of dust and foreign object protection, and the second digit indicates the degree of moisture and water protection. A higher IP number indicates a higher level of protection.
