Problems in the use of frequency converter motor

Motor is the most frequently used rotating tool so far. With the development and popularization of frequency converter, more and more motors need to be used together with frequency converter. However, many problems will inevitably be encountered in the process of supporting the use of frequency converter and motor. These exposed problems are more and more worthy of our deep thinking and communication.

1. Can the motor soft starter save energy?

The energy-saving effect of soft start is limited, but it can reduce the impact of starting on the power grid, and can also achieve smooth starting and protect the motor windings.

According to the theory of conservation of energy, due to the addition of a relatively complex control circuit, soft start not only does not save energy, but also increases energy consumption, but it can reduce the starting current of the circuit and play a protective role.

2. How about the starting current and starting torque of the motor when the frequency converter is used?

With frequency converter operation, the frequency and voltage are increased accordingly with the acceleration of the motor, and the starting current is limited to less than 150 per cent of the rated current (125 to 200 per cent, depending on the type of machine). When starting directly with the power frequency power supply, the starting current is 6 to 7 times, so it will produce mechanical and electrical impact. The frequency converter drive can be used to start smoothly (the starting time becomes longer). The starting current is 1.2~1.5 times of the rated current, and the starting torque is 70% ~ 120% of the rated torque. For the frequency converter with automatic torque enhancement function, the starting torque is more than 100%, and it can be started with full load.

3. Is there any connection between overload and short circuit of the motor?

 

There are two kinds of motor overload;
1. It is mechanical load overload: it is the overload that drives the load exceeding the rated value or the transmission system has blocking phenomenon, which has nothing to do with short circuit.
2. The load is normal: the motor current is overloaded, which may be a short circuit phenomenon between local ground and turns of the motor winding.
 


 

4. What is the application of variable frequency speed regulation? What are the benefits?

It can be applied to rotating machinery with speed regulation requirements.

Before the realization of variable frequency speed regulation (in theory, it has already been realized, but the real realization is after the invention of power electronic devices), the traditional speed regulation adopts DC, and the disadvantages of DC speed regulation are:

1. The DC motor has complex structure and high maintenance cost;

2, due to the existence of the commutator, DC motor power has not much room to rise;

Therefore, the benefits of variable frequency speed regulation are:

1. It can make the AC motor get the same excellent speed regulation performance as the DC speed regulation

2, AC squirrel cage asynchronous motor maintenance is simple and convenient

3. AC motor power does not have the limitation of commutator

5. Is it enough to use a 100KVA transformer to supply a total power of 100KW electrical appliances (up to 37KW)? How much load can a 100KVA transformer carry?

P = capacity * power factor * 80% = 100*0.9 * 80% = 72KW, generally 20% overload operation for 1 hour is allowed, so it is enough.

It mainly depends on whether the total current exceeds or not. The high voltage current of 100KVA transformer is 5.8A and the low voltage current is 150A A. It doesn't matter even if it exceeds occasionally. It mainly depends on the temperature rise not exceeding 55 degrees. The temperature rise is equal to the actual temperature minus the ambient temperature.
 
 

6. How to measure the insulation resistance of the motor?

If it is a three-phase AC motor, measure the phase-to-phase and ground insulation resistance of the three-phase windings of the motor.
If it is a DC motor, measure the armature winding of the motor to the ground, the series winding to the ground, the other excitation winding to the ground, and the series excitation winding to the other excitation winding. Select the corresponding shaking table according to the voltage level of the tested motor.
 

 

Measurement steps:

--- Disconnect the power supply

--- Discharge to ground

--- If it is the center point of three-phase AC motor opening (if it is possible)

--- If it is a DC motor, lift the brush.

--- Detect phase-to-phase and ground insulation resistance with shaking table respectively

--- Discharge to ground

--- Restore line

-Record insulation resistance and ambient temperature.
 
 

7. What is a brushless ringless starter?

Brushless ring-free starter is a kind of starting equipment that overcomes the shortcomings of wound asynchronous motors equipped with slip rings, carbon brushes and complex starting devices, while retaining the advantages of small starting current and large starting torque of wound motors. All JR, JZR, YR and YZR three-phase wound rotor AC asynchronous motors (except those with variable speed and camera) that originally used resistance starter, reactor, frequency sensitive rheostat, liquid rheostat and soft starter can be updated with "brushless ring-free starter.
 
 

8. How many kinds of capacitor starting methods are there for the motor?

There are two types of startup:

1. Capacitor start (refers to the capacitor is disconnected after the motor is started);

2. The capacitor starts and runs (the capacitor participates in the operation after starting).

Can the transformer be used as the load of the inverter?

In principle, it should be possible, but it is not practical in practice. The frequency converter does not need transformer to boost voltage, and there should also be varieties that can be used for circuits above 380V. If higher voltage is required, there are also circuits that directly use 220V or 380V direct frequency conversion and then use voltage doubling to obtain high voltage.

Frequency converter is mainly used for load drive (such as motor), rarely used for power frequency conversion, and the function of frequency converter is far from limited to frequency conversion itself, there are many additional functions, such as various types of protection, if the use of frequency converter to obtain variable frequency power supply, from the economic point of view is not desirable, it is recommended to use other frequency conversion circuit.

10. Can the frequency converter be adjusted to 1HZ and how many HZ can be adjusted to use?

If the frequency converter is used in a general AC asynchronous motor, the frequency converter is close to DC when it is adjusted to 1HZ, which is absolutely impossible. The motor will run under the maximum current limited by the frequency converter, and the motor will heat up seriously and may burn the motor.

If it exceeds 50Hz, it will increase the iron loss of the motor, which is also unfavorable to the motor. Generally, it is best not to exceed 60Hz, (it is allowed to exceed it in a short time), otherwise it will also affect the service life of the motor.
 
 

11. What is the working principle of the frequency adjustment resistor of the inverter? Why can the adjustment resistor change the frequency?

The frequency adjustment resistor of the inverter is used to divide the 10V reference voltage of the inverter in proportion, and then send it back to the main control board of the inverter. The main control board of the frequency converter converts the voltage sent back by the resistor to read the data, and then converts it into a proportional value of the rated frequency to output the current frequency, so the frequency of the frequency converter can be adjusted by adjusting the resistance value.
 
 

12. Can the frequency converter decouple the motor current?

Can frequency conversion be decoupled? No.

However, as long as the output frequency f and synchronous speed n1 keep the slip rate in a stable area 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 to be controlled by everyone to be decoupled! The inverter is the speed control device of the asynchronous motor, it is impossible to go beyond the mechanical characteristics of the asynchronous motor and the so-called any control!

13. Why is the current large when the induction motor starts? And the current will become smaller after starting?

When the induction motor is in a stopped state, from an electromagnetic point of view, it is like a transformer. The stator winding connected to the power supply is equivalent to the primary coil of the transformer, and the closed-circuit rotor winding is equivalent to the secondary coil of the transformer that is short-circuited; There is no electrical connection between the stator winding and the rotor winding, only the magnetic connection, and the magnetic flux passes through the stator, air gap, and rotor core into a closed circuit.

At the moment of closing, the rotor has not yet rotated due to inertia, and the rotating magnetic field cuts the rotor winding at the maximum cutting speed-synchronous speed, so that the rotor winding induces the highest possible potential. Therefore, a large current flows in the rotor conductor. This current generates magnetic energy that offsets the stator magnetic field, just as the secondary magnetic flux of a transformer offsets the primary magnetic flux.

In order to maintain the original magnetic flux corresponding to the power supply voltage, the stator automatically increases the current. Because the current of the rotor is very large at this time, the stator current also increases greatly, even up to 4 to 7 times the rated current, which is the reason for the large starting current.

Why is the current small after starting: As the motor speed increases, the speed of the stator magnetic field cutting the rotor conductor decreases, the induced potential in the rotor conductor decreases, and the current in the rotor conductor also decreases, so the stator current is used to offset the rotor current The part of the current that affects the magnetic flux is also reduced, so the stator current is from large to small until normal.

14. What is the effect of carrier frequency on frequency converter and motor?

The carrier frequency has an effect on the output current of the inverter:

(1) The higher the operating frequency, the greater the duty cycle of the voltage wave, the smaller the current high-order harmonic components, that is, the higher the carrier frequency, the better the smoothness of the current waveform;

(2) The higher the carrier frequency, the smaller the output current allowed by the inverter;

(3) The higher the carrier frequency, the smaller the capacitive reactance of the wiring capacitance (because Xc = 1/2πfC), and the larger the leakage current caused by the high-frequency pulse.

carrier frequency impact on the motor:

The higher the carrier frequency, the smaller the vibration of the motor, the smaller the running noise, and the less the motor heats up. However, the higher the carrier frequency, the higher the frequency of the harmonic current, the more serious the skin effect of the motor stator, the greater the motor loss, the smaller the output power.

Why can't the inverter be used as a variable frequency power supply?

The whole circuit of variable frequency power supply is composed of parts such as AC DC-AC-filter, so its output voltage and current waveforms are pure sine wave, very close to the ideal AC power supply. It can output the grid voltage and frequency of any country in the world.

The frequency converter is composed of AC DC-AC (modulation wave) and other circuits. The standard name of the frequency converter should be frequency converter. The waveform of the output voltage is a pulse square wave, and there are many harmonic components. The voltage and frequency change proportionally at the same time, which cannot be adjusted separately and 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 regulation of three-phase asynchronous motors.

16. When using frequency converter, why is the temperature rise of motor higher than that of power frequency?

Because the output waveform of the inverter is not a sine wave, but a deformed wave, the motor current under the rated torque is about 10% more than the power frequency, so the temperature rise is slightly higher than the power frequency.
 
 

Another point: when the motor speed is reduced, the motor cooling fan speed is not enough, the motor temperature rise will be higher.

17. What does the protection level of the motor mean?

For example, IP23 motor refers to the motor can prevent the intrusion of solid objects larger than 12mm, prevent human fingers from touching internal parts, and prevent the intrusion of medium-sized (large diameter 12mm) foreign objects. It can prevent the intrusion of sprayed water, or prevent the water sprayed in the direction with the vertical angle less than 60 degrees from entering and causing damage.

The IP(INTERNATIONAL PROTECTION) protection level system is drafted by the IEC (INTERNATIONAL ELECTROTECHNICAL COMMISSION). The motor shall be classified according to its dust-proof and moisture-proof characteristics. The foreign objects referred to here include tools, and human fingers, etc. cannot touch the live parts in the motor to avoid electric shock.
 
 

IP protection level is composed of two numbers. The first number indicates the level of the motor from dust and preventing foreign objects from invading. The second number indicates the sealing degree of the motor against moisture and water intrusion. The larger the Lou word, the higher the protection level.

Source: Technical Training

Disclaimer: This article is reproduced on the Internet and is copyrighted by the original author. If the text used in this article involves the copyright of the work, please inform us as soon as possible, we will confirm the copyright according to the supporting materials you provide and delete the content immediately!