Industrial inverters use seven major misunderstandings

Misunderstanding: The universal motor can only be operated with a frequency converter below its rated speed.

According to classical theory, the upper limit of the universal motor frequency is 55 Hz. This is because when the motor speed needs to be adjusted above the rated speed, the stator frequency will increase above the rated frequency (50 Hz). At this time, if it is still controlled according to the constant torque principle, the stator voltage will rise above the rated voltage. Then, when the speed regulation range is higher than the rated speed, the stator voltage must be kept constant at the rated voltage. At this time, as the rotation speed/frequency increases, the magnetic flux will decrease, so the torque at the same stator current will decrease, the mechanical characteristics become soft, and the overload capability of the motor is greatly reduced.

It can be seen that the upper limit of the universal motor frequency is 55Hz, which is a prerequisite:

The stator voltage must not exceed the rated voltage;

The motor is running at rated power;

Constant torque load.

Under the above circumstances, the theory and experiments prove that if the frequency exceeds 55 Hz, the motor torque will be reduced, the mechanical characteristics will be soft, the overload capacity will be reduced, the iron loss will increase sharply, and the heat will be severe.

In general, the actual running condition of the motor indicates that the universal motor can be speed-up through the inverter. Can the frequency increase speed? How much can be raised? It is mainly determined by the load dragged by the motor. First, what is the load rate? Secondly, it is necessary to understand the load characteristics and calculate according to the specific conditions of the load.

A simple analysis is as follows:

In fact, for a 380V universal motor, long-term operation of the stator voltage exceeding 10% of the rated voltage is possible, and has no effect on the insulation and life of the motor. As the stator voltage increases, the torque increases significantly, the stator current decreases, and the winding temperature decreases.

Motor load rate is usually 50% to 60%

In general, industrial motors typically operate at 50% to 60% of rated power. It is estimated that the motor output power is 70% rated power, and the stator current is reduced by 26.4% when the stator voltage is increased by 7%. At this time, even with constant torque control, the inverter speed is increased by 20%, and the stator current is not only increased. It will not rise, but will decline. Although the motor's iron consumption increases sharply after the frequency is increased, the amount of heat generated by it is slightly less than the amount of heat that is reduced by the stator current. Therefore, the motor winding temperature will also drop significantly.

Various load characteristics

The motor drag system is serviced by the load, with different loads and different mechanical characteristics. The motor must meet the requirements of the mechanical characteristics of the load after the speed is increased. The maximum allowable operating frequency (fmax) at different load rates (k) of the constant torque load is inversely proportional to the load rate, ie fmax=fe/k, where fe is the rated power frequency. For constant power loads, the maximum allowable operating frequency of a universal motor is mainly limited by the mechanical strength of the rotor and shaft of the motor. The author believes that the limit is generally limited to 100 Hz.

Misunderstanding: Ignore the characteristics of the inverter

The debugging work of the frequency converter is generally completed by the distribution manufacturer, and there will be no problem. The installation of the inverter is relatively simple and is usually done by the user. Some users do not carefully read the instruction manual of the inverter, do not strictly follow the technical requirements for construction, ignore the characteristics of the inverter, equate it with the general electrical components, and take care of the faults and accidents.

According to the requirements of the instruction manual of the inverter, the cable connected to the motor should be shielded or armored, preferably in metal pipe. The ends of the cut-off cable should be as neat as possible, the unshielded segments should be as short as possible, and the cable length should not exceed a certain distance (typically 50m). When the wiring distance between the inverter and the motor is long, the high harmonic leakage current from the cable will adversely affect the inverter and peripheral equipment. The ground wire returned from the motor controlled by the inverter should be directly connected to the corresponding ground terminal of the inverter. Do not share the grounding wire of the inverter with the welder and power equipment as short as possible. Since the inverter generates leakage current, the potential of the ground terminal is unstable due to the distance from the ground point. The minimum cross-sectional area of the grounding wire of the inverter must be greater than or equal to the cross-sectional area of the power supply cable. In order to prevent malfunction caused by interference, the control cable should use stranded shielded wires or double-stranded shielded wires. At the same time, be careful not to touch the shielded network cable to other signal cables and equipment casings, and wrap them with insulating tape. In order to avoid its influence by noise, the length of the control cable should not exceed 50m. The control cable and motor cable must be laid separately, using separate wireways and as far as possible. When the two must cross, a vertical crossing should be taken. Never put them in the same pipe or cable trough. However, some users did not strictly follow the above requirements when laying cables, resulting in normal operation of the equipment during separate commissioning, and serious interference during normal production, which prevented operation.

Special care should also be taken in the routine maintenance of the inverter. When some electricians find that the inverter has tripped, it will immediately turn on the inverter for maintenance. This is very dangerous and there is a possibility of personal electric shock. This is because even if the inverter is not in operation, even if the power supply has been cut off, the inverter's power input line, DC terminal and motor terminals may still have voltage due to the presence of the capacitor. After disconnecting the switch, you must wait a few minutes for the inverter to discharge before you can start working. Some electricians are accustomed to the insulation test of the motor driven by the inverter when the frequency conversion speed control system is tripped, and it is judged whether the motor is burnt. This is also very dangerous, and it is easy to make the inverter burn. Therefore, the insulation test of the motor must not be performed before the cable between the motor and the inverter is disconnected, nor can the insulation test of the cable connected to the inverter be performed.

Pay special attention when measuring the output parameters of the inverter. Since the output of the inverter is a PWM waveform containing high-order harmonics, and the motor torque mainly depends on the effective value of the fundamental voltage, when measuring the output voltage, the fundamental voltage value is mainly measured, and the rectifier voltmeter is used for measurement. The result is closest to the digital spectrum analyzer measurement and has an excellent linear relationship with the output frequency of the frequency converter. If you need to further improve the measurement accuracy, you can use a RC filter. Digital multimeters are susceptible to interference and measurement has large errors. The output current needs to measure the total effective value including the fundamental wave and other higher harmonics. Therefore, the commonly used instrument is a moving coil ammeter (when the motor is loaded, the rms value of the fundamental current and the total current RMS are not much different). When a current transformer is used in consideration of the convenience of measurement, the current transformer may be saturated at a low frequency, so it is necessary to select a current transformer of an appropriate capacity.