Poor choice of form
This should be the situation that the friends encounter the most, the big horse pulls the small car, the small horse pulls the big car's situation is everywhere. A large horse and a small cart was considered good. At least, it did not have too many problems in using it. At most, the investment had been increased, and the equipment could still run normally. However, a small horse and a large cart could not. When the load was small, it would run normally, once the load is a little bit bigger to stop, the device frequently alarm, it is very annoying.
Many friends in the choice of frequency converter like motor power to choose, such as: 15kW motor with 15kW inverter, 30kW motor with 30kW inverter. But in fact, the selection of inverter, power is only a reference, more often need to refer to the motor's operating current and the actual load.
Normally, the rated current of the inverter is corresponding to the current of the 400V class four-pole motor, the current of 7.5 kw is about 15 A, the current of 15 kw is about 30 a, which is about twice the power, the higher the power, the lower the power factor. However, if the motor is a higher number of poles, rated current and rated power is not 2 times the relationship, to be higher than this coefficient, such as 7.5 kW 8-pole Y-type motor current in 18A or so, the current of 15kW 8-pole Y-type motor is about 34A. Therefore, in the case of multi-stage motor selection, only the reference power is not enough to select the appropriate frequency converter, the motor's rated current is the necessary reference.
In addition, for lifting or heavy-duty applications, the selection of the inverter must be more careful, in most cases need to enlarge the use. If the rated current of 15kW hoist motor is 30A, at least the frequency converter with rated current above 45A should be selected to meet the application requirements. Because under the circumstances of lifting the application, the motor may appear more than 2 times the overload situation, and the frequency converter general overload coefficient is 1.5 times, according to the frequency converter 1.5 times overload calculation: 1.5 x 45A = 67.5 A, and motor double overload calculation: 2X 30A = 60A, frequency converter rated current of 45A or so to meet most of the application requirements.
Overprotective
Many users, when using the frequency converter, consider that the frequency converter is a high-value device, in order to protect it, will add a bunch of protection devices in the line side of the frequency converter, similar to fuses, thermal Protective relay, residual-current device. And so on. But in fact, the current inverter itself has a very perfect protection function, such as: over-voltage protection, over-current protection, short-phase protection, under-voltage protection... And so on.
In the installation instruction of standard frequency converter, only contactors, circuit breakers and inductors need to be installed on the input side of the converter, and more protection devices may backfire. Users in the design of the installation of frequency converter should be as far as possible in accordance with the standard installation of the implementation of the guide.
Power on frequently
Equipment in the commissioning or operation, often meet the need to restart the situation of power failure, the operator after power failure and then immediately after power failure again... ... Such an operation on the inverter is an absolute life-saving behavior . Because most of the inverter hardware design has a start resistance, it heats up quickly during the first few seconds of power-on, then when the capacitor is charged, the starting resistor cuts out the primary circuit through the bypass contactor and slowly cools down.
If the frequency converter on and off power, the starting resistance is likely to cause overheating or even burn. Therefore, the frequency converter manual will explain the power-off interval of the frequency converter, generally should not be less than 60s, different frequency converter time will be different. Please be careful not to power up and down frequently when using frequency converter!
Current leakage
We have encountered several times the user reported that the frequency converter on the power trip led to the power supply situation. After investigation, it was found that the user had installed a residual-current device on the inlet side of the inverter. In such cases, we would advise the user to either install an isolating transformer on the inlet side of the power supply or remove the residual-current device, or simply disengage/disconnect the EMC grounding terminals inside the inverter.
Because most of the converter built-in EMC filter to prevent interference into the filter must be grounded to work properly. While EMC grounding will inevitably result in leakage current generation, the general inverter in the EMC filter will produce tens to hundreds of milliamps of leakage current, the normal residual-current device threshold is set at a few hundred milliamps, which can easily break through and cause a protective action.
⚠️ Cutting off the EMC grounding terminal may result in the inverter not meeting the EMC requirements of IEC 61800-3 standard.
Be sure to save energy
In the past few years, when energy-saving and emission-reduction campaigns were in full swing, inverters really caught fire. No matter what type of application, it seemed that adding inverters could reduce energy consumption and emissions. But in fact, the use of frequency converter is not necessarily energy-saving, energy-saving depends entirely on the type of its load and operating conditions. Only in the right application, the right occasion, the inverter can play an energy-saving effect.
Like centrifugal fan, water pump such load, if before in the frequency 50Hz operation, and do not need to through the baffle or valve to adjust the air, water pressure, using frequency converter will only increase energy consumption, waste of resources; If you need to use mechanical devices to adjust the air or water pressure, you can use frequency converter to reduce motor speed, in order to adjust the air, water pressure, but also reduce energy consumption.
Poor heat dissipation
When some users design the installation mode of the converter cabinets, they consider that the value of the converter is higher and the cabinets are made more tightly. For ventilation and heat dissipation, they will add a cooling fan at the bottom or install an exhaust fan at the top, but even so, or often encounter a frequency converter overheat alarm equipment downtime.
The power loss and cooling air flow rate of this type of converter under rated operating conditions will be stated in all the manual of the converter. If the designer is designing the cabinet, did not take into account the frequency converter itself when running will have a higher heat generation, the cabinet cooling fan selection is too small, resulting in the heat generated by the frequency converter can not be distributed in the cabinet, finally, it is likely to cause the frequency converter overheating alarm, equipment shutdown.
Our suggestion is that, under the condition of low protection level, there is no need to install the cooling fan on the frequency converter, because the heat dissipation condition has been considered when the frequency converter is designed, and most of them have built-in cooling fan; When the inlet and outlet duct is well made, the converter only needs to open a large-size air inlet at the bottom, and then install the side outlet cover at the top to meet the heat dissipation requirements of most applications.
The above is only the use of the frequency converter in the error of a few cases, if you have more need to add, welcome comments.
