12 Practical Knowledge Sharing of Frequency Converter

01Inverter selection

Generally speaking, the selection of the inverter should be based on the actual working current of the motor. Of course, if you can't find the most suitable one in a short time, you can also choose a larger model, because a large inverter can drive a small motor, but a small inverter definitely can't drive a large motor!

02The significance of inverter resolution

For digitally controlled inverters, even if the frequency command is an analog signal, the output frequency is given in steps. The minimum unit of this step difference is called the frequency conversion resolution.

The frequency conversion resolution is usually 0.015~0.5Hz; for example, if the resolution is 0.5Hz, then 23Hz can be changed to 23.5 or 24.0 Hz, so the motor's action can also follow in steps.

03The principle of motor regenerative braking

When the rotor speed of the motor exceeds the rotation speed of the motor's synchronous magnetic field, the rotation direction of the electromagnetic torque generated by the rotor winding is opposite to the rotation direction of the rotor. At this time, the motor is in a braking state, which is called regenerative braking.

04How to get greater braking force

The energy regenerated from the motor is stored in the filter capacitor of the inverter. Due to the relationship between the capacity and withstand voltage of the capacitor, the regenerative braking force of a general inverter is about 10%~20% of the rated torque. If an optional brake unit is used, it can reach 50%~100%.

05What situations will trigger the protection function

When the clutch connects to the load, at the moment of connection, the motor changes sharply from the no-load state to the area with large slip rate. The large current flowing through causes the inverter to trip due to overcurrent and cannot operate, thus activating the protection function of the inverter!

06What is the motor stall prevention function?

Stall: refers to when the acceleration time is too short, the output frequency of the inverter changes far more than the speed (electrical angular frequency), the inverter will trip due to overcurrent, and the operation will stop. In order to prevent stalling and keep the motor running, the current size must be detected for frequency control.

07Causes of inverter overvoltage alarm

The overvoltage alarm usually occurs when the machine is shut down. The main reason is that the deceleration time is too short or there is a problem with the brake resistor and brake unit.

08What should I pay attention to when the motor operates over 60Hz

(1) It is fully possible for machinery and equipment to operate at this speed (mechanical strength, noise, vibration, etc.)

(2) When the motor enters the constant power output range, its output torque must be able to maintain operation (the shaft output power of fans, pumps, etc. increases proportionally with the cube of the speed, so attention should also be paid when the speed increases slightly).

(3) The life of the bearing should be fully considered.

(4) For motors with medium capacity or above, especially two-pole motors, special attention should be paid when running at above 60 Hz.

09What will happen if the inverter is not used for a long time?

(1) The lubricating fluid in the inverter fan bearing dries up, affecting its use.

(2) High-voltage filter capacitors are prone to bulging if not used for a long time, and low-voltage electrolytic capacitors are prone to leakage.

(3) Low-voltage electrolytic capacitors may also experience electrolyte overflow, and even cause corrosion to printed circuit boards.

10When should a DC/AC reactor be installed?

(1) When the inverter is connected to a power transformer of 600KVA or above or when switching the phase-advancing capacitor.

(2) When a thyristor converter such as a DC drive is connected to the same power supply system, a DC reactor or AC reactor must be installed.

11 Reasons for frequent inverter alarms

(1) If the ambient temperature is too high, the temperature of the inverter's internal components will be too high. To protect the inverter's internal circuit, the inverter will report a high temperature fault and shut down.

(2) The inverter is poorly ventilated. For example, if the air duct of the inverter itself is blocked or the air duct of the control cabinet is blocked, it will affect the heat dissipation inside the inverter and cause the inverter to overheat and alarm.

(3) The fan is blocked or damaged, and a large amount of heat accumulates inside the inverter and cannot be dissipated.

(4) When the inverter is overloaded, it will generate excessive current and a large amount of heat. Sometimes the inverter will also overheat and alarm.

12 Issues to pay attention to during the installation of the inverter

(1) Heat dissipation: The heat generated by the inverter is caused by internal losses. The main circuit accounts for about 98% of the losses in the inverter, and the control circuit accounts for 2%. In order to ensure the normal and reliable operation of the inverter, the inverter must be cooled. We usually use fans for cooling.

(2) Electromagnetic interference problem: When there are high-frequency impact loads in the system, such as welding machines and electroplating power supplies, the inverter itself will be protected due to interference, so the power quality of the entire system should be considered. In addition, if the power of the inverter is very large and accounts for more than 25% of the entire system, anti-interference measures for the control power supply need to be considered.

(3) Protection issues:

1. Dust prevention: All air inlets should be equipped with dust screens to prevent flocculent debris from entering. The dust screens should be designed to be detachable for easy cleaning and maintenance. The grid of the dust screen should be determined according to the specific conditions on site, and the joints around the dust screen and the control cabinet should be handled tightly.

2. Anti-corrosive gas: This situation is more common in the chemical industry. At this time, the frequency converter cabinet can be placed in the control room.

3. Waterproof and anti-condensation: If the inverter is placed on site, it is necessary to pay attention to whether there are pipes or other leakage points above the inverter cabinet. There should be no splashing water near the inverter. In short, the protection level of the on-site cabinet must be above IP43.