I. How to adjust the frequency of a frequency converter
For a frequency converter to operate normally, it must meet two basic conditions: a frequency signal and an operating signal. Let's talk about the first condition, which is the frequency signal of the frequency converter.
The purpose of using a frequency converter is to change the speed of the motor by changing the output frequency of the converter. So how do we adjust the output frequency of the frequency converter? The key is to change the frequency signal provided by the frequency converter, which is called the "frequency setpoint signal". The frequency signal can be obtained from the following sources:
1. Frequency Setting via Operator Panel: Frequency setting via the operator panel is the simplest method for frequency converters. Users can directly change the set frequency using the potentiometer, numeric keys, or up/down keys on the operator panel. The biggest advantage of operator panel setting is its simplicity and convenience, while also providing monitoring functionality, displaying real-time data such as current, voltage, and speed during inverter operation.
If you choose to input a digital signal using the keyboard numeric keys or the up and down keys, the accuracy and resolution are very high because it is a digital input. If you choose to input a digital signal using the potentiometer on the control panel, the accuracy is slightly lower, but it is very practical because it does not require additional wiring like an external potentiometer for analog input.
2. External potentiometer setting refers to adjusting the frequency by using a potentiometer input from outside the frequency converter.
3. Multi-function input terminals: The set frequency value of the frequency converter is changed through the multi-function input terminals. These terminals can be connected to external buttons, PLCs, or relay output points. Through function settings, two or more of the frequency converter's function input terminals can be used for frequency setting.
II. The difference between overload and overcurrent protection of frequency converters
① The protected objects are different. The overcurrent protection of a variable frequency speed control system protects the frequency converter, while the overload protection protects the motor. In a variable frequency speed control system, the capacity of the frequency converter is selected with a certain reliability factor relative to the capacity of the motor. That is, when the motor is overloaded, the frequency converter may not necessarily experience overcurrent.
Overload protection of the variable frequency speed control system is accomplished by the electronic thermal protection relay inside the frequency converter. When setting the electronic thermal protection relay, the percentage ratio of the motor's rated current to the frequency converter's rated current should be accurately set.
IM = (Ie/IN) × 100% (7-1)
In the formula, IM represents the current draw ratio.
Ie—Rated current of the electric motor, in A;
IN—Rated current of the frequency converter, in A.
② The rates of change of current differ. The rate of change of overload fault current, di/dt, is usually relatively small, meaning the current increase is gradual, with a time order of min. Overcurrent, on the other hand, is sudden, and the rate of change of current, di/dt, is often larger, with a time order of μs or ms.
③ Overload protection has inverse-time characteristics. The overload protection in a variable frequency speed control system primarily prevents motor overheating and exhibits inverse-time characteristics. Inverse-time means the operating time is related to the magnitude of the overload current; when the overload current is large, the operating time is short; conversely, the operating time is long. A relay utilizing this characteristic is called an inverse-time overcurrent relay. Its operating current and operating time relationship can be divided into two parts: one part is definite-time, and the other part is inverse-time. When the short-circuit current exceeds a certain multiple, the increase in current no longer shortens the operating time; at this point, it exhibits definite-time characteristics.
When an electric motor is overloaded, if the overload current value does not exceed the rated current value by much, the motor can be allowed to run for a longer period of time; however, if the overload current value exceeds the rated current value by a large margin, the allowed running time will be shortened.
In addition, the heat dissipation of the motor deteriorates when it operates in the low-frequency range. Therefore, under the same 50% overload, the lower the frequency, the shorter the allowable operating time.
