Suppliers of frequency converter braking units remind you that due to government policy calls, strong promotion of frequency converter technology, and strong promotion by frequency converter merchants, some industrial enterprises have subconsciously equated the use of frequency converters with energy conservation and electricity saving. However, in practical use, due to different industrial control situations, many enterprises gradually realize that not all places where frequency converters are applied can save energy and electricity. So what are the reasons for this situation and what are the misconceptions people have about frequency converters?
1. Variable frequency drives can save energy when used on all types of motors
Whether a frequency converter can achieve power saving is determined by the speed regulation characteristics of its load. For centrifugal machines, fans, and water pumps, which belong to quadratic torque loads, the motor output power P ∝ Tn and P ∝ n3 must be met, that is, the output power on the motor shaft is proportional to the third power of the speed. It can be seen that for quadratic torque loads, the energy-saving effect of frequency converters is the most prominent.
For constant torque loads, such as Roots blowers, the torque is independent of the speed. Generally, an exhaust outlet is set up and controlled by a valve. When the air volume exceeds the demand, excess air volume is discharged to achieve adjustment. In this case, speed regulation can be used for operation, which can also achieve energy-saving effects. In addition, for constant power loads, the power is independent of the speed. In these cases, there is no need to use a frequency converter.
2. Misconceptions about incorrect methods in energy consumption calculation
Many companies often use reactive power compensation based on apparent power when calculating energy-saving effectiveness. For example, when a motor is running at full load under power frequency conditions, the measured operating current is 194A. After using variable frequency speed regulation, the power factor during full load operation is increased to about 0.99. At this time, the measured current is 173A. The reason for the decrease in current is that the internal filtering capacitor of the frequency converter improves the system power factor.
According to the apparent power calculation, the energy-saving effect is as follows:
ΔS=UI=380×(194-173)=7.98kVA
The energy-saving effect is about 11% of the rated power of the motor.
In fact, the apparent power S is the product of voltage and current. Under the same voltage conditions, the change in apparent power is proportional to the change in current. Considering the system reactance in the circuit, the apparent power does not represent the actual power consumption of the motor, but represents the maximum output capacity under ideal conditions. The actual power consumption of the motor is usually expressed as active power. The actual power consumption of a motor is determined by the motor and its load. After increasing the power factor, the load of the motor does not change, and the efficiency of the motor also does not change. Therefore, the actual power consumption of the motor will not change. After increasing the power factor, there was no change in the motor's operating state, stator current, active and reactive currents. So how is the power factor improved? The reason lies in the filtering capacitor inside the frequency converter, and part of the motor's consumption is the reactive power generated by the filtering capacitor. The improvement of power factor reduces the actual input current of the frequency converter, and also reduces the line loss and transformer loss of the power grid. In the above calculation, although the actual current is used for calculation, the apparent power is calculated instead of the active power. Therefore, using apparent power to calculate energy-saving effects is incorrect.
3. As an electronic circuit, the frequency converter itself also consumes power
From the composition of the frequency converter, it can be seen that the frequency converter itself has electronic circuits, so it also consumes power during operation. Although it consumes less compared to high-power motors, its own power consumption is an objective fact. According to expert calculations, the maximum self power consumption of a frequency converter is about 3-5% of its rated power. A 1.5-horsepower air conditioner consumes 20-30 watts of electricity, equivalent to a continuous light.
In summary, it is a fact that frequency converters have energy-saving functions when operating at power frequency, but their prerequisite is: firstly, high power and being a fan/pump load; Secondly, the device itself has energy-saving function (software support); Thirdly, long-term continuous operation. These are the three conditions under which a frequency converter can demonstrate energy-saving effects.