How to Select a Variable Frequency Drive (VFD)?

With the rapid development of industrial automation, the variable frequency drive (VFD), used for frequency conversion debugging, is thereof widely used. As an important role of frequency conversion speed regulation and energy saving, VFD is mainly used to control the AC motor's power control equipment by changing the frequency of the motor's working power supply. It's not only advantageous for improving the enterprise's process and production level, but also plays an important role in energy saving. But how should we choose an appropriate VFD?

On choosing the type of a variable frequency drive, it's best to choose the control mode of the VFD according to the type of production machinery, speed range, static speed accuracy and requirements for starting torque. And the best mode means that VFD is easy-to-use and economic, while meeting the basic conditions and requirements of process and production.

Selection of motor controlled by VFD
Pole number of the motor is taken into account. In general, a motor shall not have more than four poles, otherwise the capacity of VFD shall be enlarged. As for the torque characteristics, critical torque, acceleration torque, a lower grade can be selected for variable frequency drive when the motor power keeps the same. Electromagnetic compatibility should be taken into consideration. To reduce the interference of main power supply, a reactor can be added in the intermediate circuit or input circuit of the VFD in use, or the pre-isolation transformer can be installed. Generally, when the distance between the motor and VFD exceeds 50 m, a reactor or filter shall be installed in series between them, or shielded protective cable shall be used.

How can we recognize a motor's pole number? We can make a calculation. According to the speed formula, speed = time (60 seconds) × frequency (50Hz) divided by the number of pole pairs. Here is a sample of ATO 200w brushless DC motor. The motor's rated speed is 3000 rpm, the pole pairs is (3000 rpm)/(60 seconds × 50Hz)=1. That is to say, this motor has two poles.

Selection of VFD Power
System efficiency equals the product of variable frequency drive efficiency and motor efficiency, only when both of them work at high efficiency, the system efficiency would be high. From the efficiency point of view, attentions should be paid to the following points when selecting the VFD power:

1. It's most suitable for the VFD to work at a high efficiency when the VFD power equals that of the motor.
2. If the power classification of VFD is different from that of motor, the VFD power shall be close to that of the motor as much as possible, while it should be a little larger.
3. When the motor starts frequently, brakes or starts at a heavy load, a larger class VFD can be selected to ensure the VFD to work safely for a long term.
4. Through test, the motor's actual power is sufficient, so the VFD with a smaller power than the motor could be selected, but it shall be noted that whether the instantaneous peak current would cause overcurrent protection action.
5. When the power of VFD is different from that of the motor, the energy-saving program setting shall be adjusted correspondingly, so as to achieve higher energy-saving effect.

Selection of VFD Box Structure
Variable frequency drivebox structure shall adapt to the environmental conditions, namely the factors such as temperature, humidity, dust, acidity and alkalinity, corrosive gas shall be considered.

Determination of VFD Capacity
Reasonable capacity selection is an energy-saving and consumption-reducing measure. Based on the available information and experience, there are three simple methods:

1. Determine by the motor's actual power. Firstly, measure the motor's actual power, so as to choose the VFD capacity.
2. Formula method. When a VFD is used for several motor, it shall meet the following condition: the starting current of at least one motor shall be considered, so as to avoid over-current tripping of variable frequency drive.
3. Rated current method. Determination of VFD capacity is actually to find a best matching process between the variable frequency drive and motor. The most common and safe method is to make the VFD capacity greater than or equal to the rated power of the motor, but actually, we shall consider the difference between the motor's actual power and rated power. In general, a larger power shall be selected for the equipment while the actual need is smaller, so it is reasonable to choose the VFD according to the motor's actual power, which can avoid to choose an excessive VFD and excessive investment.

For light loads, the variable frequency drive current shall be selected by 1.1N (N refers to the rated current of motor) or by the maximum motor power matched with the VFD rated output power specified by the manufacturer in the product.

Considering the Effect of Main Power Supply

1. Supplied voltage and fluctuation. It shall be specially noted to adapt to the setting value of low voltage protection of VFD, because the grid voltage is more possible to be low in actual use.
2. Frequency fluctuation of main power supply and harmonic interference. This kind of interference will increase the heat loss of the VFD system, resulting in increased noise and reduced output.
3. Power consumption of VFD and motor in the work. When designing the power supply of system main power supply, power consumption factors of them shall be taken into account.

The VFDs provided by ATO have three categories: single phase VFD, single to three phase VFD and three phase VFD. According to the different functions of ATO VFDs, their prices varies from $100 up to $10000. Because a variable frequency drive that will be chosen should perfectly satisfy our specific requirements, we must think about all kinds of cases to insure the choice to be correct.