Common Misconceptions in Variable Frequency Drive Selection
Myth 1: Choosing a frequency converter solely for energy saving. Many manufacturers and salespeople tout the high energy-saving rate of frequency converters, and users believe them, spending a lot of money to choose frequency converters simply to save electricity, only to be greatly disappointed. Whether a frequency converter can save electricity depends on the type of load it drives. For loads like fans and pumps, the energy-saving effect of using a frequency converter is significant, while for constant power loads and constant torque loads, the energy-saving effect is much worse, or even non-existent.
Myth 2: Determining the inverter selection based on the rated power on the motor nameplate.
There is a theoretical basis for selecting a frequency converter based on the rated power of the motor. However, in many actual field situations, the motor has too much operating margin or is overloaded. In such cases, the selected frequency converter may be too large, resulting in economic waste, or too small, causing motor damage or frequency converter failure.
The simplest estimation method is to select a frequency converter based on 1.1 times the maximum operating current of the motor during stable operation. If the mechanical equipment is heavy-duty, the frequency converter needs to be upgraded by one level.
Inverter Selection Analysis
The load equipment driven by the frequency converter system can be roughly divided into three types.
Constant torque load
constant power load
Fan and pump loads
The difference between constant power and constant torque: constant power is mostly used in situations with relatively light loads, while constant torque is mostly used in situations with heavy loads.
Therefore, different inverter models are selected for different load types.
>>>Constant power load<<<
Constant power speed regulation refers to a motor whose output torque is high at low speeds and low at high speeds, meaning the output power is constant. Examples include machine tools, cutting machines, and papermaking machines.
For example, machine tool frequency converter systems are constant power loads with extremely large load inertia. The frequency converter should be selected to be of the same power, with an additional braking unit and braking resistor.
>>>Constant Torque Load<<<
A constant torque load is characterized by its torque being independent of speed; the torque remains constant or nearly constant at any speed. Equipment with constant torque loads requires a frequency converter with high low-frequency torque and fast dynamic response. Examples include friction loads such as conveyor belts, mixers, and extruders, as well as potential energy loads such as cranes and hoists.
For example, the main bucket inverter system of a sand dredging vessel is a constant torque load with dual motors and a long overload time. Therefore, the inverter should be selected and configured to be two sizes larger. At the same time, the power supply of this equipment is mostly generated by generators, so the inverter needs to be equipped with an input AC reactor.
>>>Constant air blower and pump type load<<<
This is the most suitable on-site environment for variable frequency speed control. It does not have high requirements for the function and technology of the frequency converter, and the energy-saving effect is significant after using the frequency converter.
For example: In a large-scale centralized heating equipment frequency conversion system in Northeast China, the frequency converters for the blower and the induced draft fan (centrifugal fan) are selected and configured with the same power model;
Select and configure the frequency converter with the same power for the water supply pump (centrifugal pump) and circulating pump;
Select and configure frequency converters for slag removers and belt conveyors (coal conveyors) with a configuration one level larger;
Select and configure the frequency converter for the crusher (coal crusher) with a range of one or two increments.
Inverter Selection Considerations
The selection of a frequency converter is mainly determined by the characteristics of the driven load and the actual operating current of the motor. In addition, the following considerations should also be taken into account:
1 Environment
For applications involving high temperatures (above 50 degrees Celsius) and high altitudes (above 1000 meters), the selection of frequency converters should allow for a certain margin to ensure the service life of both the frequency converter and the motor.
2 distance
If the distance between the inverter installation location and the drive motor exceeds 50 meters, a reactor should be selected in conjunction with the inverter to reduce the carrier frequency.
3 One-to-many
When a frequency converter drives multiple motors, if the connecting cable is too long, the general frequency converter should be selected with a larger size, and a fuse should be installed after each motor.
4 high-speed motors
High-speed motors generate a large number of harmonics during operation, which increases the output current of the frequency converter and generates a lot of heat. Therefore, the frequency converter selected should be one size larger than that for ordinary motors.
5 explosion-proof motors
In certain mining and oil extraction environments, when the variable frequency system uses explosion-proof motors, the selected variable frequency drive should be one with explosion-proof function, and the variable frequency drive should be placed in a relatively safe location.
6 protection level
For applications with high dust and humidity, attention should be paid to the protection level of the frequency converter when selecting it.
7 Single-phase motor
The frequency converter does not work when a single-phase motor is used on site.
8-pole motor
When using a variable pole motor on site, the inverter selection should ensure that the maximum operating current does not exceed the inverter's rated current, and prevent the inverter from being damaged by idling when changing the number of motor poles.
Inverter Selection Principles and Procedures
Selecting the right frequency converter is a technical task. An improper selection can lead to converter failure and damage, and in severe cases, disrupt normal factory production. Therefore, choosing a scientifically sound and reasonable frequency converter is crucial. Below are some key principles for frequency converter selection for end-user reference.
1. Let professionals do professional things
The selection of frequency converters is best determined by the manufacturer's technical engineers based on the actual site conditions. This not only ensures accurate selection but also facilitates subsequent commissioning, installation, and after-sales service.
2. Functional requirements of load equipment
Based on the actual working conditions required on site (starting torque, response speed, speed regulation accuracy, etc.), consider whether to choose a general-purpose frequency converter, a vector frequency converter, or a special-purpose frequency converter, and whether to choose a heavy-duty or light-duty converter.
3. AC motor operating current
Inverter selection should be based on the actual operating current of the motor to determine the power rating. Generally, for loads such as fans and pumps, the motor's rated current can also be used as a basis for inverter selection. For situations where the motor may operate under overload conditions, the inverter should typically be selected with a power rating one level higher.
4. On-site installation environment
When there is conductive metallic dust on site, or the working environment temperature is too high, the selection of frequency converter needs to be carefully considered. In this case, a dedicated space is required to house the frequency converter, and the fan cooling function must be good to determine whether to select a wall-mounted unit, a cabinet unit, or an integrated industrial frequency converter.
Peripheral equipment for frequency converter selection
After selecting the frequency converter, it is also necessary to consider whether other peripheral equipment needs to be configured based on the actual site conditions. The main peripheral equipment for frequency converter selection includes circuit breakers, reactors, contactors, and braking resistors.
1. Circuit breaker
Circuit breakers can conveniently control the opening and closing of circuits. They are mainly used to automatically disconnect the power supply when the frequency conversion system experiences reverse current, overcurrent, short circuit, or undervoltage, thus providing isolation and protecting the power supply.
2. Electrical turret
Reactors are commonly used peripheral devices when selecting frequency converters. When the distance between the frequency converter and the motor is too long, or the distance between the frequency converter and the power supply is too close, DC or AC reactors must be added during the frequency converter selection process to improve the power factor and minimize the inrush current in the input.
3. Contactor
Contactors are divided into input contactors and output contactors. When a component of the frequency converter system malfunctions, the power supply is automatically cut off to prevent damage to the equipment from remote connection.
4. Braking resistor
When general-purpose frequency converters are used in inertial loads such as CNC machine tools and elevators, the selection of frequency converters requires the use of braking resistors to dissipate the excess energy generated in the frequency conversion system.
