A frequency converter is an electronic device that uses high-speed switches or insulated-gate bipolar transistors ( IGBTs ) to convert a three-phase input power supply into a variable frequency and voltage output to control the motor speed.
With frequency converters, motors can be widely used in various scenarios, enabling cross-line operations or controls that are impossible to achieve mechanically. Using frequency converter-controlled motors, users can optimize system efficiency by matching motor speeds to maintain precise system requirements. Most frequency converter applications improve system efficiency, so the investment in frequency converters can typically be recouped in less than a year through energy savings.
Like all electronic devices, frequency converters offer advanced functionality, providing enhanced system control without the need for external equipment and programmable logic controllers (PLCs). Given these innovations, it's understandable that some might assume implementing these functions requires extensive programming of the frequency converter. However, in most applications, only the most basic settings are needed to get the motor running. This is because frequency converters are designed and manufactured to simplify complex tasks.
In most cases, the default settings of a frequency converter are sufficient for application requirements, requiring no adjustments. If an application needs to adjust parameters, it generally won't exceed 12. Programming a frequency converter to suit most industrial applications only requires setting the most basic parameters to make the motor run. Understanding the following 5 important parameters can help optimize frequency converter programming.
Figure 1 : The frequency converter is pre-configured with overload protection to handle different types of motors. Image source: Yaskawa
1. Control method
Typically, the first parameter that inverter installers need to set is the control mode. The choice of control mode determines the inverter's ability to regulate motor speed. These control modes can be divided into three types: V/f (volts / hertz) control, self-induction vector control, and closed-loop vector control.
V/f control is the most commonly used motor control method. It is the most fundamental of the three control methods. When the inverter speed needs to be adjusted, V/f control can fix the inverter to output according to a predefined voltage and frequency curve, thereby causing the motor to adjust according to that curve. These V/f modes can be adjusted to provide high starting torque, or to reduce torque when a constant voltage and frequency relationship is not required, optimizing the efficiency of variable torque loads.
Self-inductive vector control is a method that allows for more precise control of motor speed. Inverters can implement this control using a variety of different and complex control schemes. Essentially, complex algorithms are used to monitor, interpret, and respond to current feedback to provide accurate motor control. However, the simplest way to view this control method is as precise motor control without an encoder.
Closed-loop vector control is the most advanced motor control method available. It uses a motor encoder to provide precise speed feedback and eliminates any deviations in inverter control caused by response current feedback. Adding an encoder allows the inverter to understand what the motor is doing and how it is responding to the load.
Figure 2 : By using appropriate acceleration and deceleration times, the inrush current during motor startup and the current surge during speed changes can be significantly reduced.
Why do we need to adjust the control method?
The control method is chosen to meet the needs of motor drive applications. Some applications are simple, requiring only approximate speed operation, while others require precise and dynamic motor control. Each control method can meet application requirements or limit the programming needed for system startup and operation.
V/f control is typically used in systems that do not require precise speed control, such as fans or pumps. In the most basic V/f control method, the motor is allowed to deviate from the speed command. Slight changes in speed have almost no impact on overall system performance because other drivers will adjust their speeds to maintain system requirements.
For example, if the fan is required to run at half speed and the demand cannot be maintained, most system configurations will increase the speed command through the inverter's proportional-integral loop or in conjunction with external devices to provide the required motor speed. Since this can be achieved with almost no programming required, V/f control is the most commonly used control method.
Based on years of practical experience, most inverter manufacturers have set the operating modes of pumps and fans to default configurations. These default configurations provide optimal energy savings and require almost no programming. Even for constant torque applications, such as compressors, the advantages of V/f control can be fully utilized in the configuration, making it very convenient.
Self-sensing vector control can improve process control and reduce maintenance. For example, it can regulate motor speed to within 1/200 of the rated speed, providing dynamic speed control, reducing high starting torque to low speed, and limiting current and torque without external equipment. To provide these advanced motor control functions, the inverter requires specific motor characteristic information, such as the motor's no-load current, resistance, and inductance.
To obtain this information, the frequency converter can be operated through simple motor tuning, by inputting basic nameplate data of the motor via a keypad, such as rated current, voltage, and speed. Applications that benefit from this control include mixers, washing machines, and punch / press machines.
Closed-loop vector control adds a speed feedback signal, maximizing process control and reducing maintenance. It enables precise speed control down to 1 RPM, high starting torque at zero speed, zero-speed control, and torque regulation. These functions are useful in scenarios where speed deviations cannot exceed a few RPMs ; otherwise, the output will not meet design specifications.
For example, many extruders use encoder feedback to precisely maintain the motor speed at the required level to ensure the product meets its specifications. Encoder feedback also ensures accurate torque monitoring, allowing the inverter to respond to high torque conditions that could clog or damage the machine. Closed-loop vector control requires the same motor adjustment as self-sensing vector control to optimize motor control and reduce the compensation needed for encoder feedback.
Figure 3: Inflection points in the acceleration / deceleration curves, appearing at the beginning and end of each ramp. At these points, the maximum torque or current is required to drive the motor to achieve the desired motion.
The more the frequency converter understands the characteristics of the motor, the better the motor will operate. This is true regardless of whether there is motor feedback. Applications such as extruders, high-speed spindles, and constant tension unwinding machines all utilize the advantages of closed-loop vector control.
2. Motor full-load current
Most frequency converters have the most commonly used application scenarios pre-configured in their control settings. Therefore, for any frequency converter installer, the first parameter that often needs to be programmed is the motor's full-load current or rated current. At rated power and rated voltage, the motor is designed to operate continuously at its rated current as stated on its nameplate. Programming the frequency converter with the motor's full-load current rating allows for configuring the frequency converter's electronic thermal overload for the running motor.
Although frequency converters are inherently soft starters, motors may still exceed their rated current for short periods, such as during startup, load surges, rapid deceleration, or over-cycle operation. However, prolonged high current can cause motor overheating, shortening its lifespan and leading to premature failure. Rotor lock-up can also occur due to mechanical damage to the load or coupling. Over time, load wear can also cause increased current consumption, potentially exceeding the motor's full-load current.
To prevent motor failure, the full-load current on the motor nameplate is set to the inverter's full-load current value. Setting the inverter's electronic thermal overload protection must meet national standards and local regulations for motor overload protection. Using the inverter's electronic thermal overload protection, users can eliminate motor mechanical overload, thereby reducing costs, eliminating potential failure points, and any maintenance needs related to maintaining the integrity of overload contacts.
The electronic overload protection function of the frequency converter can estimate the motor overload level based on the output current, output frequency, motor thermal characteristics, and time. When the frequency converter detects a motor overload, it triggers a fault, shutting down the frequency converter output to protect the motor from thermal failure. These overload curves can be set according to the motor's capabilities. Many pump and fan motors are designed for variable torque loads, meaning they are not designed for rated current at speeds below rated speeds.
Reducing continuous overload can decrease maintenance and ensure maximum motor lifespan. The inverter is pre-configured with overload functions to accommodate different motor types, including variable torque loads at 40 : 1 speed, constant torque loads at 100 : 1 speed, and unconventional motors such as permanent magnet motors.
3. Acceleration and deceleration time
Inverters are inherently soft starters. They reduce inrush current when the speed changes. To this end, inverters start and stop the motor according to preset acceleration and deceleration times. These times, or slopes, define the time required to reach the maximum frequency from zero speed. There can be a fixed speed or multiple speeds, which can be adjusted according to operating conditions or commands sent to the inverter.
Using appropriate acceleration and deceleration times will significantly reduce inrush current during startup and speed changes. This extends the lifespan of the motor (less heat) and powertrain (less dynamic high torque variations). The inverter also isolates these currents from the line. Therefore, large inrush currents from the transformer are not required, as this could cause unnecessary heat generation or affect its supply voltage, potentially impacting inverter performance or other loads on the system. Lower inrush current means utility charges for current/power fluctuations are eliminated.
Inverter acceleration and deceleration times are typically set to default values based on their intended application. Fan / pump inverters have longer ramp times, while general-purpose industrial inverters have relatively shorter ramp times. This helps simplify the installation process. However, not all default settings are suitable for every application. These ramp times need to be adjusted to keep the current within the limits of both the inverter and the motor.
Due to load inertia, the speed at which the load starts/stops may exceed the current capacity of the drive / motor. Drastic acceleration / deceleration rates will result in higher current, which can strain the inverter and motor, leading to overload or overcurrent failures. Setting the correct acceleration and deceleration times ensures proper system performance and trouble-free operation.
The inflection points in the acceleration / deceleration curve appear at the beginning and end of each ramp. At these points, maximum torque or current is required to drive the motor to achieve the desired motion. Therefore, to keep the total ramp time low, these points can be adjusted to reduce it. These points are called acceleration/deceleration or S- curve time adjustments. These settings extend the time at the high-stress points of the acceleration or deceleration ramps to reduce their impact on the overall start / stop time.
4. Speed and operating commands
A frequency converter requires two parameters at every moment of its operation: the run command and the speed reference. The run command tells the converter that the motor should start, while the speed reference provides the operating frequency. Both inputs are necessary for motor control. Otherwise, the motor will idle. Parameter setting is one of the most common troubleshooting methods used by technical support during frequency converter installation.
Setting the speed and operating commands of a frequency converter is more about choosing how to run the motor than whether you want the motor to run. Most manufacturers default to operating their frequency converters via digital and analog inputs. Contacts and relays feed the operating commands to the frequency converter. Analog inputs are then used to input speed reference values to the frequency converter. These analog references can be 0-10 V dc , +/-10 V dc , 0-20 mA , or 4-20 mA signals. Each reference source has its own advantages. Reference voltage sources are easy to generate and understand, while current signals travel further and are unaffected by nearby electrical noise. Other control methods can be achieved through direct keyboard control or network communication.
These parameters provide the inverter with the precise speed required to operate the motor. The more accurate the inverter's motor speed control reference, the higher the precision with which it can meet system requirements. Precisely meeting system requirements means the inverter can achieve greater energy efficiency. The goal of any command interface is to achieve the control required by the system to maximize efficiency, quality, and safety.
5. Fault Reset
Many external conditions can cause a frequency converter to operate beyond the conditions specified in its design specifications. To maintain product life and prevent failure, the frequency converter may trigger a fault to protect itself. Conditions that may lead to frequency converter failure include excessively fast start-up and stop times, power loss, and rotor lock-in.
Many frequency converters have an automatic fault reset function. This function allows the frequency converter to detect operating conditions that exceed its preset range and trigger a fault shutdown to protect itself, the motor, and the rest of the mechanical system. The fault reset function allows the user to detect the event and, if the fault condition has been eliminated, restore the frequency converter to normal operation.
The purpose of automatic reset is to avoid frequent downtime due to malfunctions and to maintain continuous operation. Downtime is costly, and when it is certain that no downtime is required, the automatic reset function allows the system to continue operating unless certified personnel confirm that downtime is necessary.
For example, voltage spikes caused by thunderstorms. These rare situations do not require further analysis. Under such conditions, the inverter will stop operating to protect itself. The automatic reset function allows the inverter to restart without user intervention, saving time and costs.
There are several ways to implement frequency converter technology to automate motor control needs. While setting up a frequency converter can be complex, most applications require only minor adjustments to get it up and running. Furthermore, frequency converters simplify the installation process. For example, startup routines or wizards can guide installers through a question-and-answer menu to program the frequency converter, ensuring the application meets requirements. Frequency converters are increasingly designed for ease of use, maximizing return on investment by optimizing efficiency, quality, and safety.
