Mastering these 35 inverter concepts can elevate your expertise to impressive levels!

Mastering these 35 inverter concepts can elevate your expertise to impressive levels! 

The term VFD (Variable-frequency Drive) for an inverter reflects its function of controlling AC motors by adjusting the frequency and amplitude of the power supply. In Asia, especially in China and South Korea, the term VVVF (Variable Voltage Variable Frequency Inverter) was used due to Japanese influence. VVVF stands for Variable Voltage and Variable Frequency, referring to the adjustment of both voltage and frequency, while CVCF (Constant Voltage and Constant Frequency) indicates fixed voltage and frequency.

Power sources are categorized into AC and DC. Most DC power is derived from AC through transformation, rectification, and filtering. AC power constitutes approximately 95% of all power usage, with single-phase and three-phase AC power following specific voltage and frequency standards in different countries. For instance, in mainland China, single-phase AC is 220V and three-phase AC is 380V, both at 50Hz. An inverter converts fixed voltage and frequency AC power into variable voltage or frequency AC power. This process involves rectifying AC to DC and then inverting DC back to AC, with the latter process specifically termed "inversion." Devices that convert DC to fixed frequency and voltage AC are called inverters, while those that allow for adjustable frequency and voltage are referred to as variable-frequency drives.

Inverters output simulated sine waves, primarily used for speed control of three-phase asynchronous motors, and are also known as variable-frequency speed controllers. For applications requiring high-quality waveforms, such as testing equipment in instrumentation, the waveform is refined to produce a standard sine wave, and such devices are called variable-frequency power supplies. Variable-frequency power supplies are typically 15 to 20 times more expensive than variable-frequency drives. The core component responsible for generating variable voltage or frequency in inverter equipment is the "inverter," hence the product is named "inverter." Inverters are also used in home appliances, such as air conditioners and fluorescent lights. In motor control applications, inverters can adjust both voltage and frequency, while those used for fluorescent lights mainly regulate the power supply frequency. Devices in cars that convert battery (DC) power to AC are also sold under the name "inverter." The working principle of inverters is widely applied in various fields, such as computer power supplies, where inverters suppress reverse voltage, frequency fluctuations, and instantaneous power outages.

What is an inverter?

An inverter is a device that converts utility frequency power to another frequency using the switching action of power semiconductor devices. It consists of two main circuits: the main circuit (rectifier module, electrolytic capacitor, and inverter module) and the control circuit (switching power supply board and control circuit board). The CPU is installed on the control circuit board, with the inverter's operation software programmed into the CPU. The software for the same inverter model is generally fixed, except for the Sanjing inverter, whose software can be adjusted based on usage requirements.

What are the differences between PWM and PAM?

PWM (Pulse Width Modulation) adjusts the width of pulses in a pulse train according to a specific pattern to regulate output and waveform. PAM (Pulse Amplitude Modulation) adjusts the amplitude of pulses in a pulse train to regulate output and waveform.

What are the differences between voltage-type and current-type inverters?

The main circuit of an inverter can be broadly divided into two types: voltage-type inverters convert DC voltage source to AC using capacitors for DC circuit filtering, while current-type inverters convert DC current source to AC using inductors for DC circuit filtering.

Why do the voltage and frequency of an inverter change proportionally?

The torque of an induction motor is produced by the interaction between magnetic flux and rotor current. At rated frequency, if voltage is constant and frequency is reduced, magnetic flux can become excessive, leading to magnetic circuit saturation and potential motor damage. Therefore, voltage and frequency must change proportionally. This control method is commonly used in energy-saving inverters for fans and pumps.

When an induction motor is driven by utility frequency power and voltage drops, current increases. For inverter-driven motors, if voltage decreases when frequency decreases, does current increase?

When frequency decreases (low speed), current increases to maintain the same power output. However, under constant torque conditions, current remains relatively stable.

What are the starting current and torque when operating a motor with an inverter?

With an inverter, as the motor accelerates, frequency and voltage are correspondingly increased, limiting starting current to below 150% of rated current (125% to 200% depending on the model). Direct online starting with utility frequency power results in starting currents six to seven times the rated current, causing mechanical and electrical stress. Inverter-driven motors start smoothly (with extended starting time), with starting current at 1.2 to 1.5 times rated current and starting torque at 70% to 120% of rated torque. For inverters with automatic torque boost, starting torque exceeds 100%, enabling full-load starts.

What is V/f mode?

When frequency decreases, voltage V also decreases proportionally. The proportional relationship between V and f is determined based on motor characteristics and is typically stored in the controller's memory (ROM). Several characteristics can be selected via switches or potentiometers.

How does motor torque change when V and f are adjusted proportionally?

If voltage is reduced proportionally with frequency, the tendency for torque to decrease at low speeds arises due to reduced AC impedance and unchanged DC resistance. To compensate and achieve sufficient starting torque at low frequencies, output voltage must be slightly increased. This compensation, known as torque boost, can be achieved through various methods, including automatic adjustment, selection of V/f mode, or potentiometer settings.

If the manual specifies a speed range of 60~6Hz (10:1), does this mean no power output below 6Hz?

Power can still be output below 6Hz. However, considering motor temperature rise and starting torque, the minimum operating frequency is set around 6Hz to avoid excessive heating while maintaining rated torque output. The actual output frequency (starting frequency) of the inverter varies by model, typically ranging from 0.5Hz to 3Hz.

Is it possible to maintain constant torque with a standard motor combination above 60Hz?

Generally, it is not possible. Above 60Hz (or 50Hz in some modes), voltage remains constant, resulting in roughly constant power characteristics. When constant torque is required at high speeds, careful selection of motor and inverter capacities is essential.

What is open-loop control?

When a speed detector (PG) is installed on the motor and actual speed is fed back to the control device for regulation, it is called "closed-loop" control. Operation without PG feedback is termed "open-loop" control. General-purpose inverters typically use open-loop control, though some models offer PG feedback as an option. Speed sensorless closed-loop control estimates actual motor speed based on a mathematical model of flux, effectively forming a closed-loop control system with a virtual speed sensor.

What happens when there is a discrepancy between actual and set speeds?

In open-loop control, even if the inverter outputs the set frequency, motor speed may vary within the rated slip range (1% to 5%) under load. For applications requiring high speed regulation accuracy and near-set-speed operation despite load changes, inverters with PG feedback (available as an option) can be employed.

Can speed accuracy be improved using a motor with PG feedback?

Inverters with PG feedback offer improved speed accuracy. However, the actual speed accuracy depends on the PG's precision and the inverter's output frequency resolution.

What is the anti-stall function?

If the set acceleration time is too short, the inverter's output frequency may change much faster than the motor's speed (electrical angular frequency), causing overcurrent and tripping the inverter, which halts operation. This is referred to as stalling. To prevent stalling and maintain motor operation, the inverter monitors current and adjusts frequency. During acceleration, if current becomes excessive, the acceleration rate is reduced. The same applies to deceleration. Together, these mechanisms constitute the anti-stall function.

What is the significance of inverters that allow separate settings for acceleration and deceleration times versus those that use a common setting?

Inverters that allow separate acceleration and deceleration time settings are suitable for applications requiring brief acceleration and gradual deceleration, or for small machine tools with strict production rhythm requirements. In contrast, for applications like fan drives where acceleration and deceleration times are both lengthy, a common setting for acceleration and deceleration times is appropriate.

What is regenerative braking?

When the command frequency is reduced during motor operation, the motor transitions to asynchronous generator mode and functions as a brake. This process is known as regenerative (electrical) braking.

Can greater braking force be achieved?

Energy regenerated from the motor is stored in the inverter's filter capacitor. Due to the capacitor's capacity and voltage rating limitations, regenerative braking force in general-purpose inverters is approximately 10% to 20% of the rated torque. With optional braking units, this can be increased to 50% to 100%.

What are the protective functions of an inverter?

Protective functions can be categorized as follows:

(1) Automatically correcting abnormal conditions, such as overcurrent stall prevention and regenerative overvoltage stall prevention.

(2) Blocking PWM control signals to power semiconductors upon detecting abnormalities, causing the motor to stop automatically. Examples include overcurrent shutdown, regenerative overvoltage shutdown, semiconductor cooling fan overheat protection, and instantaneous power failure protection.

Why does the inverter's protective function activate when using a clutch for continuous load?

When a clutch connects the load, the motor rapidly transitions from no-load to a region of high slip. The resulting high current causes the inverter to trip due to overcurrent, halting operation.

Why does the inverter stop during operation when large motors start in the same facility?

During motor startup, the inrush current corresponds to the motor's capacity, causing voltage drop on the transformer's stator side. For large motors, this voltage drop can significantly affect other equipment connected to the same transformer. The inverter may misinterpret this as undervoltage or instantaneous power loss, triggering its protective function (IPE) and causing it to stop.

What is inverter resolution and why is it significant?

For digitally controlled inverters, even if the frequency command is an analog signal, the output frequency is provided in discrete steps. The smallest unit of these steps is called inverter resolution. Typically, inverter resolution ranges from 0.015Hz to 0.5Hz. For instance, with a 0.5Hz resolution, frequencies above 23Hz can be adjusted to 23.5Hz or 24.0Hz, resulting in stepped motor operation. This can be problematic for applications like continuous winding control. In such cases, a resolution of around 0.015Hz ensures that for a four-pole motor, each step corresponds to less than 1r/min, providing sufficient adaptability. Some inverter models differentiate between command resolution and output resolution.

Are there any restrictions on the installation direction of an inverter?

Inverter design considers cooling effectiveness for internal components and the backside. The orientation of the unit is crucial for ventilation. For panel-mounted or wall-mounted unit-type inverters, vertical installation in a longitudinal position is recommended.

Is it feasible to directly connect a motor to a fixed-frequency inverter without using a soft starter?

At very low frequencies, this is possible. However, if the set frequency is high, conditions resemble direct online starting with utility frequency power. This can result in excessive starting currents (six to seven times the rated current), and since the inverter will trip to protect against overcurrent, the motor will fail to start.

What precautions should be taken when operating a motor above 60Hz?

When operating above 60Hz, consider the following:

(1) Ensure mechanical and related equipment can withstand operation at such speeds (mechanical strength, noise, vibration, etc.).

(2) The motor enters the constant power output range, and its output torque must sustain the workload (for fans and pumps, shaft output power increases with the cube of speed, so even slight speed increases require attention).

(3) Bearing life may be affected and should be carefully considered.

(4) For medium to large capacity motors, especially two-pole motors, consult with the manufacturer before operating above 60Hz.

Can inverters drive gear motors?

Depending on the reducer's structure and lubrication method, several considerations apply. Typically, gear structures can tolerate a maximum of 70~80Hz. With oil lubrication, continuous low-speed operation may damage gears.

Can inverters drive single-phase motors? Can they operate on single-phase power?

Generally, it is not feasible. For single-phase motors with speed controllers or switch-start mechanisms, reducing speed below the operating point can overheat the auxiliary winding. For capacitor-start or capacitor-run types, capacitor explosion may occur. Inverters typically require a three-phase power supply, though some small capacity models can operate on single-phase power.

How much power does an inverter consume itself?

Power consumption depends on the inverter model, operating state, and usage frequency. It is difficult to specify exact values. However, inverter efficiency below 60Hz is approximately 94% to 96%, which can be used to estimate losses. For inverters with built-in regenerative braking (e.g., FR-K series), considering braking losses increases power consumption, a factor to note in control panel design.

Why can't continuous operation occur across the entire 6~60Hz range?

Most motors use external fans on the shaft or blades on the rotor end ring for cooling. Reduced speed diminishes cooling effectiveness, preventing the motor from enduring the same heat generation as at high speeds. To address this, reduce low-speed load torque, use a larger capacity inverter and motor combination, or employ a specialized motor.

What precautions should be taken when using a motor with a brake?

The brake excitation circuit should be powered from the inverter's input side. If the brake activates while the inverter is outputting power, overcurrent may cause a shutdown. Therefore, ensure the brake activates only after the inverter has stopped outputting power.

Why won't the motor start when using an inverter to drive a motor with power factor improvement capacitors?

Inverter current flows into the power factor improvement capacitors. The charging current may trigger overcurrent (OCT) in the inverter, preventing startup. To resolve this, remove the capacitors and operate the motor. To enhance power factor, installing an AC reactor on the inverter's input side is effective.

What is the lifespan of an inverter?

Although inverters are static devices, they contain consumable components like filter capacitors and cooling fans. With regular maintenance of these parts, an inverter can last over ten years.

How is the cooling fan oriented in an inverter, and what happens if it fails?

Some small capacity inverters lack cooling fans. For models with fans, airflow is typically from bottom to top. When installing an inverter, avoid placing equipment that obstructs air intake and exhaust above and below the unit. Do not position heat-sensitive components above the inverter. Fan failure is protected against by detecting fan stoppage or overheating of the cooling fan.

How can the lifespan of filter capacitors be determined?

Filter capacitors, used as capacitors, gradually lose their electrostatic capacity over time. Regularly measure the electrostatic capacity, and consider the capacitor's lifespan expired when it reaches 85% of the rated capacity.

Are there any restrictions on the installation direction of an inverter?

Inverters are typically housed within panels. However, fully enclosed panels are bulky, space-consuming, and costly. Mitigation measures include:

(1) Designing panels for the required cooling of actual equipment.

(2) Increasing cooling area using aluminum heat sinks, fins, and cooling agents.

(3) Utilizing heat pipes.

Additionally, inverter models with exposed back sides have been developed.

How should inverter capacity be selected to increase conveyor belt speed to 80Hz?

Power consumption of conveyor belts is proportional to speed. To operate at 80Hz, both inverter and motor power should be increased proportionally to 80Hz/50Hz, i.e., a 60% capacity increase.

Precautions during maintenance and inspection:

(1) After switching off the input power, wait at least 5 minutes before commencing inspection (ensure the charging indicator LED has extinguished) to avoid electric shock.

(2) Maintenance, inspection, and component replacement must be performed by qualified personnel. Remove all metal items (watches, bracelets, etc.) before starting work and use insulated tools.

(3) Do not modify the inverter arbitrarily to prevent electric shock and product damage.

(4) Before servicing the inverter, confirm the input voltage. Connecting a 380V power supply to a 220V-class inverter can cause damage (capacitor, varistor, module explosion, etc.).

Inverters, composed primarily of semiconductor elements, require daily inspection to guard against adverse working environments, such as temperature, humidity, dust, and vibration, and to prevent faults arising from component lifespan limitations.

Inspection items:

(1) Daily inspection: Verify that the inverter operates as required. Use a voltmeter to check input and output voltages while the inverter is running.

(2) Periodic inspection: Examine all areas accessible only when the inverter is shut down.

(3) Component replacement: Component lifespan is greatly influenced by installation conditions.