Distinguish between heavy load and light load inverters and How to choose the appropriate discharge resistor

Heavy-duty and light-duty inverters differ in their load and torque requirements: heavy-duty inverters are used for equipment that requires high torque and continuous operation such as conveyors and crushers, while light-duty inverters are suitable for loads that require low torque such as fans and water pumps. To optimize efficiency and protect equipment, it is necessary to select appropriate discharge resistors, which help dissipate excess energy during braking and prevent overheating.

1. What is inverter?

 Figure 1: Siemens SINAMICS V20 Inverter and INVT Inverter

2. Inverter Classification

This diversity reflects the flexibility and customizability of the inverter, which helps optimize performance and efficiency in different fields. Therefore, inverters can be divided into two main types: heavy duty and light duty.
There are many different types of inverters on the market today, including AC, DC, 1-phase 220V, 3-phase 220V, 3-phase 380V, 3-phase 660V, and medium voltage inverters. In addition to general-purpose inverters, there are also specialized inverters designed to meet the specific needs of each application. Examples include inverters for pumps and fans, hoists and cranes, elevators, HVAC systems, and many other options.

 Figure 2: Fuji FRN0325G2S-4G mega inverter and Hitachi WJ200N-022SFC inverter

2.1. Same heavy and light load inverter

Both heavy duty and light duty inverters have the ability to flexibly adjust the AC current from one frequency to another.

2.2. Difference between heavy and light load inverter

3. Operating principle of the inverter

First, the inverter receives input power from the power supply, which can be single-phase or three-phase, but both have fixed voltage and frequency levels (e.g. 3-phase 380V at 50Hz). To perform the conversion process, this power source goes through a rectification and filtering stage, converting it into a flat DC source, thanks to a diode bridge rectifier and a capacitor. The capacitor not only stores but also supplies power to the inverter.
Next, the DC voltage in the capacitor is converted into a symmetrical three-phase AC voltage through an IGBT converter, which acts as a fast on and off switch, generating the inverter's output waveform using the pulse width modulation (PWM) method. The output signal frequency depends on the control signal and pre-programmed parameters in the inverter, allowing the operator to set the operating mode or control it directly.
During operation, the inverter automatically detects and warns of problems such as overload, overvoltage, voltage drop, or phase loss. If a minor fault is detected, the inverter will issue a warning message and continue to operate. However, in the event of a serious fault, the inverter can automatically stop supplying power to the motor to prevent damage to the system.

4. The role of inverter

The inverter plays an important role in controlling the motor speed by adjusting the input frequency, providing high precision and flexibility in controlling the speed of machinery and production lines. The inverter's pre-set capability when the machine runs to the end of its stroke makes the reversing or direct control process easy and flexible. In addition, the inverter is equipped with electronic devices to monitor, warn and automatically stop the power supply when problems such as overload, overvoltage, voltage drop, or phase loss occur, creating a safe system during operation. Moreover, the inverter also helps reduce the starting current compared to direct starting or star-delta starting methods, preventing voltage drops and difficulties in starting.

Communication modules integrated into the inverter allow easy control and monitoring from the center, improving system management efficiency. At the same time, starting the motor from low speed through the inverter helps to minimize sudden starting shock, reduce the risk of damage to the mechanical parts and bearings, thereby increasing the life of the motor.
Using the inverter helps to save energy significantly compared to operating the motor directly. The working principle of the inverter, converting the inverting current through diodes and capacitors, not only helps to reduce the cost of installing capacitors but also limits the loss of power on the line, bringing high efficiency. In addition, the inverter helps the motor not to be overloaded and not to operate at maximum capacity for a long time, thereby increasing the life of the motor and reducing maintenance costs. In addition, the use of the inverter also helps to reduce the noise and vibration of the motor, improve the working environment and reduce the negative impact on the health of workers.

5. Why do we need to connect a discharge resistor to the inverter?

In applications such as short deceleration times, high inertia loads, or frequent reversing loads, the motor can become a generator during the stop process, generating AC power. At this time, the IGBT protection circuit combined with the reverse current diode acts as a rectifier circuit, converting the AC voltage generated by the motor into DC voltage and feeding it back to the DC bus. However, this process can cause the DC voltage to rise above the allowable level, leading to the risk of IGBT and capacitor explosion.

6. Types of discharge resistors for inverters

6.1. Aluminum shell discharge resistor

The product is made from pure aluminum housing, providing good resistance to harsh environmental conditions and dust. With effective heat dissipation design, this product is very suitable for use in inverters, power circuits, AC Servo systems, and many other applications.

Figure 3: Aluminum shell discharge resistor

6.2. Ceramic shell discharge resistor

The tube is made of ceramic and coated with fire-resistant material, which increases the ability to withstand high temperatures. This design is often applied to loads that require quick stops or quick starts, such as cranes, centrifugal cutters, and loads with continuously changing currents.

Figure 4: Porcelain shell discharge resistor

7. How to choose discharge resistor for inverter

7.1. How to choose discharge resistor according to discharge resistor power (W)

The principle of selecting the discharge resistor is based on the amount of electricity discharged by the inverter: when the inverter discharges a lot of electricity, a large capacity should be selected, and vice versa, if the inverter discharges little electricity, a small capacity can be selected to ensure economy. For example, in the case of lifting and lowering the crane load, when the motor operates as a generator during the unloading process and the unloading time is long, a large capacity discharge resistor should be selected, usually about 1/2 or 2/3 of the motor capacity.

Figure 5: How to choose discharge resistor according to discharge resistor capacity (W)

7.2. How to select discharge resistor according to resistance value (Ohm)

The discharge resistor value needs to be adjusted according to the braking current design of each brand and type of inverter. Therefore, when selecting the resistor value, it is necessary to ensure that the value is greater than or equal to the minimum value (MIN) specified by the inverter manufacturer. Currently, most inverter manufacturers use the pulse width modulation (PWM) principle, so the discharge resistor value can be selected to be 1.5-2 times the MIN value of the inverter.

 Figure 6: How to select discharge resistor according to resistance value (Ohm)

7.3. How to connect to the inverter

- Inverter voltage input to R-S-T
- Motor output to U-V-W
- DC bus voltage to P and N1, P1-N is installed on the DC bus bar of the inverter
- Depending on the symbol of the inverter type (For example, LS H100 P1-N inverter is installed on the discharge resistor, LS-G100 B1-N ) 

Figure 7: How to connect to the inverter

7.4. Practical problems when choosing discharge resistors

- In practice, when choosing a discharge resistor, there are a few issues to keep in mind:
+ Production limitations: Manufacturers often only provide common types of discharge resistors, and cannot produce all the values ​​required by each application.
- So what is the solution?
To solve this problem, you can:
+ Combine resistors: Use multiple discharge resistors connected in parallel or in series to achieve the desired value. This allows for more flexibility in customizing the discharge resistor value.
+ Find other suppliers: Find manufacturers or suppliers who specialize in providing a wider variety of discharge resistors, including the specific values ​​you need.
+ Custom ordering: If the required value is very specific, you can consider ordering a custom discharge resistor from the manufacturer, although this may be time-consuming and more expensive.
By applying these solutions, you can overcome the limitations in choosing the right discharge resistor for your system.

7.5. Resistor pairing

- In case of installing multiple discharge resistors in parallel
+ Total capacity = Total capacity of resistors (Ptông = P1+P2+…+Pn)
+ Total resistance = Value of 1 resistor / Total number of resistors (R total = R/n)

Figure 8: Installing multiple discharge resistors in parallel

- In case of connecting multiple resistors in series
+ Total power = Total power of resistors (Total P = P1+P2+…+Pn)
+ Total resistance = Total number of resistors (Total R = R1+R2+…+Rn).

Figure 8: Connecting multiple resistors in series

7.6. Relative discharge resistance

- Rules for selecting relative resistance values ​​(R):
For applications that do not require a very fast stop, the discharge resistance value should be selected close to the value specified by the manufacturer to ensure the life of the inverter and discharge resistor. On the contrary, in applications that require a fast stop, the discharge resistor can be selected close to the minimum allowable value. However, this method will reduce the life of the inverter and discharge resistor compared to standard installation.
- Rules for selecting relative power values ​​(W)
The power value is a characteristic of the discharge resistor's tolerance. Therefore, choose a discharge resistor with a formula greater than or equal to the power value specified by the manufacturer.
It should be noted that the relative discharge resistor pairing has a disadvantage of increasing costs and increasing installation space.

8. Video to distinguish between heavy and light load inverters and how to choose the right discharge resistor?

Conclusion

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