【Daily Question 20090310】Parameter Setting of Inverters (IV)?

First, one must understand what torque boost is. Torque enhancement: This feature increases the output voltage of the inverter, thereby causing the motor’s output torque to increase in proportion to the square of the voltage, and thus improving the motor’s output torque. For a operating point at a certain frequency on the torque increase curve, either insufficient voltage (under-compensation) or excessive voltage increase (over-compensation) will result in an increased current, which in turn causes the motor to overheat. Therefore, to select an appropriate torque rise curve, it is necessary to repeatedly compare and analyze various measurement data in order to find one that truly meets the process requirements, enables the motor driven by the inverter to operate safely, and yields a relatively high power factor.

If the inverter triggers overcurrent protection during the start-up or stoppage of the motor, the acceleration and deceleration times should be reset. The acceleration of the motor during acceleration and deceleration depends on the acceleration torque, whereas the rate of frequency change of the inverter during startup and braking is set by the user. If the motor’s moment of inertia or load changes, and acceleration or deceleration occurs at a pre-set frequency change rate, there may not be sufficient acceleration torque, which can lead to motor stall. In other words, the motor’s speed does not match the output frequency of the inverter, resulting in overcurrent or overvoltage. Therefore, it is necessary to set the acceleration and deceleration times appropriately based on the motor’s moment of inertia and the load, so that the frequency change rate of the inverter can be coordinated with the motor’s speed change rate. A way to check whether this setting is reasonable is to first set the acceleration and deceleration times based on experience; if overcurrent occurs during startup, the acceleration time can be appropriately increased ； If overcurrent occurs during braking, appropriately extend the deceleration time. On the other hand, the acceleration and deceleration times should not be set too long, as longer times will affect production efficiency, especially during frequent starts and stops. . If the inverter continues to provide protection within a specified time, the start/stop operation curve should be changed, from a straight line to an S-shaped, U-shaped, inverted S-shaped, or inverted U-shaped line. When the motor load has high inertia, longer start and stop times should be used, and the type of operating curve should be set according to the load characteristics

1. Adjusting the starting torque is intended to improve the low-speed performance of the inverter during startup, so that the torque output by the motor can meet the requirements for starting up production processes. 2. In the variable-frequency speed control system of asynchronous motors, torque control is relatively complex. In the low-frequency range, the effects of resistance and leakage reactance cannot be ignored; if V/f is kept constant, the flux will decrease, which in turn reduces the motor’s output torque. To this end, the voltage must be appropriately compensated in the low-frequency range to increase torque. However, the effect of leakage impedance depends not only on frequency but also on the magnitude of the motor current, making accurate compensation difficult. In recent years, some frequency converters capable of self-compensation have been developed abroad, but they require extensive calculations and involve complex hardware and software; as a result, ordinary frequency converters are usually adjusted for compensation manually by the user. For the inverter used, it is appropriate to set the torque increase range between 1% and 5%.

(1) Torque enhancement: This feature increases the output voltage of the inverter, causing the motor’s output torque to increase in proportion to the square of the voltage, thereby improving the motor’s output torque. Technologies to improve the insufficient low-speed output torque of motors utilize \"vector control\", which enables the motor to achieve a output torque at low speeds – such as 1 Hz (resulting in a speed of approximately 30 rpm for a 4-pole motor) – that is equivalent to the torque it can produce when operated at 50 Hz (with a maximum value of around 150% of the rated torque). In conventional V/F control, the voltage drop of the motor increases relatively as the motor speed decreases, which results in insufficient excitation and prevents the motor from obtaining adequate rotational force. To compensate for this deficiency, the inverter needs to increase the voltage in order to offset the voltage drop caused by the decrease in motor speed. This function of the inverter is called “torque boost” (*1). The torque boost function is to increase the output voltage of the inverter. However, even when the output voltage is increased significantly, the motor torque does not increase in proportion to its current.

I’ve learned from it; thank you all for sharing. But is there any research on high-voltage frequency converters? :)

The V/F control method adjusts the characteristics of the motor after frequency conversion by regulating the ratio of voltage to frequency; it is commonly referred to as torque compensation or torque enhancement function. Increasing the torque boost value raises the output voltage, which in turn increases the current flowing through the motor and leads to more heat generation. Generally, the torque increase should not exceed 8%.

Simply put, it is caused by overloading. More specifically, at a certain operating frequency on the torque increase curve, either insufficient voltage (under-compensation) or excessive voltage increase (over-compensation) will lead to an increase in current, which in turn causes the motor to heat up.

Why does the motor generate more heat when the torque boost is increased? Since the load being driven requires a high starting torque, the inverter needs to have a certain starting torque boost during startup in order to enable the motor to start operating smoothly. The heat generation of the motor will not increase.