Frequency converter calculation method and selection basis - Database & Sql Blog Articles

Probe high frequency needle JTK-50L-Q has 5 needles in the high frequency needle

Photocoupler

Where: GD ² : the moment of inertia converted to the motor shaft (N·m ² )

T _L : load torque (N·m)

η, cosφ, n _M are the efficiency of the motor (take 0.85), power factor (take 0.75), rated speed (r/min).

t _A : motor acceleration time (s) is determined by load requirements

K: Correction coefficient of current waveform (PWM method is taken from 1.05 to 1.10)

P _CN : rated capacity of the inverter (KVA)

8 Selection of frequency converter for light load motor

The actual load of the motor is lower than the rated output power of the motor. It is considered that the inverter capacity can be selected to be commensurate with the actual load. However, for the general-purpose inverter, even if the actual load is small, the inverter with a smaller capacity than the rated power of the motor is used. The device is not ideal, mainly for the following reasons;

1) When the motor is idling, it also flows through the excitation current of 30% to 50% of the rated current.

2) The starting current flowing during starting corresponds to the voltage and frequency applied by the motor. Regardless of the load torque, if the inverter capacity is small and the current exceeds the overcurrent capacity, it will not start.

3) If the motor capacity is large, the leakage reactance percentage of the motor based on the inverter capacity becomes smaller, and the pulsation of the inverter output current increases, so the overcurrent protection capacity operates and often cannot operate.

4) When the motor is started with a general-purpose inverter, its starting torque is mostly smaller than that of the power-frequency power supply. It may not start depending on the starting torque characteristics of the load. In addition, the torque in the low-speed running area tends to decrease from the rated torque. When the selected inverter and motor cannot meet the required starting torque and low-speed range torque of the load, the capacity of the inverter and the motor are also Need to increase.

The above describes the capacity calculation and selection method of the inverter under several different conditions. When selecting the capacity, it is necessary to make full use of the overload capacity of the inverter, and not to overheat the device when the load is running. Some manufacturers (such as ABB) also have deterministic device quota software that can determine the output rating of the device as long as the user presents a clear load map.