What is the appropriate starting frequency setting for the inverter?

Inverter starting frequency

When the inverter output frequency is 0, the motor cannot start because the motor does not have enough starting torque. Only when the inverter output frequency reaches a certain value, the motor starts to accelerate. At the moment the motor starts to accelerate, the inverter output frequency is the starting frequency fs. At this time, the starting current is large and the starting torque is also large. The starting frequency is set to ensure that the motor has enough starting torque when starting, to avoid the motor being unable to start or causing overcurrent tripping during the starting process. In general, the starting frequency should be set according to the characteristics of the load driven by the inverter. On the one hand, it is necessary to avoid the low-frequency under-excitation area to ensure that the motor has enough starting torque. On the other hand, the starting frequency cannot be set too high, otherwise it may cause a large current shock or even overcurrent tripping when the motor starts. The size of the starting frequency needs to be determined according to the specific load conditions.

How to determine the starting frequency of the inverter

Most general motors start accelerating from 0Hz, but in some special cases, they need to accelerate directly from a certain frequency. In this case, the frequency output by the inverter at the moment of starting is the starting frequency. Usually, the starting frequency needs to be set in the following cases.

1. System with multiple water pumps supplying water simultaneously

For a system that uses multiple water pumps to supply water at the same time, since there is generally a certain water pressure in the water supply pipeline, if the water pump that is started later is started from 0Hz, it will be difficult to start running. In this case, it is necessary to start directly from a certain frequency to successfully start rotation.

2. Loads with large static friction

If the load driven by the inverter has a large friction force when static, it will be difficult to start from 0Hz. In this case, it is necessary to set the starting frequency to ensure that a certain starting impact force is generated at the moment of starting so that the system can start smoothly.

3. Conical motor

When the conical motor is started, there is a certain friction between its stator and rotor, so the starting frequency also needs to be set to ensure that sufficient magnetic flux is quickly established at the moment of starting, so that a certain air gap is maintained between the stator and the rotor, so that the motor can be started smoothly.

Inverter starting frequency setting method

(1) Constant torque load. Generally, it is appropriate for the synchronous speed of the motor to not exceed the rated slip during starting. That is:

Where p is the number of motor pole pairs; △n is the rated slip, △n=n1-ne; n1 is the synchronous speed; ne is the rated speed; (1) Square torque load. Since the square torque load has a very small resistance torque at low speed, the starting frequency can be appropriately increased.

During actual debugging, the starting frequency should be set reasonably to solve the problems of difficult motor starting and overcurrent tripping. For motors with large starting torque, the appropriate starting frequency parameters should be considered first, and then a reasonable torque boost curve should be set according to the actual load conditions. If the inverter current is too large during the starting process, or even an overcurrent trip occurs, the speed increase time can be extended to solve the problem. However, as long as there is no overcurrent, the speed increase time should be as short as possible to improve efficiency.