What is the appropriate frequency for variable frequency starting?

When the output frequency of the variable frequency speed regulator (also called inverter, English abbreviation VFD) is 0, the motor cannot start because the motor does not have enough starting torque. Only when the output frequency of the VFD reaches a certain value, the motor starts to accelerate. The output frequency of the VFD is the starting frequency fs. When the starting current is larger, the starting torque is larger. The starting frequency is set to ensure that the motor has enough starting torque when starting, to prevent the motor from failing 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 VFD.

On the one hand, we need to avoid the low-frequency under-excitation area to ensure that the motor has sufficient starting torque; on the other hand, we cannot set the starting frequency too high, otherwise it may cause a large current impact when the motor starts, or even over-current tripping. The starting frequency depends on the specific load conditions.

How to determine the startup frequency?

Generally, most 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 VFD at the moment of starting is the starting frequency. Usually, you need to set the starting frequency in the following cases:

1. A system where multiple pumps supply water simultaneously

For systems that use multiple water pumps to supply water at the same time, since there is usually a certain amount of water pressure in the water supply pipe, it is difficult to start the water pump if it is started at 0Hz. In this case, it is necessary to start directly from a certain frequency to start the rotation smoothly.

2. Load

If the VFD is driven by a load, the friction is large when static, and it is 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 starts smoothly.

3. Conical motor

During the starting process, friction will be generated between the stator and the rotor, so the starting frequency 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 start smoothly.

VFD starting frequency setting method

(1) Constant torque load. Generally, the synchronous speed of the motor should not exceed the rated slip rate during starting. That is: P is the number of poles of the motor: Δn--rated slip, Δn=n1-ne; N1--synchronous speed; ne--rated speed;

(2) Square wave torque load: Since the positive torque of square wave torque load is small at low speed, the starting frequency can be appropriately increased.

In practical applications, the starting frequency should be set appropriately to solve the problems of motor starting difficulty and overcurrent tripping. For motors with large starting torque, first consider setting appropriate starting frequency parameters, and then set a reasonable torque boost curve according to the actual load conditions. If the VFD current is too high during starting, or even overcurrent tripping occurs, it can be solved by extending the acceleration time. But generally speaking, as long as there is no excessive flow, the acceleration time should be as short as possible to improve efficiency.