GB/T 37404-2019Application guide of soft-start device for high-voltage motor (English PDF)

3.1 Bypass

A process that closes the short-circuit switch connected in parallel with the soft-start device so that no current flows through the soft-start device.

3.2 Bypass switch

A switch that conducts the bypass, which short-circuits the soft-start device and the soft-start circuit during the bypass process.

- rated current;

- rated voltage;

- rated power;

- rated speed;

- moment of inertia;

- torque-speed curve or stall torque multiple at rated voltage;

- current-speed curve or stall current multiple at rated voltage.

- Load parameters:

- moment of inertia / flywheel moment;

- static resistance torque;

- resistance torque-speed curve that is converted to the motor end.

- Power grid parameters:

- grid short-circuit capacity or short-circuit impedance;

- length and specification of incoming cable or overhead line.

- Environmental conditions:

- altitude;

- relative humidity;

- ambient temperature;

- pollution level.

- Process requirements:

- start frequency.

When the relevant parameters change, it shall recalculate the calculation results of the type selection.

5.2 Performance indicators

The parameter indicators that are specified by users and in accordance with national grid regulations, include:

- pilot differential protection (optional);

- open triangle voltage protection (optional).

NOTE: The setting values of current quick-break protection and over-current protection shall consider the characteristics of the soft-start device.

5.5 Matching type selection

5.5.1 Cable

The type selection of cable shall meet the requirements of GB 50217-2018.

Under the premise that the cable temperature rise is met in the starting circuit, the capacity can be reduced.

When it uses a high-voltage variable-frequency soft-start device and when the output side is connected to the motor at a long distance, it is advisable to calculate the damage to the motor caused by the voltage spikes generated by the output harmonics.

5.5.2 Switch

It shall select the breaking capacity of corresponding switch according to the short-circuit capacity of the power grid. The starting circuit shall choose a switch of the appropriate current level according to the starting current ratio. The bypass circuit shall select a switch of the appropriate current level according to the motor rated current.

5.6 Loss

Due to the short-term operation of the soft-start device, it is unnecessary for the soft-start device manufacturer to calculate the loss.

6.1 Typical scheme of motor stator to side-connect soft-start device

6.1.1 One-for-one system scheme

The one-for-one system scheme is shown as Figure 1.

3 - Bypass switch (contactor or circuit breaker);

4 - Starting circuit switch (contactor or circuit breaker);

5 - Soft-start device.

Figure 3 -- Typical scheme of motor rotor to side-connect soft-start device Connect the soft-start device to the rotor side of the motor. After the soft-start device and the process are ready, the signal shall be sent to the central control system. The central control system shall send the well-preparation signal. The power incoming switch shall be closed. After the site or the central system sends a command, the starting and operation switches 2 and 4 shall be closed and the soft-start device shall be put into operation. After the soft-start device detects that the motor meets the bypass conditions, the bypass switch shall close, starting and operation switch 4 shall be disconnected, the soft-start device shall stop operating and the starting shall be completed.

When the back-end circuit of the starting and operating switch is short-circuited, it shall have the ability to interrupt the short-circuit current.

7.1 Overview

After the motor is driven by the soft-start device to near the rated speed, it shall be connected to the grid for direct power operation.

7.2 Bypass requirements for buck soft-start

Before bypassing, the soft-start device shall, through a direct or indirect manner, detect the amplitudes of motor speed and motor terminal voltage. When it is determined that the amplitudes of motor speed and motor terminal voltage meet the design requirements, the soft-start device shall give bypass instruction. The specific requirements are determined by the manufacturer and user according to the site conditions.

7.3 Bypass requirements for variable-frequency soft start

Before bypassing, the soft-start device shall, through a direct or indirect manner, detect the amplitudes, phases and frequencies of motor speed and motor terminal voltage. When it is determined that the amplitudes, phases and frequencies of motor speed and motor terminal voltage meet the design requirements, the soft-start device shall give bypass instruction. The specific requirements are determined by the manufacturer and user according to the […]