Flying start (FS) is a feature that allows the drive to determine the speed and direction of a spinning motor and begin the output voltage and frequency at that speed and direction. Without flying start, the drive begins the output at zero volts and zero speed and attempt to ramp to the commanded speed. If the inertia or direction of rotation of a load requires the motor to produce a large amount of torque, excess current can result and overcurrent trips can occur on the drive. These problems can be eliminated with flying start.

Flying start is the capacity to start control at any speed other than ZERO, which is an important function in air-condition application for fan drive.

When a motor is started in the normal mode, the control initially applies a frequency of 0Hz and ramps to the desired frequency. If the drive is started in this mode with the motor already spinning with non-zero frequency, large currents are generated. An over current trip can result if the current limiter cannot react quickly enough. Even if the current limiter is fast enough to prevent an over current trip, synchronization can take an unacceptable amount of time to occur and for the motor to reach the desired frequency. In addition, larger mechanical stress is placed on the application.

In flying start mode, the drive’s response to a start command is to synchronize with the motor’s speed (frequency and phase) and voltage. The motor then accelerates to the commanded frequency. This process prevents an over current trip and significantly reduces the time for the motor to reach the commanded frequency. Because the drive synchronizes with the motor at the rotating speed and ramps to the proper speed, little or no mechanical stress are present.

The flying start function implements an algorithm that searches for the rotor speed. The algorithm searches for a motor voltage that corresponds with the excitation current applied to the motor

When the motor is spinning, the speed and position information can be estimated from the BEMF voltages. Since the stator voltage is measured, the speed and position are easily obtained by switching the inverter. A zero torque current is applied to the motor and the generated current and stator voltage is measured, then FOC module uses these signals to estimate rotor position and speed.

The block diagram of FOC with flying start is shown in Figure 3-17, the flying start module outputs a flag to enable or disable speed close loop control. A zero reference torque current is set and the speed PI controller output is disabled while flying start is operating.

Figure 3-17 Flying Start Control Block Diagram

Figure 3-18 shows the overall block diagram of sensorless FOC of PMSM using eSMO with flying start in this reference design.

Figure 3-18 Sensorless FOC of PMSM using eSMO with Flying Start

As shown in Figure 3-19, the module routine disables speed close loop control, sets the reference Iq to zero, and enables the FOC module during starting run the motor. After the phase currents and voltages are measured, the routine runs FOC and the real motor speed can be estimated. The program re-enables speed closed loop control and sets the speed reference value after flying start is completed.

Figure 3-19 Flying Start Module Program Flowchart