The principle of variable frequency speed control is based on n = 60f1*(1-s)/p, meaning that changing the stator power supply frequency f1 changes the synchronous speed n of the motor. To maintain the maximum torque Mm of the motor constant during speed regulation, the stator power supply voltage must be adjusted accordingly to maintain constant magnetic flux. Different variable frequency speed control methods exist depending on the different proportional relationships between the stator voltage U1 and the stator power supply frequency f1. Figure 1 shows the mechanical characteristics of an asynchronous motor under variable frequency speed control.
(1) Proportional control mode that maintains U1f1 = constant. In this mode, as long as U1 and f1 change proportionally, constant magnetic flux can be maintained, thereby realizing variable frequency speed regulation. However, at low frequencies, since the stator impedance cannot be ignored relative to the stator leakage reactance, the maximum torque Mm decreases as the frequency decreases, and the starting torque will also decrease. Curves 4 and 5 in Figure 1 are ideal curves at low frequencies, and curves 1 and 2 are actual curves.
(2) Constant flux control mode that keeps the maximum torque Mm constant. In this mode, in order to keep Mm constant, U1 needs to be appropriately increased as f1 decreases.
(3) Constant power control mode. When f1>f1e (stator power frequency), if U1/f1 is maintained as a constant, U1 will inevitably exceed the stator rated voltage U1e, which is not allowed. Therefore, when f1>f1e, the stator voltage no longer increases, and U1=U1e is maintained. This can be approximated as constant power speed regulation. Curve 6 in Figure 1 is the ideal curve when f1>f1e, and curve 3 is the actual curve.
When the frequency is below the rated frequency f1e, the mutually "parallel" portions of the mechanical characteristic curves 7, 8, and 9 in Figure 1 have a wide speed range, which can adapt to different load sizes.
When the adjusted frequency is too low, as shown in Figure 1, f1 < the actual capacity becomes smaller, only able to meet small load torques such as M3, but not slightly larger load torques such as M1 and M2. If the process equipment requires low-speed or even ultra-low-speed operation, the motor cannot meet the requirements due to excessive load. Therefore, the torque boosting function of the frequency converter needs to be set to meet the load requirements of the production process.
When the adjusted frequency f1 is higher than f1e, the actual mechanical characteristics of the motor deteriorate, and the load capacity decreases, as shown in curve 3 of curve 6 in Figure 1. At this time, the frequency converter changes from a constant torque operating state to a constant power operating state, as shown in Figure 2.
Figure 1 Mechanical characteristics of asynchronous motor under variable frequency speed regulation.
Figure 2 Mechanical characteristics under constant torque and constant power speed regulation
At this point, as the frequency (speed) increases, the torque M gradually decreases. However, in practical applications, it is rare to encounter situations where the speed is adjusted above the rated frequency, and sometimes it is not allowed.
The frequency converter outputs the same frequency, but due to variations in load torque, the actual motor speed deviates, as shown in Figure 1, where M2 differs from M1 and M3 by a speed deviation Δn. The greater the load variation, the larger Δn becomes. When a one-to-one correspondence between the frequency and output speed is required, the motor should be replaced with a synchronous motor. The actual motor speed is then n = 60f/p, eliminating the need for speed feedback control. Precise control of the power supply frequency is sufficient to precisely control the speed.
