AN1910 Freescale Semiconductor / Motorola, AN1910 Datasheet - Page 6

• Manufacturer Part Number: AN1910
• Description: 3-Phase AC Motor Control with V/Hz Speed Closed Loop Using the DSP56F80X
• Manufacturer: Freescale Semiconductor / Motorola

Target Motor Theory
3.3 Speed Close Loop System
6
(Omega_required)
A simple close-loop volts/hertz speed control for an induction motor is the control technique targeted
for low performance drives. This basic scheme is unsatisfactory for more demanding applications
where speed precision is required.
To improve the system performance, a closed-loop volts per hertz control was introduced. In this
method a speed sensor measures the actual motor speed and the system takes this input into
consideration. A number of applications use the closed-loop volts per hertz method because of its
simple and relatively good speed accuracy, but it is not suitable for systems requiring servo
performance or excellent response to highly dynamic torque/speed variations.
Figure 3-5
The speed closed loop control is characterized by the measurement of the actual motor speed. This
information is compared with the reference speed while the error signal is generated. The magnitude
and polarity of the error signal correspond to the difference between the actual and required speed.
Based on the speed error the PI controller generates the corrected motor stator frequency in order to
compensate for the error.
Reference
Speed
illustrates the general principle of the speed PI control loop.
Frequency
-
3-Phase AC Motor Control with V/Hz Speed Closed Loop
Figure 3-4. Volts per Hertz Control Method
Freescale Semiconductor, Inc.
Figure 3-5. Closed Loop Control System
Speed
Error
For More Information On This Product,
Volt per Hertz Characteristic
Go to: www.freescale.com
Controller
100%
PI
Motor Base
Frequency
Point
Base
(Omega_command)
Corrected
Speed
Frequency
Controlled
System
(Omega_actual)
Frequency
Amplitude
Actual Motor
Speed