Open-Loop Control (Motors And Drives)

Open loop control was presented in topic 4, under the direct current (DC) and alternating current (AC) drive sections. A brief review is given here. Figure 5-1 indicates the basics of open-loop control.
Open-loop control
Figure 5-1. Open-loop control
The open-loop control is the simplest form of drive control. The drive operates the motor, given the speed reference that is delivered from the hand speed pot. If Figure 5-1 were of a DC drive, motor-speed regulation
would be 5-7% higher. If the figure were of an AC drive, motor-speed regulation would be 3-5% higher, possibly higher depending on motor design.
Open-loop control is more typical in AC drives, where the inherent motor-speed regulation is acceptable in the 3-5% range. DC drives are almost never operated open loop because of the higher regulation characteristics. Some type of feedback is used, such as armature voltage feedback (EMF) or tach feedback. These methods will be discussed in greater detail later in this topic. Before these methods are discussed, a review of closed-loop control is required. In addition, the types of devices involved will also be presented.


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