Comparison of Variable Speed Drives

Let us now take a closer look at each of these control blocks and spot a few differences.

The first observation is the similarity between the control block of the DC drive (Figure 1) and that of DTC (Figure 4).
Both are using motor parameters to directly control torque.

But DTC has added benefits including no feedback device is used; all the benefits of an AC motor; and no external excitation is needed.

As can be seen from Table 1, both DC Drives and DTC drives use actual motor parameters to control torque and speed. Thus, the dynamic performance is fast and easy. Also with DTC, for most applications, no tachometer or encoder is needed to feed back a speed or position signal.
Comparing DTC (Figure 4) with the two other AC drive control blocks (Figures 2 & 3) shows up several differences, the main one being that no modulator is required with DTC.
With PWM AC drives, the controlling variables are frequency and voltage which need to go through several stages before being applied to the motor. Thus, with PWM drives control is handled inside the electronic controller and not inside the motor.


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