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[Part 1 begins with an overview of the operation of thyristor-fed DC motor drives. Part 2 continues with a look at converter output impedance, four-quadrant operation, single- and double-converter reversing drives and power factor and supply effects.]
CONTROL ARRANGEMENTS FOR D.C. DRIVES
The most common arrangement, which is used with only minor variations from small drives of say 0.5 kW up to the largest industrial drives of several megawatts, is the so-called two-loop control. This has an inner feedback loop to control the current (and hence torque) and an outer loop to control speed.
When position control is called for, a further outer position loop is added. A two-loop scheme for a thyristor d.c. drive is discussed first, but the essential features are the same in a chopper-fed drive. Later the simpler arrangements used in low-cost small drives are discussed.
The discussion is based on analogue control, and as far as possible is limited to those aspects which the user needs to know about and understand. In practice, once a drive has been commissioned, there are only a few potentiometer adjustments (or presets in the case of a digital control) to which the user has access. Whilst most of them are self-explanatory (e.g. max. speed, min. speed, accel. and decel. rates), some are less obvious (e.g. 'current stability', 'speed stability', 'IR comp'.) so these are explained.
To appreciate the overall operation of a two-loop scheme we can consider what we would do if we were controlling the motor manually. For example, if we found by observing the tachogenerator that the speed was below target, we would want to provide more current (and hence torque) in order to produce acceleration, so we would raise the armature voltage. We would have to do this gingerly however, being mindful of the danger of creating an excessive current because of the delicate balance that exists between the back e.m.f., E and applied voltage, V.
We would doubtless wish to keep our eye on the ammeter at all times to avoid blowing-up the thyristor stack, and as the speed approached the target, we would trim back the current (by lowering the applied voltage) so as to avoid overshooting the set speed. Actions of this sort are carried out automatically by the drive system, which we will now explore.
A standard d.c. drive system with speed and current control is shown in Figure 4.11. The primary purpose of the control system is to provide speed control, so the 'input' to the system is the speed reference signal on the left, and the output is the speed of the motor (as measured by the tachogenerator TG) on the right.
Figure 4.11 Schematic diagram of analogue controlled-speed drive with current and speed feedback control loops.
As with any closed-loop system, the overall performance is heavily dependent on the quality of the feedback signal, in this case the speed-proportional voltage provided by the tachogenerator. It is therefore important to ensure that the tacho is of high quality (so that its output voltage does not vary with ambient temperature, and is ripple-free) and as a result the cost of the tacho often represents a significant fraction of the total cost.
We will take an overview of how the scheme operates first, and then examine the function of the two loops in more detail.
To get an idea of the operation of the system we will consider what will happen if, with the motor running light at a set speed, the speed reference signal is suddenly increased. Because the set (reference) speed is now greater than the actual speed there will be a speed error signal (see also Figure 4.12), represented by the output of the left-hand summing junction in Figure 4.11.
A speed error indicates that acceleration is required, which in turn means torque, i.e. more current. The speed error is amplified by the speed controller (which is more accurately described as a speed-error amplifier) and the output serves as the reference or input signal to the inner control system.
The inner feedback loop is a current-control loop, so when the current reference increases, so does the motor armature current, thereby providing extra torque and initiating acceleration. As the speed rises the speed error reduces, and the current and torque therefore reduce to obtain a smooth approach to the target speed.
We will now look in more detail at the inner (current-control) loop, as its correct operation is vital to ensure that the thyristors are protected against excessive overcurrents.
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