AC Controllers

Figure 7-3 Curtis PMC 1221B DC motor controller .

The controller is also well matched in characteristics to the Advance model FBI4001 series DC motor, particularly in the impedance area (you read about the importance of this for peak power transfer). If the Curtis PMC DC controller characteristics sound familiar, it’s because they use the PWM IC technology you’ve been reading about throughout this chapter, and bring all these benefits to you with additional features in a rugged, preassembled, guaranteed-to-work package at a fine price. The same reputable vendor comments from the Chapter 6 also apply here.

Installation and hookup is a breeze. If you look closely at the controller terminals, you’ll notice the markings M–, B–, B+ and A2 appear (listed clockwise from the lower left when facing the terminals). You already know that the first three correspond to the –Mot, –Bat, and +Bat markings on the terminal bars in Figure 7-2. The A2 marking means Armature 2, the opposite end of the armature from the Armature 1 winding that is normally connected to the +Bat, or in this case the B+ terminal. Anything else you might want to know is covered in the Curtis PMC manual that accompanies the controller.

AC Controllers

AC overwhelmingly has benefits that make it a winner, in spite of complications involved. In general, AC motor controllers require more protection devices to isolate against noise, yet DC motors make way more noise than AC!

Chapter 6 showed that the speed-torque relationship of a three-phase AC induction motor is governed by the amplitude and frequency of the voltage applied to its stator windings (the upper left part of Figure 7-4 depicts this relationship). The best way to change the speed of an AC induction motor is to change the frequency of its stator voltage. As you can see in Figure 7-4, a change in frequency results in a direct change in speed, and if you change the frequency in proportion to the voltage (both at ¼, ½, ¾, etc.), you get the speed-torque curves shown.

Knowing the voltage and frequency ratio that you want to maintain allows you to calculate the voltage, current, and output torque relationship for any values of input voltage and frequency using vector math and lookup tables. In simpler terms, if you feed the speed and torque values you want to some sort of “smart box,” it can provide the voltage and frequency necessary to generate the proper motor control signals.