Motor Control Circuit

Prior art motor controllers typically measure the speed of the motor and the current flowing through the motor and then regulate it by suitable controlling mechanisms so as to keep the speed constant, or keep the energy input as low as possible, or avoid excessive input current surges, or achieve any other desirable objective. The problem with this approach is that the operating characteristics of motors are such that the current flow is quite erratic, noisy, and chopped up. Hence, the current flow must be averaged over some minimum interval of time, with a filter or the like, in order to establish a reasonably accurate measure of the average current flow at any one time. Experience has shown that sudden load changes on the motor, or other momentary deviations in operation, can cause surges in current flow faster than the minimum interval of time needed for practical measurement. These surges may do damage to the motor or blow fuses. My invention solves these problems.

Briefly, the instant invention avoids measuring the noisy and erratic motor current and instead utilizes a speed proportional representative voltage generated from a tachometer on the motor. This representative voltage is then used directly or in combination with resistances chosen to duplicate the characteristic counter electromotive force (back EMF) and the internal resistance of the motor being controlled. Suitable logic circuits choose either the direct voltage or the resistance proportioned voltage (to which a fixed voltage is added) to control the motor depending on the speed range of the motor. This type of control system is particularly suitable for serial commutator motors of either AC or DC design. Additional advantages and improvements will become apparent from the following detailed description.

FIG. 1 shows schematically the typical prior art approach to motor control.

FIG. 2 is a schematic diagram of the new control system of my invention.

FIG. 1 shows the typical prior art approach to motor control. Power from a supply 10 is regulated through a regulator 12 which could comprise, for example, a triac switch or the like. A motor 14 receives current through an in series resistor 16. The voltage across resistor 16 is detected by a filter 17 that smooths out the irregular current flow through motor 14. Filter 17 presents a representative averaged voltage to a maximum current detector 15. Detector 15 provides a signal to a controller 18 whenever the current reaches a predetermined maximum limit so as to prevent controller 18 from causing regulator 12 to supply excessive current to motor 14. A differential amplifier 23 provides an indication of the difference in speed from that desired by comparing a voltage from speed select 19 with a voltage developed by a tachometer 20 and a frequency to voltage converter 22. Controller 18 acts as needed to reduce the differential voltage from amplifier 23 to zero subject to the maximum limit established by detector 15. Controller 18 may be as simple as an analog electronic circuit, or more sophisticated, making use of a microprocessor following a program of operation suitable for the application involved.

The main problem with systems of the type shown in FIG. 1 is that sudden large surges in current may be drawn from supply 10 before filter 17 can smooth out and indicate to controller 18 that a change in average current flow has taken place. This transient current spike causes sensitive fuses in the supply to blow unnecessarily and shortens the life of the motor. The present invention overcomes this problem with the control circuit shown in FIG. 2.

My invention does not measure the erratic current flow through the motor itself, but instead substitutes a more stable model of the motor that uses either digital circuits, controlled by suitable software programs, or analog circuits as described in FIG. 2. In FIG. 2, resistors R.sub.1 and R.sub.2 serve as analog models of the internal resistance and characteristic back EMF of the particular motor being controlled. As before, motor 14 receives current from supply 10 and regulator 12, regulator 12 being controlled by controller 18. The output of tachometer 20 is a signal of frequency dependent on motor speed which is converted into a voltage by a frequency to voltage converter 22. When the motor is running near the desired set speed, the voltage from converter 22 is simply compared to the desired voltage from speed select 19. Differential amplifier 23 provides an output voltage E.sub.S on line 27 that serves as a control voltage for controller 18. It should be noticed that this control voltage E.sub.S is selected by a logic circuit 25 because it is smaller than an alternative voltage E.sub.K. Logic 25 compares voltages E.sub.S and E.sub.K and closes the appropriate switch to pass through the smaller voltage. In this case, E.sub.S is less than E.sub.K, so switch 24 is closed. If a sudden load causes motor 14 to slow, thus causing a sudden increase in current demand, the slowing of tachometer 20 decreases the voltage from converter 22. A corresponding reduction in voltage at point 28 passes through amplifier 30 and 32 to become voltage E.sub.K. Enough of a slow down makes voltage E.sub.K less than voltage E.sub.S and logic 25 opens switch 24 and closes switch 26 so that voltage E.sub.K is now present on line 27. Hence forth, the controller 18 tracks voltage E.sub.K and limits current flow through regulator 12 in accordance with the lower voltage E.sub.K rather than the quickly rising voltage E.sub.S. Resistances R.sub.1 and R.sub.2 are chosen to provide a voltage E.sub.K on line 27, E.sub.K being the sum of voltages E.sub.1 and E.sub.2. Voltage E.sub.2 is proportional to the speed of the motor (and thus proportional to the back EMF) and E.sub.1 represents the internal resistance of the motor such that the current supplied to motor 14 by regulator 12 is of a constant value. Only after the motor has had time to accelerate closer to its normal set speed, so as to lower voltage E.sub.S to a safer level, below voltage E.sub.K, will controller 18 again be allowed to respond to E.sub.S and operate in the normal speed regulation mode.

If the motor were to come to a stop, voltages E.sub.2 and E.sub.K would go to zero and no current would flow to the motor. To avoid this, a minimum starting current is established by a resistor R.sub.1 connected between a reference voltage 34 and ground so as to establish a fixed voltage E.sub.1 at point 36. A summing amplifier 32 adds voltages E.sub.1 and E.sub.2 so that voltage E.sub.K is always at least equal to fixed voltage E.sub.1.

It may thus be seen that resistor R.sub.1 serves as a model of the internal resistance of motor 14, always establishing a minimum control voltage E.sub.1 on line 27 to controller 18. Resistor R.sub.2 serves as a model of the back EMF of motor 14, holding motor current constant for the condition where motor 14 is turning at a speed such that E.sub.2 plus E.sub.1 is less than E.sub.S.