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DC motor speed controller having protection

  • US 4,851,743 A
  • Filed: 10/27/1987
  • Issued: 07/25/1989
  • Est. Priority Date: 10/27/1987
  • Status: Expired due to Fees
First Claim
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1. A speed controller for a DC motor connected in series with a switching device that controls the average DC current of the motor by controlling the switching duty cycle, said switching device having an input control terminal, said speed controller comprising:

  • oscillator means for generating a pulsating DC signal of controllable duty cycle;

    means for communicating said pulsating DC signal to said input control terminal of said switching device to control the average DC current of the motor;

    overload protection means for turning off said switching device in response to excessive current flow in said motor, said overload protection means comprising;

    current sensing means for sensing during at least the on portion of said duty cycle and for producing a signal indicative of the motor current;

    comparator means responsive to the signal from said current sensing means for comparing said signal with a predetermined reference and outputting an overload signal upon occurrence of an overload;

    first override circuit means communicating said overload signal to said input control terminal of said switching device, for turning off said switching device irrespective of the pulsating DC signal of said oscillator means;

    wherein said comparator means comprises integration means for integrating said motor current sensing means signal;

    wherein said motor current sensing means comprises resistor means for developing a voltage drop and having terminals and connected in series with said switching device;

    said comparator means comprises amplifier means having inverting and non-inverting inputs, gain, and an output;

    wherein each of the voltage drops at terminals of said resistor means is communicated to one of the inverting and non-inverting inputs of said comparator means; and

    ,wherein said amplifier means comprises one operational amplifier arranged to perform all of the functions of (a) amplification of the difference between said voltage drops at said resistor means terminals, (b) integration for achieving noise reduction and time delay of said difference, and (c) latching of the overload protection means in an overload signal condition, starting upon occurrence of the overload signal condition, until unlatched.

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