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Motor Tachometer Circuit

IP.com Disclosure Number: IPCOM000075890D
Original Publication Date: 1971-Dec-01
Included in the Prior Art Database: 2005-Feb-24
Document File: 2 page(s) / 33K

IBM

Related People

Henry, G: AUTHOR [+1]

Abstract

The device makes possible the unambiguous determination of the back EMF of a motor. The back EMF is directly proportional to the rotational speed of the motor, and therefore the motor may be used as its own tachometer.

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Motor Tachometer Circuit

The device makes possible the unambiguous determination of the back EMF of a motor. The back EMF is directly proportional to the rotational speed of the motor, and therefore the motor may be used as its own tachometer.

The fundamental voltage-current equation for a motor is:

V = V(B) + IR + L x dI over dT where V denotes the line voltage impressed on the motor, 1 the current flowing through the motor, R the motor resistance, and L the motor inductance. V(B) denotes the back EMF, which in a DC motor is proportional to the RPM. V and 1 are directly measurable, but R and L vary with the motor operating conditions and V(B) is, therefore, poorly determined.

The method presented here permits continuous measurement of R and L during the motor 4 operation, so that V(B) may be continuously monitored yielding motor speed without an external tachometer. A voltage whose frequency is 1-2 orders of magnitude higher than the driving voltage and of much lower amplitude is impressed upon the motor 4 simultaneously with the drive voltage by oscillator 5 and high-pass filter 3. Resultant current flowing in the motor is determined by the voltage drop across R1 and is used as the reference voltage for a phase and amplitude sensitive measuring circuit (Vector Impedance Motor) 6. Voltage across the motor is the other component for this measurement.

These signals are separated from the drive voltage by high-pass filters 1 and

2.

The resulting signal from the vec...