# What is the working principle of Tachometer ?

Discussion in 'General Electronics Chat' started by thar07, Apr 25, 2015.

1. ### thar07 Thread Starter Member

Jan 3, 2015
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What is the working principle of DC and AC ( permanent magnet and inductive ) Tachometers ?
What is the mechanical time constant and electrical time constant of tachometers ?

2. ### Papabravo Expert

Feb 24, 2006
10,338
1,850
I don't know if all your questions can be answered in a single forum post. There is a strong analogy between mechanical systems called "spring-mass-damper" and electronic systems consisting of resistors, inductors, and capacitors. Both systems are governed by a 2nd order differential equation that has solutions which are identical in form. These solutions are characterized according to whether the transient response includes damped oscillations or not. In this context a time constant it the amount of time required for an exponential solution to go from a value $e^0 = 1$ to a value of $e^{-1}= 0.367$. Does that help?

3. ### thar07 Thread Starter Member

Jan 3, 2015
71
0
No, This is beyond what I expected.
I just want to know how it works in simple with a diagram or two and the equations for the above mentioned constants.

4. ### Papabravo Expert

Feb 24, 2006
10,338
1,850
Mechanical time constants will involve mass or moment of inertia, the spring constant, and the dashpot constant. Electrical time constants will involve resistance in Ohms, inductance in Henrys, and capacitance in Farads.

The basic principle of a tachometer is to measure the rotational speed of a shaft. In the case of a vehicle this would be the crankshaft. A megnetic pickup or hall effect sensor can detect the presence or absence of a magnet and count revolutions. It is not much more complicated than that.

Last edited: Apr 25, 2015
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5. ### thar07 Thread Starter Member

Jan 3, 2015
71
0
Thank you.

What is the different between AC and DC Tachometers( permanent magnet and inductive ) ?

6. ### BillB3857 Senior Member

Feb 28, 2009
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In one form, a tachometer sending unit will generate a variable voltage by either pulse width modulation (PWM) or variable AC depending upon RPM. The PWM method uses a fixed pulse width with variable frequency. The resulting variable voltage will be indicated on a voltmeter calibrated in RPM. In another form, a tachometer sending unit will generate a variable frequency based upon RPM. The indicator will respond to the varying frequency to indicate RPM.

Permanent magnet vs inductive is rather ambiguous. Inductive types depend upon magnetism in some form, such as a bias magnet built into the sensor and a reluctor wheel. OR, it may be a permanent magnet supplying the field for either an AC or DC generator.

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7. ### MaxHeadRoom Expert

Jul 18, 2013
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There are a few types, DC which is a simple DC brushed generator, AC which truly AC would be on the lines of a Resolver, which does not posses a permanent magnet, there is also a Pulse generator, this can be magnetic or optical, which is some times referred to as AC, all are used as feedback devices.
Max.

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