Electrical Tachometer - Construction & Types | DC & AC Tachometer

The tachometer is a measuring instrument used for measuring the angular (rotary) velocity of a rotating object. They measure rotational speed in revolutions per minute (RPM). It is important to measure the rotating speed of machines in various industrial applications, automobiles, etc which can be done by tachometers.

The tachometers can be of two types mechanical and electrical type. The mechanical tachometer directly measures the angular velocity in rpm. While the electrical tachometers convert angular velocity into a proportional electrical signal (voltage).

By measuring the electrical voltage with help of measuring instruments the rotational speed or angular velocity can be determined. The electrical tachometers are classified into two different types. They are,

• DC tachometer generator
• AC tachometer generator.

These type of tachometers consists of ac or dc generators that convert the rotational or angular speed into dc or ac voltage. The electrical voltage developed in the conductor is due to the relative perpendicular motion between the magnetic field and the conductor. The magnitude of the voltage which is developed in the conductor is proportional to the angular velocity of the conductor.

DC Tachometer Generator :

It consists of a permanent magnet and a small armature. The armature is placed between the poles of the magnet. The rotating shaft whose rotating speed or whose angular speed/velocity is to be measured is coupled to the armature. The figure below shows a dc tachometer generator.

So, when the shaft rotates the armature also revolves in the magnetic field of the permanent magnet. As the armature is made up of a conducting material its rotation results in the generation of emf.

The current flowing in the armature is collected through commutator and brushes and thus voltage is generated which can be measured using a moving coil voltmeter. In the case of a short circuit, the current from the generator is limited by series resistance.

The generated emf in the armature is proportional to the rotating speed of the shaft coupled to the armature and the polarity of induced emf gives the direction of rotation. Therefore, the measure of the voltage generated gives the unknown speed and its direction of rotation.

Advantages of DC Tachometer Generator :

• The generated emf is directly proportional to speed.
• The direction of rotation of armature is known by the polarity of generated voltage.
• The range of output voltage is about 10mV/rpm and it can be measured by means of a moving coil voltmeter that contains a uniform scale.
• The presence of a series resistor limits the current from the generator in case of a short circuit on the output side.
• The relationship between input (speed) and output (voltage) is linear.

Disadvantages of DC Tachometer Generator :

• Maintenance of brushes and commutator have to be done frequently in order to avoid errors that may be produced due to any change in the contact resistance between brushes and commutator.
• Large values of armature current result in distortion of the field of the permanent magnet. So to restrict the armature current to low values the input resistance of the meter should be larger than the output resistance of the generator.

AC Tachometer Generator :

The drawbacks of dc tachometers due to the presents of the commutator are not present in ac tachometers. Because in ac tachometers the coil is wound on the stator and the magnet is allowed to rotate. The rotating magnet can be either an electromagnet or a permanent magnet.

The machine whose angular speed is to be measured is connected to the rotating magnet. When the magnet rotates the flux lines are cut by the stationary coil and thus according to the electromagnetic induction law an emf is induced in the stator coil. The amplitude or frequency of the induced voltage gives the measure of speed as both the parameters are proportional to the speed of rotation.

The circuit shown above is used when it is required to measure speed in terms of the amplitude of the induced voltage. The voltage generated by the ac tachometer generator is rectified and smoothened before it is measured by moving coil voltmeter.

Advantages of AC Tachometer Generator :

• The amplitude and frequency of voltage induced in stator winding are proportional to speed.
• The unknown speed can be measured in two ways. One is by measuring the amplitude and the other is by measuring the frequency of the induced voltage.
• The rectified output voltage can be measured by means of a moving coil voltmeter that contains a uniform scale.
• The problems of the dc tachometer generator due to the presents of the commutator are absent.
• The relationship between input (speed) and output (amplitude and frequency of voltage) is linear.

Disadvantages of AC Tachometer Generator :

• Under low-speed conditions, the resulting output voltage frequency is low.
• Under high-speed conditions impedance of coils increases along with frequency. So in order to have good linearity, the input impedance of the display device should be high compared to the impedance of coils.

Drag Cup Type AC Tacho-generator :

In this type of ac tacho-generator, the rotor is in the form of a cup shown in the figure below. The cup is made of a highly conductive material such as aluminum and it has very low inertia (i.e., the cup is thin).

The stator of the generator consists of two field windings wound on it. These two field windings are mounted on the stator, such that they are at right angles to each other. These windings are said to be in space quadrature with each other and are referred to as quadrature winding and reference winding.

The reference winding is supplied with an ac voltage and the output voltage is taken from the quadrature winding. The cup is known as the drag cup as it is connected to the object whose speed is to be measured. A reference voltage V_r cos ω_ct is applied to the reference coil. If the resistance and reactance of the coil are negligible, then a reference flux Φ_r sin ω_ct is produced which lags the reference voltage by 90°.

When the drag cup rotates in the air gap of this field an emf is induced in the cup and a current flows through it. Due to this induced emf, a quadrature flux Φ_q is produced which in turn leads to a transformer action and an emf e_q is induced in the quadrature coil.

Thus, the voltage output V_o(t) at the terminals of the quadrature coil is proportional to the speed of rotation and is a function of speed.

Advantages of Drag Cup Type Tacho-generator :

• The output of the drag cup type tacho generator is ripple-free.
• The direction of rotation can also be determined by measuring the phase of its output voltage using a phase detector.
• The output voltage of the tacho-generator can be made linear with respect to speed if the frequency of excitation voltage (i.e., the carrier frequency) is 5-10 times the frequency of speed signal (for example, 400Hz).
• They are rugged, require less maintenance, and are not expensive.

Disadvantages of Drag Cup Type Tacho-generator :

• The output voltage varies non-linearly with respect to the input voltage at high speeds of rotation. Linearity can be obtained by exciting the reference winding with higher frequencies (above 400 Hz) carrier voltage or by making some corrections.
• Since the output voltage is proportional to the product of input voltage and speed, the input voltage should be kept constant in order to have appropriate calibration. But, as the operation of the drag cup tacho-generator requires an alternating input voltage, the calibration of the drag cup tacho-generator is difficult.