In the world of industrial automation, robotics, and precision engineering, speed control is everything. Motors must rotate at the ideal speed, and control systems must instantly respond to any changes. How does the "brain" of the system know how fast the shaft is rotating? This is where an indispensable sensor—a tachogenerator—comes into play.
A tachogenerator is essentially a small electrical generator. Its purpose isn't to power light bulbs or appliances, but to convert mechanical rotation into an electrical signal. The faster the shaft rotates, the higher the output voltage.
Today, we'll examine the two main types of these devices: DC and AC tachogenerators. What are their features, pros, and cons? Let's dig in.
DC Tachogenerators (DCTG)
DC tachogenerators were the first to appear and are still widely used due to their simplicity in signal processing.
How do they work?
The device resembles a classic DC motor. Inside, there's a stator (usually with permanent magnets) and a rotating rotor with windings. Current is extracted from the rotor using a commutator-brush assembly.
When the shaft rotates, a DC voltage is generated at the output. Its magnitude is strictly proportional to the rotation speed, and the polarity (positive or negative) indicates the direction of rotation (clockwise or counterclockwise).
Advantages:
Simple signal processing: Voltage can be directly fed to a voltmeter or analog control system without complex converters.
Direction detection: A polarity reversal immediately tells the system which way the shaft is rotating.
High accuracy: Excellent signal linearity over a wide speed range.
Disadvantages:
Wear: The brushes and commutator rub against each other, wear out, and require periodic maintenance or replacement.
Radio interference: Sparking on the brushes can create electromagnetic interference for sensitive equipment.
Environmental restrictions: Due to the potential for sparking, they cannot be used in explosive or highly dusty environments.
Alternating Current Tachometer Generators (AC Tachometer Generators)
With the advancement of electronics and the need for more reliable systems, AC tachometer generators have become increasingly important. They are divided into two main types: synchronous and asynchronous.
How do they work?
Unlike DC devices, the generator winding is located on a stationary stator, and a permanent magnet (in synchronous) or squirrel-cage coil (in asynchronous) is mounted on the rotor. This means there are no brushes or commutators. The output is an alternating voltage, the amplitude and frequency of which depend on the rotational speed.
Advantages:
Exceptional reliability: No rubbing contacts (brushes), meaning no wear and tear and no need for regular maintenance.
Signal purity: No sparking means no radio interference.
Safety: Excellent for use in aggressive, dusty, or explosive environments.
Disadvantages:
Complexity of signal processing: To obtain speed data, the signal must be rectified (converted to DC) and pulsations smoothed.
Direction of rotation issue: A standard synchronous AC tachogenerator does not indicate the direction of rotation (the amplitude increases regardless of the direction of rotation). Determining the direction requires more complex electronics that evaluate the signal phase.
Brief comparison: which one to choose?
For clarity, we have summarized the main differences in a table:
Characteristics Direct Current (DC) Alternating Current (AC)
Brushes Yes No
Durability Limited (brush replacement required) Very high (maintenance-free)
Signal Processing Simple (direct voltage) Complex (requires a rectifier)
Direction Detection Yes (by polarity) Complex (requires a phase detector)
Environmental Resistance Low (sensitive to dust and hazardous areas) High
Where are they used?
Despite the rapid advancement of digital encoders, analog tachogenerators are still indispensable where instantaneous response without digital delays is required. They can be found in:
CNC machines (cutter feed control);
Elevator and escalator drive systems;
Railway transport (traction motor control);
Wind turbines and conveyor belts.
Bottom line: The choice between DC and AC depends on the application. If extreme ease of control and precise directional control at low speeds are required, the TGPT is still a viable option. But if durability, operation in harsh conditions, and maintenance-free operation are a priority, the TGPT wins hands down.
