Types of Tachogenerators: Key Features and Real-World Applications

Once you understand how a tachogenerator works, the next question is usually which one to specify. Tachogenerators are not a one-size-fits-all component — the right choice depends on the speed range, environment, and control system you are working with. This guide walks through the main types of tachogenerator available, the key features worth comparing before you buy, and some real-world applications where each type earns its keep.

The Main Types of Tachogenerators

DC (Brushed) Tachogenerators

DC tachogenerators are the most widely used type in industrial motor control. A permanent-magnet rotor turns inside a fixed field, and a commutator with carbon brushes converts the generated signal into a direct current output. The key advantage of a DC design is that the output voltage reverses polarity when the shaft reverses direction — so a DC tachogenerator tells you not just how fast something is turning, but which way. This makes it a natural fit for closed-loop DC drive systems, where both speed and direction feedback are needed. The trade-off is that brushes wear over time, so DC units need periodic inspection and eventual replacement in high-duty-cycle applications.

AC (Brushless) Tachogenerators

AC tachogenerators use stationary windings and a rotating permanent magnet, so there is no commutator or brushes to wear out. The output is an alternating signal whose amplitude and frequency increase with speed. Because there is nothing to wear mechanically beyond the bearings, AC tachogenerators typically offer a longer service life and lower maintenance than DC types. The trade-off is that a standard AC signal has no inherent direction — it cannot tell you whether the shaft is turning clockwise or counter-clockwise, only how fast. Where direction feedback isn’t required, AC units are often the more durable and cost-effective long-term choice.

Drag-Cup Tachogenerators

A less common but useful variant, the drag-cup tachogenerator replaces the wound armature with a lightweight aluminium cup rotating inside a wound electromagnetic stator. As the cup rotates, it generates eddy currents that produce an output signal proportional to speed. Because the cup has very low inertia compared to a wound armature, drag-cup designs respond extremely quickly to speed changes and load the driven shaft very lightly — a useful property in sensitive servo and instrumentation applications where the tachogenerator itself must not affect the dynamics of the system it is measuring.

Digital / Pulse Tachometers

Strictly speaking, digital tachometers (sometimes built around optical or magnetic encoders) work differently to analogue tachogenerators — rather than generating a proportional voltage, they produce a train of electrical pulses as the shaft rotates, and a controller counts those pulses over time to calculate speed. They are worth knowing about because they are often considered alongside analogue tachogenerators when specifying a new system. Digital types can offer very high resolution and excellent long-term stability with no brush wear, but they need external power and signal processing to produce a usable speed value, and very low-speed readings can lag slightly while enough pulses accumulate to calculate an accurate figure.

Key Features to Consider When Choosing a Tachogenerator

Once you know which broad type suits your application, the following features will help you narrow down the right specific unit:

Voltage gradient: usually specified in volts per 1,000 rpm, this determines how the tachogenerator’s signal will scale against your control system’s input range.
Linearity and ripple: how closely the output voltage tracks true speed across the full working range; ripple and non-linearity both introduce small errors into the control loop.
Mechanical mounting: shaft type (solid, hollow-bore), mounting flange, and overall envelope size need to match the driven equipment, particularly on retrofit or replacement jobs.
Speed range: the minimum and maximum rpm the unit is rated for — running below or above this range compromises accuracy.

Environmental rating: look at the enclosure’s IP rating, operating temperature range, and vibration tolerance against the conditions on your plant floor.
Durability and maintenance: brushed DC units need periodic brush inspection and replacement; brushless AC and drag-cup types generally need little beyond routine bearing checks.
Retrofit compatibility: on replacement projects, matching the physical footprint and electrical characteristics of the outgoing unit avoids costly rewiring or drive recalibration.
Direction sensing: only DC tachogenerators indicate direction via output polarity; confirm whether your application needs this.

Getting these specifications right the first time avoids downtime later — an undersized or mismatched tachogenerator is a common (and avoidable) cause of erratic speed control on older drive systems.

Real-World Applications of Tachogenerators

Tachogenerators are unglamorous but essential components across a wide range of industries. Some of the most common applications include:

Elevators and lifts: smooth acceleration and accurate speed regulation are essential for passenger safety and comfort on lifts, escalators, and hoists.
Cranes and hoists: precise, repeatable speed control on cranes, winches, and material handling equipment reduces load swing and improves operator control.
Printing, textile, and packaging machinery: multiple rotating sections on printing presses, textile looms, and packaging lines must be kept in sync to avoid material damage or misfeeds.
Industrial motor drives: the tachogenerator closes the feedback loop to keep a DC motor running at a set speed regardless of load changes.

Test benches and dynamometers: used on dynamometers and rolling road equipment to verify motor, engine, and drivetrain performance against real-time speed data.
Mining and bulk material handling:Conveyors, crushers, and processing equipment operating in dusty or high-vibration environments, where a rugged, low-maintenance sensor is essential.
Wind turbines and off-highway equipment: Rotor speed monitoring on wind turbines, and engine or shaft speed sensing on marine and off-highway vehicles.

From heavy industrial plant to precision servo systems, the common thread is the same: a tachogenerator gives equipment a simple, direct, and dependable way to sense its own speed.

Finding the Right Tachogenerator for Your Application

With several types available and a range of features to weigh up, choosing the right tachogenerator is easiest with some guidance from people who work with these components every day. Get in touch with the Saftronics team to talk through your speed sensing requirements — we can help you specify, source, or replace a tachogenerator that matches your motor, drive, and environment.