Dynamometer principles, features and applications

With the advancement of technology, dynamometer principle and technology have significantly evolved from the earliest hydraulic and mechanical systems to the sophisticated devices used today. Initially, hydraulic and mechanical dynamometers were designed primarily to measure force and torque through direct mechanical connections or fluid dynamics, providing foundational insights into power transmission and engine performance. The dynamometer principle relies on the conversion of mechanical energy into measurable quantities, allowing engineers to assess the output of engines and motors accurately.

However, these early devices had limitations in accuracy and adaptability, prompting the development of more advanced measurement tools that could cater to a wider range of applications and testing environments.

Modern dynamometers encompass a variety of types, including hydraulic, eddy current, magnetic powder, hysteresis, and both direct current and alternating current dynamometers. The dynamometer principle varies among these types; for instance, hydraulic dynamometers utilize fluid resistance for robust applications, while eddy current dynamometers leverage electromagnetic principles for precise load control. The dynamometer principle applied in magnetic powder and hysteresis dynamometers offers unique advantages in accuracy and versatility, particularly in laboratory settings.

Direct current and alternating current dynamometers are essential for testing electrical machines, allowing for accurate assessments of torque and power in motors. The underlying dynamometer principle ensures that these devices can provide reliable measurements essential for evaluating motor performance. Together, these advancements reflect the growing complexity of mechanical and electrical systems, underscoring the importance of reliable measurement tools based on robust dynamometer principles in optimizing designs and enhancing product development across various industries.

Versatile dynamometer types, including hydraulic, eddy current, magnetic powder, and AC, DC variants, offer tailored solutions.

What is dynamometer?

A dynamometer is a device used to measure power, torque, and performance characteristics of engines, motors, and other rotating machinery. It applies a load or braking force to the device being tested, allowing for precise measurements. There are absorption dynamometers, which absorb and dissipate energy, and inertia dynamometers, which measure acceleration and deceleration. Dynamometers are essential tools for optimizing performance and ensuring compliance in various industries.

Classification of dynamometers

Direct or alternating current dynamometer
Eddy current dynamometer
Hydraulic Dynamometer or water brake
Hysteresis dynamometer
Magnetic powder dynamometer

Direct or alternating current dynamometer principle

The current dynamometer principle is a device that measures the torque output from the shaft of various power machines using a motor and combines it with the rotational speed to determine the power. This application of the dynamometer principle provides a crucial understanding of how power machines perform under different conditions. Current dynamometers are divided into direct or alternating current dynamometers and are currently used more often.

DC dynamometer: The DC power dynamometer operates on the dynamometer principle but is unable to function effectively at high speeds due to the limitations of DC motor rectifiers, which require specialized maintenance. Additionally, the inverter feedback quality associated with DC systems is not optimal, leading to a low power factor, especially in larger power applications where failures can occur. Therefore, it is generally not recommended to use DC dynamometers in modern applications.

Alternating current dynamometers: This type of dynamometer utilizes an AC variable frequency feedback loading based on the dynamometer principle. It is designed and manufactured with a standard inverter motor or servo motor equipped with a four-quadrant inverter, allowing it to operate efficiently at both high and low speeds. The reliability of this setup is very good due to its maintenance-free design. Furthermore, the feedback quality is excellent, with waveform distortion better than 5%, achieving a power factor of 98%-100%. Due to these advantageous characteristics, combined with the recent advancements in four-quadrant frequency conversion technology, the dynamometer principle behind AC frequency conversion feedback loading has become widely adopted in various industrial applications.

Current dynamometers leverage motor-based torque and speed measurements to determine power output offering superior performance.

Direct or alternating current dynamometer features

Energy saving: due to the adoption of power generation feedback loading, it is characterized by a good energy saving effect.
Very good loading characteristics: it can maintain constant torque loading (zero speed or even reverse rotation) below the rated speed and constant power loading above the rated speed.
The stability of constant torque and constant speed control is incomparable to other loaders, it can maintain a very high stability in the whole speed (even at zero speed) and torque range.
Zero-torque and zero-speed starting can be easily realized to accomplish a true shock-free soft start for the power grid and machinery.

Dynamometer innovations enable energy-saving operation, high-precision loading, and seamless integration with power grids and machinery.

Eddy current dynamometer principle

An eddy current brake is a device that measures mechanical torque by utilizing the dynamometer principle, specifically through the generation of braking torque via eddy currents. It consists of an electromagnetic slip clutch, a force-measuring mechanism, and a speed generator. The power machine to be measured is connected to the input shaft of the electromagnetic slip clutch, which drives the armature to rotate. The magnetic pole is restrained by a force arm mounted on it, which can only swing by an angle within a certain range, illustrating the effective application of the dynamometer principle in torque measurement.

The electromagnetic torque generated between the armature and the magnetic poles can be read directly from the swing angle by using a force-measuring mechanism. When measurement errors such as wind and friction losses are omitted, the electromagnetic torque is equal to the output torque of the power machine being measured, demonstrating how the dynamometer principle allows for accurate assessments of mechanical performance.

The eddy current dynamometer operates strictly based on the dynamometer principle and can only produce braking torque; it cannot function as a motor. This characteristic limits its application to scenarios where it is used to measure power machinery whose rotational speed increases while the torque decreases or whose rotational speed remains unchanged while the torque changes. This specific use case highlights the importance of the dynamometer principle in evaluating the efficiency and performance of various power machines, ensuring precise control and measurement in testing environments.

Eddy current dynamometer features

Simple structure, stable operation, low price, easy to use and maintain.
Adopts water cooling, low noise, and vibration.
Wide input speed range, which can be used for frequency conversion speed control and other types of motor and power machinery type tests.

Eddy current dynamometer applications

It is suitable for torque and power measurement of high-speed and high-power power machinery, especially for simulation life tests and temperature rise tests of power machinery, such as three-phase motors, synchronous motors, speed-regulated motors, power tools, and high-power motors.
For performance tests, type tests, and research tests.

Water brake or hydraulic dynamometer principle

A hydraulic dynamometer or water brake is a dynamometric device that utilizes the frictional resistance generated by the movement of an object in water to absorb the power of an engine. It consists of a brake, a force-measuring mechanism, a water supply system, and an indication control part. The brake is the part that absorbs power and consists of a rotor and shell. The water enters the vortex center through the water inlet hole, and the rotor makes the water do a rotary motion in the vortex chamber, which makes the shell oscillate through the friction with the shell.

A control valve controls the amount of water discharged to regulate the thickness of the water layer. The thicker the water layer, the greater the friction torque between the water and the shell, the more work is absorbed, the greater the angle of swing of the shell, and the corresponding increase in the reading on the dynamometer.

Water brake or hydraulic dynamometer features

Simple structure, low cost
High-speed absorption of power, and smooth operation.
The disadvantages are inconvenient control, poor measurement accuracy, difficulty in realizing long-distance manipulation, and automatic adjustment.

Water brake or hydraulic dynamometer applications

Mainly used for gasoline and diesel engines with power more than 300kW (up to 30MW).
The diesel engine research and development test bench mainly focuses on high power and high torque at low speed and the endurance test does not have the requirement of fast response characteristics.
Quality control test rigs for heavy engines, locomotives, and ship engines.
Special cost-effective requirements (using seawater as the working medium to reduce cost)

Hysteresis dynamometer principle

A hysteresis dynamometer is composed of a stator with toothed poles, a hollow hysteresis cup rotor, an excitation coil, a base plate, and so on. When current is passed through the coil inside the hysteresis dynamometer, magnetic lines of force are generated and a magnetic circuit is formed to produce torque. By changing the excitation current, the hysteresis torque of the dynamometer can be changed, to achieve the purpose of controlling the load torque and completing the test of the load characteristics.

Hysteresis dynamometer features

The rotor is a hollow cup structure, suitable for low-speed and medium-high-speed motors.
High testing accuracy, high sensitivity, good load torque stability, and test repeatability.
Brushless slip ring structure, no magnetic powder friction in the air gap, long service life.

Hysteresis dynamometer applications

Hysteresis dynamometer is suitable for small and medium power motor type tests, such as starter motor constant torque with load starting, asynchronous motors, single-phase asynchronous motors (washing machine motors, range hood motors, fan motors, air conditioning motors, compressor motors), shaded-pole motors, DC motors, tandem-pole motors, stepping motors, motorcycle starter motors, low-power DC motors, series excitation motors, and power tools industry, and so on. It is especially suitable for testing the dynamic characteristic curve of various micromotors.

Magnetic powder dynamometer principle

A magnetic powder dynamometer is composed of a stator, solid rotor, excitation coil, magnetic powder medium, bracket, base plate, and so on. When the dynamometer’s internal coil through the current generates a magnetic field, the internal magnetic powder by the magnetic lines of force is arranged into a magnetic chain. The magnetic powder chain produces a pulling force which becomes a resistance to prevent the rotor from rotating, and this force is the load torque. The load torque can be changed by changing the excitation current.

Magnetic powder dynamometer features

The rotor is a hollow drum rotor, with a small inertia, and bears centrifugal force.
The torque generation of the dynamometer is formed by the pulling force of the magnetic powder chain, and the change of torque is cushioning.
The static torque moment is smooth, with no fluctuating torque of the tooth groove, and no residual magnetic torque.
Frictionless structure, long service life.
Convenient operation, only need to adjust the excitation current to change the torque size of the dynamometer.

Magnetic powder dynamometer applications

Magnetic powder dynamometer is used for testing motors with medium and large torque but low rotational speed, such as starting motors with constant torque and load, asynchronous motors, DC geared motors, single-phase asynchronous motors, washing machine motors, hood motors, fan motors, air conditioning motors, compressor motors, shaded-pole motors, DC motors, power tools, three-phase motors, synchronous motors, speed-regulated motors, stepping motors, windshield-wiper motors, window-robbing motors and automobile warm-air motors, electric vehicle motors and constant tension control used in paper making, textile and other industries.

Why do we need to use the alternating current dynamometer?

Hydraulic dynamometer can only load in one direction.
Eddy current dynamometer can load in both directions, but eddy current dynamometer has poor low-speed loading performance, and may even fail to load at low speeds.
Magnetic powder brake cannot be used for high-speed occasions, only suitable for small power or lower speed loading.
Hysteresis dynamometer does not apply to high power occasions, only suitable for small torque or small power loading.
The alternating current dynamometer has a power feed function, therefore saving energy in the experiment.

Limitations of hydraulic, eddy current, magnetic powder, hysteresis dynamometers drive the need for superior AC designs.

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