To select a suitable load for an electrical test platform, it is first necessary to understand the characteristics and working principles of various types of load actuators:
I. Eddy Current Brake
Eddy current brakes are currently the most advanced simulation loading devices in China, mainly used to simulate the output performance of various power devices. They consist of an induction disc, an armature, and an excitation section. When the excitation coil, coaxially mounted with the rotor, is energized with direct current, the magnetic flux it generates forms a closed loop through the armature, eddy current rings, air gap, and rotor. Because the outer surface of the rotor is made with uniformly distributed teeth and slots, alternating magnetic fields of varying density are generated on the air gap and the surface of the armature or eddy current rings. Therefore, when the rotor is driven to rotate, the magnetic field at any point on the inner surface of the armature and eddy current rings undergoes a change, thereby inducing "eddy currents." Under the coupling effect of the "eddy currents" and the magnetic field, a braking torque is generated on the rotor. Since the armature is fixed to the base plate by the frame, the rotor cannot drive the armature to rotate. The power output of the power machinery is converted into equivalent heat generated by the "eddy currents" on the armature and eddy current rings. This heat is consumed by the continuous cooling water entering the cooling water tanks of the armature and eddy current rings, as well as by the eddy current brake itself. For each constant current in the excitation coil, the eddy current brake exhibits a stable braking characteristic curve where the torque depends on the rotational speed. By changing the magnitude of the excitation current, the braking torque can be changed.
II. Magnetic Powder Brake
Magnetic powder brakes are a new type of transmission element that uses magnetic powder as a medium to form a magnetic powder chain when energized to transmit torque. They consist of an inner rotor, an outer rotor, an excitation coil, and magnetic powder. When the coil is not energized, the driving rotor rotates. Due to centrifugal force, the magnetic powder is thrown onto the inner wall of the driving rotor, with no contact between the powder and the driven rotor, allowing the driving rotor to idle. When a DC power supply is applied, an electromagnetic field is generated. Under the action of magnetic lines of force, the working medium, magnetic powder, forms a magnetic powder chain, connecting the inner and outer rotors, thereby achieving the purpose of transmitting and braking torque. It can transmit a certain torque regardless of slip, and has advantages such as fast response, simple structure, no pollution, no noise, no impact vibration, and energy saving.
III. Hysteresis Brake
A hysteresis brake consists of two main parts: a rotor and stator poles. The rotor is made of a special hysteresis material, and there is a certain gap between the stator poles, within which the rotor rotates. When the coil is energized, a magnetic field is generated in the gap, causing the rotor to produce a hysteresis effect. When the hysteresis rotor rotates against the hysteresis force under the action of an external force, a rated torque is generated. The torque is only related to the magnitude of the excitation current and is independent of the rotational speed, achieving non-contact torque transmission.
IV. Servo Motor
Servo motors are broadly classified into DC and AC servo motors. They enable highly precise control of speed and position, converting voltage signals into torque and speed to drive the controlled object. The rotor speed of a servo motor is controlled by the input signal and can respond quickly. In automatic control systems, they are used as actuators and possess characteristics such as a small electromechanical time constant, high linearity, and low starting voltage. They can convert received electrical signals into angular displacement or angular velocity output on the motor shaft. A key characteristic is that there is no self-rotation when the voltage signal is zero, and the speed decreases uniformly as the torque increases.
V. How to Select a Model
Secondly, it is necessary to select a suitable load based on the speed, torque, power, and other attributes of the motor being tested, as well as the applicable load scenarios mentioned above. Each type of load has its own advantages and disadvantages:
1. Eddy current, suitable for high-speed and high-power applications, with a maximum speed of 30,000 rpm supported at 5 kW;
2. Magnetic powder braking is inexpensive and mainly used in low-speed, high-power applications. However, heat dissipation issues need to be considered, and its accuracy is relatively low, with the possibility of zero drift.
3. Hysteresis brake, suitable for high-speed, low-power applications, has a very small torque range; at a power of around 5kW, the highest torque that can be measured is only 30N.m.
4. Servo motors are suitable for applications requiring high precision, offer good test repeatability, and support powered loads. The speed and torque ranges are not fixed, allowing for flexible selection of the appropriate load motor based on the characteristics of the motor under test.
Therefore, based on the testing requirements of the motor under test, the choice of which load brake to select is determined by comparing the characteristics of various types of loads.
