When it came to the eddy-current dynamometers, the wet gap machine was not considered, due to its inherent high inertia, high level of minimum torque and liability to corrosion if left static for longer periods of time (Martyr & Plint, 2007). The last mentioned point is of particular importance given the infrequent nature of the engine testing conducted at Stellenbosch University’s Biofuel Test Facility. Consequently, the following section details the evaluation of the dry gap eddy-current dynamometer.
B.2.1 Suitability of dynamometer system
Although the dry gap eddy-current dynamometer also requires a water supply, unlike the hydraulic dynamometer, the torque and speed control of the eddy- current dynamometer is not affected by any fluctuation in the water pressure. This then eliminates the need for additional hardware to control the water supply pressure. In addition, since water only flows between the loss plates of the dynamometer and is never in contact with the machine’s rotor, the eddy-current dynamometer has less drag and thus a lower level of minimum torque compared to the hydraulic dynamometer. This is particularly beneficial considering that the test engines will primarily be small capacity engines with low torque output. Similar to the hydraulic dynamometer, the eddy-current machine also does not have the ability to motor or start the test engine. This shortcoming can, however, be overcome by installing an additional electric motor behind the dynamometer as part of the drive train. This electric motor then enables the operator to motor the engine when the dynamometer is not absorbing power. However, the addition of such a motor to the system does bring about its own challenges. Firstly, in terms of mechanical design (both the dynamometer and the motor has to be coupled to the engine utilizing two concentric driveshafts, the one running inside the other). Secondly, a more sophisticated control system is required, that makes provision for the control of the additional electric motor.
B.2.2 Integration
Trunnion mounting an eddy-current dynamometer has similar requirements to that of trunnion mounting a hydraulic dynamometer. However, compared to a hydraulic dynamometer of similar capacity, the eddy-current dynamometer is typically a heavier machine (Killedar, 2012). This might complicate the mounting of the dynamometer, although supporting the mass of the dynamometer is in general not a large concern in engine test setups as the test bench can easily be designed to accommodate the additional mass. Although the dynamometer does not require a constant supply water pressure to obtain good torque control, it is
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crucial that an adequate water flow rate is maintained through the dynamometer during operation. A drop in the water flow rate will cause the loss plates to overheat and distort. In the event that these plates do distort, the air gap between the rotor and the loss plates will close up, leading to catastrophic failure of the dynamometer when the rotor comes into contact with the cooling plates (Martyr & Plint, 2007) (Killedar, 2012). If the eddy-current machine is selected, flow switches will have to be fitted to the dynamometer to ensure that a suitable water flow rate is maintained. Pressure switches alone will not suffice, as it is possible to have pressure in a closed system without having any flow of water (Martyr & Plint, 2007).
B.2.3 Cost
When it comes to the initial capital layout, the eddy-current dynamometer is slightly more expensive than an equivalent sized hydraulic dynamometer. Furthermore, since the eddy-current machine achieves load control through varying the current supplied to the field coils inside the dynamometer, it has higher electrical power consumption than a hydraulic dynamometer. This in turn, contributes to it having higher operating cost compared to a hydraulic dynamometer (Killedar, 2012).
The maintenance requirements for the eddy-current dynamometer are very similar to those of the hydraulic dynamometer. The fact that the eddy-current dynamometer also utilises a trunnion-mounted arrangement, similar to that of the hydraulic dynamometer, means that it is susceptible to similar complications in terms of brinneling of its trunnion bearings. The eddy-current dynamometer also requires that maintenance be performed on a more regular basis compared to a hydraulic dynamometer. This is due to scale formation occurring more rapidly in the loss plates of the eddy-current machine, then it does inside a hydraulic dynamometer (Killedar, 2012).
B.2.4 Operational lifetime and flexibility (future research)
As long as the required maintenance is performed and an adequate water flow rate is maintained through the dynamometer during testing (thus ensuring that the outlet water temperatures do not rise above the specified operating limits), eddy- current machines are very reliable and continue to perform satisfactory for years on end. However, if the dynamometer is not maintained properly and clogging of the water channels inside the loss plates occur, a substantial reduction in the power absorption capability of the dynamometer will be noticed.
Eddy-current dynamometers are limited to steady state testing and are not used to perform transient testing. Due to the load applied by these dynamometers only being a function of the amount of current that is supplied to field coils they are, however, capable of performing rapid load changes. They also have the capability of developing a substantial amount of braking torque at low operating speeds, which can be useful for future research projects (Martyr & Plint, 2007). Similar
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to a hydraulic dynamometer, the eddy-current machine’s biggest limitation is, however, still the fact that it lacks the ability to motor the test engine.