Historically, liquid rheostats were the preferred choice for the simplicity of the equipment and because they were more cost-effective in applications that required high starting torque, while effectively limiting motor inrush currents that could perturb the network and affect other loads. Synchronous motors equipped with air clutches were the main alternative at the time, but they were more expensive and complex to implement. Also, such systems are prone to operating errors that can cause major breakdowns. That said, synchronous motors still offer some advantages, such as providing reactive compensation as well as allowing direct coupling without the use of a gearbox for lower-speed applications.
Surely, drive systems using liquid rheostats have some limitations that need to be recognized, compared to the other solutions available on the market nowadays, specifically VFDs. These limitations include:
- Higher maintenance requirements: not only for the rheostats themselves, but also for the brushes and slip rings on the wound rotor motors (WRIM);
- Speed variation is limited and involves high energy losses in the form of heat (poor efficiency);
- Low power factor meaning that capacitor banks are usually required near the motor.
However, several technical improvements have been introduced in recent decades to mitigate part of these issues, namely:
- Using a brush lifting device that removes the brushes from the circuit after motor start-up to reduce wear (short-circuit of the slip rings directly on the rotor once operating speed has been attained). This significantly reduces maintenance requirements on the brushes, although the lifting system itself does require some servicing.
- Adding a slip power/energy recovery (SPR) drive to the rotor circuit in order to enable efficient speed variation. Indeed, a subsynchronous cascade allows energy drawn from the rotor to be restored on the stator, resulting in lower energy losses than with a full-power VFD. However, speed variation typically remains limited within a range of ± 25%.
The main advantage of liquid rheostats remains their simplicity and robustness. Not only are they less prone to breakdowns than modern VFDs, liquid rheostats are much less sensitive to power disturbances and release minimal harmonic emissions into the grid even when an SPR is introduced.
However, rigorous maintenance tactics are crucial to keep liquid rheostats working properly. Indeed, a badly tuned or poorly maintained liquid rheostat can send excessively strong jolts of torque during the start-up sequence and damage the drive’s mechanical components (shafts, gear boxes, ring gears, etc.). Effective maintenance that keeps the electrolyte solution and electrodes in good condition is usually sufficient to keep the system running properly over the long-term.
In short, given all the advantages of liquid rheostats, including their lower cost, it’s not likely that they will disappear any time soon in large-scale industrial settings. The main drawback remains that they don’t allow effective full speed variation, a feature only available with VFDs, which can be an important differentiator in some applications.
If you have questions about liquid rheostats, please get in touch; we’d be happy to share our insights with you! The BBA team has solid background in engineering and commissioning of such drive systems, including condition assessment and technical assistance for the maintenance and refurbishment of legacy units.