Oil Resistor Starters:
These workhorses manage the starting sequence of large motors. We recently serviced oil starters on a mill and a fan that revealed just how applicable proactive maintenance can be.
If you’re operating oil resistor starters manufactured more than 15-20 years ago, especially if you’re experiencing any of these symptoms:
- Unreliable starting sequences
- Timer inconsistencies
- Worn or intermittent contactors
- No visibility into oil temperature
- Previous component failures
We have a solution that can give your oil starters new life.
How Proactive Refurbishment Can Save You Downtime:
1 What We Discovered During Testing
When testing oil cooled starters installed on a Mill and a Fan, it revealed several critical issues:
1.1 Starting Current Problems
The Oil Resistor Starter on the fan was starting at almost double the rated current—and also nearly double what it should be for a fan application. According to the nameplate, the starting resistance should have been 0.687Ω, but measurements showed only 0.373Ω. This excessive starting current puts unnecessary stress on both the motor and the electrical system.
1.2 Intermittent Contact Issues
Even after servicing, several contactors made intermittent contact during startup, resulting in unbalanced currents. These brief moments of poor contact may seem minor, but they’re warning signs of impending failure.
1.3 Timer Reliability Crisis
Perhaps most concerning was the condition of the timers. These weren’t responding accurately to their settings. Some dials were missing entirely. In one case, increasing the time setting on the dial actually decreased the measured time. This level of degradation makes reliable operation impossible.
2 The Domino Effect of Delayed Maintenance
Here’s what makes this situation urgent: when contactors and timers fail, they don’t just stop working—they can cause damage to the internal resistors. Once those resistors are damaged, repair becomes significantly more difficult, time-consuming, and expensive.
We’ve already seen this in one case where a shorting contactor failed to close, forcing current to flow through resistor elements until they melted and suffered irreparable damage. The refurbishment that followed required extensive work to repair the burned internal resistor elements.
3 Modern Solutions for Aging Equipment
Our refurbishment approach transforms these aging starters into modern, reliable systems with features that would make any maintenance manager smile:
3.1 Enhanced Monitoring and Testing
- Ready To Start Indicator: A green light lets you verify all contactors are open without opening the cover
- Individual Contactor Lights: Numbered indicators show exactly which stage the startup is in
- Oil Temperature Display: Digital readout shows oil temperature in real-time, so you know if you’re approaching alarm or trip points
- Fault Indication: An amber light alerts you immediately if a contactor isn’t responding correctly
3.2 Built-in Safety Features
- Check Timer Protection: If a contactor fails to close when expected, the system trips automatically—preventing the catastrophic resistor damage we saw earlier
- Inhibit Key Switch: Test that the PLC inhibits starting without opening the cover
- E-Stop Button: Verify trip signals are working properly
- Adjustable Thermostats: Separate alarm and trip settings that you can actually test and adjust
3.3 Remote Monitoring Capability
The refurbished starters are fitted with Current Transformers and a plug for a portable recorder. This means engineers can monitor high-sampling-rate recordings of rotor currents during starting without being physically present. An electrician can plug in the recorder, and data streams to off-site engineers for analysis, support, and reporting. This feature alone can save thousands in travel costs and reduce diagnostic time from days to hours.
3.4 Improved Starting Sequence
We’ve modified the sequence so that all contactors remain engaged until the motor reaches full speed, rather than dropping the previous contactor when the next one engages. This prevents dangerous resistance jumps if a contactor’s primary contacts fail while its auxiliary contacts still function—a scenario we’ve documented in previous failures.
3.5 The Permanent Power Advantage
While the refurbished starters can operate energized only during start-up, connecting permanent 220V power unlocks their full potential. With permanent power, you get:
- Continuous oil temperature monitoring
- Ready-to-start verification before attempting startup
- Remote data collection without site visits
- Complete functionality of all testing and diagnostic features
4 Reactive repairs instead of pro-active refurbishment, means:
- Unplanned outages
- Potential damage to expensive resistor elements
- Extended repair times
- Higher costs
- Continued risk of failure
- Damage to motors and plant
The starters we refurbished demonstrates what’s possible. It went from a poor performing starter to a modern, monitored system with multiple layers of protection and diagnostic capability. The same transformation is available for every starter in your facility.
The cost of prevention is always lower than the cost of failure—and the peace of mind that comes with modernized, monitored equipment is invaluable.
Contact us to discuss a customized refurbishment program for your facility’s oil resistor starters. Our engineers can assess your equipment and develop a phased approach that minimizes disruption while maximizing reliability.
