A Hoist That Won’t Start: Sometimes For The Best
During the installation of a Vulcan electric hoist by a technician, the unit stopped after a fraction of a second. What initially appeared to be an installation issue turned out to be an important indicator of the condition of the customer’s electrical system.
What happened?
While installing a 2-ton Vulcan electric hoist (230 V / 3-phase), the technician noticed that the unit stopped working almost immediately after being switched on. The phase sequence relay, a built-in safety device, flashed red and green, preventing any operation.
After checking, the diagnosis was clear: the building’s power supply had a phase imbalance of approximately 7.5%, well beyond the acceptable tolerances for a three-phase motor. The hoist was not faulty; it was protecting itself.
What did the relay detect?
Following the Vulcan team’s recommendations, the technician measured the voltage between each pair of phases with a multimeter. The result?
| Measurement | Voltage |
| L1 – L2 | 222 V |
| L2 – L3 | 249 V |
| L3 – L1 | 249 V |
The result: a healthy three-phase system should show nearly identical values across all three phases. Here, the difference was significant. The calculation yielded a voltage imbalance of approximately 7.5%, enough to trigger the protection.
QUICK REFERENCE: An imbalance of less than 1% is excellent. Between 1% and 3% is acceptable. Above 5% is problematic. At 10%, there is a high risk of motor overheating. |
Why is this a problem?
A voltage imbalance on a three-phase motor does not translate to an equivalent current imbalance.
The current imbalance can be six to ten times greater than the voltage imbalance, meaning that a 7.5% voltage imbalance can generate up to 30–40% current imbalance.
The practical consequences:
- Motor overheating
- Abnormal vibrations
- Significant reduction in service life
- Risk of premature failure.
The phase sequence relay continuously monitors several parameters: voltage (208 to 575 V), frequency (45 to 65 Hz), overvoltage and undervoltage (±15%), phase loss, and imbalance. It trips at 8% imbalance and resets only at 7%, thresholds specifically designed to prevent this type of damage.
How the installation was completed
Unable to modify the customer’s electrical installation, the technician temporarily bypassed the relay to finalise the limit switch settings, then informed the customer that an electrician would need to correct the power supply issue before permanent commissioning.
Key Takeaway
The phase sequence relay is not just a phase reversal detector. In this case, it served a much broader purpose:
Protecting the equipment and revealing an electrical issue that could affect all devices on the site.
If this hoist were returned for repair with a burned-out motor, it would indicate that the power supply issue had not been corrected and that the protection had been permanently bypassed. In this scenario, the damage would not be covered by the warranty.
This case illustrates the value of a well-designed protection system – not as a constraint, but as a diagnostic tool. |