Detecting electrical unbalance and overloads
Thermal images are an easy way to identify apparent temperature differences in industrial three-phase electrical circuits, compared to their normal operating conditions. By inspecting the thermal gradients of all three phases side-by-side, engineers can quickly spot performance anomalies on individual legs due to unbalance or overloading.
Electrical unbalance can be caused by several different sources: a power delivery problem, low voltage on one leg, or an insulation resistance breakdown inside the motor windings. Even a small voltage unbalance can cause connections to deteriorate, reducing the amount of voltage supplied, while motors and other loads will draw excessive current, deliver lower torque (with associated mechanical stress), and fail sooner.
A severe unbalance can blow a fuse, reducing operations down to a single phase. Meanwhile, the unbalanced current will return on the neutral, causing the utility to fine the facility for peak power usage. In practice, it is virtually impossible to perfectly balance the voltages across three phases.
What we check
We capture thermal images of all electrical panels and other high-load connection points such as drives, disconnects, controls and so on. Where we discover higher temperatures, we follow that circuit and examine associated branches and loads. Panels and other connections are checked with their covers removed. Ideally, we should check electrical devices when they are fully warmed up and at steady state conditions with at least 40 % of the typical load. This way our measurements can be properly evaluated and compared to normal operating conditions.
What we look for
Equal load should equate to equal temperatures. In an unbalanced load situation, the more heavily loaded phase(s) will appear warmer than the others, due to the heat generated by resistance. However, an unbalanced load, an overload, a bad connection, and a harmonic imbalance can all create a similar pattern. Measuring the electrical load is required to diagnose the problem.
Note: A cooler-than-normal circuit or leg might signal a failed component. It is sound procedure to create a regular inspection route that includes all key electrical connections. Using our thermal imaging software we can save each image we capture on a computer and track the measurements over time. This way, we will have baseline images to compare to later images. This procedure will help us determine whether a hot or cool spot is unusual. Following corrective action, new images will help is determine if repairs were successful.
What represents a “red alert?”
We prioritized repairs by safety first—i.e., equipment conditions that pose a safety risk—followed by criticality of the equipment and the extent of the temperature rise. Immediate action is usually recommended when the difference in temperature between similar electrical components under similar loading exceeds 15 °C or when the temperature between an electrical component and the ambient air temperatures exceeds 40 °C.
What’s the potential cost of failure?
Motor failure is a common result of voltage unbalance. Total cost combines the cost of a motor, the labor required to change out a motor, the cost of product discarded due to uneven production, line operation and the revenue lost during the time a line is down. Assume the cost to replace a 50 hp motor each year is £3,000 including labor. Assume 4 hours of downtime per year with income loss of £4,000 per hour. Total Cost: £3,000 + (4 x £6,000) = £19,000 annually
When a thermal image shows an entire conductor is warmer than other components throughout part of a circuit, the conductor could be undersized or overloaded. We check the conductor rating and the actual load to determine which is the case. Use a multimeter with a clamp, a clamp meter or a power quality analyzer to check current balance and loading on each phase. On the voltage side, we check the protection and switchgear for voltage drops. In general, line voltage should be within 10 % of the nameplate rating. Neutral to earth voltage tells us how heavily your system is loaded and helps us track harmonic current. Neutral to earth voltage higher than 3 % should trigger further investigation. Loads do change, and a phase can suddenly be 5 percent lower on one leg, if a significantly large single-phase load comes online. Voltage drops across the fuses and switches can also show up as unbalance at the motor and excess heat at the root trouble spot. Before we assume the cause has been found, first we double check with both the thermal imager and multi-meter or clamp meter current measurements. Neither feeder nor branch circuits should be loaded to the maximum allowable limit. Circuit load equations should also allow for harmonics. The most common solution to overloading is to redistribute loads among the circuits, or to manage when loads come on during the process. Using our thermal imaging software, each suspected problem uncovered with our thermal imager can be documented in a report that includes a thermal image and a digital image of the equipment. This is the best way to communicate problems and to suggest repairs.
XS Engineering are expert Electricians based in Paphos and cover the whole of Cyprus.