by Ross George, Littelfuse
December 17, 2011

Today's lean maintenance staff needs pump and motor protective devices that diagnose and predict problems before they become acute.

Pumps are indispensable in mining, petroleum recovery (wellhead operations), pipelines, refining and many manufacturing operations, so their failure causes costly unscheduled downtime. In the case of an electrical ground fault the pump's drive motor can be destroyed, and it can result in high voltage potential on the pump equipment framework that is a serious safety hazard to personnel. A pump and drive motor can fail in other ways, including pumps running dry and jammed pumps causing the motor to overload, which can lead to catastrophic damage to the motor and other electrical system problems.

Automated Prevention

Most fault conditions can be mitigated with the right kind of protective device. Today's electronic motor protection relays (Figure 1) can communicate motor operating information to a programmable logic controller (PLC), motor control center or an automated monitoring system. These real-time diagnostic features are important predictive/preventative maintenance functions that indicate that a problem needs attention before a catastrophic failure.

Figure 1. Electronic pump/motor protection relay

Some motor protection relays have inputs for temperature sensors. Temperature inputs in some relays shut down the motor before it becomes too hot and causes an insulation failure. More importantly, collecting temperature data from the ambient surroundings, pump and motor bearings and the motor windings can be used for predictive/preventative maintenance.

By monitoring and analyzing motor thermal trends, operators can see that the motor may become overheated if operating conditions are not altered. Overloading the motor, ambient temperature and lack of cooling when needed may overheat the motor. With temperature sensor inputs, the relay's thermal model of the motor can be biased to reflect operating conditions or adjust for hot spots on the motor, such as windings or bearings. These diagnostic features provide more efficient troubleshooting and help avoid motor damage, freeing up staff and eliminating many repairs and replacements. Instead of reacting to failures, maintenance staff can proactively schedule corrections.

Drive Motor Ground Faults

Pumps are often used in dusty and damp environments that can compromise insulation in the drive motor and its input wiring. These conditions can increase “earth leakage” currents—persistent, small currents from a high potential point to ground that may be a precursor to a major ground fault. Some causes of phase-to-ground faults are shorts in the motor windings or input wiring due to worn or melted insulation. Ground faults can cause electrical shocks, fires or even major arc-flash events.

Thermal Overload Relays

Earlier methods of detection and protection against ground faults are still being used. These electromechanical devices include bimetallic or melted alloy (eutectic) overload mechanisms. They contain heating elements wired in series with the motor inputs that cause either a pool of solder (eutectic alloy) to melt or a set of bimetallic strips to bend and release the contacts when current becomes excessive.

Bimetallic overloads generally have a limited range of adjustable trip points. No adjustability is possible with eutectic alloy mechanisms. Some bimetallic overloads will reclose automatically when the motor cools sufficiently. Bimetallic overload design can include compensation to prevent ambient temperature changes from affecting the trip point. Ideally, a thermal overload should be installed in the same ambient temperature as the motor it protects. In practice, these devices are often installed in the motor starter, which may be in an air-conditioned switchgear room away from the motor. What's more, their simple design precludes intelligent feedback.

Solid-State Electronic Protection

Certain types of solid-state overload relays have been introduced to resolve the inaccuracies of electromechanical devices. However, many of these have pre-calculated setpoints and cannot provide the same level of protection as more recent designs. In one type, the current is measured using a set of current transformers, which may be internal or external to the device. This protection system includes monitoring and current interruption in case of ground faults, overloads, unbalance, jams, etc.

However, using a current transformer may allow some level of damage to occur before the relay trips. An alternative to current monitoring is to use an insulation monitoring relay. This device monitors phase-to-ground insulation resistance and prevents the motor from starting if system insulation resistance falls below a selected set-point, avoiding potential motor damage.

An insulation monitoring relay applies a DC voltage and measures the leakage current to determine the system's insulation resistance to ground. By establishing a setpoint for this resistance and providing a signal output when it is too low, maintenance staff is made aware of degradation, allowing for scheduled maintenance and repair.

Another benefit, compared to electromechanical relays that pass motor current through heating elements, electronic relays do not cause excessive heat buildup on a power panel. Besides saving on power consumption, this helps avoid panel de-rating and the use a larger enclosure for heat dissipation.