Protecting driven equipment, such as a large pump (see Image 1), can be a challenging and expensive procedure. In some cases, the driven equipment can be more expensive than the motor itself. Dangerous conditions such as clogged intake filters and dry running can cause serious damage to a pump system in a short amount of time.
Inefficient Motors = Higher Utility Costs and Worse
Another challenge faced by process owners is the prevention of simultaneous start-up of the motor-driven pumps, which, unless prevented, can lead to higher energy costs. With today’s intelligent motor managers, however, it is possible to use the motor itself as a sensor. These programmable electronic motor managers (EMMs) can monitor voltage, current and phase angle to detect potentially dangerous conditions. By monitoring this data stream, EMMs can prevent expensive pump damage and improve the system’s overall efficiency.
Energy costs are another critical issue for industrial pump operators. For commercial customers, utilities usually measure total energy consumption in kilowatt hours, but they also install a demand meter to measure the customer’s power usage rate.
Industrial sites usually use multiple motors. To prevent this excessive energy draw that would occur if all the motors started simultaneously, most plants use a sequential motor start-up process. Following start-up, a system process will dictate which motors run continuously and which ones cycle on and off. This often means that during the routine operation of the process, motor restarts can occur/cycle at random. At some point, two or more large-horsepower motors may start at the same time. Even if the higher energy draw only lasts for a few moments, the large inrush currents can lead to increased energy demand costs for the process owner.
Image 1. Motor-driven pumps can be expensive to protect, but an intelligent motor manager can guard against unnecessary wear and tear that can ultimately result in downtime.
Other common problems—such as clogged pump filters and dry running—not only increase energy consumption but can lead to serious problems, including system shutdown or equipment failure. Traditionally, no notification alerted end users that these problems existed. The system manager was unaware of a slow increase in motor current due to a partially clogged filter until receiving the utility bill or, worse, until the problem led to the process being down or, worse yet, having to replace a damaged pump.
Load Current Measurements
Little change occurs between low and medium currents in a motor load. The magnetic saturation makes this effect especially noticeable for small horsepower motors. The current only significantly increases in the range of the maximum load. When the load diminishes, such as when a dry run condition is about to occur, there is typically no way for the process owner to know until it may be too late. This is because most motor protection relays do not have a protection feature for under-load conditions.
Traditionally, current-dependent bimetallic, eutectic alloy or solid state overload relays have been used to protect motors from overloads. Rather than waiting for a certain amount of time, these relays detect when a specific amount of current has passed through them. When this happens, they interrupt the run command to the motor starter. These overloads cannot be programmed to ignore overload conditions for specific amounts of time. They are simple current versus time devices. Sometimes as a normal part of a process, however, overloads occur for short periods of time. It would be an advantage, therefore, for a process owner to be able to avoid these nuisance overload trips with an overload that was programmed to do so.
The curve of power factor cos φ manifests an almost opposite characteristic. Cos φ changes the most in the lower load range of the motor. If the motor power increases, the power factor changes only a small amount. With this characteristic, power factor cos φ is suited to detect load changes when the motor is close to no-load operation and, in turn, protect drive elements against under-load conditions. Unfortunately, both the power factor and the motor current are significantly influenced by voltage fluctuations, which can cause them to supply inaccurate values.
It is common for an industrial pump system to use oversized electric motors with more capacity than necessary. The advantages of using these oversized motors include:
- Longer lifespan for motor bearings
- Power reserve
- Smaller replacement motor inventory
The disadvantage is that the motor’s lower load means it does not use its full load range. If the motor current changes significantly, these changes fall outside the typical load range. The overload protection, therefore, is inefficient and more difficult to detect.
Because pumps and other motor systems tend to respond slowly to current increases, the standard overload relay response does not provide fast enough protection if a problem occurs during normal operation. During normal startup, the electric motor will have an inrush current between five and seven times the rated current. The overload relay must permit this. For seven times the rated current, it would take about nine seconds for an overload with a Class 10 trip curve—a comparatively fast tripping characteristic—to trip the motor.