Pumps & Systems, June 2007
In many pump installations, problems such as jams and suction loss can lead to serious damage to the motor or pump long before the thermal overloads trip. These problems can be quickly detected by monitoring drive motor current.
In theory, a well-designed pumping system should quietly and efficiently continue to work for years. Unfortunately, the reality is that any number of operating conditions can dramatically shorten the life of the system. Dry pump, dead head, jammed impellers and premature bearing wear can all lead to early motor and pump failure.
At the same time, to save upfront costs many pump systems are installed without any way to continually monitor operation. This means frequent inspections and visits are necessary to maintain and repair these systems. If a failure occurs without warning, it can mean irreparable damage to either the pump or the motor.
To eliminate these problems and maximize system life, simple monitoring techniques can be used to protect any pump system. A monitoring system adds very little cost at installation, and pays for itself the first time a motor or pump is saved.
One of the easiest and most often overlooked methods is to continually monitor the motor current and use the results to determine if there is a potential problem. This article covers the basics of motor current monitoring and also offers some easy tips for extended protection.
Basic Theory and Operation
In any pump installation, motor power requirements are directly related to the amount of work being done. The heavier the load, the more power the motor is required to deliver. This power is directly related to the current the motor is drawing. By monitoring the amount of current the motor is using, it is possible to determine if the system is running as expected.
Once you know the current draw for normal operation, you can determine appropriate current limits. For example, if the amount of current used increases dramatically, the pump is probably jammed. A quick drop in current indicates a probable blockage in the suction line or pin shear. These conditions can be detected within milliseconds of the occurrence, normally much faster than a thermal fuse. The system can be automatically stopped prior to serious damage to the motor or pump due to overheating.
The easiest way to monitor motor power is to use a current transducer. A current transducer (simply a coil that is inductively coupled to the AC input of the motor) provides a signal that is directly related to the current being used by the motor. This signal can be used to trip an alarm or just drive a relay to shut down the system.
Current transducers are available in a number of configurations, the most useful being the split-core. A split-core transducer can be easily snapped around one of the motor wires, and doesn't require any hard-wired connection. Installation is easy, with the transducer mounted in the same box used for the motor wiring.
Figure 1. Current transducer installation wiring.
Types of Current Sensors
The simplest example of a current transducer is one that simply outputs a signal that is directly related to the current draw of the motor. This style of transducer is normally used with a PLC, and functions as an alarm.
Upper and lower current limits are programmed in the PLC along with the appropriate motor control functions. The transducer output can be either 0-V to 5-V or 4-mA to 20-mA for easy integration with a PLC or system monitor. These transducers usually work well with VFD control systems.
Current transducers are used frequently in wastewater treatment plants. Since most of these pump facilities are remote, they may not have onsite supervision. To solve this, many wastewater treatment plants use current transducers in conjunction with SCADA systems for remote monitoring of motor conditions.
A large municipality in Georgia, for instance, is responsible for 164 lift stations. Most of these stations are remote, but still need to be continuously monitored. By using current transducers on every motor with SCADA control, the municipality easily monitors the health of every pump system from one location. The current from each motor can also be trended and used to predict potential failures.
Current Switches
A very versatile type of current transducer is the current switch. In many installations, interface to a PLC is not required, so these sensors can be used on standalone or remote installations.
A current switch has logic built-in and can be preset for specific current limits. Most current switches can be set for either a high or low threshold limit. In both cases, there is a built-in delay that will ignore the startup current surge when the motor is first turned on. When the preset current limit is reached, the current switch can either send a signal to a controller or trip an internal relay built into the switch. This relay can be used to interrupt the motor contactor to quickly shut down the system.
Undercurrent switches are the most common and are especially useful in protection against flow blockage and/or cavitation, which can lead to overheating. These transducers switch when the motor current drops below a preset threshold. Undercurrent conditions are quickly detected and the system can be shut down immediately, limiting motor damage.
Split core models are most useful since retrofitting to existing systems is simple. This type of unit also includes a built-in relay for local motor control and can be wired directly to the motor contactor. Figure 2 shows the actual installation in a motor control panel, while Figure 3 illustrates the ease of wiring.
Figure 2. Current switch installation. Transducer is installed on one power lead just prior to the starter.
Overcurrent switches are used to protect the motor from damage due to jams or bearing failure. Although not designed to replace circuit breakers or thermal fuses (fuses can detect a lower load increase over a period of time), a current switch can detect an increase in load within milliseconds. A current switch in this application functions as a locked rotor sensor. This fast response can increase motor life, as well as reduce pump damage from a jam. Installation is simple and can be easily retrofitted to any system.
Figure 3. Current switch diagram for stand alone operation.
In all cases, installation is easy and straightforward. A split core current transducer can be snapped around one of the power leads to the motor, and the output fed to a PLC. In the case of a current switch, the relay output can be connected directly to the motor contactor. The current switch is then adjusted for an appropriate switch level, and the job is completed.
In conclusion, using current monitoring can increase the odds in your favor for a well-protected pump system with an extended life. By catching problems early, you can also reduce maintenance costs. Current transducers are easily installed and, with a little planning, can substantially reduce damage and maintenance time.