System control: Current monitoring simplifies conveyor control system
By Control Engineering Staff -- Control Engineering, 2/22/2007
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| A split-core current transducer snapped around one leg of a motor's power feed provides a voltage output proportional to the motor's current draw. That signal can then provide feedback for a control system to maintain constant load or to provide alarms should the system jam or break. |
It's possible to reduce the number of sensors needed in any complex motion control system by paying attention to drive motor current and its relation to the motor load. By monitoring drive-motor current, serious problems such as jams, belt slippage and breakage can be easily detected. This technique can also increase system reliability and lower maintenance cost, as current sensors can be mounted in any number of protected locations. All that is needed is access to conductors feeding power directly to the motor.
In a conveyor installation, for example, motor power requirements directly relate to the amount of work being done. The heavier the load, the more power the motor needs to deliver. This power is directly related to the current the motor is drawing. If the current draw increases dramatically, the system is probably jammed. A quick drop in current indicates a probable belt break or pin shear. By monitoring motor current, it is possible to detect these conditions quickly—typically much faster than via sensors that monitor product flow.
The easiest way to monitor motor power is to use a current transducer. A current transducer is simply a coil that is inductively coupled to the ac input of the motor, and provides a signal directly related to the current being used by the motor. This signal can be used to trip an alarm, or 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. The current transducer output can go to a PLC programmed with upper and lower current limits, along with appropriate motor control functions.
Carrousel Grains in Wisconsin is a typical example of the thousands of grain and storage companies around the nation. Most are older with basic (if any) systems for motion control. In the case of Carousel, the grain facility happens to be an older facility that has few sensors and little interlocking control. The company was continually battling problems with coordination between their feed and transfer conveyors. In addition, these conveyor systems are totally enclosed and in locations that are not easy to see, let alone work on, when a problem arises, thus making the more commonly used position specific sensors a real problem.
Adding current transducers on the conveyor drive motor power feed lines in the motor control panels made it possible to monitor loads and conditions at the conveyor lines while minimizing piping and wiring to locations that are not easy or safe to get to. The company selected the Hawkeye 921 current sensor from Veris Industries to monitor motor currents. The sensors are available in either go/no-go style, or analog styles that provide continuous monitoring and feedback signals directly proportional to the current the motor draws.
Analog output current sensors are preferred because their proportional outputs help in preventing, and/or pinpointing potential problems before they cause damage. In many cases it is also possible to slave the speed or flow rate of one conveyor to another to keep the conveying system coordinated. By monitoring motor currents, it is possible to not only determine if the conveyors are plugging or binding, but also a number of other fault conditions, such as slipping or broken mechanical components.
In the case of Carousel, the problems involved feed (pit auger) and transfer conveyors. The pit auger is simply a pit containing an auger. Grain coming into the facility drops into the pit, and the pit auger then lifts it to the transfer conveyors. These are vertical enclosed bucket conveyors that lift the grain up above the grain bin tops. Carousel's problems involved poor regulation of the rate at which grain moved to the transfer conveyors. This frequently resulted in overloading and even conveyor damage. Installing a current transducer one supply leg in the conveyor motor starter panel and wiring the output to a variable frequency drive (VFD) that controls pit auger speed. The VFD monitors current and and speeds the pit auger up or slows it down to maintain the preset current setpoint. Since having installed this VFD and sensor, the company has not had a plug-up or breakdown as a result of over-feeding or overworking the transfer. Downtime and maintenance costs have been substantially reduced.
For more information about motor and drive control, visit Control Engineering and type 'variable speed drive' into the search box on any page.
John Hoff, automation systems project engineer, Precision Drive and Control
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