Technical Guide to Motor & Pump Monitoring
Motors and Pumps
Worldwide, approximately 60% of the electrical power used in factories is consumed by electric motors. All electric motors, regardless of application require electrical current to generate the magnetic fields necessary to create rotation. By measuring this current, useful information can be obtained to monitor the status of the motor and the equipment connected to it. Both AC and DC powered motors are used to drive pumps, fans, conveyors and a broad range of specialized equipment.
Techniques for Determining Motor/Pump conditions
Electric motors utilize current and voltage to operate. The speed of a motor is proportional to the applied voltage and/or frequency, and the torque generated by the motor proportional to the applied current. The combination of the current and voltage (torque and speed) creates the power required to operate. Thus current and voltage measurements may be used to monitor the motor operating conditions and determine motor status. Current and power transducers, as well as current operated switches, provide sensing technology to monitor current and voltage. This aides in determining the condition of an electric motor and provides a signal to direct a specific action. These sensor devices can be used for jam protection, verification of operation, load monitoring, closed loop control, and status alarms. Read more
The auxiliary contacts in a motor starter are commonly used to indicate when the motor is running. However, auxiliary contacts only indicate the position of the contactor, not the actual load status. If a downstream disconnect is open (for example, for maintenance) or the contact fails (either sticks or dirt build-up) there can be serious consequences.
At a large fish farm, failed aerator pumps resulted in massive stock losses. Because the auxiliary contacts remained closed when the pump failed, the alarm was never activated, the back-up pumps were not switched on and the fish suffocated due to lack of oxygen. Now NK Technologies’ Current Operated Switches have been installed to provide an alarm signal and a signal to automatically switch over to the back-up aerator pumps.
Closed Loop Control
Normal linear AC induction motors have a current characteristic that increases with load. Thus a measurement of current is proportional to the motor load. NK Technologies’ True RMS Current Transducers are designed to work in conjunction with variable frequency drives (VFD) and measure the current in the leg of the motor.
At an engine block manufacturing facility NK Technologies’ Current Transducers provide the input to a PLC used to position the rotating wire brushes in order to clean away burrs and debris after machining. As the current, and thus the load, changes so the brushes are positioned for optimal clean-up. Similarly Power Transducers are used to control the position of wheel wash brushes in a carwash. Power measurement provides a more linear relationship for motors and other loads with non-linear current characteristics. Current Operated Switches are used to co-ordinate operations between conveyor, washing and drying systems.
DC Motor Installations
Many applications requiring high torque at low speeds, like bridge cranes and log carriages, use DC shunt wound motors. Since it is relatively easy to control the speed of a DC motor by adjusting the voltage to the field windings, precise regulation is afforded to applications like printing press paper feeds or web control of multiple motors in the paper making process. Using an NK Technologies’ DC Current Transducer will provide an analog signal that can be used to keep the motor from over-current situations. Using an NK Technologies’ DC Current Operated Switch will insure that if the motor field supply is lost, the motor will not over-speed to the point of self-destruction.
Saw Load Monitoring
Often a band saw is used to cut logs into rough planks, before the planks are processed into finished lumber. Due to inconsistencies in the log (knots, burls and checks), the speed that the log is pulled through the saw must be varied. The operator will often reduce the speed of the cut when smoking occurs, or the blade breaks. By installing an NK Technologies’ Current Transducer the operator’s job is made easier. By watching the log and a display representing the current draw of the band saw, the operator can achieve a higher rate of output with less down-time.
A similar arrangement can be used with circular saws. The blades become brittle after excessive heating if they are worked too hard. Many automatic systems are in use whereby the current draw is used to reduce the motor load typically by reducing the feed-rate.
Crusher/Grinder/Shredder Motor Interlocks
The performance of size reduction equipment like crushers or grinders can be optimized by controlling the in-feed in order to:
- Help prevent jamming
- Improve the uniformity of the resultant product
- Enhance overall production efficiency
By installing an NK Technologies’ Current Transducer or Power Transducer to monitor the in-feed to a crushing process, the speed of the in feed drive can be slowed when the crushing motor is working too hard, or the speed can be increased when the crusher is under worked. The analog 4-20mA proportional signal is fed directly into a VFD, and the drive does the rest of the control.
Additionally, NK Technologies’ Current Operated Switches installed on other conveying motors can be used to signal malfunction (for example over-load or motor failure) to help prevent costly equipment damage.
NK technologies current sensors come in a variety of configurations for quick and simple installation either for the OEM or End-User.
Conveyor Jam Protection
When conveyors jam and the drive motor continues to run, expensive mechanical damage to the drive components often occurs. By installing an NK Technologies’ Current Operated Switch an overload condition, detected by higher than normal running amps, can be used to switch off power to the motor before mechanical damage results. The switch setting can be adjusted so it is not affected by normal changes in conveyor load. Current operated switches are the modern day equivalent of less reliable electromechanical jam relays that have been in use for many decades.
In situations where there may be large variations in load an NK Technologies’ Current Transducer may be used to feed motor amps to the conveyor control system, commonly a PLC. The PLC may be programmed to accommodate different setpoints for different mechanical loads so that jam protection is provided from light to heavy loads. In situations where the conveyor is not able to accept analog signals NK Technologies has developed a patented transducer with a frequency output suitable for the Digital Inputs of most PLCs.
Fan Status: Independent Verification of Operation
Good engineering practice suggests that every control output should have a corresponding status input for verification. On fans and pumps a current operated switch can be snapped over one motor leg and the output fed to the automation system. NK Technologies’ self-powered Current Operated Switches will close when the motor turns on and open when the motor is off.
By adjusting the setpoint pump coupling or fan belt failure can be detected. This reliable solid-state solution eliminates the need for unreliable pressure current switches and their costly installation.