Motor driven machines that continue to operate during abnormal conditions caused by a jam or blockage will result in irreparable damage, not only to the motor continuing to drive the system but also to the machine itself. In the past the extent of damage to the machine has been understated compared to the damage to the motor itself regardless of whether a variable speed drive is being used or not.
Motor management and control devices, or overloads can protect motors, but the question here lays with what is protecting your machine? In pump installations for example when there’s a jam or suction loss long before the thermal overloads trip, serious damage can be incurred by the machine as well as the motor. Reality here is any abnormal operating condition can have a detrimental effect on the life of the system. Dry pump, dead head, jammed impellers, and even premature bearing wear can lead to motor and pump failure. To save upfront costs many motor driven systems are installed without any means to continually monitor operations. As a result, frequent inspections of the motor and the operation of the machine becomes a necessity for maintenance and repair.
If failure becomes apparent without warning it can mean irreparable damage to the machine and possibly the motor. These unforeseen issues can be eliminated with the use of simple but effective monitoring and control techniques to protect the motor driven systems. Adding a small cost to the installation will pay for itself a few times over the first time a fault is detected, and the machine or motor is saved.
In an abnormal running condition, the Load Monitor measures the load drawn by the motor and compares it to what it was when normal.
The most common use would be pump dry run detection. Many pumps cannot be allowed to dry run, particularly in Petrochemical, Agrochemical and Pharmaceutical industries where hazardous chemicals are being pumped. In the water industries dry running and dead heading due to impeller being jammed will also show an abnormal motor load.
Unipower HPL Digital Load Monitors have been used in industrial motor driven applications for over 20 years. It is not possible to use motor current for this purpose to measure instantaneous increases or decreases in load and damage will occur before they can react.
The motor is acting as its sensor to detect the state of the equipment the motor is driving. UNIPOWER Load Monitors calculate Watts/Kilo Watts (kW) which is the only electrical measurement of work that is linear with and proportional to load, using the formula: P=√3×I×V×COSø
Many products are claimed to achieve the same by measuring the current. This is particularly a problem when the motor is over sized which does happen often. In such cases the motor will never reach its rated load as the motor is too large for the job and hence will operate where current is a flat line, so you cannot measure current for variations in work being done by the motor.
Phase angle detection was another way round this however, with the advancement of technology and the increased efficiency of newer motors, phase angle detection is not a reliable alternative for measuring work done by the motor.
Supervision and control are uniquely offered by the UNIPOWER family of Motor Load Monitors, replacing friction clutches, ball detent clutches, shear pins, tacho controllers etc. Typical applications include belt and screw conveyors, elevators, fans, pumps, filtration screens, macerators, etc.
UNIPOWER product designs recognise that to use power consumption as an indirect torque measurement the power must be measured by the use of formula mentioned. Measurement must be fast and accurate, reaction time must be short, measurements must be valid for none sine-shaped currents i.e. frequency inverters generating very high and short current peaks (crest factor up to 10) and include support functions such as start timer, reaction delay timer, peak detectors for max/min load, and shock Load monitoring dP/dt with voltage spike compensation dU/dt.
Available exclusively from UNIPOWER UK Ltd T/A Charter Controls.
By Amir Sami (Business Development Manager at Charter Controls)