Energy efficiency is not only important to profitability, but also to the sustainability of natural resources. Measuring motor load and efficiency enables cost effective analysis of potential energy savings opportunities. Electric industrial motors power mixers, cranes and machine tools in a wide range of applications. The efficiency of these motors drops significantly at lower loads.
In a motor system, shaft power is mechanical work the motor does (measured in units of energy). Input power is electrical energy the motor draws from the power source, which costs money. Energy losses are the difference between input and output power. Current measurements provide a linear indication of load down to around 50% of rated load, after which the amperage curve becomes non-linear due to reactive magnetizing current requirements. This is where it’s important to use accurate field measurements such as those provided by the eddy current tester.
Overloaded motors are costly and can also damage the motor. Many motors have service factors that allow occasional overloading, but the best option is to avoid running any motor at overload for extended periods. If your operation often oversizes its motors, consider using a VFD, two-speed motor or other load management strategies to maintain loads within the acceptable range. Alternatively, replace oversized standard-efficiency motors with more efficient, properly sized energy-efficient models at the first opportunity.
It is usually very difficult to locate efficiency information on a motor that has been in service for a long time or that has been rewound. In some cases the nameplate is lost or painted over. In those instances, using the “direct-read” power measurements from hand-held instruments is an option. The root mean square (RMS) current values measured at the motor terminals must be corrected for the supply voltage to determine actual load. This is accomplished with the use of a tachometer or stroboscopic meter. The AC voltage causes current to flow in a sine wave replicating the voltage wave. Inductance in the motor windings slightly delays current flow resulting in a phase shift that transmits less net power than perfectly time-matched voltage and current. This is referred to as power factor and can be corrected by the use of capacitors.
A motor’s output power varies inversely with load and is equal to the product of torque (force applied about an axis of rotation) times the rotor speed in radians per minute (o). The resulting work done is mechanical energy divided by input power, or P i = I x V. When no nameplate is present, it’s best to use direct-read power measurements from hand-held instruments to estimate the motor’s part-load efficiency and load. See the ANSI/EASA AR100 Good Practice Guide to Maintain Motor Efficiency for details on these methods.
Regardless of the method used, it’s important to remember that no electric motor is 100% efficient, meaning that some electrical energy is dissipated as heat or friction in its internal components. This is why it’s so important to choose a high-efficiency model like maxon’s ironless core and copper losses models, which eliminate the largest concentration of stray losses found in typical motors. This allows them to achieve efficiencies up to 90 percent. Get the latest scoop on used electric motors prices and promotions – click here or explore our official site.
