The notion of increasing efficiency in electric motors has been with us for some time. The US government has been pushing efficiency standards for quite some time. It’s a little like legislating gravity to stop operating. Politics and engineering don’t mix well. Engineering is all about what we can prove, politics is often about getting votes.
Our European brethren have gotten the efficiency bug too. The IEC has issued a new standard DIN 60034-30 scheduled to take effect January 1 2015. One of the interesting consequences of introducing a new standard is that a new testing protocol has to be created. After all, if you can’t measure it, does it really exist? So a whole new series of tests and equipment must be created in order for the new standard to be implemented.
This somewhat ignores the basic questions around what is possible based on the fundamental magnetics and properties of the materials themselves. In motors 100 horsepower and above, efficiencies tend to be above 90%. So the improvements that are sought are small increments above the currently available technology. To the extent that higher efficiency stator materials can be applied, tighter machining tolerances, copper rotor poles, the additional efficiency comes at a significant cost.
Is there a break-even here? Yes, but the burden of the expense is borne by the manufacturer. The regulators have no responsibility n the matter. For those engaged in engineering, and acknowledging that there is merit in being efficient, there is an interesting aspect to this question.
What is the load condition that the motor is required to move? This is not a small issue. Increasing efficiency is somewhat of an abstraction unless the load condition is considered. The typical approach is to test the motor efficiency at a constant full rated load which is normally done on a dynamometer.
In the real world full rated speed and load is not the typical load condition of an AC motor. Most real world loads are much lower RPM than the typical 1725 RPM nameplate speed of an AC motor. That’s why we have gear reducers, belt and pulley speed reducers, DC motors and speed controllers, and in today’s market, frequency drives that can adjust the speed of an AC motor.
Efficiency measurements need to be done in-situ, input to output measurements taken over at least a 4 hour period. This is the only way to get the real performance picture. Starting conditions, duty cycle variations, motor type, sizing compared to the load, there can be a variety of application specific conditions that will impact efficiency. High inertial loads can take over 1 minute to accelerate to speed. How does that impact efficiency, power factor, etc.
Forget efficiency until you have the right motor for the application.
Filed Under:Mechatronic Tips
