Electrical Motor Efficiency - Engineering ToolBox

I think it may be true that a given propeller will be more efficient (that is, turning shaft power into thrust) at lower RPM's, with allowance for a drop off in efficiency at very low speeds. Unfortunately I think that means that propeller is more efficient when the motor is less efficient. So there will be a sweet spot in the middle somewhere.

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Improving efficiency in electric motors - ScienceDirect

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Yes, the goal of motor design is different between vehicles and industrial applications. In industry, efficiency and longevity are key design parameters. That's why a 150hp AC motor weights several thousand pounds. In a vehicle, the motor weight plays a big role in the overall system efficiency. It's not just the energy efficiency of the motor that matters, but you also have to consider the weight of the motor that is being lugged around. Thus, they design for weight and power density. A 150hp motor for an electric car only weighs a few hundred pounds.

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Something that I should have emphasized is that motor designers can't seem to design for peak efficiency. They are always designing for peak power or peak torque. Often, this is known as power density (most power in the smallest volume). When you are choosing a motor for peak efficiency, you will often have a larger motor than you might normally expect for a given load.

This arrangement can give an induction motor capable of high efficiency, high power and high torques over a range of rotation rates.

Motor Energy Efficiency - Drive Technologies - Siemens

The second way to choose a motor, which applies more to drones, is to look at the efficiency curve and to select a motor/prop combination that puts you just to the right of the peak efficiency point. In this case, a load of about 0.1 oz-in would be ideal. If you select a point to the left of the peak, even light variations in load will cause vast efficiency swings (notice the steep slope) and if you select a point too far to the right, then you will be losing power continually with little recourse.

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Minimum efficiency standards for electric motors will …

Core loss is the major cause of wasted electrical energy which can result from overheating during operation, motor burnout, and physical damage such as rotor drag. Without core loss testing, it is impossible to determine that a motor is capable of operating at rated efficiency. The tester indicates the condition of core steel by measuring the watts of energy lost per pound when the core is excited to operating conditions. Also, the microprocessor works in conjunction with infrared heat scans to record “hot spot” conditions in the stator. These often occur where there has been physical contact between rotating parts and the core as well as “grounds” where the windings have arced and blown holes in the core. The computer indicates a maximum and minimum value in terms of flux, watts, and amps. If the core does not meet a minimum value, the steel laminations making up the core must be repaired, restacked, or replaced.

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One more thing. If the voltage (PWM duty cycle) is reduced, these curves also scale to the left. The scaling isn't perfectly linear, but it is close enough for estimation purposes. Also, your propeller draws torque in a very non-linear fashion (vs. speed). It is *very* likely that your max efficiency point with a particular motor/prop combination is not at maximum throttle, but at some other point. So you might need to do some testing on your current setup to find out where you are and go from there.

Everyman's guide to motor efficiency: Simplified …

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Some industrial motors have a service factor of +25% and some pumps depending on slippage and flow resistance try to grab that + 25%. In certian applications placing a resistance to flow lowers the amperage to where the motor runs efficient.