How do we find the speed of BLDC motor at which it has minimum electrical losses so that we design a gearing system accordingly. And can we change the most efficient speed of a BLDC motor to suit our requirements.?


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  • \$\begingroup\$ You don't want minimum electrical losses. You want maximum efficiency, or maybe maximum power. Normally the motor should have some kind of datasheet that provides this information. \$\endgroup\$ – mkeith Dec 27 '18 at 4:25
  • \$\begingroup\$ Typically, the motor is going to spin a bit slower than Kv * VSUPPLY. This just gets you to the right ballpark. You might want to do some further testing with a dynamometer or something. \$\endgroup\$ – mkeith Dec 27 '18 at 4:27
  • \$\begingroup\$ Thanks mkeith, commonly bldc hub motors available on ecom sites come without data sheet or Kv. All I know is 250 w, 36v bldc hub motor with magnets on the rotor outside and winding on stator on the axle. \$\endgroup\$ – Rakesh Kasba Dec 27 '18 at 11:40
  • \$\begingroup\$ Still I want to know how can I can estimate/ calculate the most efficient speed, rpm at highest torque and can I change most efficient rpm / speed by changes in bldc motor controller. \$\endgroup\$ – Rakesh Kasba Dec 27 '18 at 11:49
  • \$\begingroup\$ Basic rule of efficient electric motor operation is to use it near its highest speed, with as little torque load as you can. If you don't know its Kv, measure it. Most simply, measure its unloaded speed and operate it above 80% (preferably 90%) of that. \$\endgroup\$ – Brian Drummond Dec 27 '18 at 15:02

The maximum efficiency of all electromagnetic motors occurs at or near the maximum design speed. The efficiency variation with speed is lowest at high speeds as the speed approaches zero the efficiency declines more rapidly until it reaches zero at zero speed.

If a machine must operate at a speed below any available motor speed, it is generally best to select a motor with the lowest available maximum design speed. Not much can be done with the motor or controller design to make the system more efficient at reduced speeds.

Electrostatic motors are most efficient at low speeds. The efficiency is still zero at zero speed, but they can produce high stall torque using very little power. However, there are virtually no such motors on the market today.


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