I am calculating various values for a range of motors. However, I need to find the no load current of a motor or io to allow me to calculate other values, based on the information I have below for a sample motor can this be done?

  • Kv (RPM/v): 2100 kv
  • Power (Watts): 2100 W
  • Max Current (Amps): 140 A
  • Max Voltage (Volts): 15 V
  • Resistance (Ohms): 0.007 Ω
  • Poles: 4
  • Weight (Grams): 329 g
  • \$\begingroup\$ YOu would need no-load speed at 15V too \$\endgroup\$ – Trevor_G Dec 13 '17 at 19:31
  • \$\begingroup\$ okay if we take 2100x15 we get a no load speed of 31,500 RPM please can you tell me how i can work this out. \$\endgroup\$ – Chris James Dec 13 '17 at 19:36
  • \$\begingroup\$ YOu would only get that if there were no friction and windage in the motor. it will settle at some lower speed with no added load. Of course, in reality whatever load you are driving with this will likely be a lot more than that. \$\endgroup\$ – Trevor_G Dec 13 '17 at 19:38
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    \$\begingroup\$ Your back emf should be close to 15V.... \$\endgroup\$ – Trevor_G Dec 13 '17 at 20:01
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    \$\begingroup\$ YOu are confusing yourself... Again, if you want to know the no load current you need to know either the no-load speed or the friction and windage torque at that speed. For an ideal motor the no-load current = zero. \$\endgroup\$ – Trevor_G Dec 13 '17 at 20:19

No load current is zero amperes.

KV 2100 = 2100 RPM/V = 35 R/Vs = 220 rad/Vs

So, Ke,Kt = 1/220 = 0.00455 [Vs/rad] = 0.00455 [Nm/A]

$$U=K_e\cdot\omega + R\cdot I $$


\$T=K_e\cdot I \$ or \$I=\dfrac{T}{K_e}\$ so if T=0 then current is zero. Else you would have to know the no load torque, which could be the ball bearing friction and air drag at known speed.

The derived Ke from KV that was actually measured in laboratory consists of a test setup. This is a second motor coupled to the shaft and spin as much as the current is zero - no load current. In such situation the applied voltage equals the back emf voltage \$ U=K_e\cdot\omega\$, knowing \$U\$ and \$\omega\$ is it possible to estimate \$K_e\$ or KV in your case. So the motor is spinning at 35,500 RPM at 15V, but not by himself, rather by the help of the coupled motor from the setup.


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