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I have a 12vdc motor, a big thing connected to a hydraulic pump. I suspect its broken. It has a 40amp fuse so its designed to draw under 40 amp on average, and up to 50 amp peak. When i ohm it its 0,2 Ohm, I think that is very low. I cant find data on it, so I don't know what the ohm is supposed to be. Is it possible to calculate expected ohm based on these numbers? In advance, thank you.


The motor is connected to a gearbox, located between the handlebars and the steering arms on a three wheel motorcycle. Its purpose is to provide steering assist. There was no hydraulic pump, only gears, and the unit is not working, there is no assist. There is a tachometer under it, to know the angle of the wheels. And it also reads road speed and there is some kind of magnetic torque sensor built into it, used to determine how man amps/assist to provide. I was able to pri it loose and open it. The main board in it had a big black spot. They did not sell parts for it, so I will have to buy the whole thing with the motor new. So, if the motor is good or not does not matter much now. 1200,-Usd,, glad I have some months to save up before the winter is over:-) Thank you for your effort and reply`s.

best regards Nils

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    \$\begingroup\$ Can you expand on what makes you suspect it's broken? \$\endgroup\$ – ThreePhaseEel Dec 2 '17 at 17:19
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    \$\begingroup\$ Why do you think the motor is designed to draw only 50 amps peak? What is the fuse type? I would expect a 40 A DC motor to draw a lot more than that on startup. \$\endgroup\$ – Charles Cowie Dec 2 '17 at 17:22
  • \$\begingroup\$ "It has a 40amp fuse so its designed to draw under 40 amp on average, and up to 50 amp peak. When i ohm it its 0,2 Ohm, I think that is very low." You are measuring the motor, right? What happens when you ohm the fuse? Is it OK? \$\endgroup\$ – John D Dec 2 '17 at 17:26
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    \$\begingroup\$ If 0.2 Ohms is correct, the motor would draw 60 amps on startup. I would not be surprised if the startup current is more than that. \$\endgroup\$ – Charles Cowie Dec 2 '17 at 17:26
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    \$\begingroup\$ @CharlesCowie Not sure he's making a 4-wire measurement so the 0.2 ohms probably includes the test leads and contact resistance. The motor itself is likely less, assuming the meter is even accurate for resistances that low. \$\endgroup\$ – John D Dec 2 '17 at 17:27
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According to Ohm's law V = I * R, and if we consider that the current WILL NOT be more than 40 ampere then the minimum resistance will be: R = 12v / 40A = 0.3 ohm So I think the motor coil is shorted somewhere and there is a problem in it.

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    \$\begingroup\$ DC motors are dynamic loads. Ohm's law doesn't apply in the usually. \$\endgroup\$ – Charles Cowie Dec 2 '17 at 17:23
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    \$\begingroup\$ Except that DC motors will draw a lot more current on startup, and as they increase in RPM the back EMF will reduce the current. It's not as simple as applying Ohm's law. \$\endgroup\$ – John D Dec 2 '17 at 17:24
  • \$\begingroup\$ Sorry for my mistake, you are totally right. \$\endgroup\$ – Moutyam Dec 2 '17 at 17:25
  • \$\begingroup\$ I typed too fast. Ohm's law doesn't apply in the usual way. \$\endgroup\$ – Charles Cowie Dec 2 '17 at 17:28
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    \$\begingroup\$ @CharlesCowie it would but it would be V = I * Z and Z is also dependent on the Back EMF \$\endgroup\$ – laptop2d Dec 2 '17 at 20:43

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