Output shaft on DC motor difficult to turn by hand vs free spinning

[FeAl2613]

Hi all

I have limited experience with DC motors, but from what little experience I have some have been nearly impossible to turn the output shaft by hand, while some turned no problem just like an AC motor. Which DC motors have this resistance and which ones don't?

 

[BrianPetersen]

Permanent-magnet motors will generally have a "cogging" effect when turned by hand. Wound-rotor (essentially the rotor is an electromagnet) motors will not.

 

[FeAl2613]

@Brian

Thats kinda what I thought, I'm actually designing something where I want to take advantage of that resistance, so I got a cheap 90vdc brushed perm mag motor to play with and it spun freely. However if I tied the leads together it cogged as you describe. As could be expected the cogging was less pronounced if I tried to turn the shaft very slow and more so if I tried to turn it faster....
I used to assemble 24vdc hydraulic power units and right out of the box, without shorting them or hooking them up to anything, these motors were impossible to turn by hand. You could do it with a pair of vise grips but no way with your fingers (no they weren't gear motors or brake motors). Unfortunately there is no way for me to know brand or configuration or anything about them now. I was wondering if anyone knew what would produce that kind of effect so I would know what to order.

 

[edison123]

Brushed DC motors, especially large ones, are not so easy to turn by hand due to brush resistance. Small DC motors can be turned by hand but not as freely as an induction motor. Other reasons for an uncoupled motor not to turn freely hand are bad motor assembly (end brackets skew), bad or rusted bearings, bent or broken shaft etc.

Muthu

http://www.edison.co.in

 

[Compositepro]

Small DC motors should not be that difficult to turn.

 

[panter]

dc motor with short rotor winding coils is difficult to rotate .
Good luck