Gear Ratios/Torque/Friction??

sme

Over the weekend my nephew was up and he saw my old meccano set and wanted to play with it. When he had something built he came to me complaining that the motor wasn't working.
I removed the motor from his build and it worked perfectly, so I told him to make a gear reducer to slow the motor down and that should work. It did but it got me thinking about the basics:

What is the relationship between speed and torque? Is there one? Can you you convert speed into power? Can a small motor turning very fast turn something heavy slowly? (the motor I have is a 6v, not sure what rpm it is)

He was building a wind mill, this obviously was being driven about its CoG and it moved easily when pushed with a finger. So why would it not turn when the motor was at a high speed and would when it is at a low speed?
Is it easier to start a body moving at low speed rather than at high speed? Or is it just a case of not having enough torque to overcome static friction at a high speed?

chris85

Well the higher speeds turning a load can lead to an increase in slip in the motor which is not good. There is a certain amount of slip in every motor depending on the speed its going with relation to its synchronous speed.

The speed of a motor can be related to the torque but the weight of the motor must also be known. The distance from the motor centre to the load is also needed to be known.

The power of a motor is related to the voltage of the motor as well as the current being drawn and the efficiency of the motor

sme

Why is the weight of the motor important?

The motor weighs 55g and the distance between the motor shaft and the axle it is turning is 150mm (there is a series of gears in between)

chris85

sme wrote: »

Why is the weight of the motor important?

Got to do with the moment of inertia. its more to do with the rotor of the motor. Especially important for eccentric masses

Michael Collins

sme wrote: »

What is the relationship between speed and torque? Is there one? Can you you convert speed into power? Can a small motor turning very fast turn something heavy slowly? (the motor I have is a 6v, not sure what rpm it is)

Yes, think of how a bicycle works. On low gears you can (just about!) make it up that big hill, by pedaling fast but getting there slowly, so this is low torque, high speed being converted into high torque, low speed. And vice versa then for higher gears.

chris85

Michael Collins wrote: »

Yes, think of how a bicycle works. On low gears you can (just about!) make it up that big hill, by pedaling fast but getting there slowly, so this is low torque, high speed being converted into high torque, low speed. And vice versa then for higher gears.

That explains it much better than mine

DublinDilbert

From memory:-
Power = Angular speed x Torque

If you assume no losses in the gearing, as the gear ratio reduces the angular speed the output torque is increasing, as the power in the system is a constant....

So for a given input power you can have a high speed & low torque, or a high torque & low speed, but not both...

sme

so basically if i want more power i need a bigger motor, but for speed and torque its just a case of changing gear ratios?

mawk

sme wrote: »

so basically if i want more power i need a bigger motor, but for speed and torque its just a case of changing gear ratios?

ignoring friction and inertia, yes