Has anyone thought of running their 12v windscreen wiper motor at 24v or more? I am going to give it a go this weekend and see what sort current it pulls and how much heat is produced. I plan to run the motor with just the mill (no grain) until I get 200rpm and then load it up with grain and see what the current draw is.
Below is a good explanation of how a 12v motor will react to 24v and how to monitor it for signs for pending failure. Not my words and if you are not able to monitor current draw then I wouldn't do it. a bit of stuff on motor controllers that isn't really applicable, but I left it in anyway.
K
12v Motors on 24v
Motors are specified to run at a stated rpm at a particular applied voltage with a specified loading. The specified loading is usually that at which the motor takes its maximum continuous current. If you run the motor under a lighter load than this 'name plate rating' its current consumption will reduce and its speed will increase slightly. If you increase the load, then the motor's current consumption will increase and its speed will reduce. Obviously you are now exceeding the motor's continuous rating so it will start to get hotter than it should. The greater the overload, the quicker the motor will heat so there is a time limit on such an overload. However it is usually safe to run a motor at a 300%-400% over current for, perhaps, a minute - although this will vary from motor to motor.
If you run a 12v motor from 24v its current drain and speed will still depend on the mechanical loading. However under no load it will now run at twice the speed at which it ran with 12v. Heating in the motor is still related to the current so you can still run it at its full rated mechanical load/current. However if the motor is badly balanced you may expect noise and vibration as the general construction may be inadequate for the faster speed. There may also be a problem with brush wear since the brushes are being asked to switch the current twice as fast. These effects are, however not very likely and usually the speed increase is quite OK.
There is one caveat on this. The motor is an inductive device and the commutator and brushes are a mechanical, switch. Such a mechanical switching system will have a limit on the maximum rate at which is can work and if this is approached, the commutation breaks down. Exactly what the limits are, I would not like to say but one effect o(noise) - and extreme noise can, on occasion, cause a controller to fail. The effect is quite rare - but beware of excessive over-revving.
However, limits on motor speed are not simply bearing quality. If you rev a motor hard enough - centrifugal force will take over and the rotor will fly to pieces. Also brush and commutator design is important. Depending on the design these will have a maximum switching rate and operating above this speed will cause tremendous brush arcing. In extreme circumstances this will generate severe noise transients which can destroy the controller. This is unlikely: we have only ever seen one customer do this: he was running 12v motors on 36v and blew two controllers! These motor limits are not things a controller manufacturer can really comment on: you need to consult the motor manufacturer.
If you overload the motor, its current rises in the same way whether the motor is running from 12v or 24v. However on stall the current from 24v could be twice that from 12v, so the motor could get four times as hot (heating is proportional to the square of the current). This however won't happen when you are using a good controller as the controller will limit the current to its designed value. Also the controller varies the voltage on the motor so you are probably not going to use the motor at full voltage in any case.
Another consideration is that, if you put too much current through a permanent magnet motor, it is possible to slightly demagnetise the magnets. This is cumulative: the motor's performance will drop slightly each time you do it. However, for battery motors, is is probably fairly safe to assume that, at the rated voltage, the current drawn when the motor is stalled will not reach this demagnetisation level. If you were to run a 12v motor off a 24v battery the stall current could then be excessive if it weren't limited by the controller.
Therefore, provided you chose a controller suitable for the motor you use, you can usually run a motor 12v motor from a 24v battery with no effect except that full speed is doubled.
h34r:
