Re: What determines max rpm and amperage a motor can handle?
OK good discussion. When dealing with RPM let’s just for a moment separate the Voltage issue from Mechanical restraints issue. First we all know that RPM is directly affected by the amount of voltage supplied to it. General rule, if you double the voltage for a given torque or amperage, if it’s not stalled, you double the RPM, this is general rule that we all can safely follow. Now given that, there is a Max Voltage that a DC motor can take and it has to do with a number of factors, commutator design, like the number of bars, Turns in the field poles or stator, How well the motor is insulated, although I can't see any motor being any less the class H insulation. Motor size can dispel a lot about the voltage rang of a motor generally the larger diameter is going to mean a armature with more slots and big diameter commutator thus higher voltage capable. These are all the major factors that play maximum voltage.
Maximum RPM is really restrained by the Mechanical properties of the armature. If you have notice some of these electric Hot Rod shops will trick out an Armature by winding bands at the commutator connection and at the turn end and even have a groove in the middle of the armature with bands as well. If an Armature spins too fast it will mechanically burst or fly apart and ruin the whole motor typically speaking. So these bands are preventive ways to make the Armature able to with stand higher RPM. Another issue to High RPM is brush and commutator ware. The higher the RPM the more friction is generated and shorter the life of you brushes and commutator. This is another reason why AC motors have an advantage among other things as well.
Torque is direct function of how much current is forced through the motor, more current makes more torque. The amount of torque per amp is depended on how the motor is designed in general the bigger the motor the bigger the torque per amp again "in general" but there are other factors as well, I can give you one example of where one motor was smaller than another but made bigger torque per amp. The limiting factor to the amount of current you can for through these things is strictly depending on two things alone wire cross sectional size, and Brush Commutator size, and those two ALONE.
hope this helps