One topic that has been argued to no end is whether it is better to have a "High Speed" motor or a "High Torque" motor. For Brushless
DC Motors, here are the key terms we need to understand:
- Kv means "Motor Velocity Constant" or RPM per volt; it is also called "back-EMF".
- Kτ means "Motor Torque Constant" or Torque per Amp.
These two constants – from a pedestrian perspective – are considered inversely proportional to each other. It is the
"Motor Constant" Km that binds the two opposing constants together. Simplistically, the formulas are as follows:
- Kv = RPM/V = revolutions/minute/volt
- Kτ = τ/I (Torque/Phase Current as Amps), or as 60/(2π * Kv) = ω/Kv = 1 radians/sec/Kv
Kv and Kτ vary with respect to N (the number of Turns in a Winding) and I (the measure of Phase Current as Amps). For a
given motor and stator, Kv scales linearly with N. Therefore is falls upon the Winding Wire Gauge, the number of strands in the wire, and the
effective Stator Slot-Fill that affects R, Resistance. HEAT is waste-energy produced by Resistance, therefore we want motors that
produce the lowest amount of heat (resistance) and yet provide the best power conversion (albeit Kv speed, or Kτ torque).
The "motor constant", Km thus becomes the dominate factor in determining the motor's overall efficiency:
Generally, when we increase the Kv or speed of a motor, there is a proportional decrease the amount of torque per amp that
it can produce. This formula holds true for ALL ELECTRIC MOTORS REGARDLESS OF SIZE!
From an engineering perspective, it is better to have higher voltage and less current because it produces less heat. However,
the switching electronics of motor controllers work better at lower voltages, but then this means we need more current to create
the same amount of power, and in turn creates more heat. There is no panacea, no free lunch; we have to decide on which is more
useful: Speed or Power, or some compromise in-between.