Urs Kafader of maxon motor presents guidelines for maximising the efficiency of different elements in a drive system.
There is often the question about which motor shows the highest efficiency. In particular when talking about applications with limited power supply, such as a battery-driven tools or a solar panel powered satellite. I believe it is worth expanding the question to the full drive system consisting of controller, motor, gearhead and other mechanics. The table below gives rough estimates for typical efficiencies of different energy converting devices.
|Transformation type||Example||Typical efficiency|
|electrical to electrical||Transformer||95% and higher|
|PWM power stage of controller||95% and higher|
|mechanical to mechanical||DC motor||max 90%, depending strongly on operating conditions|
|mechanical to mechanical||Planetary or spur gearhead||approx 90% per stage|
|Ball screw||90% and higher|
|Acme lead screw||<50%|
We see that a lot depends on the mechanics. High friction in the mechanical system costs a lot of the energy. Hence optimising the efficiency of a drive train should always start with the mechanical system.
Note that DC drive systems are very often powered by a fixed voltage. Hence, it is not really the overall efficiency that must be maximised. Efficiency is a relative parameter giving the losses per input power. However, at fixed voltage it is rather the current consumption (as an absolute parameter) that needs to be minimised. Making good use of the full supply voltage reduces the current consumption – and lengthens battery life.
What follows are some rules for high efficiency and low current consumption.
Rules for mechanical transformation
- Use low-friction mechanics or avoid it all together.
- Use rolling friction (coefficient of friction <0.01) instead of gliding friction (typical friction coefficients 0.1-0.4) eg a ball screw system has a considerably lower friction than a lead screw. However, a ball screw solution is usually not self-locking; it needs to be powered to remain at a stop position. Considering the full working cycle, there are situations where an acme screw might consume less energy in total.
Rules for planetary and spur gearheads
- Use a low number of stages.
- Use small gearheads with torque ratings only slightly higher than what you need. Do not oversize the gearhead. Efficiency at low gearhead load is very bad (see graph below).
- In terms of efficiency, a larger direct drive motor without gearhead is often better than a motor-gearhead combination. However, the direct drive might be larger, heavier and more expensive.
Rules for motors
- Run the motors at speeds of several thousand rpm.
- Do not select too large a motor. Do not run it close to no-load.
- Take care when selecting the winding. Use one that makes the best use of the voltage to reach the speed under load; that is the one that needs the lowest current.
Rules for controllers
Here the rule is similar to gearheads. Load them well, do not run them at too low a current. Do not select controllers with too high a current rating. Match the size of the power stage with the required motor current.
Make good use of the full supply voltage. This reduces the current consumption and you can save money; you will need a smaller power supply, a smaller controller, less cable shielding and so on. Or as a colleague of mine put it: Voltage is for free, current costs.
Contact maxon motor for more information about efficient drive systems.