Reduce size or current
Current is not lost in a motor. It is spent in four places, and only one of them turns anything.
Knowing the split is what tells you whether the answer is a smaller motor, a different technology, a different ratio, or a lighter load.
The current split
Every ampere out of the supply lands in one of these. Measure the first one and the rest can be inferred.
- No-load current
- What the motor draws spinning free: brush friction, bearing drag, and, in an iron-core motor, hysteresis and eddy current loss in the laminations. It is the floor. It never does any work, and at light loads it can be most of what the battery is delivering.
- Copper loss
- The I²R heating in the winding. It rises with the square of current, which means it rises with the square of torque. Doubling the torque quadruples this term, which is why an overloaded small motor gets hot so quickly.
- Iron loss
- Hysteresis and eddy currents in the rotor laminations, rising with speed and flux. A coreless rotor has no iron in it by construction, so this term is essentially absent, which is a large part of why coreless motors are efficient at small sizes.
- Mechanical output
- Torque times angular velocity. This is the only term that reaches the mechanism, and the gear train then takes its own share of it before the load sees anything.
I = I₀ + T / kT
- I
- supply current, A
- I₀
- no-load current, A
- T
- motor shaft torque, N·m
- kT
- torque constant, N·m per A
Efficiency against envelope
Small motors are not just weaker. They are less efficient, and the reason is geometric.
Torque comes from the product of airgap flux and current in the winding, integrated over the rotor. Shrink the diameter and the available torque falls roughly with the square to the cube of the diameter, depending on how the length scales with it. At the same time the surface area available to move heat out falls with the square of diameter while the loss you need to remove has not fallen nearly as fast, so the temperature rise per watt of loss goes up. Continuous torque, which is set by temperature rather than by magnetics, therefore falls faster than the envelope does.
The practical consequence is that halving a motor's diameter does not halve what it can do continuously; it typically leaves a small fraction of it. The published Micro Motion motor platforms span 8 to 37 mm across 18 platforms, and the difference between the small and the large end is far larger than the diameter ratio suggests.
| Stages | Typical efficiency, published range % | What it means for the motor |
|---|---|---|
| 1 | 81 to 90 | A single reduction costs about a tenth of the power. Rarely the problem. |
| 2 | 66 to 81 | Still modest. Most small gearmotor applications live here or at three stages. |
| 3 | 31 to 73 | The spread is wide. At the low end the gearhead takes most of the power, so the motor must be sized for several times the load. |
| 4 | 43 to 66 | Adding stages for ratio buys torque with power. Check whether a slower motor behind fewer stages does the same job. |
| 5 | 35 to 59 | High ratios are for low speed, not for free torque, and the supply current reflects it. |
| 6 | 53 | A single published platform at this stage count. |
| 7 | 48 | A single published platform at this stage count. |
T_motor = T_out / (R × η)
- T_motor
- torque the motor must produce, N·m
- T_out
- torque required at the output, N·m
- R
- gear ratio
- η
- gearhead efficiency at that operating point, as a fraction
Coreless, iron-core, brushless
Three constructions with genuinely different tradeoffs. The right one depends on which of size, current, life and driver cost is actually binding.
| Property | Coreless brushed | Iron-core brushed | Brushless |
|---|---|---|---|
| Rotor | Self-supporting winding, no iron. No cogging, very low inertia. | Wound laminations. Cogging present, higher inertia. | Magnet rotor, wound stator. No brushes. |
| Efficiency at small size | High. No iron loss, and low friction. | Lower, and it falls further as the motor gets smaller. | Highest, at the cost of driver losses elsewhere. |
| Continuous current limit | Set by the winding's ability to get heat out through the airgap. Modest, and easy to exceed. | Better. The iron is a heat path and a thermal mass. | Best. The winding is on the stator, against the housing. |
| Overload tolerance | Poor. A stall can damage the rotor quickly. | Good. The thermal mass buys time. | Good, and a driver can limit current directly. |
| What ends its life | Brush wear, usually. | Brush wear, usually. | Bearings and grease, so life is typically much longer. |
| System size | Motor only. Two wires. | Motor only. Two wires. | Motor plus a driver and commutation. Often larger overall than the brushed motor it replaced. |
| Published Micro Motion platforms | 16 platforms | Not published | 2 platforms |
Send the load, the envelope and the supply
A preliminary review returns a configuration with the operating point placed, the margin stated, and the assumptions written down where you can argue with them.