With planetary gearmotors, gear ratios can range from 3:1 (in a single stage) to 2,563:1 in a four-stage planetary. So it's always easy to add to the gear ratios to decrease the final output speed, but that isn't always the best answer.
The other key consideration is torque. If a relatively large torque value is required, in addition to very low-speed sets, a four-stage planetary makes sense. That's primarily because the addition of a fourth stage not only increases the total ratio, but it increases the torque capacity accordingly — less the efficiency loss of that extra stage. However, if the lowest possible speed is desired and the torque requirement (derived from actual measured load and relative duty) is such that a three-stage planetary will suffice, then on some customized designs, it makes more sense to manipulate the motor windings to slow down the effective output speed. By increasing the back-emf constant (Ke) of the motor, and driving it at the same prescribed voltage, the motor's armature speed can be slowed, thus allowing for a decreased output speed.
Many slow-moving applications require less than 30 rpm of output speed. In fact, some are in the single digits. So, let's say we're designing a gearmotor using a three-stage planetary gearhead with maximum ratio of 308:1. Speeds as low as 1 to 10 rpm are possible with this combination. And, there are additional benefits for slowing operation in this manner. Slower armature speed results in less operational sound (noise) and vibration, and longer overall life, due to decreased wear.
Additionally, because the torque constant Kt increases right along with Ke the result is lower current draw as well. This could also reduce the cost of the motor drive control, power supply, and even conserve duration of a charge cycle in battery-powered systems. It's all a matter of optimizing the combination of motor winding and gear ratio, to obtain the best possible performance for a particular application. Sizing each application independently ensures optimization.
It is also quite possible to oversize a motor or gearmotor for a given application, carrying ‘optimization’ perhaps a bit too far. Often an OEM will select a gearmotor that is handy — logic being: If it works, why change it? And, “handy” can even mean a gearmotor that's chosen from a surplus outlet or even a junkyard — hardly good sources for OEM supplies.
Care should be observed to be certain that there is a sufficient margin of safety regarding the power required, but significant oversizing can add unnecessary costs, and ‘real estate’ consumption. Prospects often comment that they need X amount of output speed, and Y amount of torque, because that's what it says on the label of the incumbent device. However, it's always best to measure the actual current and duty profile, and establish the optimal solution from these empirical data. The space conserved and money saved can often be significant.
Otherwise, the potential drawbacks of using multiple gear stages to slow system speed include increased length, cost, inefficiency, and backlash.
Back emf
Because there's no difference between a generator and a permanent-magnet dc motor, the latter actually exhibits some generator-like traits during operation. Upon energization of the motor, current flows through its stator, or armature, its rotor begins to spin, and the motor spawns that internal voltage called back emf, possessing polarity opposing that of the input. (Because the back emf is effectively in series with that input, increasing back emf reduces the current, and thus speed.) Back emf is increased or decreased by modifying the number of turns in a motor's windings and by changing their gage of wire.
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