Most electric motors run most efficiently at higher speeds than most applications require, and that gap is bridged by mechanical speed reduction. Once you have selected an appropriate motor, choosing the best reduction ratio is the single most important factor in determining how well the motor will work in a particular application. There are several methods for implementing mechanical speed reduction. Here are some examples along with some pros and cons.



If there is too much speed reduction, the motor may have plenty of torque, but the RPM will be too slow. If the speed reduction is insufficient, the motor will need to draw much more current than would otherwise be necessary to produce the required torque.


The current drawn by the motor is an important consideration because the heat generated is proportional to the square of the current. In the first case (too much reduction), the motor is normally safe because it will draw reduced current. But if the speed reduction is insufficient, the motor can easily draw currents high enough to cause overheating. And if the reduction is insufficient, the motor may be forced to operate in an RPM range far from its high-efficiency speed. This can cause overheating at relatively low levels of torque. The benefits of proper speed reduction include:



It is good practice to use the maximum possible mechanical speed reduction. Even a small change in the ratio can significantly affect the system's performance. We always recommend erring a little on the side of over-reduction when designing a new application.



Example 1


Consider the AmpFlow A28-400-G8 motor with the AmpFlow speed reducer with a reduction ratio of 1:8.3. When operating at 550 RPM at 41 amps of current, the motor has an efficiency of 82.6% (not counting some small friction loss in the speed reducer), and the torque output would be around 2,000 oz-in. This is a very efficient operating point, but about 170 watts of waste heat are still generated.


If we try to operate the same motor at 2,000 oz-in without a speed reducer, the required current would be about 310A, and the waste heat would be over 4,000W. This is a tremendous overload condition, and the motor would quickly overheat.


The optimal reduction ratio is the one where the output shaft reaches your required top speed at full throttle while the motor itself runs at or above its peak-efficiency RPM. If the motor cannot develop the required torque for the application at this reduction ratio, or if the waste heat is too high at this output level, the motor may be too small for the application. Charts showing the efficiency, power output, and current draw at various speeds are available on our Motor Performance Calculator page.



Example 2


A mobile robot requires a top speed of 10 MPH using the A28-400-G8 motors. We want to achieve this speed when the motor runs slightly above its peak-efficiency RPM (4,500 RPM for this motor). So we choose 4,700 RPM as the motor speed. The AmpFlow speed reducer has a reduction ratio of 1:8.3, resulting in a wheel speed of about 570 RPM. So we need to choose the appropriate wheel diameter to achieve an overall reduction ratio close to optimal. To reach 10 MPH with a wheel rotating at 570 RPM, the wheel would need to be about 6 inches in diameter. To find the best combination, use our Robot Performance Calculator to experiment with different reduction ratios and wheel diameters.



Example 3


The same robot requires the same top speed of 10 MPH, but the wheel diameter is 13 inches. A 13-inch wheel revolves at about 260 RPM when traveling at 10 MPH. With the motor running at 4,700 RPM, we need a reduction ratio of about 1:18 to get 260 RPM at the wheel. The AmpFlow 19:1 speed reducer provides almost exactly the right ratio, giving a wheel speed of about 245 RPM, close enough to achieve the required top speed without any additional reduction hardware.


Here are a couple of considerations if you plan to make your own speed reducer: The largest practical speed reduction using just two gears, two sprockets, or two pulleys is about 1:5. Above that, you run into problems with oversized components and insufficient chain or belt wrap around the smaller of the two items. So in the above example, you might design a two-stage reducer with each stage having a ratio of 1:4.3 (4.3 x 4.3 = ~18), or a three-stage reducer with a ratio of 1:2.6 for each of the three stages. But if space, time, and budget are limited, we suggest opting for one of our standard gearmotors.



Electronic Speed Controllers


Do not rely on electronic speed reduction to do the job of a mechanical speed reducer. The purpose of electronic motor controllers is to control speed from 0 to 100% (and direction in reversible controllers). Using the controller to limit motor speed has all the drawbacks listed above and, in some cases, even more. The machine should be designed so that the controller is fully on at 100% throttle when the machine is operating at its top required speed.


Modern high-frequency motor controllers work by switching the power on and off thousands of times per second. To get more speed, the "on" time is increased, and to reduce speed, the "on" time is decreased, reaching zero "on" time at zero RPM. This is known as Pulse Width Modulation (PWM). The high switching speeds, combined with the motor's inductance, effectively make the system a voltage controller rather than a current controller.


Measure the motor voltage in a 24V system that is throttled to about 25%, and you will get a reading of 6V. Let's say the 24V battery can supply 100 amps, or 2,400 watts. If 2,400W go into the controller, then 2,400W must come out of the controller. If 2,400W are going into the motor at 6V, then the current must be 400A (2,400W / 6V = 400A). The math has been simplified here by omitting efficiency losses and a few other considerations, but the basic idea remains the same: the electronic motor controller can push more current into the motor than the battery can actually supply. In other words, running the motor at reduced throttle is not a good way to reduce the top speed!


Since motor heating is proportional to the current squared, you can see why using an electronic speed controller to limit top speed is no substitute for the proper mechanical speed reduction ratio.



Example 4


Here is an example showing how adding a speed reducer greatly improves performance and reduces heat.


Let's say your power supply can produce no more than 11A output at 36V (like the AmpFlow S-400-36). You want to use the AmpFlow E30-150 to drive a pump rated at 1/3 horsepower (250W) at 1,800 RPM. At 36V, this motor will produce 250W of output at 8,200 RPM with waste heat of about 80W. But there is a big mismatch between the pump RPM and the motor speed. Since the power supply is limited to only 11A, you might think that the motor would be protected from overheating.


The torque a motor produces is proportional to the current it draws. This motor will produce 51 oz-in of torque at 11A. Let's say the pump drags the motor speed down to 1,800 RPM. The mechanical power output at 51 oz-in and 1,800 RPM is only about 70W (51 oz-in x 1,800 RPM / 1,352 = ~70W). But the motor is consuming 11A at 36V, which is 400W of electrical power. All electrical power not converted to mechanical output becomes heat. So the waste heat is 330W, enough to quickly overheat the motor. And since the motor can only deliver 70W, the pump will not even reach 1,800 RPM, making things even worse.


This same motor, with a 1:4.5 speed reducer, would happily run the pump at 1,800 RPM with only 80W of waste heat.



How to Choose the Best Reduction Ratio


The optimal reduction ratio is the one where the output shaft reaches your required top speed while the motor itself runs at or above its peak-efficiency RPM. For a robot or vehicle, this means matching your wheel diameter and reduction ratio so the motor hits that sweet spot at full speed.


For machines and mechanisms, the required torque is usually known with greater precision. Use the performance charts on our Motor Performance Calculator page to confirm the motor produces sufficient torque at the chosen operating point.


AmpFlow motors can run at speeds below their peak-efficiency RPM, but the higher the torque, the shorter the duty cycle must be to allow the motor time to cool down. It is best to err slightly on the side of over-reduction rather than risk overheating the motor.



Engineering Assistance


If you need help selecting a motor or determining the optimal reduction ratio, contact us by phone at (650) 226-3560 or by email at sales@ampflow.com.


For a robot or vehicle, we will need the vehicle's weight, wheel diameter, and desired top speed. For a machine or mechanism, we will need to know the RPM and the torque. In all cases, we will need to know the duty cycle (the ratio of on time to off time), the voltage, and, if you are using a small battery or a power supply with limited output, the available current.