This is one mistake you will want to avoid. Not only is it very costly but it can take the fun right out of RC and there is no place for this! Primarily this mistake is made when selecting a brushless motor for an RC vehicle. Often times the motor that is selected by a new builder tends to not have the correct kv. Incorrectly selecting the kv for the motor can literally destroy the motor, ESC or battery. Avoid this Brushless Motor Mistake. Let’s dive into this.

Why Is kv Incorrectly Selected?

Well for starters, there are a lot of RC brushless motors available on the market. You can get one in nearly any size, colour, wind type, inrunner, outrunner, sensored, sensorless and with multiple different kv options on the same motor. Simply put, it’s quite complex and there is no one telling you if the motor you selected will work perfectly in your RC to hit the targets and goals that you have set out.

Why Kv Selection is Very Important

When it comes to selecting kv, the importance lies in the total amount of RPM that you require. Not only is the output RPM that you get out of the motor important but also the total voltage you plan to run. As we know, kv represents the RPM per volt applied to the motor. The total RPM we get out of the motor is battery voltage multiplied by the kv. You would need to know what battery you plan to run prior to being able to select the RPM per volt that you need in your RC. In addition you would also need to know the amount of total RPM that you are trying to target within your RC. This value changes depending on the type of RC that you are powering.

Rule for RPM of an RC Car, Plane, Boat

Here we have a general table that represents the total amount of RPM that you could expect for each RC vehicle Classification.

It is very possible that your RPM requirement could be less than the minimums that are above. Having too much RPM will increase the load requirement on your power system. If the system can not handle the increased load, this is where we get excessive heat and possible failure. Below we talk about how you can sanity check your selection and overall setup.

Picking a setup that runs at RPM levels above the maximums listed above is only for experienced modelers.

RC Airplane RPM Output Range

For the RC airplane in the above table, you would want to go for a higher RPM when spinning a prop close to a diameter of around 5 inches. You would want to use an RPM on the lower end when you need to spin a prop that is larger than 20 inches in diameter. Higher RPM with a larger prop will take more power from your motor, ESC and battery.

RC Boat RPM Output Range

For RC Boats, hull length comes in to play considerably when looking at the desirable RPM range to hit. Hulls that naturally have more drag (mono hull) would need a larger propeller to push them through the water with enough thrust. This will translate to requiring an RPM output on the low end of the range. However a hull with minimal drag can benefit by having higher RPM output using a smaller prop.

RC Car RPM Output Range

The RPM output range for an RC car highly depends on a few different specifications of the car. These include the overall gear ratio of the vehicle, tire diameter and speed that the vehicle would be setup to hit. See below on how you can check your selection.

How to Sanity Check the kv that you have Selected

In order to boost the confidence that you have in your selection, I would highly recommend going through a sanity check. The idea is to use the kv that you have selected as well as the vehicle specifications to calculate the expected speed that you could achieve. If you discover that your RC 1/10 scale truck is going to go 120mph (190km/h) with your conservative setup, this would be a huge indication that something is wrong.

The same approach would be used for an RC airplane or boat. Instead of using a gear ratio and tire diameter, you would be using propeller pitch and a slippage factor.

Check Out these Calculators that will help identify the speeds for your specific RC:

RC Airplane Calculator
Boat Calculator
RC Car Calculator

Conclusions

Now that you know the importance of selecting the correct kv for your RC, you will be able to pick out a reliable power system! Just make sure you continue to use a heat gun to measure the temperatures of the motor, ESC and battery. Be certain that your operating temperatures are never outside of the maximum specification for each component.

By popular request we are going to look at the idea of changing kv to go faster. Building a faster RC vehicle is a lot of fun but building a setup only to find out it burns up in the first few runs can leave you scared. It’s important to be informed to make the best most reliable decisions to help increase your chances of success.

Most electrical topics in RC can be confusing and difficult to understand. Today’s question is no exception. Let’s go through the answer to our question, does a higher kv brushless motor make your RC Car faster. Even though we are going to be talking about an RC car in this article, the same principles apply for other RC vehicles.

How kv Impacts Output RPM

A motors kv constant tells us how many revolution the motor will make in one minute per volt applied. As we increase the voltage to the motor we get more RPM. However, if we increase the kv of the motor, we get more RPM out even if voltage is held constant.

Here is a simple example showing the difference between a 2200kv motor on 4s LiPo (14.8v nom) vs moving up to a 2650kv motor operating on 4s. A 2200kv motor will spin ( 2200 x 14.8 ) 32,560 RPM where a 2650 kv motor will spin ( 2650 x 14.8 ) 39,220 RPM. Both of these values are the unloaded RPM output of the motor at full speed. As you can see, the higher kv motor spins a higher amount of total RPM.

What does higher Output RPM Contribute to?

If we want our RC car to go faster, we simply need the tires to rotate faster. A faster spinning motor surely makes sense if we are trying to get higher axle speed out of the RC car. However, the higher output RPM that we get out of the motor does not come for free. Higher rotational motor speeds suggest that the motor can make more power, but where does this power come from? Power in watts is equal to voltage multiplied by current. We know that voltage in our example above is held constant, we are not changing it. The only variable that can increase in order to make sense in our power formula is current. Current by rise.

It makes sense for current to increase. An easy way to understand when current is increasing is to consider the load that the motor would be under. Higher load translates to higher current being drawn from the motor. Asking our motor to spin faster increases the amount of work that motor can do. Since we did not change the gearing on our RC car to maintain the same speed (want to go faster) the motor must be under additional load increasing the current.

Answering the question in the header above, higher RPM output of the motor contributes to a higher potential power output. If the speed of the RC car is increasing we know it is under additional load, making more power.

Does this Mean higher kv makes more Power?

Looking at the result makes it apparent that higher kv motor does increase the axle speed of our RC car. Higher axle speeds do increase the overall RC car speed. The simplistic answer is yes, higher kv does increase the speed of your RC car. But there is a catch.

Here is where the problem lies

What if I told you that in the above example, the 2200kv motor was almost at its maximum thermal limit? Meaning it runs about as hot as it can get away with. Does this change anything for you? Well, what if I mentioned that the 2650kv motor in the above example is actually a smaller motor. Length of the 2650kv motor is 68mm long vs 75mm long for the 2200kv motor. Now does this change anything for you?

We should have a change of heart on what motor we want to use in our example RC car. If we moved to the 2650kv motor, we would certainly let the magic smoke out, rendering the motor useless to us.

How to Select the Best Option for Increasing Speed

Knowing that in our example the motor is at its maximum thermal limit means one thing for sure. We must find a larger motor to cope with the increase in power that we wish to get out of it. Going faster means more power resulting in more waste heat the motor has to get rid of. Moving to a larger motor will help dissipate this waste heat. Once we select a motor model that is physically larger, we now have options.

Increasing the kv of the Motor to go Faster!

If you can spin the motor faster without going over a maximum rotational motor limit, increasing the kv is OK. Just make sure you aren’t going to increase kv by a ridiculous number. Stay within a total maximum unloaded RPM range that makes sense for your RC. This is by far a frequently made mistake by RC hobbyists. Find out what RPM range your RC requires and stay within that range!

Change Gear Ratio to go Faster!

If you moved up to the next available size motor, you could simply select the exact same kv option as your smaller motor. To increase the speed of the axle, just adjust the gear ratio. A larger pinion gear and / or a smaller spur gear, will allow you to change the final drive ratio resulting in faster speeds! Make sure that you set your gear mesh correctly to reduce wear and maximize performance.

Increase Voltage to go Faster!

Another avenue you may be able to take is increasing the amount of cells you plan to use. One thing to keep in mind if you do plan to increase voltage is the limits. Your motor/ESC must be capable of handling the increase in voltage. In addition if you aren’t changing the gear ratio of your RC car, current will also increase. Make certain that your setup can handle the large increase in power.

Conclusion

It is a lot more accurate to think in terms of increasing your power output of the motor in order to go faster. Simply trying to increase the kv of a motor does not paint the whole picture. It is a very easy and often misleading simplification that can get you in to trouble. First, find out where your setup is at thermally. (A motor that is running cool can simply benefit in a speed increase by just swapping gears ) Then decide which avenue you would like to try. There are options, you don’t have to go straight to a kv change.

Have you ever wondered where the power ends up when you slam the brakes on an RC car or use a drag brake on an RC airplane? Energy can not be destroyed once it has been built up in a high speed RC Car. This kinetic energy is converted to other energy forms. The big question is what is the new energy form? Let’s start by looking in simplistic terms how an ESC slows down our RC car.

How does the ESC slow or apply brakes on an RC Car

Electric brakes are controlled by the ESC using the motor. Rotational kinetic energy built up in the motor creates a voltage known as back EMF that the ESC is able to read. When brakes are applied via the transmitter, the ESC uses the FETs within the ESC as a switch to short out the motor leads. This action allows the ESC to bring your RC car to a halt.

If you are even more curious as to how this works, grab a brushless motor kicking around. Take all of the leads coming out of the motor and short them out using wire terminated with alligator clips. Now try and rotate the motor with the leads shorted out. What you will notice is that there is resistance to the rotation of the motor shaft. Also, the faster that you try and spin up the motor, the stronger the resistance to motion will be. Lastly, if you remove the leads shorting out the leads exiting the motor, the added resistance to motion disappears.

A lot of energy Ends up in Heat

During the test, ran in the video above, a lot of heat is built up in the motor and ESC. During the test the motor and ESC rise in temperature quite significantly. It is estimated that about 65-85% of the energy dissipated when braking exists the system as heat. The question is, where else does this energy go? Also, how are we able to estimate 65 to 85 percent of the energy is converted in to heat?

Braking Energy Converted in to Chemical Energy

The answer to the question in the last paragraph is that the remaining bunch of power has been demonstrated to be converted in to chemicals energy. Take a look at the regenerative braking page for more information.

Conclusion

It is very interesting to be able to demonstrate where the energy ends up as we use the brakes in our RC Car. Although a large percentage of power ends up being converted to heat, a smaller percentage of energy is converted to chemicals energy.

Does this represent all forms of energy that is converted upon brake input? Simple answer is no, but the large majority consists of these 2 items. For example, sound energy is also created as brakes are applied, however, the total amount of energy would be very small making its value negligible. Knowing where the energy ends up can change the way we think about our RC setup.

Many RC ESC’s have an electronic brake function to slow down the motor where and when required. For RC Cars this is extremely important as the guys exceeding 100mph would be in the next city without brakes. For airplanes that can cruise back down from a high altitude with no throttle input, a drag brake stopping or slowing the propeller can help increase the glide ratio allowing the plane to glide further. The big question is if the ESC is able to take this energy and use it to charge a battery through regenerative braking.

You can watch this video below or skip down below the video for the answer and explanation.

First we will understand how the ESC is able to slow the motor down.

How the ESC slows the Motor down Applying Brakes

The best way to see for yourself how an ESC slows down a motor is to grab a brushless motor and use alligator clips to short out the 3 leads to one another. With the leads all shorted, rotate the motor by hand quickly. You will notice that there is a great amount of resistance to this motion. The faster you try and spin the motor, the more resistance you will feel. Removing the wires shorting out the motor leads will remove all of this added resistance.

The ESC does the exact same thing, shorting out the leads of the motor electrically. In order to manage the amount of braking force that is applied, the ESC must rapidly switch on/off the leads that are shorted out.

Does Regenerative Braking Occur

Can we recharge our battery as we use the Brake? As the leads of the motor are shorted by the ESC, power is in fact produced and sent back to the battery. An approximate 15-30% of the braking efforts ended up back in the battery through the test in the video above. This is not an overly impressive efficiency for charging the battery pack, however it doesn’t matter whether the process is 1% efficient or 100%, regenerative braking is still taking place.

Regenerative Braking Efficiency

What is interesting is that as more brake input is provided via the transmitter, a lesser amount of braking energy is placed back in to the battery. Braking that is done as more of a “drag brake” maximizes the efficiency using the data from the simple experiment completed in the video.

How is Power Generated and Pushed to the Battery?

The ESC shorting out the leads of the motor is the first part in the braking process. The core function responsible for generating the power to push in to the battery happens as the motor leads are opened and closed using the FETs (Field-Effect Transistor) on the ESC.

Back EMF is generated creating a voltage caused by the rotating motor. In turn, the motor leads are shorted by the ESC. Voltage within the windings quickly drops and approaches zero but current surges. When the FETs open the circuit, current has absolutely nowhere to go. The brushless motor windings act as an inductor which does not like any change in current. As a result, the rapid change in current causes the voltage to increase significantly. It is this increase in voltage that allows power to flow back through the circuit and into the battery pack.

Conclusion

Regenerative Braking does indeed occur in the ESC tested and any other ESC that uses the same principle for braking. Efficiency is not a strength within these systems, especially when a significant amount of braking force is required. Overall, the total amount of braking energy that makes its way back in to the battery is not significant. However, it is greater than zero and will extend the overall run time of an RC that uses brakes during the run.

Is that not cool or what?

Unfortunately the rest of the braking energy results in heat. Take a look at how hot the braking circuit can get when a small amount of brakes are applied.

A brushless motor used in RC is actually quite simple mechanically. There are only a few moving parts in the entire assembly. Those moving parts are limited to the rotor containing the permanent magnets and the bearings that support it.

When considering lubrication, the only area that could possibly require it is the bearings, right? To understand the answer to this question, we have to get a good understanding as to how the bearings are protected. The typical bearings found in an RC motor have a shield that helps protect the bearings from foreign object debris AKA dirt. The key takeaway from this is that the bearings are not water proof and can certainly allow liquids to pass by the shield.

Cleaning the Bearings

If you happen to try and clean the bearing with a product similar to WD-40, the oil can actually pass by the shield and mix with the factory bearing lubrication ultimately leading to possible damage to the bearing. To clean the bearing, my preference is to take a dry cloth and wipe any dirt away from the visible bearing surface. I do not like to use any chemicals.

Lubricating the Bearings

The quick answer to lubricating or not is that I recommend not lubricating the bearings. First off, it’s not as simple as just placing oil on to the bearings hoping to get by the shield of the bearing. Light oils may be able to make it easily past the shield of the bearing, but do not have the required lubrication characteristics. Light oils will more than likely not be able to withstand the concentrated heat and friction created within the bearing. The oil would burn off and subject the balls within the bearing to excessive frictional loads.

To correctly complete the process the shield must be removed, the balls (whole race) must be cleaned and then a proper bearing grease must be used prior to placing the shield back on to seal the deal.

Is this Process Worth the Effort

Here is another point to make. Some may say absolutely! However, I’d prefer to spend the price of a complete new set of bearings for the front and rear of my motor. It would be far less effort to replace the bearing set. I will also feel comfortable knowing that I have a fresh set (0 hours) of bearings.

One last point to be made. Out of all the brushless motors that I have ever used, only one of my motors are in need of new bearings. However, this was a motor that also lived it’s life upside down submerged under water when the boat flipped! Here’s the story. To put this in to a different perspective, the motor was significantly abused.