Three Misconceptions in RC – Brushless Motors & more

A Lower (vs higher) Kv Motor Produces More Torque

It is a common misconception that a lower Kv motor automatically produces more torque then a higher Kv motor in the same size class. What we do know is that the Kv constant of the motor is directly related to the torque constant of the motor, Kt. The relationship of Kt is the inverse of Kv. The lower a motors Kv value, the higher its Kt value will be. The torque constant of the motor tells us the amount of torque output per amp that we can get out of the motor.

Lower Kv motors do have a higher torque constant. However, by nature they are not able to sustain the same amount of current output as a higher Kv motor. Therefore a higher Kv motor can actually develop the exact same amount of torque as a lower Kv motor just by pulling more current.

Let’s take a look at an example.

Motor A – 1200Kv, 60A Max Current, Kt = 7.974 mNm/A
Motor B – 1650Kv, 83A Max Current, Kt = 5.799 mNm/A

The Maximum amount of torque output for motor A will be the Kt value multiplied by the max current. Motor A will be able to deliver 60A x 7.974 mNm/A = 478 mNm. Maximum torque output for Motor B is 83A x 5.799 mNm/A = 481 mNm. The difference of 3 mNm is negligible and discovered through rounding error.

Other Considerations when using a lower Kv motor in place of a higher Kv motor

If you are comparing a high vs low Kv motor operating at the same input voltage, the lower Kv motor will have a harder time making the same power output. This is due to the maximum current being lower on the lower Kv motor. The way that you would be able to make up for the loss in max current is to increase the voltage.

Cogging in a Brushless Motor

Cogging of a brushless motor is commonly regarded as the relationship of the motor and ESC being out of sync when starting from zero RPM. However, this is not the definition of cogging.

Cogging is a result of the iron core in a brushless motor reacting with the permanent magnet within the motor. As you rotate a motors output shaft, you can feel the “steps” of the motor through one full revolution. These steps require a certain amount of torque to overcome the resistance. Once the resistance is overcome, the rotor begins to accelerate as the torque requirement swings in the opposite direction. This creates an unbalance of torque at low speeds. Luckily for us, we can not feel the unbalance or even really notice is when operating our RC vehicles at low speeds. The only disadvantage of cogging is that it does tend to make the initial ESC to motor synchronization process a bit more difficult. To eliminate cogging, one can simply use a slotless rotor. A slotless rotor does not utilize an iron core. Visit this link to learn more on how to improve cogging.

Voltage Destroys Electrical Components

Voltage does indeed destroy electrical components. This is definitely not the misconception here. If you simply have a 6 cell LiPo ESC and run 7 cells to it, surely it will have smoke pouring out of it in no time. This misconception comes from the idea of adding a cell to a power system and having a speed control or motor blow, thinking it is directly related to the voltage increase.

Consider an example where we use the same 6 cell ESC as above and we go from using 5s LiPo to 6s LiPo. We then discover that our motor burned out. What caused the issue? Well when adding a cell extra of voltage, we are expecting our motor to turn quicker as RPM output is equal to Kv x voltage. Since our motor has the potential to spin faster and our gearing nor other RC car specs has not changed, load has increased. We know load must increase as it will take more torque for the motor to maintain a higher output speed and top speed of the car. Torque is directly related to the amount of current that our motor will pull. Increasing the voltage of the motor without decreasing the load or gearing of the RC car, will increase current. Since current is exactly what contributes to an increase in heat, this is how our motor will overheat and burn out.

Surely, Voltage Will Contribute to Motor Burnout?

When you look at why a motor would fail electrically, it really only is because of current. Not voltage. Now, a motor can fail mechanically. A mechanical motor failure can occur in the following areas. The first being the bearings that support the shaft of the motor. Mechanically over spinning the bearings can lead to bearing failure. At the same time mechanically over spinning the rotor of the motor can lead to a very similar failure. In fact a rotor failure is usually catastrophic. All motors have a rated maximum RPM and that is based on the limiting factor between either the rotor and the bearings.

The maximum RPM can be broken down in to Kv and Voltage. If you increase the voltage high enough, you will over spin the motor causing mechanical damage. The formula to determine the maximum voltage of a motor is:
Vmax = Max Motor RPM / Kv

Cavitation in RC Boat Propellers

Woops, yep, this is a fourth. We will call this a bonus!

I hear this misconception all the time not for just RC boats but even for full scale boats as well. Cavitation is the general term used to describe slippage that occurs when the prop sucks in a bunch of air causing the engine or motor to unload. However, this is not what cavitation actually represents.

Cavitation occurs when a rapid change of pressure in a liquid leads to formation of small vapor pockets in places where the pressure is relatively low. When subjected to higher pressure, the vapor pockets collapse and can generate a large shockwave. This shockwave can lead to pits found on the surface of the propeller ultimately damaging the propeller.

Now the way to reduce cavitation is by simply introducing aeration. Aeration is the process where you introduce air in to the propeller as defined in the actual misconceptions definition above. Introduction of air in to the propeller will simply not allow low pressures to build eliminating the potential for cavitation to occur.

This means that in our RC boats utilizing a surface driven prop, we will never really see the potential for cavitation to occur. Our more significant problem is we get too much aeration and are unable to get the boat on to plane quickly. The simple solution here is to move up to a larger propeller. Just make sure the larger propeller does not place too much stress on to your power system.

 

Ripple Voltage in an ESC Explained and How to Improve it

The ESC in a radio controlled vehicle is responsible for delivering power from the battery pack to the motor. Input power to the ESC is in the form of direct current. Output power is in the form of alternating current that is in perfect synchronization with the motor. The complex process of converting DC to AC power at the time a motor coil requires it, while managing motor speed is all handled by the ESC. However, the ESC is not perfect and in some cases requires help. Ripple voltage is one area that can get out of hand within your ESC. In this article we will understand what ripple voltage is, what would be an acceptable amount of ripple voltage and how it can be improved.

What is Ripple Voltage?

Ripple voltage is best defined as varying voltage at the source of power. Variation in voltage is measured at the battery input side of the ESC. It is possible to log ripple voltage within the ESC, however, not all ESC’s have this data logging capability.

You may have noticed that on the input side of the ESC are large capacitors. It is the job of these capacitors to smooth out any ripple voltage at the ESC. If the capacitors are unable to smooth out variations in voltage, the variation in voltage is felt by components further down in the circuit. FET’s also known as Field Effect Transistors are the components sensitive to ripple voltage. If it gets out of hand, these will burn up and render the ESC completely useless.

What Creates Ripple Voltage

During partial throttle usage, the ESC switches on and off the power that is sent to the motor. As the ESC goes through this cycle battery power is pulled at the same rate. The rate that the ESC is switching power on and off during partial throttle is the PWM rate. In many ESC’s, this value is programmable. The standard value or typical value that you would see is 12,000 Hz.

When the battery pack is loaded with an on cycle of the ESC, voltage sags. The amount of voltage sag that the ESC experiences depends on a few factors. One of those factors is the batteries ability to dump power quickly. As this process repeats itself multiple times per second the voltage bounces between the unloaded voltage and loaded voltage. In conclusion, it is this exact difference in voltage that is known as ripple voltage.

What is an Acceptable amount of Ripple Voltage

Now on to the important stuff. An acceptable amount of ripple voltage is less than 5-6% of the battery packs nominal voltage. Around 7-9% is OK. Around 10% is where you can get in to some permanent ESC damage which ultimately means that the lifespan of the ESC is reduced. Above 10% and your ESC is at risk. To determine the ripple voltage review a data log of your ESC while recording ripple voltage. Determine where the peaks are occurring within your run. Take the highest ripple voltage value that occurs as a peak within your run.

To calculate ripple voltage as a percentage, divide the ripple voltage by the nominal battery voltage (# cells x 3.7v/cell) and then multiple by 100.

Example:
Ripple Voltage – 1.27v / 6s Nominal 22.2v % Ripple Voltage = ( 1.27 / 22.2 ) x 100% = 5.72% This would fall in the category of good and acceptable

How to Improve or Reduce Ripple Voltage

There are a few different ways to improve ripple voltage. Let’s take a look at them.

The first area of concern is to check if the ESC to Battery leads have been extended. Having extended leads can “lead” to higher ripple voltage created within the ESC. It is important to not extend the leads of the ESC without compensating for this.

The second way to reduce ripple voltage is to add a capacitor bank. A capacitor bank is able to compensate for extending the leads on the battery side of an ESC. The capacitor bank AKA cap pack is also able to help for those beast of a power system. Cap packs are highly recommended for high powered EDF jets, high powered boats and speed run cars. You can read more on capacitor packs by visiting the Why Use a Cap Pack on an RC Car page.

One of the easiest ways to improve ripple voltage is if you are using undersized battery connectors. Check the rating of the connectors and make certain that the connectors are rated for the amount of power you plan to draw.

Lastly, using the highest rated C rating and battery pack capacity as possible will make significant improvements. The improvement comes from the batteries ability to dump more current. (amps) Under load a battery that can deliver more power will have less voltage sag. With less voltage sag there will be less ripple voltage.

Conclusion

The ESC is a very complex component found within our RC power system. However, when driven too hard or over any of its limits, the ESC can respond by turning in to a bunch of smoke. To avoid this it is highly recommended to monitor by using data logs the performance of your ESC. If you notice concerning data it is best to take action using the methods described above. Therefore, doing so will maintain maximum reliability within your power system.

Can too high of a LiPo C rating Damage your RC Car?

When you are looking to purchase a LiPo battery for your RC car, no one explains what exactly is required. Sure, you may know exactly what voltage you need as represented by the amount of LiPo cells in series. You may also know the capacity that you need as represented by the “mAh” of the pack. However what about the required C rating of the battery?

Can this LiPo Battery C rating Damage or Hurt your RC Car?
Can this LiPo Battery C rating Damage or Hurt your RC Car?

Quick Review of the C Rating

The C rating of a pack is a multiplier used to determine the maximum continuous discharge current of the battery. Typical numbers are 25,35,45 and so on. It is this Continuous rated C rating that is multiplied by the capacity of the battery pack in amp hours (Ah) to determine the maximum continuous rated current of the battery pack. A higher C rating tells us that the battery is able to provide higher current output performance.

Example:
4000mAh at 25C continuous. The maximum continuous discharge current of this battery would be 25 x 4 = 100 Amps
4000mAh at 45C continuous. The maximum continuous discharge current of this battery would be 45 x 4 = 180 Amps.

Can the C Rating Destroy your Motor or ESC ?

Now that we understand the idea behind the C rating, we can figure out if too high of a C rating can destroy your ESC or brushless motor. In the above examples we have determined that the battery pack with a C rating of 45 can deliver 80A more in comparison to the 25C rated pack. What does this mean for our brushless motor and ESC?

When it comes to battery packs and motors, the biggest concept to understand is that a battery does not “push” power to the motor. Just because the 45C rated battery can deliver 180A of continuous power doesn’t mean it will. In fact, the load placed on the motor determines how much power is going to be drawn through the motor, ESC and battery.

A higher C rating should not ever destroy any RC component. However, this does not fully mean that it is impossible. If the power system in your RC vehicle has not been correctly selected, it is possible that a component within your RC can fail. We discuss this further below.

Performance Benefit of Higher C rating

Having a higher C rated battery allows the battery to have a lower voltage drop under load. When the battery is able to maintain higher overall voltages, it is expected that the current increases as well. This increase in current is what can lead to failure within an electrical component. Power systems that fail purely due to an increase in C rating are 1) not selected correctly and 2) already operating near its thermal limits.

Here is an article that talks about choosing the best C rating for your power system. In short, it is best to select the highest C rating possible for maximum reliability within the ESC. Visit the ripple voltage page to learn more. Next we will look at how to confirm that using a higher C rated battery will not damage your RC car.

Confirming too high of a C Rating Does Not Cause Damage

There are a couple scenarios where higher or “too high” of a C rating can cause damage to RC’s and we can identify them beforehand.

Scenario 1 – Some Ready to Run vehicles use brushed motors. Brushed motors have a narrow performance range by nature and manufactures try to get the most out of these inexpensive motors. In addition, NiMh batteries are also commonly used in these setups. If your brushed motor is operating at case temperatures above 150F while on a factory setup, there is a very good change that using a LiPo battery at any c rating (NiMh batteries have a very low C rating) can cause thermal damage. It would be best to swap out the power system to a brushless setup prior to switching to LiPo batteries. If you are willing to try LiPo or a higher C rated battery, just be certain temp limits are not exceeded.

Scenario 2 – If your setup is operating near its thermal threshold, chances are significantly increasing the C rating of your pack will heat things up further. Use a temp gun to measure the temperature of each of the components within your power system. Confirm that the temperatures are within the maximum specified ranges. If you are operating within a safe temperature range on all components, try out the higher C rated battery. Measure the temperatures yet again of all components. If temps are still within spec, your system is good to go.

Conclusion

A higher C rated battery in the vast majority of setups will be best suited. Not only does a higher C rating provide better performance but it also provides higher reliability for ESC’s. Simply monitoring temperature (should be doing this already) upon any change in your setup and confirming it is within spec will guarantee reliability.

With correctly selected power system, there should never be such thing as “too high” of a C rating.

How to Avoid the #1 RC Brushless Motor Mistake

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.

Vehicle TypeMinimum RPMMaximum RPM
RC Car28,00050,000
RC Airplane5,00018,000
RC Boat22,00032,500
Table Representing the typical RPM range for general RC vehicles. Note that your RC vehicle may fall outside of this range.

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.

Does a Higher kv make your RC car faster?

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.

Does increasing kv make RC Car faster
Does increasing kv make RC Car faster

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!

MOD1 and 32P Gears of Different Sizes
MOD1 and 32P Gears of Different Tooth Counts

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.