Brushless motors come in many different winding choices. These winding choices are critical to the specific application that you are looking to place a motor in to. However, did you ever wonder how a brushless motor is able to perform equally in terms of power when Kv is changing?

Continuous Power Output Limitation

The first topic to dig in to is, what truly is limiting the power output of a brushless motor? The true electrical limitation to a brushless motor is heat. Too much heat as we’ve discovered in another article, will destroy the brushless motor.

Brushless motors do not operate with 100% efficiency. If they did, we would not have to worry about heat in any way. However, a typical brushless motor will operate between 80 to 90% efficiency.

Compare Equal Size Brushless Motors with Very Different Kv’s

We are going to follow an example throughout this article. Let’s take a look at a 1750w motor where this power can be delivered continuously.  If we assume an efficiency of 90%, our 1750w motor will be delivering 10% of 1750w in heat. This roughly equates to 175w. If you have ever touched a 100w old school incandescent light bulb while it was on, you would know what about 90w of heat feels like. Our motor has to be able to dissipate 175 watts of power in order to not over heat. Anything more and the motor should not be rated for 1750 watts.

A larger motor simply is able to dissipate more waste heat. If we look at an inrunner motor, the motor is capable of removing heat by the surface area of the case. Increase the surface area by increasing the size of the motor and you are able to remove more heat allowing a higher amount of power to be produced.

The Heat Producing Parameter of Brushless Motors

We have also learned from a previous article that heat is produced by current. Voltage does not directly impact the amount of heat that a motor will expel. The more we load our brushless motors, the more current we should expect to draw from the battery pack.

If current produces heat in our brushless motors, how is it possible that the high Kv motor (draws more current) is able to have the same output as the low Kv motor? Surely, we would expect that drawing such a high amount of current would contribute to significant waste heat.

The answer is in the data chart below. Motor one is on the top, and motor two is on the bottom.

Waste Heat produced by each Brushless Motor

We know the limitation to power output is waste heat. In order to understand how both of these motors will end up producing similar waste heats, we can use the above data set. Two forms of waste heat produced is known as copper losses and iron losses. Copper losses are found in the windings of the motor while iron losses represent the power consumption to keep the motor rotating.

To calculate the Copper Losses, we take the square of the current and multiply this by the Rm. To calculate the iron losses, we take the voltage and multiply it by the Io.

As we can see, both motors end up producing similar waste heats. These values closely match the waste heat calculated by taking 10% of the total wattage of the motor. Now the point here is not how closely we can get back to the original number we were expecting. You may try this on a motor and your results may be significantly different.

Conclusion

The point here is that motors running at higher voltage will have a higher internal winding resistance. The higher resistance results in significant waste heat. Most would expect the higher current running through the first motor would result in a lot of waste heat. However, it’s not as bad as the motor with a much higher kv has the  While on the other hand, the high no load current of motor one running at a lower voltage still produces wasted power leading to heat.

Lithium based batteries are everywhere in RC. The performance that we are able to get out of these packs are incredible compared with all past battery technologies. Within the last few years a new style of LiPo based battery has emerged. It is known as the Lithium Polymer High Voltage battery pack. The pack is commonly referenced as LiHV,  identifying that it is a high voltage based lithium battery.

High Voltage Battery Pack

LiHV Battery Pack Specifications

Lithium high voltage batteries have a higher nominal and peak cell voltage.  LiHV per cell peaks at 4.35 volts where a typical LiPo battery has a peak voltage of 4.20 volts. The nominal voltage of a LiHV battery is 3.8 volts whereas the nominal voltage for a typical LiPo is at 3.7 volts.

Voltage cut off for a LiHV battery pack is the same as a standard LiPo battery pack. The absolute minimum voltage that a cell should get to is 3.2 volts. However, in practice the battery should not be discharged past 80% of its total capacity, Discharging a LiHV further will degrade the lifespan of the pack.

In order to charge a LiHV cell, your charger must have the capability to do so. Not all chargers are capable of charging to 4.35 volts per cell. This is a must in order to receive any benefit from a high voltage Lithium battery pack. Under no circumstance should you ever attempt to charge a standard LiPo battery pack to 4.35v per cell.

Application of LiHV Battery Packs

LiHV battery can virtually be used in any RC application. There are a few notes to consider about its applications, however. The batteries do only increase the amount of voltage by a small amount. The amount actually only ends up being about 3.5% when looking at the peak voltage. However, this is not the only amount of performance that you will end up seeing with these battery packs. The other side of the coin is the current. When you are requesting the motor to spin faster RPM’s due to the increase of voltage, the result will be an increase of current. Overall you may expect a performance increase of around 8-10 percent on average.

LiHV are not going to work for every setup

A setup designed around a standard LiPo battery pack may not have the headroom for increased performance.  There are a handful of stock setups running near maximum capacity. If the setup is already pushing the limits it’s not a good idea to use the pack in your application. A good quality RC product should always have enough headroom where LiHV won’t over tax the system.

If you plan to use LiHV to grab an extra 10% more power, just be certain that your system can withstand the increase in performance. More heat buildup in the motor and ESC is a certain.

Are LiHV Batteries Worth it?

Let’s get to the point of the article. There’s no doubt about it. LiHV batteries offer better performance from a voltage standpoint in comparison to standard LiPo batteries. The voltage difference is subtle but adds up very quickly as you increase in cell cell count. Increasing voltage can certainly lead to an increase in performance by raising the total amount of output  motor RPM.

Increase RPM’s by LiHV or just a higher Kv?

I personally setup all of my RC’s specifically to the voltage that I plan to run. If I want more RPM’s out of the motor, I simply increase the kv of the motor. When I looked at HV batteries for a specific setup, my decision was to just get a new motor with a slightly higher Kv rating. I know that the motor will easily outlast the lifespan of the battery pack.(Typically 3 years) I didn’t want to have a dedicated battery for just one RC vehicle. It would be different if all the battery packs I own were LiHV based. However, these packs are not mainstream. (Yet) It’s certainly possible that the market can turn and LiHV become more mainstream.

Proven Performance

LiHV have not yet proven themselves in any RC racing league. In fact, there are many clubs and racing leagues that strictly prohibit their use.

I haven’t been able to put the HV packs truly to the test. However, reading up on some test data has shown me that HV packs haven’t yet proven themselves well enough in the performance department to warrant their use.

In conclusion I just don’t feel that LiHV are worth their value today for me. Perhaps in time things will change. This conclusion is not for everyone. I’m sure there are setups out there that will be able to squeeze a bit more power out and this is the only way to do it efficiently.

This may not be all that common, however, brushless motors can fail. There are many people out there that have seen this happen first hand.  What makes them fail electrically really only comes down to two parameters. Voltage and Current. In this article we will break down each parameter.

Does a Motor Fail Electrically due to Voltage

Short answer here is no, voltage is not responsible for electrically failing a brushless motor.

One parameter that every brushless motor will have is a kv value. A motors kv value is the relationship between voltage and total output RPM. The kv value and the voltage that the motor will run on determines the total amount of unloaded RPM that the motor outputs. RPM = kv x voltage. If you don’t know the kv of your brushless motor, this value can be calculated. Increasing the voltage of the motor will simply increase the total amount of output RPM. We do know that this of course can lead to mechanical failure if the bearings max RPM or the rotors maximum rotation speeds are exceeded. Keep in mind that we are looking at electrical failure here and not mechanical.

Increasing Motor Voltage

Assuming that the rotor or bearings will not explode, what happens if we continuously increase the voltage?In radio control modelling, voltage does fall within a specific domain. We have the ability to use anything from 1 cell LiPo up to 12 cell LiPo based on most ESC that are readily available.  As voltage is increased, the potential for the voltage to jump an insulated gap increases. This is why wire is rated to specific voltages. Luckily for us, the voltage in our RC domain do not come close to becoming an issue. The increase in voltage does increase the total RPM output but this will not destroy a brushless motor.

Does a Motor Fail Electrically due to Current

The current a brushless motor will see, is directly related to the load. As the load of a motor increases the amount of current also increases. Current is what contributes to heat. As current increases the heat built up in the motor also increases.

Brushless Motor Waste Energy

The windings that make up the stator on a brushless motor have a resistance. The motor winding resistance is typically provided by the motor manufacture but can also be determined. The current flowing through a brushless motor can be measured by using an onboard logging device. Many ESC’s have the logging capability built right in.  The equation for waste heat generated by a brushless motor is equal to the current x current x resistance. Yes current is multiplying through twice. You can really see how important current is to the waste heat generated by a motor.

All of this waste heat contributes to brushless motor failure. Heat built up in the windings can get so hot that the rotor magnet is demagnetized. When demagnetization occurs, the kv of the motor increases leading to the motor trying to hit higher RPM’s. And of course these leads to a higher current draw. When enough heat in the windings builds up, the enamel coating on the windings burn away. Once this starts to occur, the motor is freshly cooked.

Wait! Increasing Voltage Killed my Motor

If you increased the voltage of your RC vehicle and experienced a motor failure. I can assure you that it was not due to voltage that your motor electrically failed. What also must be considered is the load applied with a voltage increase. For example, on an RC car, increasing the voltage and keeping everything else equal will in fact increase the load of the motor as well. This occurs as you are demanding the motor turn the same load but at higher RPM’s. This will certainly only place more strain on your motor. This increased motor load is directly related to increased current. Current is what kills the motor, don’t be fooled.

Conclusion – What Kills a Brushless Motor

Current is what ultimately leads to the destruction of a brushless motor. Increasing the amount of current a brushless motor is consuming comes with a cost. The cost is heat. Too much heat in your motor will lead to failure of the motor windings. Don’t let the magic smoke out of your motor, otherwise your motor is toast!

RC boats and EDF jets come to mind when it comes to the top performance vehicles in RC and the most demanding on it’s components. Pulling 120A+ reliably is not as easy as plug and play. Especially if you are operating an RC that you had built. Even ready to run or fly RC’s have “watch outs” to them that will help increase reliability. Keep in mind we are talking about 120 amps continuous. Let’s look at what we can do to make certain we maintain maximum reliability in these high power demand systems.

RC LiPo battery vs High Demanding Power Systems

Every single RC vehicle (120A+) that is sold to my knowledge, does not provide a battery pack included. These are purchased separately. It is critical to the completion of the RC, to purchase a battery that can handle the load. Pulling 120A+ reliably on any battery pack is a significant load.

Generally speaking, a high demand power system that is capable of pulling 120A or more continuously will have a larger battery pack. The battery packs tend to be in the range of 4000mAh to 5000mAh in capacity. These higher capacities increase the continuous current draw of the pack in our favour. Picking the correct C rating of the battery pack to give you lots of head room will increase the reliability. For example, a 4000mAh battery with a 120A continuous current draw, selecting a 45C pack or better is ideal. A 45C battery pack will provide 50% head room in terms of the loading. The maximum continuous current draw for the battery pack is 180A. Visit the battery C rating page for more information on battery packs.

LiPo Battery Run Time

A critical component to maximum system reliability is limiting your run time correctly. A lithium polymer battery is best kept above a capacity of 20%. You can read more on maximizing the lifespan of the LiPo here. As the battery’s capacity drops less than 20%, the amount of heat that builds in the pack increases. Heat build up in the battery pack can permanently damage the pack. Avoid discharging in to this critical zone to maintain reliability.

Maintain ESC Reliability on High Powered RC’s

If you are selecting an ESC for a build, selecting an ESC is actually quite easy. Just make sure you select an ESC that can handle the correct cell count (obvious point here) and the current draw. Selecting an ESC and getting the cell count incorrect will leave you with a dead ESC immediately on power up. However, selecting an ESC incorrectly for the load that you place on it may not be immediate destruction. You may be flying your EDF jet at 100mph when all of a sudden on the 4th flight the motor shuts down when you are 600ft away and you can’t see it to land safely.  This results in a near total loss.

Even after running the calculations and doing all the research you could make a mistake. Grab a data logger and record your first run. Understand what kind of loading is placed on your power system and then compare against specifications. You never know when something might be binding and is not immediately figured out on the bench. This will show up on the meter.

ESC Temperatures vs Reliability

It doesn’t matter if you have selected an ESC that can handle 10 000A for your 100A load and the ESC is running too hot. Temperature is the ultimate killer of electrical components. Review the maximum continuous temperature that your ESC should reach in the instruction manual that came with it. Tweak your setup to be certain you can run your setup as cool as possible. Temperature is another parameter that can be logged.

Can a weak LiPo place more load on your ESC, Destroying it?

The short answer is, yes it can.  Drawing over 120 amps of power is quite significant for any power system in RC. A weak battery pack will create a more significant voltage drop for every pulse of power that is sent from the ESC to the motor. The ESC in the system sends these pulses at a high frequency. Each pulse that is sent to the motor results in the voltage sagging as the pack is under load. The amount of voltage that the battery pack sags, is dependent on the batteries specification and condition. It is this voltage difference that is known as ripple voltage within the ESC. Significant ripple voltage will place extra strain on the ESC. This strain can lead to failure within the ESC. On high powered radio controlled models, be certain to use a healthy battery pack.

Maintain Motor Reliability on High Powered RC’s

Very similar to the above, maintaining reliability on the motor is very much related to the temperature. It is best to keep motor temperatures below 60C or 140F for optimal reliability. If you are selecting a motor for your own build, you will want to size it correctly to be able to handle the load. Larger motors that can dissipate more heat will help reduce the running temperature.

Understanding how power is created in an RC power system will allow you to build or operate a more reliable power system. Power is a difficult parameter for us to understand as we can not see it or hear it. The only way we can determine its presence is by measuring it. To answer our question, does a brushless motor pull power from the ESC, let’s first look at the task of the ESC.

Brushless Motor Draws Power from ESC or the other way around?

How the Motor receives Power from the ESC

When we are operating our radio control vehicle, we use a radio to communicate with the model. Upon a control input to the radio, the receiver onboard the model sends the appropriate signal out to the ESC. When the ESC receives a signal to start the motor, it sends a pulse to the motor from the battery pack. The battery pack would be of a specific voltage and the ESC sends this to the motor. The pulse sent would be at a voltage and a specific frequency. The frequency is dependent on the RPM depending on the amount of throttle input provided. The voltage is dependant on the main battery pack.

Is the Brushless Motors Responsibility to Pull Power?

Firstly we must assume that we have a brushless motor hooked up to a power system and there is no load applied. Let’s also assume that the brushless motor does not consume power to rotate at constant speed. Newton’s First Law of motion states that an object in motion will stay in motion provided there are no external forces acting on it. Reality tells us that life is never 100% efficient hence in reality the motor would indeed have  a load applied. For simplicity of explanation, let’s ignore this.

Here is where it gets interesting. The mechanical load applied to the power system is directly related to the current drawn from the battery pack. If we have a 4 cell LiPo battery pack operating a motor at a constant speed of 1000 RPM, and zero load, that motor will draw 0 amps.  This equates to a 4 cell LiPo, ( 14.8v x 0 amps = 0 watts ) producing 0 watts of power. As we begin to place a load on the motor, current changes.

Current Drawn vs Brushless Motor Load

The load placed on the motor forces the motor to draw current. As the load on the motor increases, we can assume the current the motor requires to turn the load increases.  The motor indeed pulls power from the battery pack. Power is equivalent to the wattage that the motor is running at. Wattage is the product of voltage and current. As we load the motor, the motor will draw more current resulting in an increase of the total amount of power drawn.

https://youtu.be/LTlVWYSexBI

Conclusion

Knowing that the motor is ultimately responsible for consuming power in an electric RC model allows us to better select power system.  Many DIY power systems that fail are related to this exact thing! Modelers select a power system that draws too much current which can ultimately destroy a power system component. If current is directly related to the load on the motor, simply reducing the load can save our power systems. Keep this in mind when you build and select your next power system.