Sensored vs Sensorless Brushless Motor Applications

How a Sensored vs Sensorless Brushless Motor Works

A brushless motor receives power from the electronic speed control. (ESC) The speed control must know the position of the rotor in order to accelerate the motor from zero RPM smoothly. When it does not know the position of the rotor, the ESC must determine it. The ESC is able to determine rotor position by sending power to the motor and receiving back EMF (Electromotive Force – this really just means a voltage “signal” is produced and sent back to the ESC) as the rotor is in motion. It is this feedback loop that allows the ESC to understand the position of the rotor.

If you believe the term for the synchronization process of a brushless motor is cogging, check out the article on cogging.

In a sensored brushless motor, as the name implies, the motor contains sensors within it. These sensors are responsible for sending information back to the ESC about the position of the rotor. When the ESC knows the position of the rotor, immediate synchronization is established right from zero RPM.

Sensored Brushless Motor Setup

Sensored Brushless Motor Setup

Advantages of a Sensored Brushless Motor

Sensored motors are able to accelerate smoothly and strongly right from zero RPM. They also are able to produce more torque at slow speeds in comparison to a sensored brushless motor. Sensored motors also are able to allow safe operation of the motor by monitoring its condition. In some ESC’s, the ESC will not operate if there is something wrong in the sensor signals. Another piece of information that is captured from sensored brushless motors is the motor temperature. This is great for data logging motor temperatures.

Advantages of a Sensorless Brushless motor

There seems to be a lot of advantages for a sensored motor, but sensorless motors have their place as well. Although sensorless brushless motors do not perform well at low speed, their performance at high speed is excellent. It is so good that most ESC manufactures rely on sensorless synchronization of a sensored motor at high speed. At high speed most ESC’s will alter the timing for the sequence of power pulses that are sent to the motors windings. In turn improving efficiency and performance.

Sensorless motors also are lighter in weight, less complicated and less prone to failure. It is quite easy to damage the thin wire loom that sends all the data from the sensored motor to the ESC.

Applications of Sensored and Sensorless Brushless Motors

The number one reason why you may require a sensored motor for your application is due to requiring torque at extremely low speeds. We are talking around zero RPM. Applications where there is a requirement for torque at very low speeds are RC Car based. They include Rock Crawling, or any type of very slow speed RC Car action.

Also, cars with high gearing that require immediate torque can be quite problematic at low speeds. They may consist of high speed on road cars or high speed RC drag racing cars. The low gearing in these RC vehicles requires the motor to develop a lot of torque from a start. A request for the motor to accelerate quickly from a stop can cause a great amount of power to be drawn. When the brushless motor is not in sync with the ESC, the reliability of quick, smooth acceleration is compromised. A sensored motor in these applications would help.

When it comes to other applications, the requirement for a sensored motor is low. For example, RC helicopter, boats and airplanes of all types work very well with a sensorless motor. Most of the operation of these motors in these applications are done at higher speed. One of the most important parts is even at 0 RPM, there is a very low torque requirement in getting the system going. A propeller at slow speed does not load the motor significantly enough to warrant a sensored motor. Dropping the requirement of added sensors and wire looms to a motor in these applications saves precious weight and complication. Ultimately, this may lead to increased reliability.

Sensored / Sensorless Brushless Hybrid ESC’s

A perfect example of this system is in my 1/8 scale brushless buggy. I am using a castle system that is capable of both sensored operation and sensorless operation. What ends up happening is during acceleration from a stop, the motor/ESC combination is in sync utilizing the sensors inside of the sensored brushless motor. Once a small amount of speed is gained, the ESC switches from using the sensors to using sensorless synchronization. Here you gain all of the advantages as described above. It is an excellent solution if you are looking for smooth operation from zero RPM and excellent high speed performance.

Can a Sensored Motor work with a Sensorless ESC

Taking a sensored motor and using it with a sensorless ESC will work with no problem. The motor will simply act as a sensorless motor since the ESC will figure out rotor synchronization based on back EMF generated. In other words, a sensorless ESC will operate a sensored motor as a sensorless motor.

For obvious reasons you can not take a sensored ESC and use it with a sensorless motor. If the sensored ESC has the ability to operate as a sensorless ESC, the motor will also be treated as above – sensorless synchronization.

 

Brushless Motor Cogging Explained – Not what you may think!

Brushless DC motors used in radio control applications contain windings just like every other electric motor. However, what is more unique about them verse an AC induction motor, is that they contain a permanent magnet. The permanent magnet in the brushless motor has the opportunity to interact with the make up of the winding section of the motor. This is where the cogging begins.

Cogging in Brushless Motors used in RC

Cogging in Brushless Motors used in RC

What is Cogging in a Brushless Motor

As mentioned above, we know cogging of a brushless motor starts with the interaction of the permanent magnet on a brushless motor and the windings. Looking more specifically at the winding section of the motor, cogging occurs in a slotted stator. A slotted stator is made up of thin steel lamination’s that are stacked up and the motor windings are placed in the created slots. It is these iron slots that create the cogging torque on brushless motors.

The permanent magnet as it is rotated by the motor shaft, passes by these slots. The magnet then interacts with the steel lamination’s creating that bump that you know of.  If you don’t know what I am referring to here, you can grab any slotted brushless motor and try it out. Grab a brushless motor and slowly rotate the shaft. Depending on the motor, you will feel the strength of the cogging torque applied by the interaction of the iron core and the permanent magnets. I have noticed that 6 Pole inrunners tend to have the strongest cogging torque.

Does Cogging Exist in a Sensored Motor?

Sensored motors have been designed as a result of sensorless motors not being able to determine rotor position. It is critical for the ESC to know exactly where the rotor is relative to the windings to accelerate the motor from a stop. As a result of a sensorless motor, the motor stutters on takeoff. It does seem that most would contribute this phenomenon to cogging. In fact this is not the definition of cogging. We described cogging as the result of the interaction between the slotted stator and permanent magnets in a motor. A sensored motor only adds a sensor to aid in determining the rotors position. Therefore cogging does in fact exist in a brushless sensored motor.

How Does Cogging Torque Effect Performance?

The only area in our RC applications where cogging torque can effect performance is in the startup from zero RPM. This is more than likely where confusion of the term cogging all began. In a sensorless application the ESC must overcome the additional torque caused from the magnetic field interacting with the stator slots.  The ESC must do this as it is figuring out the position of the rotor. As the ESC sends a pulse to the windings of the motor and moves the rotor as a result, the cogging torque applied changes the position of the rotor slightly. This slight positional change is something that the ESC must overcome and figure out. Most ESC’s have no problem overcoming the effect of cogging torque applied. Once the motor is rotating, cogging does not effect the performance any longer.

Can I see Cogging Torque acting on my RC

Yes, you can. If you have an RC car that is geared very high, it will be easier to pick out. (On Road Application) The easiest way that you would be able to see cogging in action is to get your RC car rolling under its own power and then let off the throttle completely. As the car begins to slow to a crawl you can hear cogging in action. You will only be able to hear it just before the car comes to a stop.

Another way to hear cogging is to push your RC car very slowly along the ground. The car should sound very smooth as you push it across the ground. However, where there is cogging present, you will hear rattling and backlash of the gears as positive and negative torque values are applied to the motor shaft as a result of cogging.

5 Ways to Maximize LiPo Battery Lifespan

LiPo batteries is one area of the RC hobby that can be fairly expensive. Especially if all the RC vehicles that you prefer to run, uses a different pack. The typical lifespan that we can expect from a LiPo battery pack can on average about 2 to 3 years.

For certain RC applications, the LiPo battery lifespan may very well be outside of this range. For example RC EDF jets or RC boats may have a shorter lifespan as these vehicles draw a lot of power from the pack. On the other hand trainer style RC airplanes, or scale RC boats pull significantly less power. These packs may last many years. For these reasons specifically, it is very important for us to maximize the battery lifespan so that we don’t need to be purchasing new batteries every year or two. That can be very expensive! Prolonging the life of your LiPo battery pack is not hard, follow these 5 ways:

Maximum Discharge Rate – LiPo Battery Lifespan

LiPo battery packs have a rating that allows one to understand the maximum amount of current the pack can discharge continuously. This is one of the most important specifications on a battery pack that you must consider for your application. The specification that we are referring to is known as the C rating. The C rating of a battery pack is explained further in this article.

In order to maximize the life of your battery, it is  important to be certain that you have a lot of head room in the C rating of your pack. My recommendation is to only draw 50% of the maximum continuous power output that your battery specifications specify. If you have the ability to reduce the percentage, indeed do so! The smaller the load factor of the battery pack, the more that you are able to prolong the life of the battery pack.

Maximum Discharge or Run Time – LiPo Battery Lifespan

Maximum discharge refers to the maximum amount of capacity that you should pull from your LiPo battery pack. The magic rule that helps prolong the lifespan of a LiPo is 80/20. The meaning behind this rule is that the maximum discharged capacity should be no more than 80%. This leaves a minimum capacity in the pack of 20%. Following this rule will help increase the lifespan of your battery pack.

Be certain that you know the maximum run time that you have available in your RC to not discharge more than 80% of the capacity. The resting voltage of a battery pack that doesn’t exceed 80% discharge is approximately 3.7v.

Storage of a LiPo Battery – LiPo Battery Lifespan

Storing a LiPo battery pack is defined as not using the battery pack for more than 10 days. If you don’t plan to use the battery pack in the next 10 days, it is highly recommended to place the battery pack in to storage mode. What does this mean? Well let’s go through it. To maximize the lifespan of the battery pack, it is critically important to store the pack correctly.

To store a LiPo battery pack correctly, many chargers on the market can help with this. However, if your charger does not have a charging mode specific to storing the pack, you can do this manually.  The voltage of the battery pack is a good indicator for an optimal storage specification. The recommended storage voltage for a LiPo battery pack is 3.80 to 3.85 volts. Battery capacity will be approximately 40-45% at the correct storage voltage.

Leaving your battery pack at maximum charge is one of the worst things you can do to the battery when you are not using it. At the same time leaving the battery near empty is also far less than ideal. Be certain to leave your battery pack at the storage voltage specification to maximize your LiPo battery lifespan.

Maximum LiPo Battery Temperature – LiPo Battery Lifespan

This factor that effects LiPo longevity is fairly straight forward. It doesn’t matter if you are or are not following any of the above tips if your LiPo is getting too hot. Like all electronic components, LiPo battery packs are no exemption. The amount of heat that your LiPo generates while being discharged is directly proportional to the lifespan of the pack. A pack that constantly gets hot will not last as long as a pack that stays cooler.

The maximum recommended temperature for a LiPo battery pack while in use is 60C or 140F. This is an absolute maximum. You can be certain that if your LiPo is achieving this temperature every run that you are degrading the lifespan. To maximize lifespan, decrease the amount of heat that your battery pack builds. You can do this by reducing the load of the battery pack. To reduce load, increase the amount of LiPo battery packs being used by wiring them in parallel. Or decrease the amount of load on your battery by reducing prop size or gearing.

Battery Charge Rate / Peak Voltage – LiPo Battery Lifespan

Battery packs being sold on the market today are advertising 5C and upwards of even 10+C charge rates! These are crazy! I believe anything more than 3C is crazy. 3C charging is very fast. What I have found is the most optimal battery charge rate when considering both quick charge and lifespan is 2C. Within about 30 minutes you can have a pack fully charged and you are not pushing the pack. You can be more conservative towards battery life by charging at a 1C charge rate.

The maximum specified fully charged voltage of a LiPo battery pack is 4.20 volts. Charging to this voltage actually degrades the lifespan of the LiPo battery. If you are to charge the battery pack up to a maximum below the 4.20v mark, you can increase the lifespan of the pack. Tesla actually uses this method to increase the lifespan of their lithium based batteries. What I do is take the battery pack off the charger as I see the pack starting to ramp down in current closer to end of charge. If your charger allows, you can also set the maximum peak voltage of a LiPo. Setting the maximum charge voltage to something less than 4.20 volts will prolong the lifespan. Setting anywhere from 4.10 to 4.18v will help, and obviously the lower you go, the longer the lifespan potential you will see.

Unfortunately everything in life is a trade off. Trading run time for battery life is one thing that we can do to increase the lifespan of our batteries. Ultimately, you get to decide if this is worth it.

All about RC Brushless Motor Windings

Every Brushless motor has a series of windings that are an essential part of its operation. The windings in a brushless motor are considered a core item withing the motor. The purpose of the motor windings is to produce a magnetic field controlled by an electronic speed control to interact with the permanent magnets causing the motor shaft to rotate.

Number of Turns on a Brushless Motor

When we refer to a winding, we are talking about a motor turn.  This is where a turn represents the wire that is wrapped in full loop along the direction of the permanent magnet. A manufacture that sells electric motors, typically has a range of windings available for purchase. The availability of different windings or turns is what allows RC hobbyists to precisely select the best for their application.

Number of Turns vs Performance

We will investigate turn performance by stating that all other motor parameters are unchanged. As the number of turns increases within a motor, the Kv of the motor decreases. One can assume that the amount of current that the motor may consume decreases and the torque potential of the motor increases. A lower Kv allows a higher voltage to be used in order to maintain the same amount of overall output power. (watts)

The exact opposite is true when the number of turns is decreased.

How to select the best Turn Count

In our RC application, knowing the amount of turns is not all that important relative to the performance of a motor. However, understanding the amount of Kv that a motor has to offer is very important. The Kv value of a motor is the value that we must be using to select the correct motor.

A turn does not always consist of One Wire Strand

This is an important point to make as many months ago, I received a question. That question asked why someone’s one turn motor appears to have a million wires in it. How could it possibly be a one turn motor. Motor manufactures will typically wind a motor with not just one strand of a particular gauge of magnetic wire. What they will do is use a smaller wire consisting of a higher gauge number in order to create one turn.

Using smaller wire to create One winding turn

Using smaller wire to create One winding turn

The primary reason this is done, is that you can more effectively pack smaller wire in a tighter space vs having larger wire. A very important part of winding a brushless motor is making certain that you can pack in the most amount of copper as possible. Doing so will increase the efficiency of the motor. These smaller diameter strands of wire will allow the windings to be packed very tight decreasing the amount of voids in the windings.  Take a look at this motor below. This is a 2.5 Turn motor. It looks like there is a lot more than 2.5 wires. In fact you can nearly see the groupings of each turn and how they are weaved.

Only a 2.5 Turn on a Brushless Inrunner

Only a 2.5 Turn on a Brushless Inrunner

Slotted vs Slotless Stator in a Brushless Motor

A slotted motor is a motor containing thin steel lamination that are stacked together. Copper windings are placed in the open slots and wrapped around the steel lamination’s. In a slotless motor, the windings are placed in to position and must be self supporting. There is no structure to help the placement of the windings in a slotless motor.

Slotted Brushless Motor

Slotted Brushless Motor

Advantages of a Slotted Motor

  1. Creates stronger magnetic fields resulting in higher torque
  2. Lower Kv motors
  3. Windings do not need to be self supporting
  4. Inexpensive to produce compared to Slotless motors
Slotless Brushless Motor

Slotless Brushless Motor

Advantages of a Slotless Motor

  1. No cogging torque caused by the permanent magnets lining up with the iron lamination’s.
  2. Best sensor-less low speed operation
  3. Reduced  core losses at high speed operation

Difference between Wye and Delta Winds

There are two wiring patterns to terminate the windings on a brushless motor. One termination method is known as the Wye wind. The other termination method is known as the Delta wind. In a Wye wind, each motor connector is attached to the start of a phase. All three legs or loads are terminated together as below in the top diagram. In a delta wiring pattern, each motor phase is connected to two brushless motor connectors. Refer to the bottom diagram below.

Kv of a Delta Wound Motor vs Wye Wound motor

In general terms, a Delta wound motor would suggest the motor is of a hotter wind. This is of course when all else is equal. Kv of a Delta wound motor would be the square root of 3 or 1.732 times higher than a Wye wound motor. This is the most significant characteristic difference between the two terminations types. You can expect the current load of a Delta wound motor vs Wye wound motor when all else is equal, to be 1.732 times higher. This is exactly what is meant by a “hotter” wind. The motor is going to be more power hungry delivering a higher Kv.

Sensitivity of “hot” Delta Wound Motors

In my own personal experience, I have noticed that the electronic speed controls that I use are less sensitive to changes on a Wye wound motor. I have noticed this primarily when adjust the timing of the motor. Increasing the timing on a Delta wound motor can result in a lot more heat build up within the motor. I have noticed this more commonly with very hot setups running inrunner motors. More commonly 1 turn Delta wound motors.

Termination point of Motor Winds

In some motors, the wire termination point exists where the three leads exit the motor. It’s important to not cut or shorten these wires that are outside the motor case. in other words, doing so may cause your motor to become useless.

My Wind Preference

I don’t have a significant preference when it comes down to the type of motor wind for an application. I am more interested in finding the correct Kv for my application. However, with this being said, I am sensitive to selecting a Delta wind for a high performance race type application. I’ve had a greater amount of luck being able to dial in performance with a Wye wind. Just keep in mind this is my personal experience and there are many people out there who experience excellent race results using a Delta wound motor.

Wye vs Delta Winding Diagram

Wye vs Delta Winding Diagram (Delta on Bottom)

How to make your RC Car Faster

If you are anything like me, you are probably wondering what you can do to make your RC car faster. Then you complete your first modification and run the RC car for a few days. That is the point in time where you wonder can I make it go even faster. This process never gets old, it only gets more addicting! Let’s look at what you can do to increase the top speed of the average RC Car. In each case below you can use the RC Car Calculator to estimate the top speed that you can expect to see with your modification. Video Here.

Increase top Speed of your RC Car

Increase top Speed of your RC Car

No More Brushed Motors – Making Your RC Car Faster

OK, I had to get this one out of the way. If your RC car has a brushed motor, your RC Car is way too slow. Replace the brushed motor with a brushless motor. Doing this will increase the speed that you were use to seeing with your RC car drastically. To find the perfect brushless motor for your RC car, my suggestion is to start researching what others are using for that specific model. Knowing that others are using the same brushless power system is one way to make certain that you are picking a reliable, proven power system to accomplish your goals.

Change the Gear Ratio – Making Your RC Car Faster

Changing up the gear ratio on your RC car is one of the easiest things you can do to increase top speed.  You have a choice between changing the motor pinion gear or the mating spur gear. The third option is that you can actually change both gears and get twice the benefit.

To increase top speed, the most common place to start is with the pinion gear. The goal is to decrease the overall gear ratio. For every turn the motor completes, we require an increase in turns that the wheels have to make. To decrease the gear ratio, we must increase the number of teeth on the Pinion Gear. To decrease the gear ratio by changing the spur gear, we would decrease the tooth count of the spur gear.

If you were to only increase, for example, the size of the pinion gear, you would eventually hit a diameter issue. There is only so much room that your RC Car will allow adjustment for. In this case you must decrease the size of the spur gear in order to make room to increase the pinion gear size. Changing both the spur gear and the pinion gear together can make a big difference in terms of the speed potential.

Increase the Cell Count – Making Your RC Car Faster

Increasing the LiPo cell count is another way that you may increase the top speed of the RC car. If your car is running on a 2 cell LiPo, moving up to a 3 cell LiPo will increase the speed significantly. In theory increasing cell count from 2 to 3 will increase speed by approximately 50%. This simple modification can make the biggest difference with the least amount of changes required. However, increasing the cell count is not always possible depending on the power system of your RC Car.

You must only do this if you know that your ESC can handle the additional voltage and amperage. The amount of current consumed by the motor is directly related to the amount of load it must overcome. Increasing the cell count, increases the expected amount of total RPM of the motor. More RPM translates in to more load. An increase in load on the ESC could destroy your ESC (and more) if it was not designed for it. Check those specs!

Increase the Motor Kv – Making Your RC Car Faster

This may be the most complex type of modification in order to squeeze more speed out of your RC car. Increasing the motor Kv essentially is an ask to swap the brushless motor for a different brushless motor that is wound with a higher Kv.

If your current setup before modifications was pushing the motor very hard, moving up to a larger motor will help. You will know if the motor was pushed very hard depending on the temperature that it reached. The next step is to find an appropriate sized motor with a higher kv value. Here again, it is best to hop on a forum and find out what someone else has been running in order to be certain the setup that you choose will remain reliable. Increasing kv is a great way to increase the speed of your rc car. Just make certain that you understand the setup well and it’s ability to maintain reliability.

Change Tire Diameter – Making Your RC Car Faster

Changing your tires on the RC car is yet another way that you can gain speed. When we are trying to increase the speed of our RC car, every little bit makes a difference to add up to the new top speed. In order to increase speed with a tire change, you must look at the diameter. Note what the diameter of the tire is on your RC car. Increasing the diameter of the tire is increasing the amount the rollout of a tire. Rollout is essentially the tire circumference. If the motor rotates the wheel 1 turn, the car will travel further when the circumference of the tire is increased. The RC Car Calculator can also help you out in determining the difference between your tire diameter selections.

How to maintain a reliable power system

It’s fun and all modifying our RC cars until the unthinkable happens. That’s when one of the components of your power system decides to just say NO to your modification. You will know it’s too late when that component is smoldering with smoke pouring out of your RC Car. Demanding more power from your power system is what you are doing when trying to achieve a higher top speed. More power mean more heat to the electrical components. Pushing them too hard and they will overheat and ignite.

To prevent this rather unfortunate situation, make certain to monitor the temperature of the motor, ESC, and batteries. I would recommend not exceeding 60C or 140F in temperature for the motor and batteries. My recommendation for the ESC is 70C or 160F. Refer to the owners manual of the particular ESC for exact specifics. Being certain that you are not overheating the components will reduce your risk.

Good Luck