Firstly, I have to state that cogging is actually not the term to use here. However, since this term is commonly used within the RC community, I want to be sure that all can still find the improvements to the motor hesitation that we will be covering.
For a deeper understanding as to what cogging is, visit the cogging explained page. That leads us to our next question, if the brushless motor is experiencing hesitation on startup or acceleration from 0 speed, what is this called?
Typically this is referred to as the syncing process between the ESC and motor at extremely low speeds. Once the motor is in sync with the ESC, this issue is resolved entirely until the next time the motor comes to a stop or enters extremely slow speeds.
How a Sensorless System Works
A sensorless motor is exactly as it sounds. It is a system that does not use sensors to understand what position the rotor of the motor. In other words the ESC has no information as to the position of the rotor.
The ESC figures it out by first sending power to the motor, rotating the rotor slightly. While the rotor of the motor rotates, what is known as Back EMF is produced. This back EMF is read by the ESC. Back EMF is just a fancy term for voltage fed back to the ESC. This process of power sent to the motor and then back EMF sent to the ESC happens several times over a short period of time until the ESC and motor are fully in sync. During this process is where you would experience motor hesitation, chugging, poor synchronization, unsmooth operation or whatever you’d like to call it. Now we will figure out how to improve it or even completely remove it if you’d like to go that far.
It’s important to note that this occurs most commonly in RC land vehicles.
How to Completely Eliminate ESC/Motor Sync Issues
Switching to a sensored system will be able to completely eliminate hesitation within your setup. A sensored motor will be able to tell the ESC the exact position that the motor is in. Understanding position, the ESC will send power at exactly the correct time it’s required.
Does this mean that you need to switch all of your setups to a sensored system? My answer, is simply no. 9/10 of the radio controlled RC’s I have use a sensorless system. There would be very little point to run a brushless boat, plane or helicopter with a sensored system. It would be added weight and complexity for not much benefit. However, if your RC vehicle happens to be a rock crawler, then a sensored system is nearly required. Just think about how often the motor on a rock crawler would be close to 0 RPM with quite a high load to overcome.
Improving Startup Hesitation
Inspect for Binding / Unnecessary Load
Inspect gear train to be certain there is no binding or unnecessary load in the drivetrain.
This is very easy to do and well worth the check. If you find areas of binding, removing this will improve the amount of start up hesitation that your ESC/Motor combination experience. Areas to check are around dogbones, CV joints, and meshing gears.
The reason why this is important is that any additional load the motor has to rotate will make it more difficult for the ESC and motor to sync up.
LiPo Battery Discharge Strength – Improve Motor Hesitation
Be certain the battery that you are using can supply the current required for a brushless motor. The startup synchronization process takes a lot of power to initially get the motor in Sync with the ESC. Weaker Battery packs will struggle to supply the necessary power required for startup.
NiMh batteries contribute to ESC / motor synchronization problems as they generally can not deliver the power required to get moving effectively. In this case swap them out for a more reliable LiPo battery.
3 Wire Connection from the Motor to ESC
Ensure there is a good connection between the 3 wires from the motor to ESC. A good strong connection is an absolute must. As mentioned above, there is quite a bit of power that must be consumed in order to get the motor started up and in sync with the ESC. Having good connections on all 3 motor to ESC wires will allow transfer of power at the least amount of resistance. Lowest resistance means the motor receives maximum power to get going!
Gear Ratio – Improve Motor Hesitation
The gear ratio is important for hesitation within the motor at startup. Having a higher gear ratio increases the load on the motor at all speeds. What is most critical is at 0 RPM. High Gearing can make motor hesitation much more exaggerated. This may be especially true for the guys who are doing speed runs well over 160km/h. (100mph) With such high gearing to achieve these speeds, motor hesitation during startup may be significant.
Often times with my high speed RC Car, I will give it a shove to assist off the line. Once moving I engage power and the synchronization of the motor/ESC is nearly instant. Dropping a few teeth on your motors pinion gear, thus, moving to a lower gear ratio will help with hesitation.
Switching to a Motor with More Poles
In general, motors with more poles at the same kv tend to (but not always) have a smoother startup. This is due to having more ESC / Motor switching steps for every one revolution of the motor. If you have the opportunity to move up to a good quality motor with a higher pole count, try it out.