How Brishless Motors and ESCs work

[Si Goodchild]

Guys,

I've been here for about a year now and am loving my heli flying and am starting to get in to EDF Jets. My understanding of brushless motors and ESCs really comes from listening to you guys so I thought it was about time to check my understanding before I build or buy any more kit to throw in to a jet or heli. Can you please confirm that the following is correct:

• The speed that a motor turns at is a function of voltage delivered and KV rating so a 3000KV motor with 10v will turn over at 30,000rpm
• In doing so, the motor will draw the current it needs to sustain that speed depending on the load (within reason)
• The ESC will deliver whatever current is required up to its rating to sustain the speed that you set via the throttle.
• Provided that the ESC and motor are rated for the voltage you are using, there are a number of ways to achieve the same power requirement: eg to achieve 2000rpm headspeed in a hover with 3.5 Kg TREX 600E, you could use a low voltage (6s) with a high kv-rated motor and (for example) 10:1 gearing. Equally you could use a high voltage (12s or 10s) with the same gearing and a lower kv rated motor. In the latter example the amps pulled will be less so less MAH batteries will be required, in the former example you'll need more MAH to deliver the current.
• If all this is right, then to achieve my 2000rpm headspeed at 10:1 I'll need 20,000rpm motor speed. I could do this by using a 2000kv motor at 10 volts (about half power at 6s) or a 1000kv motor at 20 volts (about half power at 12s)
• However, I could also run a 12s set up with the 2000KV motor provide I changed the gearing to 5:1. (assuming that motor is happy turning at that crazy speed!)
• So, with the same overall performance, I could run a TREX 600E with a pair of 6s 2800mah batteries in two entirely different ways. Either put the batteries in series to work as 12s 2800mah or put them in parallel to work as a 6s 5600mah. (I accept that the 6s solution is easier to implement on a standard TREX600E as a 12s solution will require new components. I'm just trying to understand the maths here)
• Or, using a TREX 500 as an example, a 6s 3000mah setup will have similar endurance and performance to a 5s 3600mah setup provided that the gearing is optimised. (Volts * mah the same in both cases)
• And now a bit on ESCs: an ESC will deliver between 0% and 100% of the LIPO voltage available. So when the battery starts to fade, the max rpm of your motor will drop too. So in crude terms (provided the ESC is rated for the voltage) 50% stick when working on 3s gives out about 5.5 volts (11.1 volts on 6s) and at 100% 11.1 volts (22.2v on 6s). All voltages are nominal. (I've seen a video where an EDF jet model was flown on 4s even though the motor and ESC were only rated for 3s. The extra voltage gave brilliant take-off performance but risked a burn-out if used excessively. My theory is that provided the throttle was kept below 3/4 then you could run like this indefinately because the voltage would be the same as 3s)
• I can fit a massive rated ESC (say 200A) in to a TREX 450 (usually 30-45A) with no fear because it won't "push" amps, the motor will "pull" them. The bigger ESC is only a problem in terms of weight and cost.

Guys, sorry for the long post, but this is crucial stuff and I really want to learn how to build my own set-ups so I'd also be grateful for any links to good websites or guides. Any corrections to the above will also be appreciated as I know there are some really technically-minded dudes out there.

Many thanks,

Si

[unwind-protect]

All AIUI:

The speed that a motor turns at is a function of voltage delivered and KV rating so a 3000KV motor with 10v will turn over at 30,000rpm

Kind of. My understanding is it's actually calculated in reverse (the voltage generated by turning the motor at x rpm), as this has fewer variables.

In doing so, the motor will draw the current it needs to sustain that speed depending on the load (within reason)

Very approximately, yes. As the load increases, the motor slows and the back voltages the motor generates drops, allowing more power into the motor. Additionally, the pack voltage will "sag" under load, further reducing the theoretical top speed. Also, an ESC can only switch windings so fast, limiting the top speed.

The ESC will deliver whatever current is required up to its rating to sustain the speed that you set via the throttle.

Not quite. The esc will let through pretty much whatever the motor demands, but above its rating it will quickly overheat and die.

Provided that the ESC and motor are rated for the voltage you are using, there are a number of ways to achieve the same power requirement: eg to achieve 2000rpm headspeed in a hover with 3.5 Kg TREX 600E, you could use a low voltage (6s) with a high kv-rated motor and (for example) 10:1 gearing. Equally you could use a high voltage (12s or 10s) with the same gearing and a lower kv rated motor. In the latter example the amps pulled will be less so less MAH batteries will be required, in the former example you'll need more MAH to deliver the current.

Yes. AIUI there are small efficiency gains to be had from the higher voltage setups, though.

If all this is right, then to achieve my 2000rpm headspeed at 10:1 I'll need 20,000rpm motor speed. I could do this by using a 2000kv motor at 10 volts (about half power at 6s) or a 1000kv motor at 20 volts (about half power at 12s) (various other setups elided)

That seems correct.

And now a bit on ESCs: an ESC will deliver between 0% and 100% of the LIPO voltage available. So when the battery starts to fade, the max rpm of your motor will drop too. So in crude terms (provided the ESC is rated for the voltage) 50% stick when working on 3s gives out about 5.5 volts (11.1 volts on 6s) and at 100% 11.1 volts (22.2v on 6s).

Note quite. The ESC gives the appearance of only delivering some of the voltage by switching a winding on and off very quickly. If it's only on 50% of the time that it "could" be on, then it only applies 50% (or is it 25%?) of the force it would if on all the time. However, this process is less efficient than switching the winding on 100% of the time, for reasons I don't really understand, which is why it's advised to run at close to 100% throttle when possible.

(I've seen a video where an EDF jet model was flown on 4s even though the motor and ESC were only rated for 3s. The extra voltage gave brilliant take-off performance but risked a burn-out if used excessively. My theory is that provided the throttle was kept below 3/4 then you could run like this indefinately because the voltage would be the same as 3s)

Don't see why not, though I have read of ESCs overheating when used extensively at low throttle settings as it's acutally harder on them than staying "open" the whole time.

I can fit a massive rated ESC (say 200A) in to a TREX 450 (usually 30-45A) with no fear because it won't "push" amps, the motor will "pull" them. The bigger ESC is only a problem in terms of weight and cost.

Yes.

[Si Goodchild]

Top man. Many thanks for taking the time to read and respond.

Si

[Gareth-71]

Hi

This is right up my street as I have designed and produced sensor less BLDC motor controllers (ESC’s) and therefore have a pretty good understanding on how they work.

These simple three phase DC motors are used for a number of reasons the important ones are cost to produce, simplicity of design, efficiency and ruggedness.
The down side is driving complexity and they can get very complex.

Basically, the controller needs to know the exact position of the stator, as these motors have no sensors on them to give position, the controller uses the reverse emf (the voltage created when a magnet passes a winding) to determine position – very clever and very tricky to do.
Up to about 100RPM, this voltage is not large enough to detect, so for startup, ALL ESC’s startup up in a dumb mode – it just outputs drive signals to the motor without knowing the position at all, this is why some esc’s have a jerky startup.

Anyway, your questions have already been answered very well, so I will just add a bit on each of your points:

· The speed that a motor turns at is a function of voltage delivered and KV rating so a 3000KV motor with 10v will turn over at 30,000rpm Si

In an ideal world, yes - and they do run pretty close.

· In doing so, the motor will draw the current it needs to sustain that speed depending on the load (within reason)

Yes.

· The ESC will deliver whatever current is required up to its rating to sustain the speed that you set via the throttle.

Uh! well yes, but in a non linear way. This has to do with efficiency, inductance and many other more technical parameters.

· Provided that the ESC and motor are rated for the voltage you are using, there are a number of ways to achieve the same power requirement: eg to achieve 2000rpm headspeed in a hover with 3.5 Kg TREX 600E, you could use a low voltage (6s) with a high kv-rated motor and (for example) 10:1 gearing. Equally you could use a high voltage (12s or 10s) with the same gearing and a lower kv rated motor. In the latter example the amps pulled will be less so less MAH batteries will be required, in the former example you'll need more MAH to deliver the current.

Again, in an ideal world, yes. A few simple notes on this:
1. A motor typically does not have a maximum voltage rating (well there is, but for very different reasons and almost much greater than any ESC)
2. The real advantage (as dictated by Ohms law), is that for the same POWER, the higher the voltage, the lower the current, this is directly proportional. So at double the voltage only half the current is required, this means much lower C ratings, thinner cables, much less heat produced, lower current requirement on the ESC etc etc. The same reason why power companies distribute power to us at 330KV or higher – much lower losses due to physical limitations such as cable size.
3. You would use a much lower KV motor otherwise your speed will be too high!

· If all this is right, then to achieve my 2000rpm headspeed at 10:1 I'll need 20,000rpm motor speed. I could do this by using a 2000kv motor at 10 volts (about half power at 6s) or a 1000kv motor at 20 volts (about half power at 12s)

Don’t understand your “half power” statements, but otherwise yes.

· However, I could also run a 12s set up with the 2000KV motor provide I changed the gearing to 5:1. (assuming that motor is happy turning at that crazy speed!)

Yes, but your mechanical efficiency would go way down and therefore heat would go way up (because more power will be required to do the same job).

So, with the same overall performance, I could run a TREX 600E with a pair of 6s 2800mah batteries in two entirely different ways. Either put the batteries in series to work as 12s 2800mah or put them in parallel to work as a 6s 5600mah. (I accept that the 6s solution is easier to implement on a standard TREX600E as a 12s solution will require new components. I'm just trying to understand the maths here)
· Or, using a TREX 500 as an example, a 6s 3000mah setup will have similar endurance and performance to a 5s 3600mah setup provided that the gearing is optimised. (Volts * mah the same in both cases)

Yes, but the series config (12s) would give you better efficiency.

And now a bit on ESCs: an ESC will deliver between 0% and 100% of the LIPO voltage available. So when the battery starts to fade, the max rpm of your motor will drop too. So in crude terms (provided the ESC is rated for the voltage) 50% stick when working on 3s gives out about 5.5 volts (11.1 volts on 6s) and at 100% 11.1 volts (22.2v on 6s). All voltages are nominal. (I've seen a video where an EDF jet model was flown on 4s even though the motor and ESC were only rated for 3s. The extra voltage gave brilliant take-off performance but risked a burn-out if used excessively. My theory is that provided the throttle was kept below 3/4 then you could run like this indefinately because the voltage would be the same as 3s)

NOPE! The way it works is that full battery voltage is applied to the motor, but for shorter duration (PWM – pulse width modulation). The voltage rating should not be played with – the semi-conductors used in the ESC will have a voltage rating (usually limited by the bridge MOSFETs), exceeding this rating will cause rapid failure. The same is not true with current. The limiting factor with current is heat. So if you found a way to remove the heat, you could run more current (a quick note on this – the ON resistance of the MOSFET is the primary limitation, much more expensive MOSFETs have a much lower RDSon (ON resistance), meaning smaller ESC with higher current capability – but at a significant cost – enter ‘quality’ esc’s ).

· I can fit a massive rated ESC (say 200A) in to a TREX 450 (usually 30-45A) with no fear because it won't "push" amps, the motor will "pull" them. The bigger ESC is only a problem in terms of weight and cost.

absolutely, however electrical efficiency would go down because the quiescent current will be higher (the amount of power used by the esc itself basically).



Gareth

[Si Goodchild]

Very, very many thanks for your comprehensive answer.

Now to think about that super-fast jet!

[Ashley Davis]

don't forget that in calculating RPMs you can't just multiply kv by voltage as there are other losses in the system. Lower voltage systems run higher amps to achieve the same power but with higher amps you generate greater heat. Also, not all ESC are born equal and depending on components used (quality) you can get a voltage drop across the ESC which will directly impact your output voltage to the motor, reducing the overall attainable motor speed.

[Si Goodchild]

. A motor typically does not have a maximum voltage rating (well there is, but for very different reasons and almost much greater than any ESC)

So does that mean that I can get more thrust out of my jet simply by upping the voltage (which means an ESC change to handle it) knowing that the motor will probably be alright? I guess the limitation will be that when running at the higher RPM the motor may try to draw more current to overcome the additional load and could potentially destroy itself?

[Gareth-71]

Hi

YES! However, your motor is designed to run at a certain power. And power is directly proportional to heat generated.

Heat is the enemy of BDC motors, and asides from driving more current than the windings can handle, you can run it as hard as you like, just keep it cool.

Once the motor reaches the point of thermal run away - thats it!

Most BDC motor failures are from heat evaporating the oil/grease from the bearings (enter high temp grease) and they sieze, which causes the stator to hit the magnet cage and puf!

i.e. If you used liqued nitrogen to cool the motor then you could get HUGE amounts of power out of it! (that and when conductors near 0 kelvin strange things happen - enter super conductors - but thats a topic for another day )

[robgt]

Gareth, I bet it's cool to sit and chat with you


Interesting stuff
Cheers,
Rob

[Hangar-7]

Hi


......Basically, the controller needs to know the exact position of the stator, as these motors have no sensors on them to give position, the controller uses the reverse emf (the voltage created when a magnet passes a winding) to determine position – very clever and very tricky to do.
Up to about 100RPM, this voltage is not large enough to detect, so for startup, ALL ESC’s startup up in a dumb mode – it just outputs drive signals to the motor without knowing the position at all, this is why some esc’s have a jerky startup.

Again, in an ideal world, yes. A few simple notes on this:
1. A motor typically does not have a maximum voltage rating (well there is, but for very different reasons and almost much greater than any ESC)
2. The real advantage (as dictated by Ohms law), is that for the same POWER, the higher the voltage, the lower the current, this is directly proportional. So at double the voltage only half the current is required, this means much lower C ratings, thinner cables, much less heat produced, lower current requirement on the ESC etc etc. The same reason why power companies distribute power to us at 330KV or higher – much lower losses due to physical limitations such as cable size.
3. You would use a much lower KV motor otherwise your speed will be too high!


NOPE! The way it works is that full battery voltage is applied to the motor, but for shorter duration (PWM – pulse width modulation). The voltage rating should not be played with – the semi-conductors used in the ESC will have a voltage rating (usually limited by the bridge MOSFETs), exceeding this rating will cause rapid failure. The same is not true with current. The limiting factor with current is heat. So if you found a way to remove the heat, you could run more current (a quick note on this – the ON resistance of the MOSFET is the primary limitation, much more expensive MOSFETs have a much lower RDSon (ON resistance), meaning smaller ESC with higher current capability – but at a significant cost – enter ‘quality’ esc’s ).

absolutely, however electrical efficiency would go down because the quiescent current will be higher (the amount of power used by the esc itself basically).

Gareth

Sorry to hijeck your post - but I thought you have explained "back emf" very well which is the heart of all brushless motors.

It is a very informative post - Thnx again.

[Gareth-71]

Gareth, I bet it's cool to sit and chat with you


Interesting stuff
Cheers,
Rob

LOL, I usually grab my coat (or is that anorak) and run.....

Seriously though, with anything there is the theory and the practice, and more often than not someone with loads of practical experience but limited theoretical knowledge will be able to offer better advice.....and I am a relative neewb to heli's.

Gareth

[Si Goodchild]

Don't diss yourself - a combo of theory and practical knowledge is needed in most aspects of life.

Si

[Dedcalm]

I believe early brushless motors had 6 wires due to the sensors for the stator position, then some clever chap managed to use the back EMF to determine the position thus reduing the complexity and thus cost.

clever people are great arent they

sadly although I understand the principles and might possibly be able to build an very basic BDC ESC (given time and google), I am not clever enough to have muntz'd (http://en.wikipedia.org/wiki/Madman_Muntz) the original design though.

[Gareth-71]

Hi

Well, there are two flavours of brushless DC motors, sensored and sensorless.

Both are widely available, although the sensored version is by far the most used for a number of reasons:

1. Full Speed Control, Torque and efficiency can be achieved from 0 rpm
2. They can yield better efficiency
3. Many industrial appilcations require more precision than sensorless can give
4. They have a greater speed range
5. The controllers are MUCH simpler and therefore cost less, not an issue with £50 motors and ESC's, but a BIG issue with controllers costing thousands.