Understanding Brushless Motors

Lucien Miller of Scorpion Motors wrote an extensive Motor Tutorial that should help your next motor purchase.

About Scorpion Motors

Scorpion motors are made on the most modern CNC equipment available, so from a purely mechanical standpoint, they are manufactured as well as a Hacker or AXI motor. The fit and finish is second to none, and there a few nice touches like the laser engraved part number and company logo, and the angle cut cooling holes on the front housing that act as an air pump while the motor is running to pull cooling air across the stator and motor windings.

The biggest difference in Scorpion Motors is the magnets that are used in the construction of the motor. Before I go into the details, I think it would be best to give a little bit of background information about the magnets used in brushless motors in general, so you will understand how important this is. This will take a little bit of time, but once you get to the end, you should have a very complete understanding of how a motor works and how power, heat and magnets interact with one another.

Motor Magnets in Detail

The magnets used in our brushless motors are a ceramic material that is comprised of Neodymium, Iron and Boron, and are often referred to by their chemical symbols from the periodic table as NdFeB magnets. Since Boron is a fairly toxic substance, the magnets are chrome plated after manufacture to seal them up. This serves three purposes:

1. Protect the users from Boron exposure;
2. Seal the magnets so they do not absorb moisture, which can cause the magnets to break down over time;
3. Strengthen the magnets, since the ceramic material is fairly brittle.

The magnets have two major properties, their magnetic strength, usually expressed in the units of MegaGauss-Oerteds, and the temperature rating, which is expressed as a letter suffix to the strength value. The strength value of NdFeB magnets varies from a low of around 28 MGO to a high of 50 MGO. The majority of high-end name-brand brushless motor use magnets in the 45-50 MGO range.

The one downside to NdFeB magnets is that when compared to other types of magnets, such as Samarium Cobalt, they have a relatively low operating temperature. The majority of magnets used in motor construction have a maximum operating temperature of 100ºC or 212ºF. Operation above this value will cause a permanent and irreversible loss of magnetic strength in the magnet. The amount of magnetism lost increases as the temperature increases, and once you reach the Curie temperature of the material (around 310ºC), all magnetism is lost.

NdFeB magnets are available in several different compositions with varying maximum temperature ranges. These temperature ranges are signified by a letter code after the strength value. Here is a list of the ones that are currently available, along with the max temp:

• 80C – No suffix
  
• 100C – M
  
• 120C – H
  
• 150C – SH
  
• 180C – UH
  
• 200C – EH

The entire part number of a magnet normally starts with a letter "N" to signify that it is a NdFeB magnet, which is then followed by the magnet's strength, and finally its temperature rating. So common magnet part numbers would be N48, N45M, N42H, N35UH N30EH and so on.

Typically, as the magnet's strength goes up, the maximum operating temperature goes down, so you end up with the following commonly available magnet strength-temp ranges:

Magnet Strength – Temperature Ratings

 

Suffix	Temp Rating	Part Number
No Suffix	80ºC (176ºF)	N48-N50
M	100ºC (212ºF)	N45-N48
H	120ºC (248ºF)	N42-N48
SH	150ºC (302ºF)	N38-N42
UH	180ºC (356ºF)	N30-N35
EH	200ºC (392ºF)	N28-N33

Scorpion Motor Magnets

Now with that understanding about the magnets, I can move on to explaining the magnets used in Scorpion motors. As I stated earlier, most of the better motor manufacturers are using N50 or N48M magnets in their motors. Using an N48M magnet gives you a motor that can withstand 20ºC more temperature and still have 96% of the magnetic strength of an N50 magnet, so it is a good trade-off. For cost reasons, the majority of the cheap Chinese No-name import motors use N35 or N38 magnets in either 80ºC or 100ºC temperature ratings.

Scorpion wanted to build a motor that in normal use was virtually impossible to burn up, so they went to the best magnet manufacturer on the planet and looked at magnet options. After going over all the options, Scorpion discovered that it is possible to create an N50 magnet with an EH rating, that would be good for 200ºC operation.

Currently there are only two magnet manufacturing companies on the planet that have the equipment and the technical know-how to produce such a magnet. To produce these magnets, it takes some extremely rare trace elements to mix in with the Neodymium, Iron and Boron, and in order to make it economically feasible, a LOT of them need to be made in the production run.

Since Scorpion wanted to make the best motor available, they decided to go ahead and have N50EH magnets custom made for them, and they are the ONLY motor manufacturer that has them. Because of this, Scorpion motors are the only motor made that can operate at temperatures exceeding 150ºC and sustain no damage whatsoever.

To complement the incredible magnets used, Scorpion uses wire to wind their stators that has an insulation which is rated for 180ºC. These two components combine to produce a motor that can handle at least 50% more power than any comparable motor without burning up.

Most people will tell you that, "I never burned up my magnets, it was the wire that fried". What they don't understand is the reason the wire fried is because the magnets DID start to de-magnetize. When a motor fails due to the windings burning up, in most cases it is due to the magnets getting too hot. Let me explain:

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Brushless Motors Generate Power

When a motor is running under power, it not only functions as a motor using up the electricity being sent to it, it also functions as a generator, supplying power back to the source. This is why a motor draws less current when it is running than when it is stopped. Here is an example to explain this:

Let us assume we have a motor that is running on 10 volts, has a Kv of 1000 and has an Rm value (motor resistance) of 0.1 ohms. In an ideal situation, if you put 10 volts on this motor it would spin at 10,000 RPM. We all know that it will spin a little slower than that due to the drag of the bearings, the air on the motor and other factors. Let's say that in this case it spins at 9,800 RPM.

Now since the motor can act as a generator just as well as a motor, a motor with a KV of 1000 will also generate 1 volt for every 1000 RPM that it is spun by an external source, even if that external source is the motor itself.

In the case just described, the motor has 10 volts applied, and since it is spinning at 9,800 RPM, it generates 9.8 volts. When you take the difference between these two voltages, you get 0.2 volts; if you divide that value (0.2) by the Rm of 0.1 ohms, this yields a no-load current (Io) of 2 amps. This is a simplified version of the Io calculation, but it explains the point. Now, as you put a prop on this motor and load it down, the motor turns slower, and as a result it generates less internal voltage.

Let's say that you prop the motor so it now turns 9,000 RPM with 10 volts applied. In this case, the motor will only generate 9 volts. When this is subtracted from the 10 volt supply, you have a difference of 1 volt. Now, if you divide this 1 volt by the Rm of 0.1 ohms again, you will see that the motor will now draw 10 amps of current. If you put on a bigger prop and slow the motor down to 8,000 RPM, it will draw 20 amps of current, and so on.

Motor Heat and Failure

So know you know why motors draw more current as the prop load increases and the motor RPM decreases, so how does this all pertain to burning up magnets?

The heat that is generated inside the motor is proportional to the square of the current.

Since the formula for power can be expressed as

P = I x I x R

where P is power, or in our case heat, I is the current flowing through the motor, and R is the resistance of the motor, or Rm in this case. Every motor has a mass of metal inside it, namely the stator, and this makes up a large portion of the total weight of the motor. Since the stator is in direct contact with the wire motor's windings, it takes the brunt of the heat that is generated by the motor windings. In most of our motors, the magnets are only a few thousandths of an inch away from the stator, so as the stator heats, a large portion of this heat is passed on to the magnets, since they are so close.

The amount of heat a motor can take varies with the size of the motor, but a generally accepted rule of thumb is that a motor can safely handle 100 watts of power per ounce of motor weight. Some can take more, and some can take less, but this is a good middle-of-the-road value.

For our example that we have been using so far, let's say that the motor we have been using weighs 3 ounces, so it can safely handle 300 watts of power. With 300 watts of power in at 10 volts, the motor would be drawing 30 amps of current (Watts = Volts x Amps, so Amps = Watts / Volts). In this condition and using the numbers we derived earlier, a motor pulling 30 amps of current will generate 7 volts. Since 7 volts is 3 volts less than the supply voltage of 10 volts, and the Rm value is 0.1 ohms, 3 volts divided by 0.1 ohms is 30 amps of current [((10-7.0) = 3.0)/0.1 = 30 Amps]; since the motor has a Kv of 1000, then it must be turning at 7,000 RPM (1000 x 7 volts) with a 10×6 prop.

Voltage, Kv, RPM and Current – How They Work Together

I know that this is a lot of numbers, but if you can follow along you can clearly see how Voltage, Kv, RPM and Current all interact in a motor as the prop load changes. Now with all that said, we can finally address the heat issue of the magnets!

With our motor now spinning at 7,000 RPM and drawing 30 amps of current, we are putting in 300 watts of power (30 Amps x 10 Volts). So how much of this power is going to heat? Since power is equal to I x I x R, in this case it would be 30 x 30 x 0.1, which is 90 watts (remember this motor has an Rm value of 0.1 ohms, the 0.1 in the equation). This means that for the 300 watts we are putting in, 90 watts are going up as heat, and the remaining 210 watts are available to spin the prop. This means that the efficiency of the motor in this condition is equal to 210/300 or 70%.

Now that we have established the maximum operating condition for this motor, we can look at how the generated heat can effect the magnets. We have determined that our motor uses 300 watts of power and 90 watts of that is turning into heat. To get an understanding for how much heat 90 watts is, grab hold of a 100 watt light bulb that has been running for a while. Don't actually do it, but I think you get the picture. Light bulbs are only about 10% efficient in converting electricity into light, so a 100 watt bulb makes 10 watts of light and 90 watts of heat. This is how much heat the hunk of metal called your stator has to dissipate while running at full power.

Fortunately for us, our motors spend most of their life strapped to the front of an airplane that is going through the air while the motor is producing this power, so most of the heat blows out and everything stays cool enough to operate. Let us assume that this motor is using N50 magnets, which are rated at 80ºC. And for the sake of this example, let's say that when the motor is running at 300 watts, the stator temperature is 70ºC. At this temperature, the magnets are still happy, since it takes 80ºC before they begin to get damaged.

Since normal room temperature is 25ºC (77ºF), with 300 watts of power going into the motor it is at 70ºC, so the motor is now 45ºC hotter than it was at the beginning of the flight. Since there is 90 watts of heat being dissipated into the stator, this means that for every 2 watts of power the stator has to dissipate, the temperature will go up 1 degree C. With this value, we can calculate the stator temperature for any current draw of the motor.

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Prop Selection And Its Impact on Motors

I know, it has been quite a journey since we started this, but now we can finally see how all this comes together. Let's say that you have been flying your plane with your 10×6 prop for a couple flights, and you land, flip the plane over and break your last 10×6 prop. Bummer! You go to your tool box and discover that you do not have any more 10×6 props, but you do have an 11×6 prop. So you figure "Oh well, close enough", bolt it on and go flying again. Now what you have done is put into motion a chain of events that will lead to the destruction of your motor, so like an episode of CSI, we will go through all the steps that will lead to the demise of your favorite motor.

The amount of drag that a prop puts on a motor is what I like to call the Prop Load Factor. The load factor of a prop is proportional to diameter cubed times the pitch, so the load factor of a 10×6 prop is 10 x 10 x 10 x 6 or 6,000. An 11×6 prop has a load factor of 11 x 11 x 11 x 6 or 7986, which is 33% greater than the load factor of the 10×6 prop. Since the current draw on an electric motor is proportional to the load factor of the prop, we can estimate the current draw based on the load factor:

The 10×6 prop, with its load factor of 6000, pulled 30 amps from the motor. This means that for every 200 units of load factor, the motor will draw 1 amp of current. Using the same ratio, the 11×6 prop, with it's load factor of 7986, will cause the motor to draw 39.93 amps, which is a 33% increase in current (7986/6000 = 1.33, or 33% increase).

Now let's see what this does to the motor: Since the power dissipated in the stator is equal to the current squared times the resistance, or I x I x Rm (I= Amps), the new power dissipated in the stator is equal to 39.93 x 39.93 x 0.1 which is equal to 159.4 watts. So now instead of the motor pulling 300 watts with 90 watts (30 x 30 x 0.1) going to heat, we are now pulling 399.3 watts (10 Volts * 39.93 Amps = 399.3 Watts) of power into the motor with 159.4 watts going up as heat. This leaves 239.9 watts to spin the prop and yields an efficiency of 239.9/399.3 which is 60.1%. So the efficiency of our motor has gone down from 70% to 60% and the internal heat has gone up from 90 watts to over 159 watts.

Earlier we calculated that for this motor, every 2 watts of heat energy in the stator would cause a 1 degree C temperature rise – with 159.4 watts, this means a 79.7ºC temperature rise. Since the day we were flying it was 25ºC, if we add the new temperature rise we see that the stator temp has climbed to 104.7ºC! So now, the simple act of changing from a 10×6 prop to an 11×6 prop has caused your stator temp to rise from a safe value of 70ºC to the now dangerous level of 104.7ºC!

Heat and Motor Failure

At this point you are probably asking yourself "Why would that hurt the wire? It is most likely rated for at least 120ºC if not 150ºC, surely the wire will not burn up." As I said earlier, the problem is not the wire, although it will die in the end as well, it is the magnets that will start the ball rolling.

So now your magnets are no longer happy campers, since they have a max operating temperature of 80ºC and the stator, only a few thousandths of an inch from the magnets, is at over 104ºC. Now the permanent and irreversible damage is starting to take place. The magnets begin to lose some of their strength because they heat up beyond their operating temperature. This has two detrimental effects:

• First, it raises the Kv of the motor slightly
• Second, it reduces the ability of the motor to function as a generator

If you continue operating the motor in this condition, a run-away thermal condition will take over and the motor will quickly burn up.

What happens is that the weaker magnets can no longer generate as much voltage. From our earlier calculations, we know that if our motor was pulling 39.93 amps, then it was turning just a hair over 6,000 RPM and was generating 6.007 volts. With the weakened magnets, let's assume that it is now only generating 5.5 volts. This means that now the motor will pull 45 amps. This will cause the heat level to go up to over 200 watts, raising the temp of the motor to 125ºC, and then the magnets will demagnetize some more and the motor will only generate 5 volts, and the current will go up to 50 amps.

Now the heat generated will go up to 250 watts and the stator temp will go up to 150ºC. Now the magnets are really hurting and the motor can only generate 4 volts, so the current goes up to 60 amps, the heat generated goes up to 360 watts and the temperature of the stator rises to 205ºC and, Whoops, all the insulation just melted off the wire and the motor windings short out and the big trail of blue smoke starts pouring out of the back of your motor.

Now since your ESC is working into a dead short, it more than likely let out its magic smoke as well, and if the failure mode was a shorted FET, your battery will follow along behind the other two rather quickly!

And that is exactly how a motor burns up when you push it too far. In the end, the wires burning up caused the ultimate death of the motor, but the root cause was the magnets losing their strength because they got too hot.

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Scorpion Motors

Now, having said all that, let's go back to the Scorpion motors. With magnets rated at 200ºC, if you had the same scenario of switching the 10×6 prop to an 11×6 prop, the stator would still be at over 104ºC, but the magnets couldn't care less, because they can happily exist at 200ºC. And since the wire is rated at 180ºC, it is happy to run at 104ºC as well. In fact, in order to cause the catastrophic chain of events to occur as with the earlier motor, the stator core would have to heat up to 180ºC, and the wire would fail first, leaving the magnets intact. Working the numbers backwards, this would require a power level in the stator of 310 watts, which would require a current level of 55.7 amps and a power input level of 557 watts.

Now comparing the two motors -the other Name Brand motor with 80ºC magnets versus the Scorpion motor with its 200ºC magnets – increasing the load of the prop by only 25-30% can quickly cause a thermal run-away condition that results in the complete destruction of the motor with 80ºC magnets. On the other hand, the Scorpion motor, with it's 200ºC rated magnets, can take an overload of 86% and the only thing that happens is that the wire burns out without affecting the magnets at all.

If you pushed the motor that hard, you could re-wind the stator and the motor would be good as new. With the other motor, there would be nothing salvageable other than the metal parts. The motor would need to have all the magnets replaced and be re-wound to get it going again, which is so much work that it would not be worth it.

So there you have it! I know that it was a VERY long explanation, but I think that it was needed to explain the advantage the Scorpion Motors have over every single other motor out there. They truly are a revolutionary product, and the best part of all is that they are about 2/3 the cost of a comparable Hacker motor and only 1/2 the cost of a comparable AXI motor. When you put all that together and add in the 2-year manufacturers warranty that Scorpion has, it is pretty much a no-brainer as to which motor to choose.