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Understanding R/C Brushless Motor Ratings

From: http://www.hotslots132.com

kV Ratings

Ok, lets start with kV ratings. The letters kV stand for the the RPM of your motor per volt with no load. For example if you own a brushless motor with a kV rating of 4600 and 12V. Take the 4600, multiply by 12 to get 55,200 RPMs. This is the max RPMs that this motor can reach under no load. Once you get it inside your vehicle, this will come down due to friction.

Almost all brushless motors will have the kV ratings stamped somewhere on them. Some motors will have kV ratings on the motor can, others on the motor leads, but some you will only see on the motor�s spec sheet.

Ok. now you are this far, so what does this mean to you really?

  • A motor with a higher kV will have more top end speed, but not as much acceleration/torque.
  • A motor with a lower kV will not be as fast, but will accelerate faster.

So, now you can decide which one works best for your kind of racing. You have the room to really crank it up and reach top speeds? A higher kV will get you there. But maybe you are on a shorter track, and what you want is acceleration out of the corners, then look for a lower kV number. Still not sure which way to go? Try something in the middle!

Note: If motor heat is an issue then a lower kV rating with a higher voltage battery will give you the same effect.

The big thing to remember when using kV for your Brushless Motor Ratings is that your Brushless Motor and ESC will each have a maximum input voltage (battery cell count) that is allowed. So if either your motor or ESC has a lower maximum voltage then you must use this to calculate your top RPMs. If you go over the recommended voltage then you have a high chance that something will fry in your setup.

Motor Turns

Motor Turns is the same for brushed motors and brushless motors. The word turns stands for the amount of wire windings around each of the motor's rotor poles.

  • The higher the number of wirings/turns means less top speed, but higher acceleration/torque.
  • The lower the number of turns equals higher top end speed and lower torque/acceleration.
  • A motor with a turn rating of 5.5 will have less acceleration/torque but higher top speed than a motor with a 12 turn rating.

Current Rating - Amps

It is a great idea to find an ESC that has a current rating that is higher than your motor�s by at least 20%. It will be a good safety cushion to make sure that you don�t burn up your brushless power plant.

Here's why: the max current rating is the maximum amount of current that a motor is able to handle safely. This current is measured in Amps. The continuous current rating of a motor is the Amps that a motor can handle safely over a long period of time.

The estimated current rating of a motor is usually on the factory specs sheet. However other factors affect the actual current that a Brushless motor will draw. Things like the kV rating, battery voltage, how heavy the RC vehicle is, and gear ratio or prop size. The harder a motor needs to work to reach it's top speed, the higher the Amp draw is.


Watts

 Watts are the power rating or the horsepower equivalent of your brushless RC Motor. The math here is Amps x Volts = Watts. You will see a watt rating in the brushless motor specs. Your brushless motor should have a watt rating on its spec sheet, something like "180W". This is the amount of "horse power" that it should produce safely. Running anything over this rating could damage your motor, especially over a long period of time.

Motor Efficiency

The efficiency of a motor determines its quality. Higher efficiency means better design and high quality components. The higher the efficiency of the motor the more power it can produce before it overheats. A 70% efficient motor produces 70% power and 30% heat. A 85% efficient motor produces 85% power and 15% heat. If your battery is sending the ESC 180 watts, your motor will produce 153 watts (85%), the rest is gold ole heat. 27 Watts of heat will melt solder with some soldering irons, so, that is a lot of wasted watts!

A cooler running motor will give you much less trouble. To reduce heat you can change your gearing or prop size, use a more efficient motor, reduce your voltage or amps, or try a motor heat sink and motor fan. Keeping the heat down on your motor allows it to run longer, and give you the power it needs.. Efficiency matters folks.

 

 

 

From: http://www.rcgroups.com/forums/showthread.php?t=1652450

Same KV brushless motors, different weight?

My Question:
If I have two motors that are both rated at a given KV, then why would there be a difference in weight. The only reason that I can think of would be to handle more current, but this doesn't seem to be the case...Please enlighten me...Thanks nige838

 

While Martyn and Larry's suggestions have some merit, it has more to do with the mass/weight if the quality is about the same.
Here are two equal quality motors from the same manufacturer, same materials.Look at this motor (Cobra C-3510-24 Brushless Motor, Kv=820)
http://www.innov8tivedesigns.com/pro...db535b087e8eeeand this motor (Cobra C-3520-12 Brushless Motor, Kv=820)
http://www.innov8tivedesigns.com/pro...db535b087e8eeeLook at the specifications. The only thing in common is the 820 Kv.
The C-3510-24 is a 141g, 24T (means smaller diameter wire), 26 amp motor
The C-3520-12 is a 216g, 12T (meaning larger diameter wire able to handle more current), 56 ampsIf the generic name of diameter in millimeters and length in millimeters followed by dash then Kv comma weight in grams they are;(C-3510-24) Cobra 4336-820, 141g
(C-3520-12) Cobra 4346-820, 216gLook at the prop tables for both motors using a 3S Li-Poly:
(C-3510-24) http://innov8tivedesigns.com/Cobra/C...0-24_Specs.htm
(C-3520-12) http://innov8tivedesigns.com/Cobra/C...0-12_Specs.htmC-3510-24: APC 11x10-E, 11.1v, 23.87 amps, 264.9 watts, 6,485 RPM, 61.4 mph pitch speed, 1013 grams thrust, 35.73 oz. thrust
C-3520-12: APC 11x10-E, 11.1v, 29.98 amps, 332.8 watts, 7,169 RPM, 67.9 mph pitch speed, 1256 grams thrust, 44.30 oz. thrustKeep in mind that the battery/power supply and prop are identical. These ARE measured numbers, not calculated numbers by some motor/power system calculator.Motors with the same or approximate Kv, as they get heavier, can handle more power and actually produce more power because they are more efficient.Kv really doesn't tell very much about a motor, except suggesting what size (diameter & pitch) of prop might be suitable for those that understand Kv. It is really the mass the determines the power and efficiency in MOST outrunner motor cases where the quality of the components and workmanship are about equal.The RPM is the Kv times the voltage OUT of the motor, NOT the voltage in! This is the hardest thing for some folks to understand. There is a voltage drop through the power system. Vnet (which is the net voltage) = V [volts in] - (I [current] * R [resistance])C-3510-24: 6,485 RPM / 820 Kv = Vnet 7.909 volts
C-3520-12: 7,169 RPM / 820 Kv = Vnet 8.743 voltsHere is one more similar Kv motor
Cobra C-4120-12 Brushless Motor, Kv=850
http://www.innov8tivedesigns.com/pro...db535b087e8eee
Generic Cobra 5052-850, 293g
Prop chart:http://innov8tivedesigns.com/Cobra/C...0-12_Specs.htm
APC 11x10-E, 11.1v, 40.48 amps, 449.3 watts, 7,873 RPM, 74.6 mph pitch speed, 1528 grams thrust, 53.90 oz. thrust
7,873 RPM / 850 Kv = Inet 9.262 volts
Remember this is all with the same 11.1 volts in and an APC 11x10E

Ken Myers

 

 

POLES

 Poles are the magnets in the rotor. 2 pole motor means 2 magnets,4 pole means 4 magnets. Obviously the more magnets the greater the magnetic pull/resistance when the rotor rotates inside the stator.

From: http://www.radiocontrolinfo.com/Articles/index.php
Brushless Motor Magnetic Poles

When one refers to How many poles an electric motor has, they are commonly referring to how many magnetic poles there are. Many often wonder the differences between motors having more or less poles. Here, we will focus on a brief discussion concerning Brushless motor magnetic poles. Main focus will be placed on In Runner Motors, where the rotor spins inside of the stator.

Common in runner motors have 2 or 4 magnetic poles. There are some motors containing 6 poles as well. Outrunner motors may have in excess of 6 magnetic poles.

Poles vs Motor Characteristics

The largest characteristic that a motor with more poles displays when all else is equal is a drop in the KV value. The KV value as we all know is the amount of RPM per volt. When all else is equal between 2 brushless motors, the one containing a higher pole count will have a lower KV value. This is the number one important characteristic of differences in a motors pole count. All else that we will talk about is based off of this.

A drop in KV, what does that do?

When a motors KV value drops when we keep the motor the same physical size and the number of motor electrical winds (wind / turn) the same, we gain motor torque. The torque is gained as a result of lower KV. Depending on the exact application this may be an advantage or disadvantage. More torque may mean greater acceleration but a lower KV value will reduce maximum RPM achievable.

How Does a Manufacture maintain a usable KV?

There is one route a Motor Manufacture may take in order to bring the KV back up to a usable amount. The best option is to decrease the amount of winds in the motor. A 2 Pole motor for example will typically have many more winding than a 4 Pole motor. When a 4 Pole motor has the amount of windings reduced, KV will increase back to the amount needed. When a winding from the motor is removed, the physical size of the motor remains the same resulting in a void. The best solution to this created void is to add more copper. More Copper is added to the winding. This is done in such a way similar to moving down in the Gauge type. In other words the wire used as a wind is now increased in diamater.

As we know from electrical theory, when the cross section of a wire is increased the resistance decreases. This decreased resistance would allow a greater current load to pass through the motor.

Advantages and Disadvantages

When comparing a 4 Pole motor that has the same physical size as a 2 Pole motor and the same KV as a 2 Pole motor, the 4 Pole motor in theory would be able to handle greater current loads. The greater current loads will result in greater power. This is similar to saying a 4 pole motor has better overall efficiency when compared to a 2 pole motor.

Although it sounds like there are many advantages, there are also some down falls to this as well. It is very common for 4 pole motors to not have as many motor options as a similar 2 pole motor. Less motor options may make it difficult to get the exact KV value you need. This is even with 4 pole motors using both wind types. A wind type is the difference in how a wind is terminated before it gets to the 3 wires hanging out the brushless motor that you typically see. These wind types are commonly known as the Delta wind and the Wye wind. Using one wind type over another does provide different motor characteristics yet again. We will note this as a possible disadvantage as well.

Which Pole Count Works best for me

Overall, it will come down to testing, testing, and testing. These differences explained above may be very subtle to the point where you don't even note any differences. There may be many reasons to this including your driving habbits. In order to determine which works best for you, it is recommended to give both types a try. You may prefer either one of them.

 

 

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