Understanding Motor Specifications

Haha~

Do you know different motor specifications's mean?

Today, i will tell you the motor specifications's mean.

Most electric airplanes either come with a motor or the manual will tell you exactly what size motor it needs. If you're converting from a brushed motor or glow engine to a brushless RC motor, understanding what these designations mean will be important.

1.)Voltage Constant
Kv is the "Voltage Constant". This is how many RPM's the motor turns for each volt applied. Actually, it's a tad less because even brushless motors aren't 100% efficient.
For example, if you apply 12volts to a 200Kv motor, it will turn at just under (12 x 200) 2400 RPM's.

2.)Torque Constant
Kt is the "Torque Constant". For all motors, Kt= 1355/Kv. You'll have to ask someone smarter than me where the 1355 comes from, cause I don't have a clue! Just accept it for what it is.
In a nut shell, this means the faster the motor spins for a given voltage (Kv), the lower the output torque will be (Kv).

Outrunners generally have a lower Kv, which in turn produces more torque at a slower speed for spinning those larger props. The opposite is true with inrunners.

3.)No Load Current
Io is the "No Load Current". This is the amount of current it takes to spin the motor with no prop.

For example, if your motor is pulling 25Amps, and the Io is 2Amps, then you really only have 23Amps turning the prop.

4.)Terminal Resistance
Rm is the "Terminal Resistance". This is the internal resistance of the motor measured in Ohms. The higher the Rm, the less efficient the motor is.

5.)Current and Power
The maximum current and power is what determines how large of a prop and what size plane can be used with the motor. Simply multiply the current by the battery voltage to get power.

The larger the propeller's diameter and pitch, the more current the motor will draw for a given RPM. For example, the 200Kv motor will run at 2400 RPM's regardless of whether it has a 10x6 prop or a 11x5 prop. But, the 11x5 prop will cause the motor to draw more current. Drawing too much current will destroy the motor.

www.rcecho.com

How does Timing effect the power of an electric motor?

In general terms this describes motor action:-) But (bet you knew that was coming) more specifically you need to first visualize that the permanent magnets create a fixed field in magnitude and position (for this discussion). This field has a shape with a maximum intensity exactly between the magnets (in a 2 pole motor - most DC motors - directly opposite each other). To generate the maximum torque from the motor, the electric field generated by applying current to the windings must be located 90 ELECTRICAL degrees from the center of the magnet field (90 physical deg. in a 2 pole motor). This is defined as the max. Kt (torque constant - oz.in./A)


What happens in a real motor under load is that the electric field lags the magnet field due to inductance effects in the windings and possibly some saturation effects in the steel parts (magnetic iron). The amount of lag varies with load (and rpm), so is a complex variable.


We try to preset the motor timing (brush position) so that at a selected load current, the fields are at the 90 E deg point for max. Kt., and max. magnetic power conversion efficiency. At no-load (without a prop) this will be an advanced position and since it is ahead of the 90 E Deg. point, will have a reduced Kt (take the SIN of the angle x KT90, eg SIN108 x 1 = .951 x Kt), which requires more current to turn the shaft (at no-load. The current will pull into phase at the selected load current. This will actually look like the current decreasing as the load increases, up to a point.


In a brushless motor the same thing is happening, but we advance the hall sensors for this effect. The big difference is that we have to visualize the analogy while sitting on the rotor (stationary frame of reference) and look at the stator field.


This is explained in most texts of ac and dc electric machines. It's a lot more complex than this, so I hope I've given you some insight as to the goings on in that little bundle of wire and magnets.

www.rcecho.com

Choosing the Right Brushless Electric Motor for Your RC Airplane

Today i wnat to teach you ho to choose a Right Brushless Electric Motor for your RC Airplane

Finding the brushless motor that is the best choice for your plane or helicopter can seem to be a daunting task due to the large number that are available. There are a few important considerations you should keep in mind when choosing. This article will help you identify these issues so you can spend more time flying and less time trying to find that “perfect” motor.

Ultimately, you want to swing a certain size prop at a certain RPM. In fact, the freedom you have in choosing propeller size and operating RPM can lead to huge performance gains over comparable glow motors used in many remote control airplanes. Prop and RPM selection determines how much power you need, it is important to choose a motor that is almost at its limits when running at that power level. A motor that is too small will overheat and ruin itself, a motor that is too large will be a detriment to performance, due to the added weight.

Translating propellor size and RPM into power requires some help. This help can come from a computer-based prop simulator. You can also find data posted by someone who has done what you are trying to do; find out what prop they used, what RPM it spun, and how much power was used.

Your list of potential motors should now only contain motors that can comfortably (but not ‘in their sleep’) put out the power you need. Now you’ll have to make decisions on the other things: battery voltage and capacity, direct drive or geared, outrunner or inrunner, and kV.

The easiest choice is whether to use direct drive or a gear box, so make that one first. If you want to turn high RPM (greater than 10,000 RPM) you’ll probably want a direct drive inrunner. For lower RPM, you can run an outrunner in direct drive or an inrunner through a gearbox. The outrunner motor is simpler and quieter, but the inrunner motor in a gearbox can be more adjustable and slightly more efficient. In some cases, the outrunner motor can be quite a bit cheaper. Each has its advantages, so consider them both.

At this point you know what RPM your motor needs to turn. It is either the same as you want the prop to turn(direct drive), or at a ratio faster than the prop when using a gearbox. Motor RPM is going to determine your specific motor and battery choice, by the following approximate formula (assuming lithium polymer batteries).

Motor RPM = 0.8 x 3.5V x Series Cell Count x Motor kV Rating

You need to select the right motor and battery combination that will satisfy the motor RPM formula. You can do it with a low kV motor and a high series cell count battery, or vice versa.

Lithium polymer battery packs are ideal for use with brushless motors in radio controlled airplanes and helicopters due to their low weight and high capacity compared to NiMH and NiCd packs. Along with a brushless motor and battery, you will also need a brushless speed control (ESC) with an amp rating equal to or greater than the peak current drawn by your motor.

Make your choice, order the parts, put them together.You want to make sure that you are near the RPM and power levels you were aiming for. Remember, though, the most important test is how it performs in the air. Fly it, and fine tune with prop selection. Hopefully this article has brought you close enough that a motor or battery change isn’t required.

Hope this information can help u~
in the last month , i brought a AXI Gold Line 4120/18 Outrunner Brushless Motor from rcecho.com.
It is very suitable for my helicopter. And it is powerful~~

www.rcecho.com