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Electric motor help

Old 02-08-2007, 07:39 PM
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Default Electric motor help

Hi all,

What does the Kv in electric motor stats mean? Also, why is this important to know?

Thanks for any help.

Old 02-09-2007, 08:21 AM
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Default RE: Electric motor help

RPMs per volt.

Depending on your application (geared, direct drive, inrunner, outrunner ) you will need this info.

For example:
On a recent 40 size model I just finished I wanted to use an outrunner motor with a 12x8 or 12x10 prop turning ABOUT 8,000 - 9,000 rpm to generate the thrust I needed.

Using other criteria I determined the wattage of the motor I needed.

Then I determined I would need to use a 4 cell lipo battery (14.8 volts) for this motor. Dividing the rpms by the voltage I determined I needed a motor that had a kv between 540 and 608 rpms per volt to provide the rpm range I wanted.

Old 02-09-2007, 05:15 PM
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Default RE: Electric motor help

Thanks for the info... Helped out a lot.

Old 02-10-2007, 08:50 AM
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Default RE: Electric motor help

Motor dimensions are another confusing topic. Generally, some manufacturers list their motors as glow equivalents. E-flight lists theirs as Power 10 or Power 25. With 3 Lithium cells (nominal voltage under load is around 10 volts) they are equivalent to a .10 and .25 glow engine respectively. Others list the dimensions such as AXI. For instance, a 2208-34. The first two numbers (22) represent the diameter of the stator (the fixed part in the middle of the motor) in millimeters. The second two numbers (08) represent the length of magnets (attached to the rotating case) in millimeters. The third set of two numbers (34) represents the number of wire winds, also called turns. Generally a smaller number of turns represent a higher Kv motor. Well, shouldn’t we get the highest kv motor we can get? Not so fast, nothing is free. Think of a high kv motor like a high RPM pylon racer, small prop tons of speed but no torque. A low kv motor is like a big four-stroke, not a lot of speed but tons of torque for those big draggy Cubs! If the kv is known, then we can determine another constant called kt.
Kt is the torque produced per amp. kv and kt are proportional as shown: kt = 1355 / kv This relationship between kt and kv is a law for every motor.
Motor constants are used to define the characteristics of a motor in quantifiable terms. Every motor can be accurately defined using exactly 3 motor constants: kv (rpm/volt), rm (terminal resistance), and io (no-load current).
The kv constant is the RPM's produced by a motor per volt applied. Thus we would expect that a 1kv motor to spin at 1000RPM if we applied 1 volt through the speed controller, but there are still frictional and other losses in the motor that make this unattainable.
Maximum current- This is the amount of current (amps) that the motor can safely handle without damage. Current draw from a brushless or brushed motor is determined by the choice of propeller (or fan). A brushless motor has a “happy” current point, i.e. the point where it is most efficient, this is usually given in the operating instructions as the Max efficiency current. Try and prop your plane or heli at this point using a “Watt-Meter”.
The rm constant is called the "terminal resistance" of the motor. This is the loss inside the motor due to the wiring in the armature. The rm constant represents a loss of power due to imperfect materials inside the motor. The final constant, io, is the no-load current. Think of electricity like water.Volts = pressure Amps = flow
Volts is like pounds per square inch, psi. Says nothing about how much water is flowing, just how hard it is being pushed. You can have 100 psi with zero water flow.
Amps is flow, like gallons per hour. You can have flow at low pressure and you can have flow at high pressure.
Amp-hours is how much flow can be sustained for how long. It is used as a way of measuring how much electricity is in the battery. Like how many gallons of gas in your tank. It is a capacity number. Says nothing about flow or pressure, it is about capacity.
Amps and milliamps? We are just moving the decimal point around.
1 amp, short for ampere-1000 milliamps (milli means 1/1000)
So a 7 cell NIMH or NICAD pack provides about 8.4V (pressure).
The motor will draw electricity from the pack at a certain flow rate, or amps.
If you have a have a 650 milliamp hour pack, it can deliver a flow of
.650 amps (650 milliamps) for one hour. If you draw it out faster, it doesn't last as long. So your motor might pull 6.5 amps for 1/10 of an hour, or about 6 minutes.
A 1100 mAH pack has double the capacity of the 650 mAH pack, so it should last "about" twice as long.
What is C in relation to batteries?
C ratings are simply a way of talking about charge and discharge rates for batteries. 1C, = 1 time the rated mAH capacity of the battery. So if you charge your 650 mAH pack at 1C, you charge it a 650 milliamps, or .650 amps.
1C on a 1100 pack would be 1.1 amps.
2 C on your 1100 pack would be 2.2 amps
Motor batteries are often rated in Discharge C and charge C.

So a 1100 mAH pack (1.1 amp hour) might be rated for 10C discharge, but only 2C charge, so you can pull 11 amps ( flow ) without damaging the battery, but you charge it at 2.2 amps
Things should be making a bit more since now.
If you have a 500 mAH pack - any kind - and it is rated at 16C that means it can deliver 8 amps.
If you have a 1000 mAH pack - any kind - and it is rated at 12C that means it can deliver 12 amps
One last point. Motor batteries vs. receiver batteries
Some batteries can sustain high discharge rates. Others can not. Those used as transmitter/receiver packs typically are made for low flow/amp
rates while those made for motor packs can sustain higher rates.
So, having a 600 mAH pack does not tell you if it is a motor pack that can put out 6 amps, or if it is a transmitter/receiver pack that would be damaged if you tried to pull power at 6 amps. It is enough to say that they are different.
Watts is a measure of power. There are two kinds of watts that we are concerned with: watts "in" and watts "out". The watts coming "in" is the power consumed by the motor/speed control and associated wiring resistances, and watts "out" is the power generated by the motor and absorbed by the propeller. Watts "in" is what most modelers measure, as it is much easier. Watts "in" is simply the product of volts and current drawn. A NiCad or NiMh cell will produce about 1.2 volts without load, but less at maximum efficiency. You can count on 1.0 volt per cell, which means that: watts in = # of cells x amps drawn. With better Li-Po cells, figure about 3.7 volts/cell under load for any calculations. Watts "out" will therefore be some fraction of watts in, depending on the total power system's efficiency

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