Motor Parameters

MP02.gifThis is a motor constant , expressed in RPM/Volt, that indicates how fast the motor would turn for a given voltage if there were no internal resistance. Stored in Motors database. It is inversely proportional to Kt, the torque constant. Kv is usually measured by chucking the motor shaft in a drillpress (hold the motor!) with a DVM connected to the motor leads. Run the drillpress and measure the DC volts from the motor (generator). Divide the drillpress RPM by the measured voltage. This should be done with neutral timing.

MP03.gifThis is a motor constant , expressed in milliohms, which characterizes the equivalent internal resistance of the motor. Stored in Motors database . Rm is the most difficult motor constant to measure. You typically need to supply about 10 amps to the motor while keeping the motor shaft from turning while measuring motor voltage and current. You need an accurate DVM to measure the voltage and an Astro meter is fine for the current. A battery pack with a load (power resistor or headlight) could supply the current. The motor timing does not matter, but shaft position can vary the reading. We like to take several readings while slowly rotating the shaft.

MP04.gifThis is a motor constant (Izero), expressed in amps, that indicates the amount of current necessary to turn a motor without load, or how much current does not contribute to the output power. Stored in Motors database . Io is easy to measure but it generally requires the motor to be at neutral timing. In fact, we say we have neutral timing when we adjust for minimum Io. This must be done quickly, since a motor gets surprisingly hot under these conditions.

MP05.gifThis Is the computed motor current. You can also enter a desired current and the program will determine the prop diameter or pitch or motor gearing to obtain this current. This value is not stored in any database. If you enter a desired Amps value, the Diameter, Pitch, and Gearing text boxes will "light up". If you double-click on one of these, the program will recalculate its value to that which gives the Amps value entered.

MP06.gifThis is the manufacturer or reseller of the motor. Stored in Motors database .

MP07.gifThis is a description of the selected motor. Stored in the Motors database . Many of the European motors and the Aveox motors have a numbering system that describes the length and diameter of the motor. Unfortunately this standard is far from universal.

MP08.gifThis is the Motors database control. If you click the big button, you jump to the Motors database. The smaller buttons step you back and forth through the database. Use CTRL_M as a shortcut to access the database. This selects a motor/gearbox combination, including Kv, Rm, Io, gear ratio, manufacturer and description.

MP09.gifThis is the rotational speed of the motor in thousands (K) of revolutions per minute. If there is a gearbox, it is input side RPM. The motor tries to get the RPM to the value determined by Kv and the input voltage. The power required to turn the prop is proportional to the cube of the RPM. This load causes current to be drawn which reduces the voltage seen by the "ideal" motor. This ultimately results in an equilibrium where the motor is "happy".

MP10.gifThis is the percentage of energy to the motor that is delivered to the propeller or gearbox. This does not include any propeller or gearbox losses, but includes losses in the motor itself. Motor efficiency is generally higher for rare-earth (cobalt and neodymium) magnets than for ferrite. However, high efficiencies can be attained with ferrite motors that are designed to run at very high RPM. These require a high-ratio gearbox and brush life is an issue. Brushless motors have somewhat higher efficiencies than brushed motors.

MP11.gifThis is the power loss in the internal motor resistance and Io. This is what makes the motor get hot. This number is difficult to measure without a dynamometer.

MP12.gifThis is the power supplied to the motor. It is the motor voltage multiplied by the motor current. This is the denominator in determining motor efficiency. This is easy to check with an Astro meter or equivalent. It is a good idea to do this with a new setup and compare it with ElectriCalc predictions. This frequently points out wiring and other problems such as a weak battery pack.

MP13.gifThis is the voltage across the motor terminals. The RPM of an ideal motor model should be Kv times the difference of the motor voltage and the motor current times the internal resistance.

MP14.gifThis is a number based on certain motor parameters that is an indicator of the "quality" of a motor. If you have two motors of about the same size, the one with the higher motor constant would generally be better. Larger motors generally have higher motor constants. Don't worry about this number. This number is obtained by taking the motor Kt and multiplying by the square root of Rm. The torque constant Kt is 1352 divided by Kv.

MP15.gifThis is the motor input current that results in the greatest motor efficiency. It is higher than the current for best system efficiency. The motor efficiency curve is zero at Io and peaks rather quickly. It falls off gradually and is zero at Istall. It is generally better to prop higher than the max. eff. current as long as you keep within the rating of the motor.