Metal Construction
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Selection of ElectromotorsWith the drive of metal construction models the selection of an adequate motor often causes problems. Data of existing motors mostly are not present and also the acquisition of an "especially heavy" motor with well-known data guaranties itīs qualification in no way.But in practice the evaluation of a motor requires only few specifications which normally simply can be determined. Conclusive is the proper adaption of the (desired) model properties to the motor data by means of a gear. At vehicles aggravating is added that different loadings can occure, simple models however usually are not equipped with a change speed gear. So the adaption has to be realized by only one gear shifting and therefore this only can be a compromise. The most frequently mistake is to make too great demands on a motor. If the gear reduction is too low the motor is overloaded; in this case it canīt reach itīs nominal values and seems to be too weak - aside from this eventually it can be destroyed because of overheating. The selection of a DC-motor for the 1½-Deck-Bus subsequent is (simplified) physically analysed and illustrated by means of different motors; but the principle can also be applied to other models. The input values can be limited to the following data: - mass of the model - radius of the driving wheels - assumed final velocity - assumed acceleration and/or climbing ability. Input values The model has a mass of 16 kg, and the radius of the driving wheels is 50 mm. The vehicle shall be accelerated within 1 s onto a velocity of 3,6 km/h (1 m/s) and shall be able to override a climbing angle of 10°. With these values firstly the required rotation speed and the required torque of the driving wheels can be estimated. From comparison with the nominal rotation speed and the nominal torque of a considered motor follows the gear reduction. Because of simplification effects like inertia, gear losses and rolling resistance shall be unconsidered. Rotation speed of the driving wheels This follows from the correlation
and can be calculated to Torque of the driving wheels from assumed acceleration The required torque can be calculated with the equation
to Torque of the driving wheels from the assumed climbing ability The for this purpose required torque can be determined both empirical and analytical. For the experimental determination the (freewheeling) model can be placed onto an inclined plane with the assumed climbing angle and the downhill-slope force can be measured with a spring balance. Multiplication with the radius of the driving wheels results into the torque which the drive at least has to produce. Alternatively also the equation
can be used, which for the assumed case delivers a torque of this value still will be necessary later for the cross-check. 1. Example: Estimation of a low-power low-cost motor (price approx. 8 EUR) For an estimation only few data are required and these are to determine if necessary (see "Characteristics of Direct Current Motors"). The model is provided with an IGARASHI SP 3650-65 with the following rated values: More data are not needed. The power of the motor in every loading case can be calculated with the product
and related to the rated values it is It is to notice that not the power is transformed by a gear but the rotation speed and the torque; aside from losses for the power the same value can be assumed at the input and at the output of a gear. For adaption of the motor rotation speed to the rotation speed of the driving wheels a gear reduction of is necessary. Comparison of the motor torque and the torque of the driving wheels requires a gear reduction of This result indicates that a vehicle with only one gear shifting either can speed or can override hills. The compromise is to choose an optimal reduction for both cases. Here it shall be determined to an average of For interpretation of this result the operation diagrams of the motor are helpful (see "Characteristics of Direct Current Motors"). These indeed show the behaviour of the motor at a voltage of 6 V, but of course they are also valid to characterize the motor with 8,4 V supply voltage.
The rated values normally define the operating point at the maximal efficiency; in a vehicle because of different loadings however the motor operates within a range around the nominal ratings. Because of the larger reduction (u=39 instead of u=31) the model will not reach the assumed final velocity. Otherwise on a flat bottom the motor mostly works below the nominal rating, because after acceleration only the rolling resistance is to compensate. So the rotations speed exceeds the rated value and according to this the model rides a little faster. At an uphill grade the rotation speed falls and the motor works above the nominal rating; both the power and the torque increase. To override the assumed climbing angle the motor has to produce a torque of M = 136 Ncm / 39 = 3,5 Ncm, which according to the above operating diagrams is possible without appreciable overload. In practice the model will pass the hill without trouble - even thought significant slowlier (assuming that the other driving components withstand the load). In the range above the power maximum the power decreases with large current consumption. This case corresponds to an overload with a large torque but also with a large heat production and should be avoided as permanent load. With a gear reduction of u=39 the motor without more ado is suitable for the intended application. In the 1½-Deck-Bus a gear reduction of 36 is used which also still allows satisfactory operation. 2. Example: Estimation of a special motor (price approx. 40 EUR) Such a motor predominantly can be used for the drive of minitruck models. For comparison the input values of the 1½-Deck-Bus shall be valid. With a supply voltage of 12 V the motor has the following rated values: which corresponds to a rated power of The necessary gear reductions are Remarkable is that this values are nearly the same, and with an average reduction of u = 17 both the rotation speed and the torque are adapted very well. In this case the motor works only in nominal operation and with regard to the climbing ability so this motor has substantial torque reserves. Alternative to this the reduction could be reduced to reach a larger velocity and a lower climbing ability. For the assumed application in the 1½-Deck-Bus the motor can be considered as oversized. For comparison: With a supply voltage of 8,4 V this motor would have only a nominal power of approx. P = 9 W and so would be inferior to the above estimated low cost motor. 3. Example: Estimation of a high-power low-cost motor (price approx. 8,50 EUR) Also for this example the input values of the 1½-Deck-Bus shall be valid. With a supply voltage of 8,4 V the motor has the following data: corresponding to a rated power of The required gear reductions can be calculated to Compared to the 1. example here the reverse situation exists. The torque of the motor is large enough to operate it with a gear reduction of u = 17 and the velocity of the model reaches the 3-fold of the assumed value. For the application in the 1½-Deck-Bus this motor is multiple oversized. Further aspects Especially for radio controlled vehicles further technical details are to consider. An oversized drive during overload produces an enormous torque and so it can endanger the other driving components important. Also the operation time of the accumulator according to experiance decreases with "better riding quality". For 12 V motors is added that because of the higher voltage an accumulator with the same capacity will be larger and heavier. Moreover not all speed controllers are suitable for a 12 V operation. Mechanical size and the diameter of the motor shaft also can be important like price and durability. |
