Metal Construction
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MÄRKLIN SolenoidLarge MÄRKLIN kits of the fifties and sixties contained a solenoid to build up various motors and other electromagnetic devices.Because this solenoid was lost over the time it was to buy second hand - or to make a self made reproduction. For the reproduction firstly the mechanical data were determined using the construction plans; at this the inner coil form diameter was estimated to 8 mm, because it´s not visible on any picture. By means of this values a plastic coil form was formed on a lathe and an angle bracket was made. More difficult was the determination of the electrical data. For the operation of the solenoid 16 V DC or AC voltage is specified. Further on the plans one can see, that the winding stretches to approx. 1 mm below the outer coil form diameter, what´s important for the filling volume of the winding. On some construction plans a relative large wire diameter (approx. 1 mm) is shown. But this was unrealistic, because in this case a very high current would flow. On the most illustrations in contrast no single turns are visible, what indicates a small wire diameter. The deciding tip was, that the diameter is 0,35 mm, which less insulation coating corresponds to a copper diameter of approx. 0,3 mm. With these data a winding with approx. 1000 turns and 10 Ohms resistance can be calculated, corresponding to an ampere-turn of 1600 A at 16 VDC. However the reproduced solenoid should not be operated with 16 V, but at the same magnetic induction with 8,4 V (7 cell accu). Therefore a short (simplified) insight into theory is necessary: The magnetic induction of a solenoid is calculated to
The product n · I (ampere-turn, also magnetic potential difference) had to be constant while using new and interdependent values of n and I. Without details one can show, that in this case at given volume of the winding the number of turns has to be proportional and the wire cross section has to be inversely proportional to the voltages; the wire diameter thus is in inverted proportion to the square root of the voltage ratio.
Motor-DriveIn the first models a MEKANIK-motor from the sixties was used, which in series connection particular at stillstand has a strong torque and furthermore can be provided with different gear reductions.A little complicated at such kind of motor is the reversion of the rotation direction. The reverse polarity of the supply voltage changes the polarity as well of the rotor current as of the exciting current and the original direction is preserved (whereby the motor also is usable for alternating current). So for reversion of rotating the polarity of either the rotor winding or the exciting winding has to be changed, what´s not possible without more ado with operation at an electronic speed controller. This problem can be solved e. g. by using a polarity dependent relais circuit consisting of some electronic components. This circuit detects the polarity of the controller output voltage and switches the direction of rotor or exciting current before the motor starts.
After this procedure the motor operated without problems again. But the abrading operation because of driving the large models has been stopped soon in consideration of conservation of the motor. Manufacturers of construction kits admittedly offer drives with low cost motors, but often these are too large or improper because of other reasons. Necessary for a small construction e. g. is a convertion of the power train from the longitudinal to the transversal direction of the drive, what mostly is been realized by a worm gear transmission. A worm gear transmission however is self-locking, a vehicle in contrast has to roll out.
Accrued is a drive which is not only size compatible to the MEKANIK-motor, but also stronger at about the same rotation speed using a commercial low cost model motor. Cylindrical Gear DifferentialIn metalkit models a differential often is not usable, because they are either too large or not stable enough. In this case mostly the model is driven by only one wheel and the other wheel rotates independent on the same axle.Proposals for build up bevel gears differential are available in instruction books, but even this models because of the necessary separation of the axle in 2 semiaxles are particular instable. Furthermore brass gears can be unsuitable for driving heavy models, but commercial steel bevel gears are relative expensive.
For building up this differential is to notice, that all gear wheels of course are in contact, and therefore the number of teeth is to match to the arrangement. That means, that an arrangement in 60° steps requires that the number of teeth allows a 60° gearing. In the actual case cylindrical gears with 12 teeth were used; by using gears with 13 teeth this differential could not work. Drive UnitOn the base of the proven motor-drive a complete drive unit was constructed and built up.
Except for the 90° V drive modul 1 steel gears were used which not only look very solid, but also dont´t require too much precision. Relating to the bearing of the differential also has been deviated from the known MÄRKLIN- and MECCANO-proposals: Because the driving force acts onto the cage of the differential as a result the cage was supported, whereby the semiaxles concentrical stick out of the bearing pins. The benefit of this is, that also with strong forces the differential cannot be pushed aside.
RFI (Radio Frequency Interference) Suppression of Collector MotorsGeneralMECCANO models often are driven by electromotors. These usually are low cost collector motors, which naturally generate high frequency (HF) interferences because of sparking; therefore as a general principle they should be RFI suppressed. Manufacturers resp. distributors of such drives at least should point out the suppression because last but not least also regulations exist for RFI suppression. Particularly radio controlled models respond very sensitive to interferences, up to complete loss of operation. The same applies if collector motors are operated by electronic stabilized power supplies; in dependence of the motor power also a power supply damage is not improbable. If such effects occur during motor operation, at first the RFI suppression should be checked and possibly upgraded. There are several ways for RFI suppression. Usually capacitors are used because they are small and (can be) able to work properly. Also choke coils can be used for suppression, and in very persistent cases complete filters consisting of a combination of capacitors and chokes can be applied. The following proceedings are universally valid for RFI suppression of electric and electronic components; in EMC (electromagnetic compatibility) engineering they are an essential standard. RFI suppression with capacitors This kind of suppression bases on that, to short the generated HF ideal as possible immediate at the source (thus at the motor) before it is spreaded. Usually ceramic capacitors are used because they have excellent HF properties. Film capacitors e. g. are wounded and hence they have more inductance; for HF Suppression therefore they are less suitable.
As guide values for x- resp. y-capacitors 100 nF (nanofarad) resp. 47 nF are used. The suppression would also work with other capacities; much more important than the values are the connecting leads of the capacitors! They have to be short as possible, because otherwise the suppression cannot work optimal. The reason for this is, that for a wire an inductance of approx. 10 nH (nanohenry)/cm can be assumed. A multiplication with the angular frequency of e. g. 40 MHz (typical radio control frequency) results into an inductive resistance of approx. 2,5 Ohm. With other words: Every cm wire at 40 MHz has a resistance of approx. 2,5 Ohm; the for HF suppression necessary (ideal) short circuit is not existing. Particularly y-capacitors are affected, because they have to absorb the high components >1 MHz. The wires of x-capacitors are less critical, and often a x-capacitor can be omitted. If there is doubt at the function of a suppression, at first the connecting leads of the capacitors should be checked and possibly shortened.
In the simplest case a selfmade coil without core (air-core inductor) with 5 to 10 turns can be connected into the motor supply lines. Because of the missing core and the resulting low inductance this coil however will damp only very high frequencies; on the other hand also at a high motor current this coil will not be ineffective because of saturation. For all chokes of course an adequate wire cross section is to consider. A better RFI suppression is possible by using ferrite core chokes. As the name says, this chokes have a ferrite core and this results into more inductance at the same size; because of the large "air gap" between the poles of the core also this coil will not be saturated. A further alternative is the use of a current compensated choke. At this choke both windings are placed on one toroidal core, whereby the magnetic fields of the motor current are compensated; with this a saturation of the core is avoided. But this works only, if the choke is connected proper; otherwise the magnetic fields will be added and the choke will be ineffective because of saturation. ery elegant is the RFI suppression with a clamp ferrite. These are available in different sizes and will be clipped together on both motor supply lines. The operation corresponds to a current compensated choke, but without soldering. Certainly the reachable inductance usually is significant less than of "true" chokes, because on principle it is a choke with only one turn. If the ferrite is big enough, the motor supply line can be feed through several times, whereby the inductance increases (theoretically) with the square of the number of turns. But also this is limited, because the side by side placed turns result into a capacity which increasing shorts the inductive resistance. RFI suppression with complete filters This is the combination of the 2 above described proceedings. Hereby capacitors and chokes can be connected in manifold methods, without deepening at this place. For industrial applications a large number of different filters is available; for model motors such a complex filter probably only in particular cases is justified. |
