: Bert, thanks so much for your great insights on Tesla.
Thanks for the kind words!
: I'm currently
: building a motor utilizing flat coils (8"W, 23awg, 2" air core,
: 300turns) and N48 magnets. Would it be best to use a standard winding flat
: air core coil or use Tesla's parallel winding method that he patented in
Tesla was actually looking for a winding method that would reduce the effects of "self capacitance" between turns within a winding. This was an important factor when dealing with high frequency circuits. Using a single layer spiral (as in the patent you mentioned) is only one technique that can be used - there are others as well. However, in your application, winding self-capacitance is really not a problem, so you are free to use any winding technique that is easiest for you. The magnetic field will increase with increasing turns or increasing current.
: I am pulsing the coil with about 300vdc through a commutator but
: am looking to pulse the dc to the coil by electronic switching rather than
: draw extra load by physical contact.
: My questions are these: 1).What does the magnetic field look like in a
: flat coil? I thought of using Ferrofluid to view it in 3 dimensions but
: the wife has me on a tight budget.
The field will consist of loops that pass through the center region of the coil, then loop around the coil and back through again. The actual shape will be a function of the shape of the coil winding and the size of the open center space. For a solenoid shaped coil, the field looks very similar to that of a permanent magnet. For example:
: 2). Will stacking the coils (and connected in series) increase the
: magnetic field?
It depends on the current flowing through the single and series connected coils. If you force the same amount of current through the series connected coils, then the magnetic field will double. However, if the current flow is limited ony by the resistance of the coils, then connecting the coils in series will result in only half the current flow. In this case, the ampere-turns is the same as before, so the overall magnetic field will also be the same as before.
: 3). When permanent magnets are stacked, at what point (er how many) will
: adding more not increase the field? E.E. is is better to buy one
: 2"x2" rather than four 2"x.5"?
It really depends on how the magnets are stacked and which way they are magnetized. If you stack them in "series" (N-S-N-S), then a pair of magnets can be significantly stronger than one of the magnets, but NOT 2X as much. Eventually, stacking no longer increases the field strength - how quickly this occurs is a function of the magnet pole area versus its thickness. For typical magnets, the point of minimal increase may be reached with a stack of only 3-4 magnets. Generally, a single larger magnet will outperform a similar sized stack of smaller magnets.
: 4). What is the best way to switch 300vdc @ about 50ma?
A power transistor, such as a MOSFET, IGBT, or a bipolar junction transistor can be used. All of the above transistor styles are available in voltage ratings that will easily do this job. The type of transistor you select is mainly a function of how you intend to turn the transistor on and off. MOSFET’s and IGBT’s are turned on by applying a voltage to the Gate lead of the device. A bipolar transistor requires applying current into the Base lead. Irrespective of which device type you choose, you also need to provide a protection circuit to handle the voltage that’s developed across the coil when you attempt to turn off the current. A diode, or a combination of a diode and resistor, connected "backwards" across the coil, will do the job. Trying to switch off the current flowing through a coil without the protection circuit will cause a high voltage transient that may damage, or even destroy, the transistor.
: 5). And lastly, when a flat coil (or any coil)is de-energized, the
: magnetic field's polarity changes is that correct?
No. The polarity of the field does not change unless you actually reverse the current that flows through the coil. If all you are doing is turning DC power on and off to the coil, the field will increase and decrease, but will not reverse polarity. If you also have a moving magnet nearby you may induce current into the coil. The field created within the coil will always be in a direction that opposes that of the moving magnet.
: Thanks very much,
Good luck and best regards,