Re: Rotary spark gap
Posted by Bert Hickman on 11/5/2008, 11:28 pm, in reply to "Rotary spark gap"
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: Does anyone know if I can use a VFD on a regular induction motor to reach a
: sycronous speed? I think that rpm's will always lag behind the waveform
: because of slippage. Any help would be greatly appreciated.
Virtually all VFD controllers provide 3-phase outputs to directly drive true 3-phase motors. There are two major types of three phase motors - induction and synchronous. Three-phase induction motors use three identical stator windings instead of "start" and "run" windings seen in smaller single-phase split phase, capacitor start motors. The rotors of these consist of a series of shorted turns which generate the rotor field (sort of like a rotating transformer). The short circuited windings on the rotor "slip" to a speed that's lower than the field rotation rate in the stator. The amount of slip varies from approximately 0% at no load, to around 3-7% less than the stator field speed under low to moderate load. Even with slip, by driving a VFD at a suitably higher frequency, the shaft speed can be synchronized to the incoming line frequency, effectively making it "look" like a synchronous motor. However, most coilers using VFD-driven spark gaps usually operate theirs at variable speed in asynchronous mode.
In addition to stator windings, high power three-phase synchronous motors also use a set of three rotor windings, powered via slip rings. As with a single phase synchronous motor, the output speed of a 3-phase synchronous motor is "locked" to the incoming frequency driving the windings - they have zero slip.
By adding feedback from a tachometer, the VFD can maintain a fixed output shaft speed, under varying loads, using either induction or synchronous motors. Surplus three-phase motors are relatively inexpensive, perhaps because home consumers do not have direct access to three phase power, and most of them are unfamiliar with VFD's and/or phase converters. I've often seen surplus VFD controllers and drive motors priced at little more than their scrap metal value on eBay and at scrap yards.
A number of experienced coilers use VFD controllers and 3-phase motors to drive their rotary spark gaps since they can be operated in either asynchronous or synchronous modes, providing a great deal of flexibility. Also, most three phase motors tend to be larger (5 HP or more), making them ideal for larger, high-power rotary gaps.
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