[Prev][Next][Index][Thread]

Why Triac/SCR Controllers Don't Work Well

Controllers based on Triac and SCR light dimmers or motor
controllers do not work well on Tesla coils. There
are several reasons for this.

The controllers work by turning on at some phase
angle past the zero-crossing point of the sine wave
shaped AC input.

 (a) Fine for resistive loads like light bulbs.
     (Light bulbs will often emit noise, however, and
     useful lifetime is often reduced)

 (b) Pretty good for AC/DC style motors.
     These series-wound motors have DC style armatures
     and speed is a function of average voltage.

 (c) AWFUL for true AC only motors.
     These rely on the rotating magnetic field
     and the speed is based on line frequency
     more than average voltage or current.

 (d) HORRIBLE for AC synchronous motors.
     These rely on AC line frequency primarily.
     Changing the average voltage only affects
     their torque.

 (e) *DISGUSTING* when used with a transformer!
     The transformer has a totally different kind
     of response to the sine wave and the sharp
     rising edge of a Triac/SCR gated waveform.
     At 50% the waveform presents a sharp rising
     edge that goes from 0 to 170 volts (peak)
     in less than a millisecond. The transformer
     will often make a loud thumping noise at
     this point. The transformer does not like
     the waveshape, and the sharp rising edge gives
     rise to some nasty inductive effects. The
     result is nasty spikes that often cause the
     Triac/SCR to trigger falsely. At this point
     the controller is not controlling at all.
     You have electrical chaos.

 (f) **POSITIVELY PUTRID** when trying to control
     a Tesla coil. You have all the problems of
     (d), plus the RF that the Tesla coil is
     generating to contend with.

Use a variac. These really work well.

Fr. Tom McGahee




-----Original Message-----
From: Tesla List <tesla-at-pupman-dot-com>
To: tesla-at-pupman-dot-com <tesla-at-pupman-dot-com>
Date: Thursday, February 03, 2000 10:14 AM
Subject: Re: Cheap Variac Substitute?


>Original Poster: Ed Phillips <evp-at-pacbell-dot-net>
>
>Tesla List wrote:
>>
>> Original Poster: "B2" <bensonbd-at-erols-dot-com>
>>
>> Hi All,
>>     I recently got a catalog from Eagle America tools.  On page 49 is a
>> router motor speed controller rated up to 3 1/4 HP or 115 Volts/15
Amperes.
>>  It is on sale for $34.99.  The part # is 421-1300.  Web page is at:
>>
>>  http://www.eagle-america-dot-com/
>>
>> Question:  Has anyone tried this particular unit successfully on a Tesla
>> coil?  Did filters improve the performance of this unit (aka prevent it
>> from being cooked)?
>>
>> Cheers,
>>
>> Barry
>>
>
> Probably won't work at all.  Those units (at least most of them) are
>intended to work with universal (AC/DC) motors, and depend on the
>counter emf of the armature for speed sensing.  With a transformer there
>won't be any real counter emf, so control will be wacked up.
>
>Ed
>
>
>