The most unusual 'anchor that's ever followed me home

[Mike Dinolfo]

Joe Buch wrote:
>
> At 05:43 5/27/98 -1000, you wrote:
> >Yesterday, a Federal Signal Corp. "air-raid" siren somehow ended
> >up in my garage - all 400 pounds of it. Model SD-10, 10 hp 240
> >VAC single phase motor driving an aluminum rotor.
> >The plate also says "F.L. Amps 54." (What is "F.L."?)
> >
> My guess is that the motor is rated to draw 54 Amps at "Full Load".
>
> >One question - how do I apply 3-phase 240vac to a single-phase
> >motor?
>
> There is no such thing as 240VAC 3 phase in standard USA electrical
> systems.

I disagree.  240 VAC 3 phase power may not be encountered very often in
residential work, but it is found in commercial and industrial projects
(although I think the preferred system voltage for most customers has
been 208 volts for the last few decades.)  In this area (mid atlantic
states), for example, 240 VAC three phase power is available from PEPCO
(Potomac Electric Power Company), and possibly also from BG&E (Baltimore
Gas & Electric) and Virginia Power.  The fact is that most power
companies will provide whatever voltage you request, if you are willing
to pay for it.

 If you only have 3 phase power available, connect the motor across
> the hot legs of two phases.  This will get you 208VAC. Good enough for
> government work, but put an equivalent load on the unused phase to avoid
> large neutral currents.

There's no neutral imbalance in connecting a single phase motor to a 3
phase system, since there's no neutral connection at the motor.
However, if you connect a single phase motor on a 3 phase system, and
then you make a neutral-to-third-phase power connection in order to
balance the phase loads, you _will_ create a neutral imbalance (from the
added load).

>
> The motor was intended to run on single phase power like that you probably
> have coming into your house.  For residential service you have a
> transformer belonging to the power company with a center-tapped secondary.
> The secondary is rated at 240 VAC with 120 VAC on each side of the center
> tap that is normally grounded at the pole and again at the service
> entrance.  If you have this kind of service, connect the motor across the
> two hot legs of the feeder.

Yes, you can do this, but I recommend (for your own safety) that you do
not.  I've been designing this type of system for decades (I've been a
licensed professional engineer since 1976, and a ham, I might add, since
1965)- believe me when I say that you're playing with fire if you think
that skills developed in connection with radios are adequate to qualify
a person to make this kind of connection on their own.  These are two
entirely different fields of expertise.

>
> Make sure you have a 100 Amp service properly fused or do not try this at
> home.  You will really be a hit in your neighborhood at 3 AM.
>
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> Joe Buch                                |  @  @ |     |  @    |
> Amateur Radio N2JB                      |       |_____|       |
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