[Amps] Use of 60Hz blowers on 50Hz mains for cooling ceramic triodes/tetrodes

Jeffrey Madore K1LE@ARRL.NET
Wed, 14 Aug 2002 09:41:01 -0400

----- Original Message -----
From: "Michael J. Tubby B.Sc. (Hons) G8TIC" <mike.tubby@thorcom.co.uk>
To: <AMPS@contesting.com>
Sent: Tuesday, August 13, 2002 3:43 PM
Subject: [Amps] Use of 60Hz blowers on 50Hz mains for cooling ceramic

> All,
> As I recently posted, my Commander-II amp went up in smoke
> after an extended period operating WSJT meteor scatter during
> Perseids, even though I was running it at under half max power.
> I've now stripped it down to get a new transformer made, and
> while I was at it I decided to take a look at the blower that cools
> the 3CX800 (and the whole amp). The blower is made by Revcor
> in the USA and is model Q30B (guess it might be 30 CFM looking
> at its size and model number).
> The thing is that it has a synchronous motor on it, and while the
> Commander-II was strapped for 234VAC operation, the motor
> is 110/115VAC and was across half the primary and operated in
> auto-transformer mode - this is okay as far as obtaining the right
> volts, but doesn't address my other concern...

Mike, my guess is that it is not a synchronous motor, but an induction
motor. A synchronous motor locks right into the AC synchronous speed and
stays there. Hence, a 2 pole synchronous motor @ 60 Hz, would turn at 60
cycles per second - which equals 3600 cycles per minute, or 3600 revolutions
per minute. This type of motor also requires a more sophisticated starting
circuit, field excitation, etc.

An induction motor attempts to approach synchronous speed but cannot attain
it, as slip is required to produce rotor current and thus torque. So, a 2
pole induction motor operated at 60 Hz will often have a full load speed of
something like 3450 rpm.

My concern is, that an induction motor will not only run slower but will
draw more current (run hotter) as the frequency is reduced below nameplate,
without also reducing the voltage. This is simply due to the reduction of
inductive reactance.

Since an induction motor is basically a power transformer with it's
secondary on bearings, my question to the group is: is the same power
transformer used for 60 Hz and 50 Hz? If this be the case, I would think
that the core area and/or coil turns would need to be increased, to avoid a
significant increase in the "watt" component of the excitation current.
Possibly this is figured into the transformer design and rating? If not,
might this be a cause for increased heating / loss of life, of USA power
transformers operated overseas?

73, Jeff - K1LE ><>