[Boatanchors] Are 400 Hz variac's good for anything but a real 'boat anchor'

[John Hurst]

The magnetizing current @60 Hz will *much* higher than 7 times the current
at 400Hz. The transformer iron will be driven into hard saturation (non-
linear) and the average inductance will approach that of an air core
transformer. The current will certainly burn out the winding with or w/o any
load on the wiper.


John, KU6X

-----Original Message-----
From: boatanchors-bounces at puck.nether.net
[mailto:boatanchors-bounces at puck.nether.net] On Behalf Of Fuqua, Bill L
Sent: Sunday, July 17, 2011 1:41 PM
To: jfor at quik.com; doc at kd4e.com
Cc: 'Boatanchors List'
Subject: Re: [Boatanchors] Are 400 Hz variac's good for anything but a real
'boat anchor'

The inductive reacatance is about 1/7  at 60Hz.
With no load the transformer will draw 7 times the current from the 
120 volt ac supply.  This this means the the power dissipated in the 
coil during no load alone is 49 times greater than at 400Hz.
That means that there will be lots of heating just due to primary current at
60 Hz.
Any current drawn from the tap will increase this considerably, unless it is
set at 100%,
since that current is  not just reflected to the "primary" but shared with
it 1:1.
So the part of the primary that carries the output current is also carring
the no load
input current. At 50% setting and 8 or 10 amps it is really going to get
hot. Again don't forget
power is proportional to current squared.
  Hotter than a firecracker on the 4th of July.

73
Bill wa4lav

________________________________________   
From: boatanchors-bounces at puck.nether.net
[boatanchors-bounces at puck.nether.net] On Behalf Of J. Forster
[jfor at quik.com]
Sent: Sunday, July 17, 2011 1:05 PM
To: doc at kd4e.com
Cc: 'Boatanchors List'
Subject: Re: [Boatanchors] Are 400 Hz variac's good for anything but a  real
'boat anchor'

> I am math-challenged, or more precisely, algebra-challenged.
>
> Are you saying that a 400Hz device operating at 60Hz has
> to be downgraded 6.5:1?

The allowable input voltage has to be decreased by 60/400.

> So if he stayed with 120vac and the device was rated 65A @ 400Hz
> it would be good for 10A @ 60Hz?

The current rating stays the same as for 400 Hz and 60 Hz. The max
allowable current is determined by the wire size, not the core.

> So one rated at 10a @ 400Hz would be good for ??? @ 60Hz?

10 A @ (60/400)*120 V at 60 Hzs.

> I am interested because I have a pair of selsyns/synchros that are
> rated at 400Hz and finally have a purpose where I can see if they
> are functioning correctly and can handle a little torque (turning
> a big roller inductor).

You can apply 60/400 of the allowable 400 Hz voltages at 60 Hz.

A big rollerductor is likely to need more torque than a selsyn can supply.

-John

===============

>> The issue is core saturation. If you run it at a lower input voltage, 18
>> V
>> or so you can still run it at 10 amps out. It's still not too useful,
>> but
>> you could use it with a transformer for a variable filament supply or
>> low
>> voltage power supply.
>>
>> 60/400*120V = 18V
>>
>> John,KU6X


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