Re: Transformer Soft Start

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Bob Simon <bsimon@q...> wrote:

>
> >
>
> Here's what's happening;
>
> When a transformer is switched on, a transient current flows. The

value of

> this current over the first half cycle of mains voltage exceeds the

normal

> magnetizing current by a very large factor, and is often sufficient

to blow

> the primary fuse or lead to excessive tripping of the input circuit
> breaker. This current decays exponentially over several cycles

until normal

> magnetizing current flows.
>
> This phenom is common in machine tool and welding transformers,

even with

> no load on the secondaries. ie; Even though the panel on the

machine says

> 50FLA, a 100A circuit breaker feeding the machine trips out when

the

> cabinet switch on the machine is thrown.
>
> Standard breakers and fuses react too fast to the inrush current.

What's

> needed is a slower disconnect to allow the current to settle to

normal levels.

>
> We usually install a high inrush delay circuit breaker that can

handle a 3X

> overcurrent for a couple of cycles. No more problem trips. Dual

element

Bob, You are totally right. The amount of inrush current to a
transformer is a factor of the magnetization curve of the core and
the angle (sin) of the voltage wave at turnon. Problems occur
because the inrush voltage at an improper phase angle causes the
magnetic core of the transformer to saturate and draw many times the
normal current. A large inductor like the power transformer shows
peak current when it is switched on at the zero crossing point of the
AC waveform. This is exactly opposite of a resistive or capacitive
load which operates best at zero crossing. If the transformer is
switched on at peak (and there is no residual core magnetization)
then the inrush current is minimal.

I have seen a scheme (EDN I think) where a slave relay circuit is
used to limit inrush current. They put a resistor in series with one
of the AC transformer legs and a NO relay's contacts in parallel.
The small DC supply for the relay coil (derived from the ac line) has
an RC factor that causes it to delay turning on and shorting out the
resistor for a few cycles. While it is more complex than using a
thermistor it has the advantage of being a low loss device and it
will operate without regard to a cool-down cycle required by the
thermistor device.

Delay fuses will work but the stress of magnetic saturation on the
other components can take its toll.