How do you turn on a three phase Variac with a large rated Current?

How do you turn on a three phase Variac with a large rated Current?

Variac Inrush Problem:

Looking at the photos below of a 55A per phase 3 phase variac: 0-470v with 415v input 50Hz air cooled.
Now, this variac is rated at 55A, but we wanted to run it from 32A per hase outlet with a 3 phase CB rated at 32A per phase with ‘C’ curve characteristics.
Initially, when we connected it to the 3 phase supply( with the 55A 3 phase variac in circuit), it blew the CB on turn-on due to the inrush every time we turned it on.

So we had to either change the CB or control the inrush current?

See the CB characteristic of the ‘C’ curve and ‘D’ curve CB.
We decided to control the inrush current via the use of 35A 2 ohm (cold ) NTCs. These are large NTCs made by AMETHERM USA Part # 570-1027-ND (MS35 2R035)
The resistance of the variac was about 0.5 ohm. (phase to phase resistance)
So with 415v input voltage; the max inrush was 415/.5+.5 =415A rms
With the NTCs in circuit in each phase on the input side: the max inrush was 415/2.5+2.5=83A rms
83/32=2.6 times rated current. This is < 5 times rated current. (This is very pessimistic calculation as it does not allow for inductance limitation.)

Now, see the current characteristics of the ‘C’ curve and ‘D’ curve breaker. We need the inrush current to be less than 5 times rated current for a ‘C’ type breaker
For a D type breaker it must be less than 10 times rated current.
Hence we design the three NTCs to be housed in a large grey enclosure and used large terminal blocks the make sure they can carry the current. We protected all components from excess heat which is generated when the NTCs are running at rated current . We used NOMEX heat shields.

RESULT: no need to change the protection CB or setting of the CB.

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