Reducing transformer insurge

a very common means is called "step start". resistors are placed in series with the primary to limit the current for the time it takes a relay to close. the relay contacts then short the resistors.

high power equipment (loads of about 10 kW) typically uses three 1 ohm resistors for 3 phase or two 2 ohm for single phase. 25 to 50W resistors usually suffice as the on time is brief.

another method is starting in delta then switching to Y.

in article 41713eb5$0$29944$ snipped-for-privacy@news.dial.pipex.com, esb at snipped-for-privacy@dsl.pipex.com wrote on 10/16/04 8:33 AM:

Breakers are available with various tripping characteristics. Do you know for sure that your breakers is not an instant trip breaker instead of one that allows for short time surges? For example, there are breakers that allow short time overloads for induction motor starting.

Bill

transformer from a 240V

switch-on always

unit to

works

thinner cable

larger cable to

to increase the

for both the

breaker should be

the transformer

installation, or a

field has built

running the

A tricky set of trade offs. I think you have it thought out correctly.. the lighter wire though is probably illegal for the rated load.. you could go to a 25 or 30 amp breaker if the thats legal or run a larger wire size.. why not?

If you were real serious about protecting the transformer the breaker is not the best way to go.. an added *dual element fuse ... sized close to peak load would work the best for that..

Phil Scott

I think resistance is the factor helping you. On smaller cable sizes, R tends to be >> X.

cable to

Yes, larger cable size is good.

increase the

A 20A breaker is maybe a little tight for a 4500VA txf on 240V?

installation, or a

A larger breaker size could solve the nuisance trip problem. You also could try starting the transformer with a load on it (with a load on, it draws more current, which must flow through upstream Z thereby decreasing V seen at primary, thereby decreasing flux magnitude (driving txf 'less far' into saturation).

The 20 Amp breaker is already too big for proper protection of this transformer.

As Repeating Rifle previously mentioned, there are circuit breakers specifically designed with transformer inrush in mind. I have even seen examples where a breaker with electronic circuit protection curves will work most of the time on a transformer.

4500/240*125%=23.5 amps allowed to go to 25 amps if available or 30 if not

PCK, your reply seems more in line with my calculations. The following is my reply to snipped-for-privacy@home.com - I was unable to reply to his email address.

I assumed that a 20A breaker should be about right as 4.5kVA transformer operating at full load from a 240V supply should be drawing about 19A. Are my calculations wrong? A delay type breaker may be what I require - I've used delay fuses (HBC / HRC) for motor start circuits in the past, but have not used delay breakers.

My understanding of the regulation miniature circuit breakers available in the UK is that there are three basic types - Type B,C and D, each with higher breaking current multiples, Type A will break in the required time with 5 x current rating and TypeB with 10 x rated current. That is, a 20A, Type B MCB will be guaranteed to break in 0.04 to 5.0 seconds when carrying an overload current of 200A or more, but with a current of 30A, it will take about 14 minutes to break the current. This is still within the max allowable overload for my 6mm sq cable of about 46A, but probably beyond the max overload current of the 4.5kVA transformer.

Eric

Do you concur with my reply? What type and size of breaker would you suggest for my installation? By the way, I am starting the transformer on load to try to reduce the in-rush as suggested by Operator Jay in a previous post. Once the MCB is correctly sized, I think a suitable soft start device would be the best option. Tim Perry's suggestion seems a good one, but I want to avoid switching large currents with mechanical relays - I may look at a system using a solid state relay. Do you have any suggestions? Thanks.

Eric

----- Original Message ----- From: "PCK" Newsgroups: alt.engineering.electrical Sent: Monday, October 18, 2004 4:54 AM Subject: Re: Reducing transformer insurge

Sorry, I misread your transformer ratings. PCK is correct. The email is not valid on purpose.

Pertaining to your question about miniature breaker ratings. Type B is usually for electronics and is 6-10x (varies by mfg.)

I just had to replace a type B on a 5 kVA transformer feed, 480 volt,

10 Amp 2 pole CB. The type B would trip randomly (10% of the time) on energizing. I replaced it with a D, although a C probably would have worked.

Your secondary protection should be selected to prevent low level primary overloads. The primary protection is mainly to protect the rest of your electrical systems from a fault on your 4.5 KVA transformer. If appropriate, faster acting protection may be used on the 420 volt circuit.

4500 VA/420=10.7 amps x 1.25 = 13.4 amps If you need the full 4.5 kVA then 14-15 amp CB or fuses for secondary.

My opinion on the other current limiting suggestions, are the fewer mechanical parts, the better. Less to fail.

Unfortunately that will not change the initial inrush much although it may cause a faster decay of the inrush current. Accept the fact that there will be inrush and use a proper breaker which can handle it and still provide protection.

what is your load in watts? is it constant?

Are

one transmitter i maintain uses an 11 KVA single phase HV transformer. it runs at about 35 A at 202 Vac. the manual calls for a 75 A breaker. the nearest available locally obtainable breaker value is 80 A. the primary is slowly ramped up with big SCRs configured as a TRIAC.

it works just like a big lamp dimmer.

the relay soft start method uses the response time of a relay closure to allow the power supply to stabilize so a SS relay might operate too fast (unless further delay is added to the control element or a time delay relay is used).