Hi, i have a circuit that consist of transformer to step down input ac voltage from 220 to 100v for servo motor driver.

I'm taken inrush current for transformer only (without any load connected ) by using inrush current meter, the inrush current get is not consistent, within 10~200A. The higher inrush current always cause the C curve type MCB to trip.

Is there any method to tackle this issue without changing the MCB? Such as add some component to lower down the inrush current draw by transformer to energizing?

 

You may be able to solve the problem by connecting a suitable negative temperature coefficient thermistor in series with the primary. I had to do this once with a 200 watt (240v to 12v) toroidal lighting transformer. It was intermittently tripping a 6 amp MCB. I have found that toroidal transformers seem to give this problem more than transformers with E & I laminations.

Les.

 

You may be able to solve the problem by connecting a suitable negative temperature coefficient thermistor in series with the primary. I had to do this once with a 200 watt (240v to 12v) toroidal lighting transformer. It was intermittently tripping a 6 amp MCB. I have found that toroidal transformers seem to give this problem more than transformers with E & I laminations.

Les.

What is in my mind is that, in order to energize the transformer, an inrush current need to be draw. If an NTC thermistor is connected in series with transformer, it will only act as a protective equipment for transformer that prevent high inrush current to flow into transformer which might cause harm to transformer.

This is because initially the NTC thermistor have low temperature (high resistance). When switch turn on, inrush current cannot flow through the NTC thermistor due to its high resistance. Then, energy will dissipate at NTC thermistor cause its temperature to increase, as well as resistance to decrease. Then, the steady state current can only pass through the NTC thermistor and flow into the transformer .
(https://en.wikipedia.org/wiki/Inrush_current_limiter)

Since in order to energize the transformer, high inrush current will be drawn no matter an NTC thermistor was connected or not and the NTC thermistor will only act as protective component. This will still cause the MCB to trip.

So, beside than this method, is there any possible method can be used?

 

I do not agree that it will still cause the MCB to trip. Another method I have used that will only work if the load on the secondary is low at switch on is to have a resistor (Or suitable rating incondescent lamp.) in series with the primary and a relay coil in parallel with the primary. The contacts on the relay close shorting out the resistor or lamp. when the initial current surge has ended as the volage drop across the resistor decreases enough for the relay to pull in.

Les.

 

Change it for a D type mcb...

 

Les.[/QUOTE]

I do not agree that it will still cause the MCB to trip. Another method I have used that will only work if the load on the secondary is low at switch on is to have a resistor (Or suitable rating incondescent lamp.) in series with the primary and a relay coil in parallel with the primary. The contacts on the relay close shorting out the resistor or lamp. when the initial current surge has ended as the volage drop across the resistor decreases enough for the relay to pull in.

Les.

Ok sir, so maybe i should give a try on using the NTC to limit inrush current. So, is there any link that help me to select a suitable NTC?
Below is my transformer specification:
1)2kVA
2)200VAC, 50Hz input source
3)Steady state current=~10A
4)Highest Inrush current get=~200A

 

Change it for a D type mcb...

Sir, my problem is that customer refuse to change their factory MCB, the customer need us to find a way to decrease the our product's inrush current, so that it willn't cause their factory MCB to trip

 

Hello,

Perhaps these might do the trick:
https://www.ametherm.com/inrush-current/high-inrush-current-protection-for-industrial-equipment
Or the "light" version:
https://www.ametherm.com/inrush-current/ms35-inrush-current-limiters


Here is an application note special for transformers:
https://www.ametherm.com/inrush-current/transformer-inrush-current.html

Bertus

 

I think the links that bertus has supplied should answer you question. When I needed to solve the problem I just looked through the range of NTC thermistors the Farnell stocked and selected one that had enough resistance when cold to limit the current to the required value vut had a low enough resistance when hot not to drop much voltage at the normal operating current.

Les.