transformer wave Figure 1
  • A transformer draws inrush current that can exceed saturation current at power up.
  • The Inrush Current affects the magnetic property of the core.
  • This happens even if the transformer has no load with its secondary open.
  • The magnitude of the inrush current depends on the point on the AC wave the transformer is switched on.
  • If turn-on  occurs when the AC voltage wave is at its peak value, there will be no inrush current drawn by the transformer. The magnitude of the current in this case will be at normal no load value.
  • If at turn-on, the AC wave is going through its zero value, then the current drawn will be very high and exceed the saturation current (see Figure 1).

In this scenario, the transformer has to be protected from inrush current.


Transformer Protection Guide - Typical Inrush Current Limiters for Select Transformers
Transformer
KVA
Single phase Input Voltage
Vac
Continuous Current
A
Inrush Current
A
Impedence X
(Ω)
Inductance Xl
(µH)
F
(HZ)
Energy
(J)
Min R
(Ω)
Recommended Part
.50 120 4.16 104 1.63 4328 60 23.4 4.9 SL12 10006
1.0 240 4.16 104 3.26 8642 60 46.7 9.78 SL22 10008
2.0 240 8.33 208 1.63 4328 60 93.62 4.89 SL32 10015
3.0 240 12.5 312 1.09 2881 60 140.6 3.26 AS32 5R020
5.0 480 10.42 260 2.6 6913 60 234 7.83 MS32 10015
10.0 480 20.83 521 1.3 3457 60 469 3.92 2x MS32 2R025 or
1x MS35 5R025

 Inrush Current Limiters for Transformer Applications - Try It Now - In Stock
Part UL R SSI
Max
Joules
Max
Voltage
Max
Digikey Mouser Farnell
SL12 10006 Y 10.0 6 40 240 570-1078-ND 995-SL12-10006 72J6734
SL22 10008 Y 10.0 8 90 240 570-1034-ND 995-SL22-10008 72J6819
SL32 10015 Y 10.0 15 150 240 570-1058-ND 995-SL32-10015 72J6844
AS32 5R020 Y 5.0 20 300 240 570-1106-ND 995-AS32-5R020  
MS32 10015 Y 10.0 15 250 480 570-1014-ND 995-MS32-10015 9006052
MS32 2R025 Y 2.0 25 300 480 570-1019-ND 995-MS32-2R025 72J6622
MS35 5R025 N 5.0 25 600 680 570-1029-ND   72J6634


Protection of the Transformer

This application note provides a convenient solution (see Figure 2) to deal with the problem of inrush current exceeding saturation current in transformers.

The solution uses an NTC Thermistor in series with the primary.

This NTC Thermistor offers high resistance at the beginning of switching and limits the inrush current.

After a short time, the NTC Thermistor resistance decreases to a low value due to self heating and does not affect normal operation.


Figure 2

Each transformer rating: 1000 VA, transformer step-down: 30 V
Total transformer rating: 2000 VA
Filter capacitors used: 30V, 2300 μF



NTC Selection Criteria #1: Energy

Energy required for the NTC: Inductive reactance of the transformer

transformer react

Note:

  • Peak Inrush Current occurs in one cycle = 564 A, as measured on the oscilloscope
  • Input Voltage = 120 VAC
  • Frequency = 60 Hz



NTC Selection Criteria #2: Steady State Current

Assume, Efficiency of transformer: 70%, Ambient Temperature: 75ºC, Minimum input voltage: 90 V



Normally thermistors are rated up to 65ºC for their operating current, and then a de-rating factor must be taken in to account.

Decision criteria: choose an NTC Thermistor that can provide at least the steady state current as calculated above:

Using the de-rating curve at 75ºC , use corresponding 90% of max rated steady state current,
= 0.90 X 36 A = 32.40 A

You can use any of the NTC Thermistors that are rated up to 36.0 A to meet your Steady State Current and Energy Requirements. See the charts for part numbers.



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