[Amps] Step Start And Transformer Surge Current

[Will Matney]

A step start circuit is used for one thing, and that is to reduce the surge current when a transformer is first fired up (being magnetized). This current can be several times the operating or magnetizing current, and can last up to +/- 1/2 second. For amps that use a C-core (cut core) type transformer, a step start shouldn't be needed at all. I have seen some manufacturers use them though. By tests done by William McLyman at JPL, a gapped C-core does not have such surge current as compared to a transformer without gaps (IE interleaved construction or solid wound toroids). Interleaved construction does have the surge current phenomenon as mentioned earlier. The trade offs are that a C-core transformer has a larger magnetizing current (current at idle) than an interleaved transformer. This is due in part to the C-core having a lower permeability than an interleaved or solid wound core plus other losses. In other words, you'll fill the effects of a C-core in your wallet over the power bill. However, there was some tests done on what is known as a composite core. These can be made in the C-core or interleaved designs. The composite type meaning that 1/2 of the core is solid or interleaved, and the other 1/2 is gapped or butt stacked. This, from tests at JPL, showed that you get the best of both worlds. Both being reduced surge current, and reduced magnetizing current. They used composite cores mainly in some impedance transformers, but now are coming into the power transformer business. If one wanted, they could order this type of transformer and specify the core be a composite. I will stress this though, a C-core composite will be very much higher in price than an EI core which can be stacked. This is due to the fact that the C-cores matching solid core has to be wound similar to a solid toroid by special time consuming machines. Since the EI composite should act the same as the C-core composite, then the EI would be the best bet as it would be way cheaper. The weight will go up some, but it's not as much as one would think.

Information about this type of composite core, including the tests can be found in the book titled; Transformer and Inductor Design Handbook 2nd edition, by Col. William T. McLyman. The ISBN number is; 0-8247-7828-6.

I said I would not comment on the wiring debacle but one thing to mention. The neutral (white wire), and the ground (either naked or green) go to the same set of ground terminals (IE ground terminal strip) in the breaker box. Also, an electron is color blind.

Best,

Will