Current transformers and vacuum cleaners

Q: In the recent unforeseen question and answer, I mentioned the current transformer. What is it and how to use it?

A: Current transformers are just different uses of ordinary transformers. You can find it in textbooks, but many engineers seem to turn a blind eye! I use a current transformer to start the vacuum cleaner.

The transformer in the general concept consists of two magnetically coupled coils: a primary coil and a secondary coil with a 1:N turns ratio. If the voltage of the AC signal on the primary coil is V, the signal voltage N×V will be generated on the secondary coil.

When the transformer is used as a current transformer, the alternating current will pass through the primary coil. For the legendary "ideal transformer", the short-circuit secondary current is proportional to the input current and inversely proportional to the turns ratio.

If the secondary coil is not short-circuited, but connected to a resistor (assuming the resistor is not too large), the above relationship still holds. This provides a way to measure the alternating current in the secondary coil, from which the alternating current in the primary coil can be derived and allows us to measure the alternating current with an isolated current.

Current transformer design is usually not important, but in extreme cases (AC and DC measurements, high voltage, low loss measurements, etc.) still need attention. For power lines and audio frequencies, small core transformers found in dustbins or produced by Radio Shack are slightly modified, ie a new 1:5 primary coil is wound around the existing coil with an electronic wire (assuming there is enough space to make Such a coil) is generally sufficient to meet the needs. Commercial current transformers usually consist of a wire-wound toroidal core, the primary coil being a wire that passes through the center of the torus. The ring is sometimes hinged and can be opened to insert the primary coil.

The secondary resistance must be small enough to prevent the transformer core from saturating. The lower the resistance, the lower the possible operating frequency of the transformer. If the transformer output is applied to the inverting input of an operational amplifier with resistive feedback, the resistance will be very low and the minimum operating frequency is also very low. Usually, people only need to use miniature current transformers. The rated operating frequency is 10 kHz for 200 Ω loads and 50/60 Hz for op amps as loads.

In addition to measuring AC current, current transformers can also detect currents for control purposes. For example, a current transformer is used to detect the current in the woodworking saw to operate the vacuum cleaner. In fact, current transformers have been widely used in home power monitors.