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In the book I'm studying from Power Electronics - Mohan he says:

(...) has two drawbacks (1): In practice the load would be indictuve. Even with a resistive load, there would always be certain associated stray inductance (...)

This is in reference to a basic buck converter where for the sake of simplicty he explained the operation using a very basic circuit:

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Then, he said what I just quoted. He said that an inductive load is a drawback. Not only here, but in previous chapters they speak contemptuously about inductive circuits in general. So, my question: why is it so bad to have an inductive circuit or an inductive load in general (they can't be bad always, can they?)?

Miguel Duran Diaz
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  • Switching supplies use inductors all the time. Many loads are inductive, but if you know how to drive them, providing a path for the inductive current you're fine. If you want to draw out the circuit you are referencing we could comment on it. There's nothing inherently bad about inductors, inductive circuits or inductive loads. The reference is probably to trying to switch off an inductive load with no path for the current, but without a circuit diagram I'm just guessing. – John D Aug 31 '18 at 20:32

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Adding to Dave Tweeds good answer, it is the act of switching OFF a reactive load that can cause major problems. Most common would be a large inductor or solenoid or motor with large inductive values.

They store energy as a magnetic field and when the power is removed the field collapses, returning the same current back into the wires. If there is no path to discharge this energy it will normally arc across the switch or relay contacts, causing the need to replace them every few months or years. The crux of the problem is that the voltage (back-emf) will rise until it finds a discharge path.

If the source of power is an electronic device such as a UPS then damage can occur over time, as they have just simple 20 mm MOV protection against voltage spikes. Any electronic device or AC outlets that drive a heavy reactive load should have strong surge protection.

NOTE: A UPS in general is NOT designed to drive heavy reactive loads, even though it may have twice the current rating of the load. A UPS is designed to drive wall-wart transformers or switch-mode supplies with little to no reactive loading.

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It's actually an issue with reactive loads in general, although inductive loads are the type most commonly seen in industrial applications (lightly-loaded motors, etc.)

With a resistive load, the current always has the same polarity as the voltage, and this assumption vastly simplifies the design of power circuits in general.

With a reactive load, the inductance or capacitance sometimes returns energy to the source, which means that the current can have the opposite polarity from the voltage at times during the AC cycle. This is much more difficult to handle in a power circuit.

Sometimes it's well worth the extra effort — for example, in an electric vehicle, the same power circuit can handle driving the motor from the battery as well as the regenerative braking.

Dave Tweed
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