Discharging an inductor

Question

I wanted to know if it's practically possible to use inductors the same way we use capacitors , in the sense of charging it using a charging circuit then discharge in another circuit.

I thought it could be possible but then i read on the web that when an inductor is disconnected the magnetic field starts collapsing inducing a very high voltage, wouldn't this voltage breaks down any transistors used in the switching from the charging to the discharging circuit? If i understand this correctly, this only happens when there is no way for the current to go so voltage builds up, but then again there is a small split of second between the switching between the 2 circuits, will that cause such a problem?

Edit:

If I used this circuit to charge and discharge the inductor, will there be any voltage spikes?

Answer 1

I find it helpful to think of capacitors and inductors to be complimentary.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. Ideal and imperfect components.

• Capacitors store energy in an electric field. Inductors store energy in a magnetic field.
• A capacitor holds energy when open circuit. An inductor holds energy when short circuited.
• Capacitors lose energy through parallel leakage resistance. Inductors lose energy through series resistance.
• Capacitors "like" to keep the voltage across them constant. Inductors like to keep the current through them constant.
• When a capacitor is short circuited the resultant current is very high. When an inductor is open-circuited the resultant voltage is very high.

... when an inductor is disconnected the magnetic field starts collapsing inducing a very high voltage, wouldn't this voltage breaks down any transistors used in the switching from the charging to the discharging circuit?

Yes it would but there's a simple solution:

Figure 2. A simple buck converter. Source: All About Circuits.

In Figure 2 S is the transistor switch similar to that mentioned in your question. When it is switched the inductor tends to maintain current in the direction of I. Since the right side of L is "held" by C and current is to keep going then the left side of L goes negative to try to maintain current. Whe the voltage reaches -0.7 V D starts to conduct and maintains the current through L keeping it "happy" and avoiding a transient high voltage.

You will see this arrangement more commonly in snubber diodes on relay coils.

^{simulate this circuit}

Figure 3. A typical relay control circuit. Without D1 the inductance of the relay coil would generate a large negative voltage on switch off. This would be likely to destroy Q1 . The diode limits the negative excursion on Q1 collector to -0.7 V.

Answer 2

Disconnecting inductor is the same like shorting a capacitor in the way they are similar.

A perfect inductor, if it would be shorted, would store energy. But this is only possible for superconductors. In reality losses on wire resistance waste the energy quite quickly.

But not instantly. In fact in switching power supplies this is exactly what inductor does: charges from the source and discharges to the load. Just fast enough so losses are small (and for several other reasons)

Answer 3

Superconducting inductive energy storage has been used commercially to some extent, so it can be considered to be "practically possible." It appears that they probably need to be of the megawatt hour or tens of megawatt hour scale to be practical. Even at that scale, commercial viability is questionable. For additional information look at the Superconducting magnetic energy storage Wikipedia article and the references cited there.

Answer 4

Lets consider storing Energy in both a capacitor and an inductor.

For a capacitor we charge it to some voltage by storing a charge between its plates and we we can easily show \$E = \frac{1}{2} C \cdot V^2\$. in principle if we can take a charged capacitor out of one circuit and put it in another we can make use of this Energy in another circuit or in the same circuit at a later time. One common use for this is to detect when power is failing and use this to store last settings and ensure a clean shut down on products with microprocessors in them. We can't store energy in a capacitor forever however as real capacitors have leakage and will eventually self discharge.

For an inductor we store energy in a magnetic field and we can easily show \$ E = \frac{1}{2} L \cdot I^2 \$ To store this energy having charged it we need to keep the current flowing so need to place a short across the inductor. Because real inductors have resistance this makes them less useful than capacitors for storing energy long term as they tend to be big and lose this energy relatively quickly. However inductors are used to store energy in several circuits. One obvious example is a buck type switch mode power supply

^{simulate this circuit – Schematic created using CircuitLab}

In this circuit we apply a positive voltage at V1 greater than the output. This causes the current in the inductor to increase, ramping up. When V1 disappears or goes negative current continues to flow in D2 and ramps down. The inductor is continually storing and releasing energy to provide a DC output voltage.

Answer 5

Yes, inductors can be used to store energy. That's the basis for many switching power supplies, just to mention one example.

However, the problem with storing energy in a inductor is that the current has to be kept circulating. Our current technology makes that quite lossy for long term storage. The inevitable resistance of whatever conductor is used to make the inductor will dissipate the energy rather quickly.

There have been such things as inductors using superconductors. Those can store energy "long term", but such things are difficult and expensive to do with our current state of technology.