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Why can't transformers be powered by DC power?
Time：
2022-10-31 14:10
Source：
A transformer is a device that increases or decreases alternating current or voltage. It can only work under alternating current (AC), not direct current (DC).
But what happens if we connect DC power to a transformer?
01. What happens when the transformer is connected to the DC power supply?
We all know that the working principle of the transformer is electromagnetic induction, only in the primary coil through the changing alternating current, the secondary coil will produce continuous induction electromotive force, and then achieve voltage change (two coil turns ratio is equal to the voltage ratio). And connected to the DC power supply, although it can also generate induced current, but only in the moment of just connected to the power supply and just disconnected power supply to generate induced electromotive force, so can not change voltage. But that's not the only reason transformers can't connect to DC power.
Next, let's connect a transformer to the DC power supply and see what happens.
Suppose:
Primary voltage V= 230V
Resistance R= 10 Ω
Inductance L = 0.4H
The frequency f is equal to 0
According to Ohm's law, the current in the primary coil of the transformer is:
I= V/R=230V/10Ω=23A
In the case of DC supply, the current of the primary coil is 23A. What does 23A mean?
High current!!
You know, 50mA can kill someone. Now we connect the transformer to the AC power supply, we can clearly contrast the huge difference in their primary coil current.
02, transformer connected to AC power supply current is how much?
Let's make AC power supply and DC power supply have the same voltage:
Primary voltage V= 230V
Resistance R= 10 Ω
Inductance L = 0.4H
Frequency f=50Hz
Let's see how much current flows through the primary coil of the transformer in the case of alternating current.
We know that AC resistance = impedance (Z) :
Z=√ (R2+ XL)2
Intermediate inductance XL：
XL=2πfL
XL=2*3.1415*50Hz*0.4H=125.67Ω
Integrate the two formulas to get:
Z = √ (102 + 125.672 )=126.1Ω
So the primary coil current is:
I = V / Z=230V/126.1Ω=1.82A
It can be seen that when the transformer is connected to AC power supply, the inductive reactance is large, so the AC impedance is large, so the primary coil current is small. However, if DC power supply is connected, the inductive reactance is 0 directly and the resistance is small, resulting in excessive current of the primary coil.
And the primary coil current at 23A is still a steady current. Because it is a DC power supply, the current of the primary coil will not change, resulting in a constant magnetic flux, flux change rate is zero, so the induction potential is zero. Due to the low resistance of the primary winding, the primary winding will consume most of the current. As a result, the primary coil will continue to overheat and burn out, and burn out fuses and circuit breakers, and even cause the whole transformer to explode!
Therefore, be sure not to connect the primary coil of the transformer to the DC power supply. However, transformers are not absolutely impossible to use DC power supply, under certain conditions, can also be safe to use.
03. Under what conditions can the transformer be connected to DC power supply?
There are two situations in which we can connect the transformer to the DC power supply:
● Pulse DC power supply as input
● Primary coil with high resistance and series
Pulse DC power supply as input:
When pulsed direct current is fed into the primary coil of the transformer, the negative period resets the magnetic flux and the time integral of the voltage is zero in a full cycle, thus resetting the magnetic flux in the coil. This case is used for SMPS(switching power supplies).
Primary coil with high resistance and in series:
We can run the transformer on a DC power supply by connecting a high resistance in series to the primary coil of the transformer.
When the primary coil of the transformer is connected to the DC power supply, the high resistance is connected to the primary in series, which limits the primary current to a safe value, thus preventing the primary coil from burning out.
Conclusion:
To sum up, the transformer can only work under alternating current. If use DC power supply, is very dangerous!! May cause the primary coil of the transformer to smoke and burn. So if not specific requirements, do not let the transformer connected to DC power supply.
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