DC keeps one polarity.
A DC voltage keeps the same polarity between its reference points. Its magnitude may be steady or vary. In a simple resistive circuit supplied by steady DC, current flows in one direction.
Batteries are familiar DC sources. A supply labelled DC need not be perfectly smooth: electronic power supplies can have some ripple.
AC reverses polarity.
An alternating voltage repeatedly changes polarity. A sine wave is an important example, but AC waveforms are not all perfect sine waves.
The plots compare steady DC with sinusoidal AC. The centre line is zero voltage; crossing it changes polarity.
Converting between AC and DC.
A rectifier converts AC into DC. Additional circuitry can smooth and regulate the result. An inverter uses DC to produce an alternating output.
These are functions within a system, not simply different names for the same device. A typical VFD uses both rectification and inversion.
Several types of supply in one installation.
A lift can have an AC incoming supply, DC control circuits and battery-backed equipment. A drive can create a controlled AC output for the motor.
A stopped lift may still have live supplies or stored electrical energy. AC versus DC describes electrical behaviour; neither label makes equipment safe to touch.
A closer look
For a pure sine wave, RMS voltage equals peak voltage divided by √2. RMS is useful for comparing heating effects in a resistor. It is not the average signed value of the wave, which is zero over a full symmetrical cycle.
Check your understanding
Must a DC voltage have a perfectly constant value?
Reveal the explanation
No. DC can vary in magnitude while keeping its polarity. “Steady DC” is the special case with a constant value.