AC vs DC
Alternating current reverses direction many times a second; direct current flows one way. Why it matters on site.
What they are
Direct current (DC) flows steadily in one direction — like from a battery. Alternating current (AC) reverses direction many times a second, rising and falling in a smooth wave.
Why the difference matters
AC is used for power distribution because it's easily transformed up and down in voltage and works with simple, robust motors. The constantly-changing field is what makes transformers and induction motors work — neither does anything useful on DC.
DC is used for electronics, control signals, battery systems, and the internal bus of a VSD. Many control circuits and sensors run on DC (e.g. 24V DC), so knowing which you're measuring — and that your meter is set correctly — matters.
On site
A transformer or contactor coil needs the right kind and level of supply. Measuring DC on an AC range (or vice versa) gives misleading readings. A VSD turns AC into DC and back to make variable-frequency AC.
Related faults
No control voltage in the panel
Nothing in the control circuit will operate — contactors won't pull in, indicators are dead, the PLC may be off. The control voltage that should be there simply isn't.
VSD powered up but won't start the motor
The drive is energised and the display is alive, but it won't run the motor. No fault may be shown — it just sits in 'ready' or 'stopped' and ignores the start command.
SSR not switching on (load won't energise)
A solid-state relay won't turn its load on even when commanded — the heater/load stays off because the SSR isn't conducting.
Related definitions
Transformer
Transfers electrical energy between circuits by magnetic coupling, stepping voltage up or down.
VSD (Variable Speed Drive)
Controls the speed of an AC motor by converting the supply to a variable frequency and voltage.
RCD (safety switch)
Detects earth leakage and disconnects fast to protect people from electric shock.
Control vs power circuits
Low-power control logic decides what happens; the power circuit carries the load — kept separate for safety and clarity.