Contactor coil voltage: 24 V DC or 230 V AC?

A contactor is specified twice: once for the load side, and once for the control side. The load side gets all the attention - AC-3 rating, kW at 400 V, thermal current - and the coil is left to whoever is writing the bill of materials. That is how machines end up with a 230 V AC coil driven by a 24 V DC PLC output, or with a contactor that chatters every time the control cable runs more than 50 metres.

The coil is a separate circuit. Its voltage is whatever the control circuit provides, and it has no relationship to the 400 V the main contacts switch.

What the coil actually has to do

Three things, in this order:

1. Pull in at the lowest voltage the control supply can deliver. IEC 60947-4-1 requires an AC coil to pick up reliably between 85 % and 110 % of its rated voltage Us.
2. Hold without chattering at that same low voltage, across the temperature range of the panel.
3. Drop out cleanly when the control circuit opens - by 75 % Us for AC coils, and it must not drop out above 20 % Us.

The gap between the pull-in and the hold requirement is the reason coil data sheets list two power figures. An AC coil draws several times its holding power for the first few cycles, until the magnetic circuit closes. A 9 A contactor with a 230 V AC coil might hold at 7 VA but pull in at 70 VA. Size the control transformer on the pull-in figure, not the holding figure, and add the inrush of every coil that can energise simultaneously.

DC coils do not have this inrush peak in the same way, which is one of the reasons they are easier to size.

The four voltages you will actually meet

If you are replacing a coil, read the printed voltage on the coil body, not the label on the contactor front. Coils are interchangeable within a family - the same frame accepts 24 V DC and 230 V AC - so the front label describes the frame, not what is inside it.

Why 24 V DC wins on new machines

A 24 V DC coil can be driven straight from a transistor output, which removes the interface relay from the drawing. Three practical consequences:

• Wiring is simpler. One common, one conductor per coil.
• It is SELV. No 230 V in the same duct as the sensor wiring.
• It is silent. No mains hum, which matters on machines installed next to operators.

Two details are easy to get wrong. First, the freewheel diode: a DC coil is an inductor, and interrupting it produces a reverse spike that will eventually destroy the output transistor. Most coil families offer a built-in suppressor variant - take it. Second, a diode makes the coil release more slowly, typically adding a few milliseconds. On a press or a safety-related stop, that delay belongs in the risk assessment, not in a footnote.

With long control cables, DC coils have the opposite problem to AC: cable capacitance holds the coil energised after the contact opens. Runs beyond a few hundred metres, or cables with high capacitance per metre, may need a bleeder resistor. AC coils do not suffer from this, but they lose voltage to cable impedance instead, and a 230 V coil at the end of a 300 m run may never see the 85 % it needs.

Matching the coil to the overload relay

A contactor and its overload relay are usually ordered as a pair, and the coil voltage is the one field that has to match the rest of the panel. If the panel is 24 V DC controlled, fit 24 V DC coils throughout - including on reversing pairs and star-delta combinations, where three contactors have to be electrically interlocked.

On a reversing starter, both contactors must drop out before the other can pull in. A DC coil with a freewheel diode extends the release time; if the interlock timing is tight, check the release delay in the data sheet rather than assuming it is zero.

Reading the coil data sheet in 60 seconds

Four numbers matter:

• Us - the rated control voltage, with the tolerance band (usually 0.85-1.1).
• Pull-in VA / W - the figure to use for transformer and power supply sizing.
• Hold VA / W - the continuous draw, which sets heat inside the panel.
• Release voltage - the point at which the contactor drops out; matters for undervoltage behaviour.

Everything else on the page is marketing.

When a contactor buzzes

A contactor that hums instead of holding is almost never a coil failure. In order of likelihood:

1. Dirt or corrosion on the armature face. The magnetic circuit cannot close, so the coil stays at pull-in current and overheats.
2. Voltage below 85 % Us. Long cable, undersized transformer, or a 230 V coil on a supply that sags when a drive starts.
3. AC coil with a failed shading ring. The short-circuited ring in the pole face is what keeps an AC magnet from chattering at twice line frequency. When it cracks, the contactor buzzes and then burns out.
4. Wrong coil fitted. A 400 V coil on a 230 V supply will sometimes pull in and always overheat.

Replacing the coil fixes case 4 and hides case 1. Clean the mating faces before ordering anything.

A short selection rule

• New machine, PLC controlled: 24 V DC, with suppressor built in.
• Retrofit into a 230 V AC panel with no 24 V supply: match the existing coil voltage exactly, do not "modernise" it.
• Long control runs (over 200 m): prefer 230 V AC or add a bleeder to the DC coil circuit; check the volt drop first.
• Safety-related drop-out: check the release time for the coil variant you are ordering, not the family average.

The coil is the cheapest part on the contactor and the one that stops the machine. It is worth the two minutes it takes to read the four numbers on the data sheet.