Contactors Selection Guide
A contactor is an electromechanical switch that opens and closes its main contacts with the control voltage applied to its coil. Types, AC-1/AC-3 categories and selection steps.
What is a contactor? A contactor is an electromechanical switch that opens and closes its main contacts remotely with the control voltage applied to its coil. It is used to switch loads such as motors, heaters, lighting and capacitors frequently and safely. It provides no short-circuit protection; in a motor circuit it is used together with a thermal overload relay or a motor protection circuit breaker.
This guide explains what to look at when choosing products in the Contactors category.
Contact arrangement and designation
A contactor has main contacts that carry the load current, a coil that pulls them in, and auxiliary contacts that give position information to the control circuit. The main contacts are almost always normally open. Auxiliary contacts are designated NO (normally open) and NC (normally closed); the same NO and NC abbreviations also appear in product names. One NO contact is needed for sealing-in (latching), and one NC contact for the electrical interlock between two contactors.
Types of contactors
- 3-pole power contactors: used in motor feeders and referred to by their AC-3 current (as in 9A AC3 in Schneider Electric product names).
- 4-pole contactors: used for resistive loads such as heater banks and in distribution circuits where the neutral is also switched. In TeSys Deca LC1DT types the current is stated according to AC-1; the ABB AF09-40-00 and Chint NC1-1204M also have four main poles.
- Auxiliary Contactors: types that carry no power contact and multiply the control signal.
- Lighting Contactors: modular installation contactors mounted on a DIN rail. The Schneider Electric iCT and Easy9 silent contactors are also modular.
- Capacitor contactors: the product name of the Siemens 3RT26 series states the kVAr value. The power factor correction types of other brands are in the Power Factor Correction Contactors category.
- Medium-voltage vacuum contactors: the ABB ConVac series is not a low-voltage product; it is selected according to the medium-voltage project.
What the utilization category means
| Category / Criterion | Load Type and Switching Condition | Selection Rule | Catalog Link |
|---|---|---|---|
| AC-1 | Non-inductive or slightly inductive loads (heaters, distribution) | Size according to AC-1 thermal current rating at panel temperature | Contactors |
| AC-3 | Squirrel-cage motor starting and switching off during running | Match motor full-load current (In) and kW rating at operating voltage | Contactors |
| AC-4 | Inching/jogging and plugging of squirrel-cage motors | Upsize frame according to electrical durability curves | Contactors |
| Related Comparisons | High-cycle heater PID control or motor overload protection | Compare SSR vs Contactor and MPCB vs Thermal Overload Relay | SSR vs Contactor · MPCB vs Thermal Relay |
IEC 60947-4-1 classifies the load a contactor switches by utilization categories, and the rated current is given separately for each category:
- AC-1: non-inductive or slightly inductive loads; heaters, resistance furnaces.
- AC-3: starting squirrel-cage motors and switching the motor off while it is running.
- AC-4: starting squirrel-cage motors, plugging (reverse-current braking) and inching.
- AC-6b: switching of capacitor banks.
The same contactor carries a different current in different categories. In the ABB AF09-30-10 datasheet, the AC-3 current is given as 9 A (4 kW) at 380-400 V, and the AC-1 current as 25 A at 40 °C ambient with a 4 mm² conductor and 22 A at 70 °C. For this reason, in a heater circuit the manufacturer's AC-1 table is read together with the ambient temperature and the conductor cross-section. In AC-4 duty the contacts wear faster because they break the inrush current; the contactor is selected according to the manufacturer's electrical endurance curve.
Contactor selection steps
- Determine the load and its category. For a motor, compare the rated current and power on the nameplate with the contactor's AC-3 value; for a heater, compare the line current with the AC-1 value. If you only have the power value, you can calculate the current with the Current and Fuse Calculation from Active Power tool.
- Take the coil voltage from the control circuit; whether it is AC or DC is a separate choice. The 100-250 V coil of the ABB AF series is common to both 50/60 Hz and DC; the TeSys Deca types with the BNE (24-60 V) and KUE (100-250 V) codes also appear as AC/DC in the product name.
- Select the number of main poles and the auxiliary contacts in the housing; if they are not enough, see the Auxiliary Contacts category for add-on blocks.
- Add overload protection: a thermal overload relay or motor protection circuit breaker that matches the contactor's series and frame size.
- For forward-reverse and star-delta arrangements, select the mechanical interlock and connection kits from the Contactor Accessories category according to the contactor series.
How to read the type code
- Schneider Electric TeSys: in the code LC1D18E7, 18 indicates the AC-3 rated current and the E7 at the end the coil voltage. In the products in this category, B7 is a 24 V AC, E7 a 48 V AC, F7 a 110 V AC, M7 a 220 V AC and Q7 a 380 V AC coil; BD is a 24 V DC, ED a 48 V DC and FD a 110 V DC coil. The difference between LC1K1210 and LC1K1201 is the auxiliary contact in the housing: 10 is one NO, 01 is one NC.
- ABB AF: in the code AF09-30-10, 09 indicates the size of the contactor (9 A at AC-3), 30 the three NO main contacts and 10 one NO auxiliary contact. The AF116-30-11 has one NO and one NC auxiliary contact, and the AF09-40-00 is four-pole without auxiliary contacts.
- Chint NC1: the NC1-1810 is 18 A with one NO; the NC1-5011 is 50 A with one NO and one NC auxiliary contact. The Z at the end indicates a DC coil.
The same contactor is made with many coil voltages; before ordering, compare the last part of the code with the coil voltage on the product card.
Brands in this category
- Schneider Electric: TeSys Deca (LC1D), TeSys K and EasyPact TVS power contactors, iCT and Easy9 modular contactors.
- ABB: 3- and 4-pole AF block contactors, ESB installation contactors, NF auxiliary contactors.
- Siemens: 3RT and 3MT7 power contactors, 3RH2 auxiliary contactors, 3RT26 capacitor contactors.
- Chint: NC1 and NXC series; the product name states the number of poles, current, coil voltage and auxiliary contact.
- Tense: UEC1 and SGC1 series AC contactors.
Order notes
The sales unit is piece. For contactors listed as made to order, the lead time is confirmed before the order. The quickest way to reach the model with the coil voltage you are looking for is to type the manufacturer code, such as LC1D18E7 or AF09-30-10, into the site search.
Frequently asked questions
What is the difference between a contactor and a relay?
A contactor is a type of relay with high current and power ratings, designed to be connected directly to loads such as motors and heaters. Control relays with sockets switch signals and small loads and mostly have changeover contacts. In a contactor the main contacts are mostly normally open and the current is given with a utilization category such as AC-1 or AC-3 according to the load type. A relay or an auxiliary contactor is used as an intermediate element between a PLC output and the contactor coil.
Can a contactor with an AC coil be operated with DC voltage?
A standard AC coil and a DC coil cannot be used in place of each other; the coil must be supplied with the voltage and current type on its label. The windings are designed differently because the impedance of the winding limits the current in an AC coil and its resistance does so in a DC coil. Types that work with both AC and DC are made separately: the ABB AF series has an electronic coil interface and its 100-250 V coil is common to 50/60 Hz and DC; in Schneider Electric TeSys Deca the models with the BNE and KUE codes are stated as AC/DC in the product name.
Why does a contactor hum?
In a classic contactor with an AC coil, the pull-in force drops at every zero crossing of the current; the shading ring in the core delays the magnetic flux and keeps the core from vibrating at twice the mains frequency. If the ring is broken, the core surfaces are dirty or rusty, or the control voltage is low, the core does not close fully and the contactor runs noisily. This vibration does not occur in contactors with DC coils. Silent-operating modular contactors for residential and office enclosures are listed separately.
Why is an interlock placed between two contactors in a forward-reverse motor?
Changing direction is done with a second contactor that swaps two phases. If both contactors pull in at the same time, these two phases are short-circuited directly. To prevent this, the NC auxiliary contact of each contactor is connected in series with the coil circuit of the other; this is called electrical interlocking. In addition, a mechanical interlock that physically prevents one from closing while the other is pulled in is fitted between the two housings. Interlock and connection kits are specific to the contactor series.
Can a standard motor contactor be used on a capacitor step?
When a capacitor is switched in, it draws a very brief, high inrush current, and in a standard contactor selected according to its AC-3 value this current can cause the contacts to weld. Capacitor switching is defined in IEC 60947-4-1 by the AC-6b category. Power factor correction contactors limit this inrush current with early-closing contacts with pre-insertion resistors and are selected not by their rated current but by the kVAr power they can switch; the value is read from the manufacturer's table according to the mains voltage.