Contactors

Contactors are chosen based on the following criteria:

• Application-based on IEC utilization category
• Load current and voltage
• Control voltage for selection of coil voltage
• Type of contact (Normally open NO / Normally Closed NC)
• Number of poles

Contactors are categorised based on the type of load which is generally based on IEC utilisation categories – IEC 60947, current and power rating. Ratings of a contactor are given according to the pole of the contactor. It also relies on factors like fault withstand current, coil voltage and many more. According to their rating, contactors are categorised into the following:

• AC1: Non-inductive or slightly inductive and resistive heating type of loads
• AC2: Contactors for starting of slip-ring motors
• AC3: Starting of squirrel-cage motors and switching off only after the motor is up to speed
• AC4: Starting of squirrel-cage motors with inching and plugging duty
• AC11: Auxiliary control circuits
• AC-15: Control of AC electromagnets
• AC-56b: Switching of capacitor banks
• DC–1: non-inductive or slightly inductive and resistive heating type of loads
• DC-2: Starting, inching and dynamic breaking of DC shunt motors
• DC-3: Starting, inching and dynamic breaking of DC series motors
• DC-13: Control of DC electromagnets

More often than not, contactors are used in three-phase applications while relays are more generally used in single-phase applications. A contactor does not have a common between the phases and joins 2 poles together. A relay uses a common contact that connects to a neutral. Contactors are usually rated up to 1000Vand relays are typically rated up to 250V. Most applications that have a rated current of 10A or more normally use contactors.

With conventional contactor technology, different contactors were needed for different network voltages. Thanks to the wide operating range of the AF contactor, it can operate just as well in all parts of the world from Europe to Asia and North America. The core coil of the AF contactor range covers a voltage rating ranging from 100-250 V, AC/DC and operational frequency of 50/60 Hz.

It also caters to surge suppression. With traditional contactor technology it is recommended to use an external surge suppressor, an accessory that could cost as much as half the contactor itself. With the AF technology the surges are managed by the contactor itself and the surge never extends to the control circuit. Neither the surge suppressor nor the actual surge has to be considered anymore. One less product and one less complication to worry about.

The cause of the no-action failure of the thermal overload relay may be due to the following reasons:

• A very large current setting
• Burnt or sealed off thermal element

In order to correct this, the setting current can be adjusted appropriately according to the load capacity, and the thermal element or the action mechanism can be overhauled.

The cause of malfunction of the thermal overload relay may be due to the following reasons:

• Current setting value below the feasible minimal value
• Unmatched thermal overload relay and load
• Motor starting time is too long or too many times of continual starting
• The thermal overload relay is subject to strong shock or vibration,

In order to correct this identification of the possible cause and fixing it in terms of adjusting the current setting range, or circuitry fault or working environment is crucial.

In case of the thermal overload relays provided by IPD its resistance to shock is rated at 25g / 11ms and its resistance to vibration is rated at 3g / 3 ... 150 Hz.

There are two ways to reset the thermal overload relay, namely manual reset, and automatic reset. The first way involves a manual reset, and the second is an automatic reset.

Manual Reset
After the thermal overload relay’s protection action, the reset button needs to be pressed by hand to make the normally closed contact return closed. Manual reset should be performed 2-3 minutes after the tripping because the internal bending heating sheet needs cooling.

Automatic Reset
After the thermal overload relay’s protection action, the normally closed contact is automatically closed, and the time of automatic reset is usually no more than 5 minutes. The reset method can be selected through the reset adjustment screw.

Most importantly the ampere-second characteristic of the thermal overload relay should be as close as possible or even coincide with the overload characteristic of the motor, or under the overload characteristic of the motor. Meantime, the thermal overload relay should not be affected and not act at the moment of short-term overload and startup of the motor.

Thermal overload relays are chosen based on the following criteria:

• Ampere setting range
• Fuse type (short circuit protective device)
• Tripping class
• Rated operational voltage
• Number of poles
• Suitable contactor
• Operating frequency without early tripping

• The thermal overload relay does not offer short circuit protection, it can only be used for motor overload and phase failure protection.
• Place of installation
• The temperature difference between the installation place of the thermal overload relay and the protected equipment should not be too large
• Very low to non existent source of vibration at the point of installation

The binding screws of the thermal overload relay should be tightened, if not, the contact resistance and the temperature of the heating element will rise, which will affect the thermal overload relay operation and cause it to malfunction.