1.Introduction to Contactors

In a battery system, contactors serve as switches that can be manipulated by a control system, essentially functioning as relays. 

These contactors are engineered to disconnect (or turn off) the circuit even under full load conditions, meaning at maximum current and system voltage.

There are two primary types of contactors: Normally Open (NO) and Normally Closed (NC). A Normally Open contactor has its contacts in an open position when the coil is de-energized. Upon energizing the coil, the NO contacts close, making them ideal for circuits that require connection when the coil is energized. Conversely, Normally Closed contacts are in a closed position when the coil is de-energized, and they open once the coil is energized. NC contacts are typically used in circuits that need to be disconnected when the coil is de-energized.

Consequently, we utilize EV contactors configured as NO to guarantee that the circuit is turned off in case of a power loss to the drive circuit.

The selection of a contactor is an iterative process that involves considering system specifications and fault scenarios. Most contactor manufacturers share the necessary information in datasheets. Therefore, a sensible approach is to create a comparison table that covers the initial basic specifications, such as thermal and mechanical characteristics.

Seven Steps for Contactor Selection:

• Check if the contactor is suitable for both thermal and mechanical loads.
• Determine the absolute maximum voltage and moving average current of the battery system to ensure that the battery system's current carrying time curve stays below the contactor's curve.
• Analyze the maximum short-circuit current that the contactor can withstand.
• Review fuse selection to ensure that the contactor can disconnect incomplete short circuits if the fuse fails to blow.
• Inspect the busbars and adjacent components to determine if temperature derating will affect the selection.
• Consider whether a contactor with auxiliary contacts is needed based on system requirements, for remote control, status indication, or interfacing with other devices.
• Choose the appropriate contactor based on installation space and method, such as panel mount, rail mount, etc.

During the DC breaking of DC contactors, the magnetic field energy stored in inductive loads is instantly released, generating high-energy arcs at the breaking point. Therefore, DC contactors are required to possess certain arc-extinguishing capabilities.

Medium/large-capacity DC contactors often adopt a single-breaking point planar layout with an integrated structure, characterized by a long arc distance during breaking and an arc-extinguishing chamber containing arc-extinguishing grids.
Small-capacity DC contactors employ a double-breaking point three-dimensional layout structure.

When selecting contactors, attention should be paid to the following points:
The rated voltage of the main contacts should be greater than or equal to the rated voltage of the load.
The rated current of the main contacts should be greater than or equal to 1.3 times the rated current of the load.
Coil rated voltage. When the circuit is simple and there are few electrical appliances used, 220V or 380V can be selected; when the circuit is complex, there are many electrical appliances used, or in unsafe locations, 36V, 110V, or 127V can be selected.
The number and type of contactor contacts should meet the requirements of the control circuit.
Operating frequency (number of contact opening and closing operations per hour). When the breaking current is large and the breaking frequency exceeds the specified value, a contactor model with one rating higher should be selected. Otherwise, the contacts may heat up severely or even weld together, causing loads such as motors to run with phase loss.

3.Pre-Charge Resistor

When the battery pack contactor is closed onto the motor and inverter, there will be an inrush current in the inverter capacitors. This extremely high current can at least cause aging of the contactor and may even permanently damage it.
Therefore, when we close the contactor on the battery pack, we do it in three steps:
Close the main negative contactor
Close the contactor with a series resistor
Close the main positive contactor
A simplified schematic diagram illustrates the basic principle.

4.The fault of the contactor

The contactor is the sole moving component within the battery pack, and while it is simple in design, there are numerous contactor malfunctions that can halt the operation of the battery pack.

The faults are typically:

• Permanently closed
• Permanently open
• Overheating
• There are various reasons for contactor failures:
• Poor contactor design
• Quality issues in manufacturing
• Incorrect sizing and selection
• Electrical considerations
• Both the LV (Low Voltage) and HV (High Voltage) sides need to be considered
• Thermal sizing
• Mechanical vibrations and shock loads
• Incorrect design of associated busbars and fixtures
• mproper control