The main contactor is to safely isolate the high voltage traction battery from the vehicle electrical system in general running as well as in case of malfunction. Generally, two main contactors are used in EV as positive main contactor and negative main contactor. The main contactors must be able to carry and safely switch considerably higher currents in electric vehicle.
Working characteristics of main contactor
As shown in Figure 1, the main positive contactor charges the unfilled capacitor at the moment of pull-in, so the load form controlled by the main positive contactor is a capacitive load;
The working characteristics of the main contact of the main positive contactor are: due to the small circuit resistance, which is equivalent to a short circuit, the main positive contactor has a large inrush current through the main contact at the moment of pull-in. The impact current of the main positive contactor at the moment of pull-in is large and is inversely proportional to the value of the capacitor precharge voltage;
During the driving of the vehicle, the main positive contactor has been kept closed. At this time, the current carried by the main positive contactor mainly comes from the operation of the main motor, air conditioning operation, PTC heating, air pump operation, etc.;
When the vehicle fails, the main positive contactor should be able to be reliably disconnected under load.
In summary, the following points should be fully considered when selecting the main positive contactor:
- Rated Voltage
The rated voltage is a voltage value that used in combination with the rated current to determine the purpose of the appliance. It is related to the corresponding test and use category.
Usually, the rated working voltage of the main contactor should be no less than 1.25 times the rated voltage of the working circuit. Our EVQ Series rated voltage is 12-900Vdc, and EVH Series rated voltage is 12-1500Vdc, surely they all meet the requirements of the current electric vehicle application.
- Rated Current
When the contact is closed, the current can continuously pass to the on-off part, and the temperature rise limit of other parts of EV contactor stud is not exceeded.
Generally, rated current of the main contactor should be no less than 1.25 times the rated current of the working circuit. For example, if the rated current of working circuit is 200A, technically, we should select a 250A rated main contact.
- Make and Break Power Switching Capacity
The main contactor should be able to make and break normal loads and overload currents without failure under the conditions specified. Switching cycles (electric life) and mechanical life should be specified clearly in data sheet.
It’s almost impossible for the main positive contactor to switch on load in normal operation of the electric vehicle, unless the vehicle is malfunctioning. So the requirements of this indicator of the main positive contactor is not particularly high
- Temperature rise of the main circuit
The main circuit should be able to carry the agreed heating current with temperature rise does not exceed the specified limit value. The temperature rise of the main circuit of the sealed DC contactor is related to the conductor’s cross-sectional area, connection method and system heat dissipation
- Capacitive load switching capability
The inrush current generated by capacitor charging shall be switched on and off by main contactor without malfunction under the conditions specified. Switching cycles (electric life) and mechanical life should be specified clearly in data sheet. Based on the warranty period of the vehicle electrical control system, we should test the capacitive load switching capability according to the actual precharge status, use environment and use frequency.
- Overload current duration
The ability to withstand overload current is overload current duration. If the current exceeds the limit, the contacts will separate, and immediately an arc is formed. And the contact temperature will increase, the contacts are then either welded together or the switching device is thermally destroyed after a contact opening with a burning arc.
In addition, select a fuse that matches the withstand overload current of the DC contactor, and ensure that the fuse opens before the DC contactor is damaged when the vehicle is overloaded or short-circuited.