How to Choose EV Contactors: Ultimate guide to select precharge contactor and heating film contactor
EV contactor is essentially an electromechanical switch, or relay, which reliably switch DC loads in electric vehicle (plug-in hybrid, pure electric, or fuel cell). The high-voltage battery must be connected or disconnect from the power train by EV contactors.
If you need DC contactors for your electric vehicle application, your EV system voltage is 450V or even 1000V, you expect EV contactors be able to carry and safely switch high current 10A in pre-charge circuit, 200A in main circuit. Also switching capacity and switching cycles (electric life) of contactor must be considered in case any malfunction……so many aspects have to consider, how can I size the right EV contactors?
No worry, below guide will help you save time and money to get the suitable one. Let’s dive in.
Pre-charge contactor
Pre-charge contactor is a secondary switch that is connected in parallel with the main contactor, the purpose of pre-charge contactor is to prevent a large inrush of current when main contactor closes. The power inverters for operating electric motors have large filter capacitors at the input stage, making it difficult for the contactors to close the main circuit. Initially, the filter capacitors are not charged, which could cause extremely high switch-on inrush currents. These would overburden the main contactor’s switching contacts and could lead to contact welding. Precharge contactors are used to prevent this. We use a precharge resistor that precharges the filter capacitors to a voltage of around 80% to 95% of the battery voltage. Precharging takes only a few tenths of a second and ensures that the resulting switching-on inrush current for the main contactor amounts to only a few hundred amps.
Working characteristics of Pre-charge contactor
Figure 1
As shown in Figure 1, when the pre-charge contactor is closed, the capacitor is charged, so the load form controlled by the pre-charge contactor is a capacitive load;
The working characteristics of the main contacts of the pre-charge contactor are: instantaneous current is greater when connected than disconnected, and the energization time is shorter. As shown in figure 1, the current of pre-charge contactor at the moment of pull-in is 10A, when the capacitor voltage reaches 90% of the system voltage, the current of the contactor at the moment of disconnection is 1A, and the energization time is only 1.72 seconds.
The following points should be considered when choose pre-charging 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 EV 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 precharge contactor should be no less than 1.25 times the rated current of the working circuit. For example, if the instantaneous current of the capacitor is 10A, we should select the EV contactor with a rated current of 12.5Amps. But the common models on the market are: 10A, 20A, 40A, so you can choose 20A EV contactor
- Make and Break Power Switching Capacity
The pre-charge 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.
Please note that the electrical life cycles provided by various brands on the market are different, you should not use it as the only basis for selection. Switching capacity tests shall be carried according to the application load conditions, frequency and environment.
- Due to the short energization time of the pre-charge contactor, there is no need to consider the temperature rise of the main contact.
Heating Film Contactor
As shown in Figure 2, the heating film contactor is loaded when it is connected and disconnected. The load form controlled by it is resistive load, there is no inrush current, and the switching frequency is not high, so damage is less. The following points should be considered when choosing a heated film contactor
Figure 2
- 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 precharge contactor should be no less than 1.25 times the rated current of the working circuit. - Make and Break Power Switching Capacity
The heating film 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.
Please note that the electrical life cycles provided by various brands on the market are different, you should not use it as the only basis for selection. Switching capacity tests shall be carried according to the application load conditions, frequency and environment. - 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.
