Main Contactor for Electric Vehicles
What is a Main Contactor?
A Main Contactor is a heavy duty relay which is normally open. When a small current is applied (to the low current circuit), it is energizes and closes the contacts allowing high current to flow (through the high current circuit).
How does a Main Contactor work?
A Main Contactor works by having a solenoid which when energized by a low current, low voltage (and hence low power electrical source) causes the plunger of the solenoid to move a piece of metal (usually copper) to make physical contact with 2 metal posts, thus forming a connection between the 2 posts. These 2 metal posts are the terminals for the high current circuit, also known as the traction circuit.
How to use a Main Contactor in Electric Vehicles?
To use a Main Contactor in Electric Vehicles, the energizing, low power circuit is wired up to the 12 volt system of the Electric Vehicle or Electric Car where the key switch will energize the Main Contactor. Often, a protection diode may be placed across the terminals of the low powered circuit. The high powered terminals of the Main Contactor are connected in line with the high current, high voltage (hence high power) circuit. Usually, one terminal of the high powered current will be close to the B+ connection of the Motor Controller whilst the other terminal will be after the high current fuses, circuit breakers (please check Motor Controller manual for details – some Motor Controllers are not compatible when a Main Contactor is used). It is separate from the 12 volt electrical system of the Electric Vehicle or Electric Car. Some controllers may specify that a high resistance resistor (sometimes referred to as a precharge resistor) be placed across the terminals of the high current terminals of the Main Contactor. The purpose of this precharge resistor is to keep the capacitors of the Motor Controller charged, even when the Main Contactor is off. You need to check the manual for the Motor Controller you are using to see what type of precharge resistor it required and also how they specify things like the Main Contactor etc should be connected.
What You Should Look For When Sourcing a Main Contactor?
When sourcing your Main Contactor, you need to know how much current draw and voltage is required for the low current circuit. For the typical Electric Car Conversion, the low current circuit is activated by the key switch, hence, it needs to be able to accomodate 12 volts. The other thing you need to know is the current and voltage rating of the high current circuit of the Main Contactor. If your Electric Vehicle is for example 144 volts DC and drawing a peak current of 300 amps DC, then you need a Main Contactor that can handle those DC voltages and currents. These are sometimes referred to as voltage and current rating. Also, you need to know if those ratings are for AC or DC. For example, a Main Contactor rated for 300 amps AC may not be suitable for 300 amps DC. You need to know if the ratings quoted are for AC or DC.
When to use a Main Contactor in Electric Vehicles & Electric Cars?
Main Contactors are used as a switch in the high current circuit of Electric Vehicles and Electric Cars. They are connected in series with other components like fuses, circuit breakers, etc of the high current circuit. In some Electric Car Conversions, where the battery pack is split between the front and rear the car, there are two Main Contactors – one in between the battery packs and the other close to B+ of the Motor Controller. When using more than one Main Contactor, you will want to check what resistance pre charge resistor needs to be put across the high current terminals of each Main Contactor (if the Motor Controller specifies that a precharge resistor must be used). As mentioned above, it is best to check the Motor Controller manual when wiring up your Main Contactor and other parts when doing your Electric Vehicle Conversion. As mentioned above, some Motor Controllers can’t use a Main Contactor.
This is Al Bunzel signing out.
P.S. Please watch for my next article regarding how I use a Main Contactor for my modified 24 Volt Starter Motor.