Wiring Diagrams

Isolated Systems

The figure below is a simplified diagram that illustrates how the electrical systems in an electric vehicle are isolated from each other.  There are two main isolated systems:


1.       12-V automobile electrical system

2.       High-voltage system based on the battery bank


The figure below shows two isolated battery chargers - one for the 12-V battery and one for the HV battery bank. The 12-V battery charges as needed from the high-voltage bank during use as the HV to LV converter/charger takes the place of the alternator.  Although not shown in the diagram, it's a good idea to place a relay on the high-V side of the converter and another one on the 14-V side of the converter - each relay has a 12-V coil and are activated by the key switched 12 V.  These relays prevent the converter from drawing current from the HV bank when the vehicle is off and also prevents the converter from drawing a small current from the 12-V battery when the vehicle is off.  The relay on the LV side must have high-current contacts that are rated greater than the max current output of the converter/charger.  I use a 600 Watt converter (SWS60015 TDK-Lambda) that puts out about 40 A.


The Control Box (black) will be discussed in detail, but it's purpose is to ensure safe turn-on and operation of the vehicle.




Motor Wiring Options



The series-wound motor allows many wiring options that produce the same direction of rotation.  Series motors are also called Universal Motors because they will actually run on AC or DC.  If you reverse the applied polarity to the motor, it continues in the same direction.  Changing the polarity of both the armature and the stator at the same time causes the motor to continue in the same direction.  The only way to reverse a series-wound motor is to change the polarity of either the armature or the stator, but not both.  This is why a reversing contactor is used to change the polarity of the stator winding to create bi-directional operation. Note that this is not necessary when a standard transmission is used because it has a reverse gear.


Did you know that 120 VAC electric drills, saber saws, reciprocating saws and circular saws all have universal motors and will run directly from your 96 to 120 V battery bank?  It's true, and so will all 120 VAC incandescent light bulbs.


System Control


The system control circuit, in the black box, is an interface control and safety circuit between the two systems.  The control circuit contains relays that energize according to operational rules.



1.       The high-voltage motor and control system must not energize if the parking brake is on.

2.       The high-voltage motor and control system must not energize if the operator has his/her foot on the gas pedal.

3.       The high-voltage motor and control system must not energize until the gas-pedal foot (right foot) is on the brake pedal.



If the control circuit is designed to follow these rules, safe operation can be ensured.  The following wiring diagram shows the control circuit that I designed for my Chevy S10 conversion:


Note: The following control circuit diagram is for the Curtis 1209, 1221 and 1231 controller or any controller that requires the high-voltage connection from terminal 7 of the control box (blue line).  




Control Circuit Description of Operation


When the parking brake is set (ON), terminal #2 is grounded. When the key switch is turned ON, RL-1 energizes and the normally closed contact shown in the diagram is opened. This breaks the ground path for RL-2, so it cannot energize. When the parking brake is released (OFF), the normally closed contacts on RL-1 are closed and the ground path for RL-2 is established. Note that RL-2 will not energize even though a ground is provided to its coil. That is because +12V is not available at the opposite coil terminal because the positive path is interrupted by the open red contacts of RL-2. RL-2 will only energize when your foot is placed on the brake pedal. The brake pedal switch passes the +12V through the inline diode, through the closed contacts of the throttle microswitch to terminal #4 and to RL-2. RL-2 then energizes and stays energize because the red contacts of RL-2 now close (self-holding contacts). With RL-2 energized, the +HV is passed through its black contacts to the precharge resistor and out terminal #7 (blue wire) to the low-current +HV terminal on the controller. The system is ON and the controller is ready for use. When the accelerator is pressed (gas pedal), the throttle microswitch switches to pass +12V from terminal #4 to the heavy-duty contactor coil. The heavy-duty contactor pulls in to bridge the +HV to the B+ terminal of the controller.


Why is the inline diode needed? The inline diode prevents +12V from going from terminal #4 back to the brake lights when the key is turned on and RL-2 is energized. This would cause the brake lights to be ON all of the time. Not good!


Why is there a diode across each relay coil and the heavy-duty contactor coil? When a relay coil is de-energized, the coil produces a high voltage that arcs across the controlling relay contacts or switch. This deteriorates the switch or relay contacts. The diodes forward conduct and prevent the high-voltage arcs from occurring. 


The other diodes are traffic cops that restrict flow in only one direction. They are an important part of the overall control logic.


The photo below relates the PB6 micro switch connections to the diagram above.





The small black box is the control box that contains the circuit just above.

The gray box, with diamond-plate cover, contains the heavy-duty contactor. See below.



Note that the above control circuit is actually safer than what is shown in the Curtis Instruments controller manual (1209B/1221B/1221C/1231C Manual).  Curtis recommends that a pre-charge resistor be placed across the heavy-duty contactor terminals.  If this is done, high-voltage from the battery bank will be present at all terminals of the motor and controller even when the key is off.  The user would be required to remember to turn off the main circuit breaker before working on the motor and controller system, which of course is a good practice anyway.  The modified circuit above helps you avoid unnecessary shock if you forget.


Disclaimer: Neither EVhelp nor Mark E. Hazen is responsible for injury or losses of any kind resulting from use or misuse of information contained herein. 



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