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Engineering Series
Controller Engineering – To help in the Selection  

The controller  in an EV is similar to the carburetor on a internal combustion engine; it controls the amount of energy going to the electric motor by using an input signal.  The input signal is from the accelerator pedal, joystick throttle, thumb switch, twist grip, etc. through a potentiometer or inductive throttle.   

The controller is the key component in the renewed interest in EVs.   There were a number of EVs in the 1970s but most of them used voltage switching thru contactors or resistance circuits with great inefficiency.  The voltage switching created imbalances in the batteries.  The solid-state controller produced in the 1980s definitely renewed the interest in EVs.  

Typically, there are two sections to a controller: the Power Section and the Control Section.   The Power Section takes the energy in the batteries and delivers it to the motor.  This can be thru MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) or thru IGBTs ( Insulated Gate Bipolar Transistors).  Most controllers that are designed for 200V and less use MOSFETS.  The advantages are cost and efficiency. The power loss in the MOSFETs is proportional to the square of the current; so the drop at 400 amps is 4 times the loss at 200 amps.  IGBTs have a constant voltage drop (~2V) which makes them more efficient at high power levels but less efficient at low power levels.  IGBT controllers typically utilize water cooling.         

Remember it is the current (amperage) that heats up components and that the losses are proportional to the Current (amperage) squared. This not only affects the controller, but the also the motor, contactors, cables, lugs and the batteries.  It is this heat that contributes to the deterioration of the components and their failure. Naturally, if you double voltage, the current will be cut in half for the same load (vehicle hp).  It appears then that the obvious solution is to double the voltage, but this ignores economics.  Doubling the voltage can double or triple the cost and require more space in the EV.  The battery space will increase.  Smaller batteries are not always the solution because they may not be able to sustain the currents required for acceleration and hill climbing.  It is for this reason, that we encourage our customers to investigate thoroughly the Drive System and Battery System as they design their EV.    

The Logic Section of the controller has the brains. Most controllers have:

· High Pedal Disable to prevent the vehicle from operating if the controller is receiving an input signal prior to it being the Power Section being turned on.

· Overcurrent Protection to protect the motor.  Remember, we recommend an inductive coil across the motor to minimize high spikes.

· Overvoltage Protection to protect the motor and controller from high voltages beyond the limits of their design.

· Thermal Protection to protect the controller if a it is overheating due to high loads.

· Throttle Protection ( runaway protection) in case the input signal exceeds established parameters.

· Undervoltage Protection to protect the Drive System from high currents when the battery pack voltage is low.

Naturally, each controller manufacturer identifies their protection circuits in their manual.  We recommend that it be read thoroughly.

In conclusion, the controller is the key component in any EV.   It should be sized properly for the application (weight, speeds, terrain, etc.) and the voltage utilized.  Selecting an undersized controller (e.g. 72V for a truck conversion) will decrease performance and eventually damage components.  Selecting an oversize controller for the application increases the cost and decreases the efficiency of overall operation.  


New plug-in conversion credit. A new plug-in conversion credit of 10% of the cost of converting any motor vehicle (new or used) to a qualified plug-in electric drive motor vehicle. The maximum credit is $4,000 per vehicle. This credit is claimed on Form 8910, Alternative Motor Vehicle Credit, and applies to property placed in service after February 17, 2009.
March 2010