Posted 12/31/15 in News by Steve
Tesla Motors has revised the horsepower ratings for their dual-motor Model S performance sedans. Previously, the automaker had stated that the horsepower ratings for the electric cars with the D designation were calculated by adding the HP ratings for the front and rear motors to arrive at a combined figure.
A battery-powered electric vehicle has three main components that make up its power system: the battery pack, the controller and the motor. Tesla revised their horsepower ratings to account for total system output under more realistic conditions.
The company's entry level 70D is rated at 328HP. The 85D model uses a higher capacity battery and is able to produce higher output resulting in 417HP. The P85D incorporates a higher performance rear motor resulting in a 463HP rating. P85D models enhanced with Ludicrous mode are rated at 532HP, a staggering amount for a 4-door sedan that can haul 5 adults. The top-of-the-line P90D horsepower ratings are not listed on the Tesla website, but in tests by Motor Trend, it was the fastest sedan they had ever tested and faster than all other street cars except for the Porsche 918 and Ferrari LaFerrari hypercars.
This change in horsepower rating does not change the Model S performance numbers. They still accelerate as before. They reach 0-60 in the same time. The car remains completely unchanged except for the stated HP numbers.
In September 2015, before officially revising the horsepower numbers, Tesla’s chief technical officer, J.B. Straubel, addressed the questions related to the company's HP and torque ratings. In the blog titled “Tesla All Wheel Drive (Dual Motor) Power and Torque Specifications” he attempts to clear up “…some confusion about our methodology for specifying “equivalent” horsepower ratings for our all-wheel drive, dual motor vehicles – the “D” versions of Model S.”
The blog post describes the problem with defining electric power in terms of horsepower. He begins the blog by stating “...directly correlate horsepower ratings in petroleum burning vehicles to horsepower in an electric vehicle is a difficult challenge. The physics of an electric vehicle propulsion system are very different from a gasoline one. In an EV, electrochemical reactions in the lithium ion cells create electricity. That electricity flows through power electronics that control the voltage and current, then it flows to electromagnets in the motor that create powerful magnetic fields rotating the shaft to turn the wheels. The power required to rotate this shaft has the most correlation to traditional measures of horsepower. However, the chain actually begins in the electrochemical reactions that happen in the battery pack. Depending on the battery’s temperature, state of charge and age, the amount of electricity extracted can vary widely”
Straubel notes that because the battery electric horsepower rating varies and is not a precise number and should not be used for specifying the physical capability of an EV. Instead, he explains that the horsepower reading of the electric motor's shaft is a more consistent rating, and it is only this motor shaft horsepower rating that is legally required to be posted in the European Union.
The blog entry goes into a detailed explanation of how battery horsepower can fall below or be above the electric motor's capabilities. This is where some confusion occurs because the 85D and 70D vehicles' combined motor shaft power is very similar to the battery electrical horsepower under many normal conditions. Under some extreme examples, like heavy acceleration, that battery output can exceed the motors output. Straubel closes by saying “With the P85D the combined motor shaft power can often exceed the battery electrical horsepower available. The dual motors utilize the battery horsepower in the widest variety of real world conditions. The true measures for any performance EV driver are acceleration times and driving performance of the vehicle”