This is partially correct, it's the bearings. Specifically the fluting that occurs in the bearings if the shaft is not grounded properly. My P85+ is going to the SC to have the drive unit replaced. The technician said that this is due to the bearings not being properly insulated and thus the high discharge voltage travels through the bearing and damages the bearing races. Ideally the current path travels through another path and not the bearing.
-
Want to remove ads? Register an account and login to see fewer ads, and become a Supporting Member to remove almost all ads.
gavine
Petrol Head turned EV Enthusiast
Yep:
Bearing Protection - Bearing Currents | Electro Static Technology, Inc.
They make shaft-grounding rings for bearings.
Jashev
Member
Lex
Member
Well personally I'm well over 25k miles now and still on my original motors, with no new sounds to report fore or aft.
If you google it you will find many references to a "gentlemen's agreement" among the large German auto-makers (other than Porsche) where they voluntarily agreed to limit their cars to 155 mph or 250 kph in order to prevent government regulation and speed limits on the autobahn.
That may or may not be correct.
Jashev
Member
I asked a BMW manager about that a few years ago. He said the cars come with V speed rating tires which are rated to 149 mph, so the cars were limited to a speed that the tires could safely handle. As you said however, that may or may not be correct ;-)
seems like a good contestant for a cvt. I know they are not generally known for torque handling but the new wrx handles 290 ft/lbs, not too far off the base S. It would allow the engine to stay at its perfect happy place for efficiency and longevity.
I suppose the reason for not using is same as traditional gears: cost/reliability not worth the gain. Anyone w further info I'd be pleased to hear it.
I suppose the reason for not using is same as traditional gears: cost/reliability not worth the gain. Anyone w further info I'd be pleased to hear it.
jelloslug
Active Member
You need a transmission with an engine because of the way it's torque curve is shaped. The torque curve in EVs is basically flat so a transmission is just about useless unless your goal is to try and have a higher top speed.
What type of engineer are you? Do you operate a diesel locomotive? Or diesel engines aboard a ship? By mechanical engineering definitions a transmission has multiple gear ratios available (making "multi-gear transmission" redundant) while a gearbox has a fixed ratio. Since an ICE produces peak torque over a relatively small RPM ratio a transmission is nearly always required to keep an ICE engine operating near its peak torque. There is a Koenigsegg exception (One:1 - Koenigsegg) but many ICE sports cars have at least 6 speed transmissions, while some have as many as 8 (Dodge Charger, Cadillac CTS, BMW X1, etc). Since a multiphase induction motor can produce high torque at even zero RPM (where no ICE could even operate) no transmission is required.
Transmissions, even manual ones, are less efficient than gearboxes since there are more gears in mesh; each pair of meshing gears loses about 2% in efficiency (using straight cut gears). So adding one or two transmissions (for 2 motor AWD) increases the weight and cost, reduces the efficiency, and is more difficult to control than the already necessary VFD(s) for the motor(s).
No, that is not the main advantage of an electric motor, diesel engines can clearly produce high torque at low RPM. Electric motors have an essentially infinite working RPM ratio with relatively simple control (e.g. include the Tesla traction control and vehicle stability control for example).
That is also incorrect since electric motors do not work the way that you imagine. HP is proportional to Torque times RPM. At low RPMs induction motors operate in a current limited mode since keeping the HP constant all the way down to zero RPM would imply infinite drive current. Hence at zero RPM you have zero HP but a large torque, where the latter is limited by the motor winding resistance (heat generated) and the power supply maximum current (a battery in the case of the Tesla).
As the RPM increases the HP increases while the drive current could remain at its maximum continuous rating. Above the motor's maximum HP speed the current starts reducing due hitting the maximum continuous HP rating, so the torque must fall as RPM increases. The increasing back EMF of the motors with increasing RPM also naturally reduces the drive voltage on the stator windings. Since the rate of change of the current in the stator windings is proportional to the drive voltage, that means that at a sufficiently high RPM the VFD will no longer be able to sustain enough drive voltage across the stator windings to keep raising the RPM, since the required dI/dT cannot be achieved when using too low of an effective drive voltage (supply voltage minus the back EMF).
The end result is that the induction motors exhibit constant torque at low RPM range, constant HP in the middle RPM range and reduced HP at the very upper RPM range. Tesla makes their own motors to control these characteristics, and the rear drive motor of a P100D has a working speed range of zero to 16K RPM(!), while the front motor has a working speed range of zero to 18K RPM(!). The Tesla's 155 MPH top speed is purposely electronically limited, as it is for many ICE cars (e.g. Audi S8, Bentley Continental R6., BMW 3 and 5 series, etc) by a "gentleman's agreement" among most car manufacturers.
Gearing the Tesla for a higher top speed would only reduce the lower speed performance and not increase the efficiency at all. Since it requires a specific HP to overcome the aerodynamic drag and frictional losses at a specific vehicle speed, lowering the motor RPM requires increasing the stator drive current to maintain that same required HP. Tesla has already optimized their vehicles; the model S has among the lowest drag coefficient of any general purpose 5 (or 7) passenger sedan and yet even the P100D has a nearly 100 MPG rating. Fuel Economy of the 2017 Tesla Model S AWD - P100D
gavine
Petrol Head turned EV Enthusiast
This is the part that I think is most-overlooked when comparing to ICE. ICE engines are more efficient at lower RPM, but not necessarily the case in an electric motor. If you "geared-down" an electric motor, you would need to feed more kW to it in order to maintain the speed thus equaling-out the energy usage vs higher RPM. Higher RPM isn't necessarily much less efficient in an electric motor.
With the dual motor Tesla, the two motors operate at different gear ratios. The computer chooses which to use and when. This gives a car with the gear ratio optimized for two scenerios.the front and rear motors are selectively slept when the choice is made. That is why the dual motor Tesla has a slightly better efficiency and range than the single motor version of the same car.
So the dual motor car is a much more elegant design than a design that incorporates a transmission. You get the efficiency benefits that you otherwise seek with the transmission. Plus there's a lot more.
Well done, Tesla, doing without a transmission, giving me all of the benefits and none of the headaches.
So the dual motor car is a much more elegant design than a design that incorporates a transmission. You get the efficiency benefits that you otherwise seek with the transmission. Plus there's a lot more.
Well done, Tesla, doing without a transmission, giving me all of the benefits and none of the headaches.
God forbid. The biggest reason I like my MS is lack of the satan incarnate, the worst thing is the world -- the automatic transmission. If you are talking about having a three pedal Tesla with a manual gearbox, then my apologies.
Only the PxxD cars have different gear ratios in the front and rear. The two small motors in the D cars have the same gear ratio.
Nope, you can refer to the EPA filings that clearly show the different gear ratios: "This is a Dual Motor Base configuration with firmware limited 75kWh Battery pack - MY2016 Running Change Individual HP is 193kW Front / 193KW Rear Axle Ratio Front Motor 9.34 Rear Motor 9.73"
There are all kinds of typos in that report.
Here's some max power data that llavalle just gave me for his 85D during a zero to sixty run.
Measured Power, Battery power, Battery Voltage, Battery Amps, Torque, RPM, Speed(kph):
F 139.7, 348.3, 302.9, 1150.0, 311.7, 4366, 59.7
R 131.6, 348.3, 302.9, 1150.0, 295.5, 4339
T 271.3
Notice that the front and rear motor rpms are almost identical and that the rear is actually lower than the front. If the rear and front ratios were 9.73 and 9.34, respectively, the rear rpm should be 9.73 / 9.34 * 4366 = 4548, not 4339.
Last edited:
Similar threads
