So the remark that the P90DL consumes 1,500 Amps at peak acceleration got me thinking about the voltage... Do all Tesla motors consume the same voltage regardless of 70D, 85D, P90D etc., and is the torque controlled by the Amps fed into the motor? Is the voltage the same for the front and rear as well?
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Voltage of Model S Motors
- Thread starter MsElectric
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Isn't the battery voltage around 375-400? So does this mean the electric motors gets whatever the voltage is of the battery pack without any step up or step down? For the 90kwh (and eventually higher capacity batteries) pack does that mean the motor will receive a higher voltage? I assume the torque of the motor is controlled by the amount of Amps of the current? Just trying to understand the science behind the battery pack electricity and the motors.
The motors are fed with three phase AC power, whereas the battery outputs DC power. The inverter electronics (found right next to the motor) takes the DC and turns it into three phase AC. And I'm 90% sure that the torque is controlled by varying the voltage (and consequently amperage) of the three phase AC.
Of course, if the inverter only receives 400 kW of DC power from the battery, it can only output 400 kW of three phase AC (less when you consider losses). Upgrading the fuses and main contactors means you can feed more DC to the inverter, and then the inverter can take this power and crank up the voltage of the three phase AC.
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mspohr
Well-Known Member
I always assumed that the voltage of the battery packs were all the same with a nominal voltage of 370 volts. When charging, I have seen the battery voltage goes from about 350 to 400 volts. I don't think it would make sense to have a voltage converter stage. I assume that the motor controller just takes whatever voltage the battery is delivering (range 350 to 400 volts) and adjusts the amperage to control the power.
Has anyone noticed that their battery pack has a voltage different than nominal 370 (350 to 400) during charging?
The voltage of the battery pack will vary with state of charge. Here's a graph showing how the voltage changes for a generic cell:
The battery pack has a bunch of these cells in series, thus boosting the voltage to a much higher level, but each cell will act pretty much like this, and when the battery is almost empty, you will have a much lower voltage on the battery pack. When charging, the voltage one would measure on the battery would be higher than if you are not charging, this is because you need to apply a higher voltage to the battery to get the current to flow into the battery and thus charge it.
mspohr
Well-Known Member
I assumed that the Tesla battery packs were made up of sets of 100x 3.7 volt batteries in series giving a nominal 370 volts for each set. If each battery has a 2000 to 3000 mah capacity, then 100 of these cells would have a capacity of about 7 to 10 kwh. "Larger" battery packs would have more of these sets connected in parallel. I don't think they change the number of cells in series to change the voltage since that would complicate charging and the motor control electronics.
I was wondering if anyone had noticed that their Model S battery pack had a voltage different than the nominal 370 volts that I have observed.
I don't recall the exact number of cells in series, but it may be close to 100. As the graph shows, the battery pack voltage would with 100 cells in series vary between ~350V and ~400V depending on state of charge. (The far left and the far right of the graph would be hidden in the inaccessable margins at the top and bottom of the battery.)
It's 96 series cells for the 85 and 90kWh battery and about 84 series (don't quote me) for the 60 and 70kWh packs
Torque is varied by motor current which is defined by voltage amplitude and frequency. Frequency is varied in software. Voltage is also varied in software - by varying the width of the ON pulses. (Pulse-Width Modulation.) The drive inverter will attempt to approximate a sine wave, using the motor's parasitic inductance to reduce ripple current.
In the case of Tesla, the waveform probably does not saturate near the peaks until the battery voltage gets low. This will be one factor in the power limit near low SOC. The other factor will be limiting voltage droop, and also reducing heating near low SOC which may impact battery lifetime.
Torque is varied by motor current which is defined by voltage amplitude and frequency. Frequency is varied in software. Voltage is also varied in software - by varying the width of the ON pulses. (Pulse-Width Modulation.) The drive inverter will attempt to approximate a sine wave, using the motor's parasitic inductance to reduce ripple current.
In the case of Tesla, the waveform probably does not saturate near the peaks until the battery voltage gets low. This will be one factor in the power limit near low SOC. The other factor will be limiting voltage droop, and also reducing heating near low SOC which may impact battery lifetime.
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You can see the pack voltage and the corresponding SOC when supercharging.
Supercharging Tesla Model S 60 kWh vs 85 kWh - YouTube
To my knowledge, there's no such video for the 70kWh battery. But we've had screenshots which proves they have the same number of cells in series as the 60, so the same voltage. (It has more capacity as the 70kWh pack uses the same modules as the 85kWh pack, whereas some cells in the modules for the 60kWh were replaced with ballast.)
Supercharging Tesla Model S 60 kWh vs 85 kWh - YouTube
To my knowledge, there's no such video for the 70kWh battery. But we've had screenshots which proves they have the same number of cells in series as the 60, so the same voltage. (It has more capacity as the 70kWh pack uses the same modules as the 85kWh pack, whereas some cells in the modules for the 60kWh were replaced with ballast.)
djp
Model 3 Performance
This old Roadster blog has a good explanation on how torque is generated in electric motors - the same principles apply to the Model S. Torque is controlled by changing the frequency of the 3-phase AC waveform. The AC waveform creates a spinning magnetic field in the motor. If this field is spinning faster than the motor (higher frequency) then you get positive torque, if the field is spinning slower than the motor (lower frequency) you get negative torque (ie regen braking).
Induction Versus DC Brushless Motors | Tesla Motors Canada
Anvesh Sameer
New Member
But why the range of battery pack voltage 350 to 400? why not more than 400 or less than 350?
Individual cells will suffer if the voltage is too high or too low. Of course you can the change the number of the cells in series, but that will change the voltage permanently.
Tesla has 96 cells of 4.2 V maximum voltage in series producing maximum pack voltage 403V. If you would want to charge this package above the 403V, it would result the individual cells voltage above 4.2V, which would fast destroy them. The same goes depleting the pack too low.
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The large and small motors are both rated at 320volts. The voltage is limited in part by the breakdown voltage of the wire insulation. The semiconductor devices in the inverter also have maximum voltage ratings that cause catastrophic failure if they are exceeded.
The 1520 amps is what is drawn from the battery pack at full power. This is not the current that the motors are drawing. The torque requested by the accelerator determines the current required in the motor. The rpms of the motor determine the voltage requirements of the motor. This is all handled by the inverter. At full acceleration from standstill, the motors draw large amounts of current, but because the rpms are low the power is also low. In this case the battery only has to supply this low amount of power, so only a small amount of current times the 400volts of the battery is required.
On the p90DLv3 the voltage sags from 400volts to about 321 volts at full charge when this much current is drawn. So the battery is providing 321 * 1520, or 488 Kw, of power. The motor/inverter is about 81 percent efficient so the power at the shaft of the motor is 488 * 0.81, or 395 Kw. In my testing, I've found that about another 6 percent is lost in getting power from the motor shaft to the wheels. So 395 Kw * 0.94, or 371 Kw. This is 371 * 1.341 = 498 hp at the wheels.
The 90 Kwh battery is made of 16 modules . Each module has 444 cells arranged as 74 cells in parallel and 6 of these 74 cell units in series. Each module has an output voltage of 4.2 * 6, or 25.2 volts. These 16 modules are then connected in series to provide 16 * 25.2 , or 403.2 volts. The lower capacity batteries only have 14 modules in series, so their output is 14 * 25.2, or 352.8 volts.
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