This is beyond my A-level physics! Why can't you find an electrical engineer when you want one.
I'm mechatronics engineer and I will give my 2 cents
One possible problem I see in this situation might be the lack of torque multiplier in the transmission. Let's do some math:
A regular ICE-only car:
a) First gear reduction: around 3.5:1
b) Differential reduction: around 4:1
c) Engine torque: suppose 18Kgfm
That means a torque delivered to the wheels around 250 Kgfm. Besides that, we have more torque multiplication in the Torque Converter (automatic gearbox) or in the Clutch (manual gearbox), which can multiply this torque a few more.
- Please some more experienced person correct me if I'm wrong -
In the case of the PHEV, the front motor has 14Kgfm and the rear motor 20Kgfm, summing 34Kgfm. I have no idea about its gearing system to calculate the total torque delivered to the wheels, but I guess it is a lot lower in comparison to a regular ICE car with gearbox. The lack of a gearbox also lowers the maximum delivered torque.
That might be
an explanation of why an EV car has a start torque on the wheels lower than a car with a gearbox.
About how the electric motors work and how the maximum torque is achieved, let's start with some concepts:
a) The motor torque
proportional to the current
drawn by the motor;
b) The motor rotational speed
proportional to the voltage
to the motor voltage;
c) The motor electric Power
is Current * Voltage
d) The motor mechanical Power
is Torque * Rotational Speed
When the car is stuck and you try to start the movement, the motor inverter is supplying a huge current to the motor - probably the maximum allowed by the system - but the motor voltage is low, because it is stopped. Do not confuse battery voltage with the motor voltage, they are completely different. That means the total electric power drawn by the motor is quite low, not even close to the maximum power (120kW).
In the case you are requesting the maximum torque from the motors and even though it is not enough to move the car, the motor control unit just stop feeding the motors because it is useless (the motors are not moving) and all that energy is being converted to heat. By the way, a warm electric motor has less torque than a cold one.
I hope I could explain why a stalled motor can provide full torque at low power consumption.
If there is any further question, let me know.