Discussion
Pretty much all EV use 3phase brushless motors, either with a permanent magnet (expensive but higher efficiency/smaller size) or using inductive energisation (cheaper, but less power/efficiency) in the rotor. These motors need to have their stator windings supplied by a suitable 3phase waveform, that is "locked" to the rotors position as it revolves. As they pretty much all have a DC voltage energy store an Inverter system is needed to generate this 3phase AC waveform. The inverter uses high power solid state silicon switches (IGBT's usually) to alternately switch each phase end to either the battery +ve or battery -ve terminal. By doing this at high speed (typically say ~10 to ~20kHz) it creates the appropriate voltage across each stator phase in the motor, and the inductance of that phase moderates that voltage to generate the correct current waveform. (Where the average voltage is proportional to the rotating speed of the motor, and the average current proportional to the torque produced). There are various mathematical control methods to do this, but the classic Field Oriented Control(FOC) method has become somewhat industry standard. With the proliferation of low cost, high performance microcontrollers, running complex maths based control algorithms with high enough bandwidth has become extremely easy.
Max_Torque said:
Pretty much all EV use 3phase brushless motors, either with a permanent magnet (expensive but higher efficiency/smaller size) or using inductive energisation (cheaper, but less power/efficiency) in the rotor. These motors need to have their stator windings supplied by a suitable 3phase waveform, that is "locked" to the rotors position as it revolves. As they pretty much all have a DC voltage energy store an Inverter system is needed to generate this 3phase AC waveform. The inverter uses high power solid state silicon switches (IGBT's usually) to alternately switch each phase end to either the battery +ve or battery -ve terminal. By doing this at high speed (typically say ~10 to ~20kHz) it creates the appropriate voltage across each stator phase in the motor, and the inductance of that phase moderates that voltage to generate the correct current waveform. (Where the average voltage is proportional to the rotating speed of the motor, and the average current proportional to the torque produced). There are various mathematical control methods to do this, but the classic Field Oriented Control(FOC) method has become somewhat industry standard. With the proliferation of low cost, high performance microcontrollers, running complex maths based control algorithms with high enough bandwidth has become extremely easy.
I understand that as i speak nerdMany don't
The fundamental purpose of the vast majority of the world's electric motors is to electromagnetically induce relative movement in an air gap between a stator and rotor to produce useful torque or linear force.
According Lorentz force law the force of a winding conductor can be given simply by:
\mathbf{F} = I \boldsymbol{\ell} \times \mathbf{B} \,\!
or more generally, to handle conductors with any geometry:
\mathbf{F} = \mathbf{J} \times \mathbf{B}
The most general approaches to calculating the forces in motors use tensors.[81]
Power
Where rpm is shaft speed and T is torque, a motor's mechanical power output Pem is given by,[82]
in British units with T expressed in foot-pounds,
P_{em} = \frac {rpm \times T}{5252} (horsepower), and,
in SI units with shaft speed expressed in radians per second, and T expressed in newton-meters,
P_{em} = {speed \times T} (watts).
For a linear motor, with force F and velocity v expressed in newtons and meters per second,
P_{em} = F\times{v} (watts).
In an asynchronous or induction motor, the relationship between motor speed and air gap power is, neglecting skin effect, given by the following:
P_{airgap}=\frac{R_r}{s} * I_r^{2}, where
Rr - rotor resistance
Ir2 - square of current induced in the rotor
s - motor slip; ie, difference between synchronous speed and slip speed, which provides the relative movement needed for current induction in the rotor.
According Lorentz force law the force of a winding conductor can be given simply by:
\mathbf{F} = I \boldsymbol{\ell} \times \mathbf{B} \,\!
or more generally, to handle conductors with any geometry:
\mathbf{F} = \mathbf{J} \times \mathbf{B}
The most general approaches to calculating the forces in motors use tensors.[81]
Power
Where rpm is shaft speed and T is torque, a motor's mechanical power output Pem is given by,[82]
in British units with T expressed in foot-pounds,
P_{em} = \frac {rpm \times T}{5252} (horsepower), and,
in SI units with shaft speed expressed in radians per second, and T expressed in newton-meters,
P_{em} = {speed \times T} (watts).
For a linear motor, with force F and velocity v expressed in newtons and meters per second,
P_{em} = F\times{v} (watts).
In an asynchronous or induction motor, the relationship between motor speed and air gap power is, neglecting skin effect, given by the following:
P_{airgap}=\frac{R_r}{s} * I_r^{2}, where
Rr - rotor resistance
Ir2 - square of current induced in the rotor
s - motor slip; ie, difference between synchronous speed and slip speed, which provides the relative movement needed for current induction in the rotor.
yes thanks!
It of course depends on a lot of factors, such as system voltage, the architecture of the inverter and rectifier(for charging) used. Percentage losses are also somewhat dangerous to quote for EVs, because as they run with regenerative braking, the total energy moved for any given cycle can approach 150% (or more) of a conventional car (which cannot recapture energy when slowing, instead wasting it as heat in the brakes or powertrain friction). So an EV, which says shunts 1kWhr out of the battery, stores it in it's KE, then shunts say 0.5kWhr back into the battery has "Moved" 1.5kWhr through it's powertrain, whereas an ICE vehicle has only moved 1Kwhr. So, for the ICE, the negative torque powerflow is always 0% efficient, whereas for an EV, high efficiency is desirable (to capture as much energy from the KE as possible)
Having said all that, if you include all the on-vehicle losses from the charger circuit onwards, over say the EUDC cycle, a typical EV will be over 65% efficient. More importantly for EVs is really there absolute energy consumption, in Whrs / km for example.
It of course depends on a lot of factors, such as system voltage, the architecture of the inverter and rectifier(for charging) used. Percentage losses are also somewhat dangerous to quote for EVs, because as they run with regenerative braking, the total energy moved for any given cycle can approach 150% (or more) of a conventional car (which cannot recapture energy when slowing, instead wasting it as heat in the brakes or powertrain friction). So an EV, which says shunts 1kWhr out of the battery, stores it in it's KE, then shunts say 0.5kWhr back into the battery has "Moved" 1.5kWhr through it's powertrain, whereas an ICE vehicle has only moved 1Kwhr. So, for the ICE, the negative torque powerflow is always 0% efficient, whereas for an EV, high efficiency is desirable (to capture as much energy from the KE as possible)
Having said all that, if you include all the on-vehicle losses from the charger circuit onwards, over say the EUDC cycle, a typical EV will be over 65% efficient. More importantly for EVs is really there absolute energy consumption, in Whrs / km for example.
Max_Torque said:
yes thanks!
It of course depends on a lot of factors, such as system voltage, the architecture of the inverter and rectifier(for charging) used. Percentage losses are also somewhat dangerous to quote for EVs, because as they run with regenerative braking, the total energy moved for any given cycle can approach 150% (or more) of a conventional car (which cannot recapture energy when slowing, instead wasting it as heat in the brakes or powertrain friction). So an EV, which says shunts 1kWhr out of the battery, stores it in it's KE, then shunts say 0.5kWhr back into the battery has "Moved" 1.5kWhr through it's powertrain, whereas an ICE vehicle has only moved 1Kwhr. So, for the ICE, the negative torque powerflow is always 0% efficient, whereas for an EV, high efficiency is desirable (to capture as much energy from the KE as possible)
Having said all that, if you include all the on-vehicle losses from the charger circuit onwards, over say the EUDC cycle, a typical EV will be over 65% efficient. More importantly for EVs is really there absolute energy consumption, in Whrs / km for example.
Thank you for the detailed response.It of course depends on a lot of factors, such as system voltage, the architecture of the inverter and rectifier(for charging) used. Percentage losses are also somewhat dangerous to quote for EVs, because as they run with regenerative braking, the total energy moved for any given cycle can approach 150% (or more) of a conventional car (which cannot recapture energy when slowing, instead wasting it as heat in the brakes or powertrain friction). So an EV, which says shunts 1kWhr out of the battery, stores it in it's KE, then shunts say 0.5kWhr back into the battery has "Moved" 1.5kWhr through it's powertrain, whereas an ICE vehicle has only moved 1Kwhr. So, for the ICE, the negative torque powerflow is always 0% efficient, whereas for an EV, high efficiency is desirable (to capture as much energy from the KE as possible)
Having said all that, if you include all the on-vehicle losses from the charger circuit onwards, over say the EUDC cycle, a typical EV will be over 65% efficient. More importantly for EVs is really there absolute energy consumption, in Whrs / km for example.
AER said:
I was of the understanding that induction machines offer efficiency advantages at part load over equivalent permanent magnet machines. At least this is what Tesla and AC Propulsion claim.
In my experience, the jury is still out on that one. Also, it's worth noting that "efficiency" as far as Emachines is concerned isn't actually the be-all and end-all of things, because even frankly rubbish ones are still surprisingly efficient. I.E. a poorly designed modern SPM machine might be say 97% efficient at full rated load, but only say 88% at very low load. But at very low load, a 12% loss of f-all is even less than f-all..... Ie what you care about is the total energy losses, rather than the efficinecy per-say.I can see how a pure AC induction machine could be slightly more efficient at high speed low load, but as you could design a PM machine to make the same output at a lower speed (or with a smaller volume device) i suspect the actual losses would be very similar. Swings and roundabouts imo. Of course, the major benefit is a low cost Emachine, and one that "could" (depending on stator wiring spec/insulation) be run at high temps than a PM machine (which would have to use expensive magnet materials to avoid de-sat at elevated temps etc)
For someone like Tesla, using a unique induction machine makes some sense, as at low volumes it is certainly easier to develop/manufacture, and i believe they also hold some US Patents on the control of induction motors for EV useage, which give them some leverage of their IPR etc. Pretty much everyone else (GM, BMW, VW, Nissan, Renault etc) all use PM machines for there greater power density, and i guess the buying power of larger volumes helps with PM material sourcing. (For ultra high performance apps (P1, 918 etc) then a PM machine is a no brainer)
Also worth noting that one of the claimed benefits of induction machines, the fact that you can safely spin them without any active commutation and suffer no uncontrolled generation (because you actively need to create the rotor currents that can then generate the stator currents), isn't that big a deal in reality, as all automotive HV DC systems have to have a "Hardware" isolator (i.e. a feck off big relay!) to be able to safe the system in the event of a HV system fault. So as long as your wiring insulation rating is higher than the peak Bemf of the motor (at max rpm) all is good, and you just open the main contactor in the case of a fault. (at full load, that may only be possible once, due to the arcing damage caused from breaking the high current, but the contactors are often housed in the removable "safety disconnect link" housing, and are so easily (if expensively) replaceable.
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