Electric Machine



The energy conversion of electric machines is expressed on the mechanical side by means of forces, whereas on the electrical side as induced voltage. In principle, each electric machine can work as an engine or a generator. Depending on the vehicle concept, different designs of electric machines are applicable.

Within an electric motor, the immovable part of the functional arrangement is called stator, and the rotary part is called rotor. The basic principle of all machines is the following: In the stator, electrical power is given or withdrawn; in the runner, mechanical power is given or withdrawn. The energy conversion takes place in the air gap, where losses in the stator and in the rotor, however, develop simultaneously. In principle it is distinguished between the following types of machine:

D.C. machine: Relative to the stator resting air gap field, runner rotates

Rotating field-/three-phase machine:

  • Synchronous machine: air-gap rotating field rotates with synchronous speed, rotor follows synchronously
  • Asynchronous machine: air-gap rotating field rotates with synchronous speed, rotor follows asynchronously




Principal design of electronic energy converter

Classification of electronic-mechanical energy converter



Operating limits


M/n-operation diagram for electric-machines

For each electric machine, operation limits in the M/n operation diagram exists. Thereby, it has to be differentiated between nominal parameters and maximum parameters. Nominal parameter such as nominal moment Mn and nominal power Pn can be permanently adjusted. Maximum parameters such as maximum torque Mmax and maximum power Pmax can be adjusted for short time only. Limiting parameters are thereby temperature, mechanical strength and life span. If a machine is overloaded beyond the permissible values, a thermal overloading of the machine due to excess current occurs. For example, the coil insulation melts at approx. 180°C. With too high speeds, the gear reaches its thermal limit, thus shortening its life span. By higher speeds, direct mechanical damage can result, which can then lead to the malfunctioning of the machine. In this case, high centrifugal forces acting on the rotor may loosen some parts, which can eventually wreck the mechanism. Therefore, depending on design and dimensioning, overloading capabilities of factor 1 to 4 are selected. Thus, the machine temporarily withstands up to the quadruple of its nominal load.

  DC motor synchronous motor asynchronous motor transversal motor SR-motor
electr. exc. perm.exc. electr. exc perm.exc.
power density 0 + + ++ + ++ ++
reliability 0 + + + ++ + ++
efficiency -- - + ++ 0 ++ ++
controllability ++ ++ + + 0 + ++
overload capacity + + + + + + +
noise level - - + + + + +
thermal overload protection - - + ++ + + +
costs (price) 0 - 0 - + -- +
costs of the machine -- - - + -- ++
costs of the control ++ - -- - 0 0
development state ++ 0 0 + - 0


In each machine, two fundamental operation areas are to be differentiated. Initially, there is the basic speed range. This range is characterised by the fact that with each engine speed, starting with 0, the nominal moment Mn or the maximum moment Mmax can be adjusted. Increasing the speed at constant moment MN, the mechanical power rises linearly, until the nominal power is reached. At this point, the nominal speed nN is automatically adjusted. For a long-term operation, the nominal power should not be exceeded. In order to achieve nevertheless higher engine speeds, the torque must be decreased by a simultaneously increasing engine speed. This area is called the range of constant power. This behaviour is reached by the attenuation of the magnetic field; therefore this range is also called weak field range. Regarding the relationship between moment and speed for the four-quadrant operation of an electric machine. Four-quadrant operation means, that the machine brakes or drives in both rotation directions (forward and backwards).



Metrics

METRIC SUB-METRIC UNITS RATING DATA TYPE
Technology Accessibility Compatibility with existing consumer technologies 0-4      
Number of companies selling the technology number      
Probability of market co-existance with current (competing) technology 0-4      
Global Environmental Impact GHG- emissions at full load g / kg fuel& 0-4 (Rating)   no CO2, except when recharged from electric power grids working on fossile fuels Electric vehicles[1]
  30-40 mg/mile plug-in mid sized HEV[2]
GHG- emissions at part load g / kg fuel      
Local Environmental Impact Air quality impact (consider NOx, PM, CO, NMHC) 0-4   No Electric vehicles
  20-30 mg/mile plug-in mid sized HEV[2]
Noise or perception of noise from the technology (SPL, loudness,etc.) dB(A), sone      
Design / product appearance impact 0-4      
Efficiency Part load efficiency of technology %      
Full load efficiency of technology %   Full hybrid: 85% Siemens hybrid (experiment)[3]
  Full electric: 96% Siemens hub motor concept[3]
Efficiency of auxiliary components %      
Capacity & Availability Capacity to meet user's needs (for eg. Performance and acceleration of vehicle) 0-4   sustained speed: 97-98 mph, top speed: 120 mph (for 2 mins)Acceleration: 0-60 mph in 8.9 s, 50-70 mph in 5.2 secFuel economy: 110 to 120 mpeg (in electric only mode) plug-in mid-sized HEV[2]
  Output power of electric motor: 75 kW
Acceleration of vehicle from 0-100 km/hr in 7 secs
Siemens hybrid (experiment)[3]
Number of hours per year during which technology is available hours/year      
Durability of technology hours      
Cost(click here for more datails) Capital investment for technology EUR   LSVs: $ 6000 to 14000 electric vehicle[1]
  HEVs: $ 27,000 to 37,000 plug-in mid-sized HEV[2]
Cost of ownership for consumers (for eg. Maintenance) EUR / year      
Cost per unit of energy from technology EUR / kW      
Safety Technology breakdown (including misuse) no. / year      
Severity of failure 0-4      

References

  • H. Wallentowitz
    Alternative Vehicle Propulsion Systems
    August 2003

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