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 The basic principle of all machines is the following: In the stator, electrical power is given or withdrawn; in the runner, mechanical power is given or withdrawn.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 P_n can be permanently adjusted. Maximum parameters such as maximum torque M_max and maximum power P_max 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 M_n or the maximum moment M_max can be adjusted. Increasing the speed at constant moment M_N, the mechanical power rises linearly, until the nominal power is reached. At this point, the nominal speed n_N 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