Optimized control of the electrical machine - hofer powertrain GmbH

Challenges of the Series Production of Electric Drive Systems 

Berlin, 2.12.2009 

Your Partner for 

energy-efficient powertrain systems 

www.hofer.de 

hofer powertrain GmbH 

A company of hofer AG 

• 72644 Oberboihingen • Nürtinger Strasse 78 

• E-Mail: info@hofer.de 

• www.hofer.de


Content 

Introduction 

Electrical machines 

Challenges of the electrical drive system 

Conclusion 

9th International CTI Symposium 

„Innovative Automotive Transmissions“ 

www.hofer.de 2.12.09 / Schäfer 

2


Introduction 

The e-mobility will play a major role in the near future 

Especially electrical axle drives for hybrid- and electrical vehicles are in 

focus nowadays 

Because an electrical axle drive can be considered as a high dynamic 

torque-source/sink, special attention is required during the operation mode 

as well as in the failure mode 

The mass production costs for an electrical axle drive and the availability of 

the related material will be very important 




3


Introduction 

Electric drive system for a typical axle drive 




4


Introduction 



Typical performance characteristic 

This kind of characteristic can be achieved by using different electrical 

machines. 


5



General requirements on electrical machines 

Electrical machines integrated into the powertrain have to fulfil the following 

requirements: 

low cost 

low weight / small installation space 

high efficiency 

long durability / low wear 

high availability of the material 

low influence in case of failure 

low noise and vibration level 

high grade of protection 




6



Three-phase machines 

Electrical three-phase current machines are particularly suitable for use in 

hybrid – and electrical vehicles. 

Asynchronous 

machine 

with cage rotor 



Three-phase machines 

Synchronous machine 

with permanent 

magnet excitation 

with separate 

excitation 


7



Asynchronous machine 

Block circuit diagram for such a drive 

Rotor design of an asynchronous machine with squirrel cage 

Electrical drive based on asynchronous machine 



Conductor bars 

Short circuit ring 

Rotor 


8



Permanent magnet excited synchronous machine 

Block circuit diagram of a permanent excited synchronous machine 

Rotor design of permanent excited synchronous machines 

Electrical drive based on permanent excited synchronous machine 



Permanent magnets 

Bandage 

Rotor 

PSM 

Permanent magnets 


9



Current excited (separate excited) synchronous machine 

Rotor design of a separately excited synchronous machine 

Electrical drive based on a separately excited synchronous machine 



SM 


10



Utilization of electrical machines 

Important equations 

1. Equation: 

Fast running machines therefore have a smaller size than machines with lower 

speed at the same performance 

2. Equation: 

⋅ n 

The size of an electric machine will be determined by the required torque, if 

will be constant. 



f 

f 

f 

s 

s 

s 

S 

π 

⋅ ⋅ D 

4 

⋅V 

= T 

Pmech 

= 

V ⋅ 2π 

2 

⋅l 

= T 

f s 

= Bˆ 

⋅ Aˆ 

Pmech: Mechanical power 

fs: Specific tangential force 

ns: B: ˆ 

Rotor speed 

Flux density peak value 

Â: Electrical loading peak value 

D: Diameter of the rotor 

l: Length of the rotor 

V: Rotor size 

T: Torque 


11 

f s



Development process for an electrical machine 

expert knowledge 

In addition to the above mentioned performance simulation also a dynamic simulation concerning 

the dynamic behavior of the electric drive – especially in case of failure – is required. This 

simulation is very important regarding the influence on the vehicle dynamic. 



FEM 

mechanic 

calculation 

requirements 

E-drive 

EM 

analytical 

calculation 

FEM 

electro magnetic 

calculation 

no 

system 

simulation 

prototype 

test bench 

vehicle 

yes requirements 

fulfilled 

boundary conditions 

• power electronic 

• battery 

• installation space 

thermal simulation 

EM-control strategy 

power electronics 

control 

structure dynamic A. 

acoustics 


12



Optimized control of the electrical machine 

In principle all mentioned electrical machines can be used. But especially for axle 

drives a high speed asynchronous machine in connection with a transmission 

with a high gear ratio is well suited for the following reasons: 

Very high robustness 

No safety issues in case of failure 

Needs aluminum inside the rotor instead of rear earth magnets, therefore no problem 

with availability of magnet materials 

Low cost solution compared to other solutions. Therefore well suited for mass 

production 

High overload capability 

Well suited for high speed applications 

No problems with acoustic noise 

Very low torque ripple compared to other solutions 




13



Optimized control of an ASM 

Temp 

_ref 

Motor/Generator/ 

passive mode 

is_max 

Temp. 

compensation 

n_ref 

n_obs 

Map for optimal 

efficiency 

Flux on line 

optimization 



Rs 

Rr 

feed forward 

rd_ref isd 

+ 

isd_ref 

- 

isd_real 

_ref 

1 isq_ref 

isq 

+ 

n rd 

- 

isq_real 

feed forward 

2 

is = isd + isq 2 

is Us Usq 

Usd 

2 

Uu 

Uv 

3 

isq 

isd 

Speed 

Observer 

Uw 

2 

3 

n_obs 

iu 

iv 

Control for optimal efficiency in 

complete speed range 

Air gap flux must be adjusted 

permanently to the driving situation 

(dynamic- efficiency mode, selection 

via CAN) 

Best voltage usage in field weakening 

(Optimal Pulse-Patterns ) 

ϕrd = f ( U dc, 

Treq 

, ω) 

ϕ rd – Rotor flux 

U dc – dc-link/battery voltage 

T req – Reference torque 

ω 

– Rotor speed 


14



Optimized control of a PSM/IPM Control of IPM Machine with consideration 



of non-linear E-Machine Parameters 

Parameter Definition strategy Ld, Lq, Psi_p 

Best voltage usage in field weakening 

(Optimal Pulse-Patterns ) 


15



Optimized control of the electrical machine 

Typical simulation of an electrical drive including a battery model 

Typical system simulation including electrical machines, power electronics and battery 

based on a driving cycle (NEDC) 



Tq_req_asm 

N 

2.5 

Battery model 

U_batt 

+ 

- 

U_batt = 270 V 

R_batt = 0.4 X 

R_batt 

Gear 

Tq_req_psm 

N 

U_dc 

N_isg 

I_dc 

PSM 

ASM 

Tq 

I_ph 

P_dc 

I_dc_psm 

Tq 

I_ph 

P_dc 

U_dc I_dc_isg 

+ + 

I_dc_psm 

I_dc_isg 

Tq 

I_ph 

P_dc 

I_dc 

Tq 

I_ph 

P_dc 

I_dc 

Tq [Nm] 

Idc [A] 

200 

150 

100 

50 

0 

-50 

160 

140 

120 

100 

80 

60 

40 

20 

0 

-20 

Torque PSM 

-100 

900 950 1000 1050 1100 1150 

t[s] 

-40 

900 950 1000 1050 1100 1150 

t[s] 

I_dc PSM 


16



Electrical axle drives can be applied in hybrid vehicles as well as in pure electrical 

vehicles. In a hybrid vehicle normally only one axis is electrified. In a pure electrical 

vehicle all axis, even all wheels can be separately driven by an electric drive. 

B 

B 

G E E G 

Chassis 

G E E G 

Electrical vehicle with all wheel drives 

With this configuration it is possible to influence the longitudinal dynamic as well as the 

transversal dynamic of the vehicle. With this concept it is possible to have a specific 

acceleration – or brake torque on each wheel, so called “torque vectoring”. Furthermore 

it is possible to compensate over steer or under steer with a brake intervention but also 

with a power boost in a range of some mille seconds. 



B 

B 

Front 

wheels 

Rear 

wheels 

E: Electrical machine 

B: Brake 

G: Gear - Box 


17



System behaviour in normal mode of the electrical drive 

As earlier mentioned only an axle drive will be considered. 

Mechanical model of an axle drive 

This model can be used for different investigations: 

Acoustic noise 

Bonanza – and jerk effect 



Chassis 

Axle bearing 

Stator EM 

Rotor EM 

- 

EM 

T 

Gearbox 

Drive 

shaft 

c 

Vehicle 

- 

F 


18



Acoustic noise 

The main reason of acoustic noise of an electrical axle drive is normally the torque 

ripple of an electrical machine. The torque ripple depends on the kind of the electrical 

machine but also on the design of the machine. 

The torque ripple will be transferred via different ways: 

From the rotor shaft to the gearbox 

From the stator housing via axle bearings to the chassis 

In both cases the torque ripple can generate acoustic noise. 

Furthermore the stator housing of the electrical machine can also generate structure 

borne noise caused by radial magnetic forces. 




19



Dynamic drive control 

The main task of the dynamic drive control is the active damping of oscillations within 

the drive train. The following boundary conditions have to be taken into account: 

On the one hand, an oscillation excitation by the driving 

torque of the electric drive shall be compensated. This is 

the case if a fast torque demand is required e.g. "Tip in". 

On the other hand also outer disturbances, e.g. caused by 

the wheels have to be considered. 

The reduction of the oscillations, shall not lead to a noticeable 

reduction of the torque dynamics which the driver perceives asbeing negative. 

In case of failure the vehicle dynamic should not be influenced noticeable. 



Chassis 

Front 

wheels 

Rear 

wheels 


20 

B 

B 

G E E G 

G E E G 

B 

B




Classical approach 

T ref 

filter 

Block circuit diagram of the controlled system with a classic control concept 



- 

feed-forward 

control 

controller 

T* em 

control plant 

inverter 

+ ASM 

filter 

T* 

disturbance 

drive train 

TS Ωwheel Ω Rotor 


21




State variable control 

In the control engineering the state variable control represents an alternative to the 

classical methods. Starting point for this is a description of the system to be controlled 

in the state space in which a linear time-invariant behavior is presupposed. 

T ref 

Model of an axle drive with state variable control 



VV 

preliminary 

filter 

T em 

- 

• 

x (t) 

1 

S 

x(0) + z (disturbance variable) 

x (t) 

BB CC 

RR 

A 

measurement 


22 

Ω R



Safety relevant aspects 

Possible failures within an electric drive 




23



Safety relevant aspects by using a PSM/IPM for an axle drive 

Three-phase short circuit Two-phase short circuit 



•n= const 

Braking torque Oscillation torque 

Idle speed / 

no load operation 

Dragging torque 

B 

B 

G E E G 

Chassis 

G E E G 

Front 

wheels 

Rear 

wheels 


24 

B 

B



Safety relevant aspects 

By using an electrical axle drive based on an asynchronous machine or a separately 

excited synchronous machine there are no special measures in case of failure 

required 

By using an electrical axle drive based on a permanent magnet excited synchronous 

machine (PSM/IPM) the following measures are required in case of a three- 

phase/two-phase short circuit in order to avoid safety problems with the vehicle: 

Disconnecting the mechanical drive train by a fast switching clutch 

Disconnecting the terminals of the machine or disconnecting the star point of the threephase 

winding of the machine 




25


Conclusion 

Some important items have been considered in order to avoid serious problems with 

the mass production of hybrid– and electrical vehicles 

It is very important to consider the total vehicle system behavior in the operation 

mode as well as in the failure mode of an electrical axle drive 

It was shown, what kind of an electrical axle drive has a big potential for the future 

to fulfill the economic and technical requirements of hybrid- and electrical vehicles 




26


Contact 

Your contact: Dr. Heinz Schaefer 

e-mail info@hofer.de 



tel. +49 931 359335-0 

fax +49 931 359335-129 

Visit us at: www.hofer.de 


27

Optimized control of the electrical machine - hofer powertrain GmbH

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