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Wound Rotor Synchronous Machine for Traction Application

Motors and generators with MotorSolve

This example illustrates wound rotor synchronous machines for use in traction applications. An 8 pole 48 slot wound rotor motor is considered based on the design of a similar interior permanent magnet (IPM) machine. An advantage of wound field motors is that an additional degree of freedom (due to rotor windings) for applying optimal field weakening may be utilized. The results presented below demonstrate this as well as a simple example of vector control of the wound field motor. The model was designed and analyzed using MotorSolve.

Some of the advantages and disadvantages of the motor compared to that of an IPM are highlighted. With advances in power electronics based control, it is now amongst the other motor types as contenders for traction application, as this example illustrates.

An 8 pole 48 slot wound rotor brushless machine

METHODS and RESULTS

REQUIRED MOTOR SPECIFICATIONS

The design specifications of the machine are presented in this table. An 8 pole 72 slot wound rotor machine has been selected so that direct comparisons could be made with an IPM with similar pole/slot combination. In fact, the stator of the two machines is kept identical so that only the rotor designs are optimized to generate similar performance.

Design Specifications  
Frame size (mm) 200
Frame length (mm) TBD
Drive Type Sine
Voltage (DC rail) 650
Power output (cont./int., kW)) 55
Rated Speed (rpm) 4000
Rated Torque (Nm) 100
Peak Torque (Nm) 190
Rated rms phase current (amps) 170

INITIAL MODEL & DESIGN ITERATIONS

The initial model requires the selection of machine configurations including pole/slot combination (selected already for direct comparison), lamination materials and rotor windings (field current and number of turns). The initial model is obtained by iterations to obtain the desired flux density distribution levels as well as a comparable back emf to that of the IPM. The flux density distribution is shown here at no load (zero stator current).

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BACK EMF of the INITIAL MODEL

Comparison of the back emf between the wound rotor and IPM model is presented next. The two designs show reasonable agreement between their back emf.

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TORQUE & POWER vs. SPEED

The torque-speed characteristics of the wound field and the IPM are presented here. It shows that the wound machine performs comparably to the IPM in the speed range of interest.

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FLUX WEAKENING

The flux weakening capability of the wound field machine is demonstrated in the output power vs. speed results presented here. The results show the feasibility of applying field weakening for a large constant power speed ratio.

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PERFORMANCE SUMMARY

After iterations of the geometric and other parameters of the model to satisfy the desired performance criterion a final model is obtained whose rated performance summary are shown here.

Performance Summary at 4000 rpm, 30 deg Gamma  
Input Power (kW) 59.2
Output Power (kW) 57
Efficiency (%) 95.1
Average Torque (Nm) 99.3
Air gap stress (N/mm2) 0.0324
Rotor core mass (kg) 6.1
Stator core mass (kg) 15
Rotor copper mass (kg) 2.22
Stator copper mass (kg) 4.72

LOSS COMPARISONS

Loss comparisons are presented here between the IPM and the wound motor model. As expected, the IPM has lower winding losses compared to the wound rotor model. The core loss comparison shows similar levels. The disadvantage of high copper losses may be offset by added flexibility motor control options in wound field motors as well as the ability to extend the constant power speed range to high values.

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VECTOR CONTROL of a WOUND FIELD MOTOR

From MotorSolve, an FEA based equivalent circuit model of the motor can be exported to SIMULINK for realistic drive simulation. A simple example of d-q or vector control of the motor model is illustrated here. The PI simulation circuit along with the equivalent motor model is shown in this figure. Load driven transient simulation of the model has been performed and some results are presented.

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SPEED vs TIME

Speed vs. time and d-q axis currents vs time are presented here from transient simulations of the PI controlled load driven simulation of the wound rotor model. The results show that it is easy to implement a simple PI control strategy for this machine to achieve the desired level of speed control. The variation of the d-q currents control the torque and behave as expected.

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