MOTORS FOR TRAM DRIVES

This paper compares five different motor types used in variable speed drives: a dc motor with a mechanical commutator and with electromagnetic or permanent magnets excitation, a cage induction motor, asynchronous cascade with a slip-ring motor and a brushless motor with PM excitation. These motors are to be used for tram drive and they should all be characterised by identical external dimensions and a cooling system. Rated power and efficiency are the principal comparison criteria.


Introduction
The paper investigated different electric motors used in variable speed drives. It is in these drives only that it is worthwhile to use dc motors with an electronic or mechanical commutator. The variable speed drives are used in the following cases: • when changes in speed are required by the duty algorithm of the drive (e.g. roll mill drive), • when the drive should operate at minimum energy consumption -energy-saving drive. Energy-saving drives are preferable from the viewpoint of environmental protection.
Energy-saving operation is achieved when the drive is run at minimum speed compatible with the requirements of the engineering process.
Variable speed drives can be equipped with the following motor types (Fig 1): • dc motor (Ma) with electromagnetic excitation supplied from a power electronics converter (rectifier) ac/dc, • dc motor (Mb) excited by permanent magnets (NdFeB) placed in the stator supplied from a power electronics converter (rectifier) ac/dc, • cage induction motor (Mc) supplied from a power electronics converter (inverter) ac/dc/ac, • asynchronous cascade consisting of slip-ring induction motor (Md) and inverter/transformer set used for transmitting electrical energy from the rotor to the power network, • brushless motor (Me) excited with permanent magnets (NdFeB) placed in the rotor supplied from power electronics circuit called an electronic commutator ac/dc/ac. The comparison of the rated power and efficiency of these motors is given in the paper. The comparison criteria are: • external overall dimensions D 400 = mm, l 660 = mm (Fig 2) (Fig 3) has been constructed by modifying motor existing design. Permanent magnets (NdFeB) have been glued to the pole shoes on the air gap side.  The dc motor with electromagnetic excitation (Ma) is rated at 40 kW, 300 V, 1800 rpm, efficiency 89 % is the reference base for the comparison.

MOTORS FOR TRAM DRIVES
In Poland dc voltage is used for traction purposes. In railway it is 3000 V and in tram catenaries the voltage is 600 V. Traditionally, the trams ere equipped with dc series motors with resistor starters. This technology has become an anachronism, however, for economical reasons mostly. Most of the existing drives are still in operation. It is worthwhile to investigate possible drive modernisations, assuming that the mechanical gearbox should not change and that only the motor and possibly a supply and starting system get altered. Hence, the dimensions of the alternate motor should be the same as those of a motor currently in operation. The following analysis of different drives is based on this assumption.
Dc motor (Ma) and induction motor (Mc) are currently manufactured and used in 105 N tram drive, their parameters are available. Dc motor (Mb), brushless motor (Me) and slip-ring motor (Md) parameters have been determined by analysis. mm and commutation winding parameters have also been left unchanged. Indexes "a" and "b" relate to Ma and Mb motors, respectively. Since excitation winding is absent in Mb motor, the window cross-section between the main poles and commutation poles may be decreased, since this window contains commutation poles winding only.

Dc motor with a mechanical commutator
Hence, rotor diameter of Mb motor can be increased. The rated power Nb P of Mb motor can be estimated from the formula [1]: Diameter b D of the new motor will be greater, since the window cross-section between the main poles and commutation poles will be decreased. In Ma motor excitation coil two flat copper wires are placed near the pole. The height and width of these wires are Maintaining the air gap induction and assuming that air gap width is equal to b 2 = δ mm (in Ma motor the air gap is equal to a 3 = δ mm), NdFeB PM length should be equal to l m 6 = mm. This is determined by calculating the induction at rotor surface when a magnetic circuit is excited with permanent magnets [2].
Mb motor rotor diameter and rotor volume will therefore be equal to: Mb motor rated power at continuous duty ( 1 S ) is determined by equation (1). Its rating is P Nb 56 = kW. Mb motor efficiency will go up, since excitation losses f P ∆ are non-existent. The power losses in the main poles pole shoes will also be less, since the air gap for alternating components of the flux will be increased from a 3 = δ mm to mm. These losses are due to: • slot pulsations of excitation magnetic flux, • armature reaction flux pulsations due to power electronics converter. These losses are neglected in the overall power balance.
Armature power losses in Mb motor will increase in proportion to rotor volume: These losses ( P 5416 = ∆ W) are greater than total power losses in Ma motor. In order to keep the motor heat balance, total losses should not exceed 4940 W (value for Ma motor). This can be achieved by decreasing Mb motor rated power Nb P by 5 %, i.e. from 56 kW to 53 kW. The power losses will go down to P 4852 = ∆ W. Mb motor efficiency is equal to: Efficiency is determined by equation (5). Its rating is % ,6 91 = η . To summarize, using the casing of Ma motor it is possible to design Mb motor with greater rated power and with higher efficiency.
Dc motor Mb excited with NdFeB permanent magnets will be a separately excited motor with one speed control range at constant torque.
Ma and Mb motors characteristics are presented in Table. 3

. Cage induction motor Mc
Cage induction motor Mc has been designed by the authors with identical dimensions as Ma motor and is currently being manufactured and employed as the main drive motor for N 105 N type trams. It is often installed in the tram during vehicle general overhaul, when the drive is modernized. Since there is no commutator, the active part of the winding is longer (i.e. stacking is longer) l c 300 . However, usually vector control is used since it improves the drive dynamics and brings it close to dc motors dynamic properties.
Mc motors characteristics are presented in Table. 4

. Asynchronous cascade Md
Asynchronous cascade consists of a slip-ring induction Md motor and a frequency converter ac/dc/ac connected into rotor circuit -see Rated power is determined by equation (7). It rating is P Nd kW. Iron losses, while induction remains the same, will also decrease by the same ratio kVAr. Md motor power factor is: ( 11) Power factor is determined by equation (10)  Md motors characteristics are presented in Table. 5. Dc brushless motor Me excited with permanent magnets The magnetic circuit of brushless Me motor with electronic commutator is shown in Fig 4. Motor stator and stator windings are identical as in the induction Mc motor. The stacking length may remain unchanged and equal to l e 300 = mm and the stator inner diameter may be equal to D e 215 = mm. It has also been assumed that air gap e 1 = δ mm and magnetic length of permanent magnets l m 4 = mm. The brushless Me motor with an electronic commutator, at load power 53 kW (continuous duty 1 S ) will be characterised by better operating parameters than an induction motor [2].
Current flowing in the winding will possess the active component only: . The efficiency of a brushless Me motor with an electronic commutator is higher by 4,2 % than the W, and rated power subsequently increases up to P N 77 = kW. Hence a brushless Me motor with an electronic commutator can be designed on the basis of Ma motor dimensions. This new motor will be excited with permanent NdFeB magnets and it will be rated at 77 kW. Its rated efficiency is determined and its rating is % ,1 94 = η . This motor type makes possible the achievement of the highest power and efficiency at the given volume. The motor operates as a dc motor excited with permanent magnets, i.e. only one range of speed control is available ( const = T ); the speed varies as supply voltage changes.
This motor is also characterised by high torque overload capacity depending on allowable transistor currents and mechanical strength of the shaft, coupling and transmission.