J. Joller

Research of
Trams Traction Drives

The research conducted is based on the contract concluded between the Tallinn Tram and Trolleybus Company and Tallinn Technical University in January 1998. The measuring program, an automatic measuring system for the tram KT4 (C KD), theoretical calculations and analysis results are presented in this paper. The possibilities for the modernisation of traction drive are proposed.


The Tallinn Tram and Trolleybus Company owns trams of type T4 (56, average age of 19.5 years) and type KT4 (75, average age of 12 years). Type KT4, the most recent ones (36, age up to 12 years), have a DC drive with starting rheostats and waste much energy. To improve their performance, they could be equipped with modern power converters. After modernisation, their energy consumption will probably decrease about twice. In spite of that, it was necessary to prove in terms of technical and economic considerations that the modernisation will be reasonable in Tallinn's traffic conditions and that the investment will have a reasonable pay-off time. Another aim of the measurements was acquisition of data for new converter design.


To determine power distribution, the following automated measurements have been carried out:

The measurements have been carried out on a study tram (line 4) and an passager loaded trams (line 4 and 2) at different times in real traffic conditions (104 424 results, respectively). Safety requirements of the measuring system were extremely high to guarantee the security of passagers.

The simplified measuring system and tram power circuit is shown in Fig. 1. The voltage dividers (PJ) of the multimeters DT 830 E (UNI-T) were used for adapting the voltages (DC net voltage:

600 V ) with the data logger.

The non-intrusive current transducer APPA32 (AC/DC 100/600 A) was used for current measurements. The frequency of the speed signal was measured via adapting circuit (SL) and a digital input of the data-logger. The time step chosen for measurements was 1 s because in general, accelerations and decelerations have longer duration. All the signals were sampled simultaneously. The measurement data were stored in the data-logger and off-loaded with a PC after measurements. The total accuracy of the measurements was ± 1.5 %. The currents of the motors were equalised by the tram's circuits.

The speed and current of the tram on the Viaduct of Pärnu Road are shown in Fig 2.


The following formulae were used for calculations:

The speed of the tram, m / s

, (1)


is the the output frequency of the tacho,
p is the the number of poles (),
is the perimeter of the tram's wheel, is the transfer ratio between motor and tacho ( = 9 / 19),
is the transfer ratio between motor and wheel ( = 1 / 7.43).

The consumed power of the traction drives (two pairs of motors) at acceleration ( > 0)

Fig. 1. The simplified measuring system and power circuit of the tram KT4.

Fig. 2. The speed and current of the tram on the Viaduct of Pärnu Road.


. (2)

– the armature current of the motor (see Fig. 1).

The armature voltage


The voltage coefficient of the DC motor with series-excitation, Vs / rad

, (4)

where is the angular velocity of the motor and is motor resistance.

The voltage coefficient must be calculated again for every measurement, because the magnetic flux of the series excitation DC motor depends on the current and on the excitation shunt resistance.

The angular velocity of the motor is


The motor resistance is

where is armature resistance and –resistance of the compensation windings.

The torque of the motor is

, (7)

where is the torque coefficient.

. (8)

The power of the motor is


The power of the start rheostat is


where is the voltage drop on the rheostat.

The energy consumed by the motor is

where n – the number of measurements,

i – the counter (i = 1 … n),

t – measuring interval, sec.

The energy consumed by the rheostat is


Total consumed energy is



The main results of the tram KT4 (line 4) are shown in the table 1.

The main results of the measurements and analysis of tram KT4 (line 4)

Table 1
Maximum starting current 
511 A
Maximum braking current
493 A
Average power (per motor)
Maximum power (per motor)
– duration 1 s
– duration 10 s
3.19 kW

92.5 kW
44.6 kW

Average speed
14 km/h
Consumed energy
52.1 kWh
Losses in rheostat, total
37 kWh
– at acceleration
13.5 kWh
– at electrical breaking
23.5 kWh
Reduction of energy
consumption using IGBT
– without regenerative braking
– with regenerative braking, 
average in real conditions
theoretical maximum


26.3 %

46 %
60 %

The energy balance (Fig. 3) gives a good overview of power division in traction drive and mechanics of tram KT4.

Fig. 3. Energy balance of tram KT4 without converter.


The energy balance after replacing the rheostat with IGBT step-down converter is demonstrated in Fig. 4.

Fig. 4. Energy balance of tram KT4 with IGBT step-down converter.



The motors (4 x 40 kW) have a relatively low load. The average power is only 3.19 kW per motor. The maximal

power was 92.5 kW (duration only 1 s). The rated power appears only for short periods of one to 15 seconds.

The modernisation of tram KT4 traction drives is necessary. Replacement of the starting rheostats with IGBT step-down (buck-) converter results a 46 % decrease (1.85 times) in energy consumption in the existing DC grid conditions. If the DC net could swallow the whole braking energy, the energy consumption of trams will be reduced maximally by 60 % (2.5 times). To achieve this, accumulators or inverters in substations (or in trams) must be used.

The pay-off time of the IGBT step-down converter will be approximately six years. The amount of energy, which can be saved in the remaining 10 – 15 years of exploitation, cannot pay off the costs of changing the whole drive system with AC motors.

The analyis showed that it is rational to apply the new IGBT step-down converters with existing DC motors of trams KT4. The starting rheostat must be replaced with the IGBT step-down converter.


1. Joller, J., Arukivi, A., Mälter, M., Power Measurements of Trams and Trolleybuses in Tallinn's Traffic Conditions. Report of research work No. 677 L, Tallinn Technical University. Tallinn, 1998, 216 p. (in Estonian).
Jüri Joller, MSc.

Lecturer at the Department of Electrical Drives and Power Electronics, Tallinn Technical University. Special scientific interests: power electronics, microelectronics, and electrical drives.

(Department of Electrical Drives and Power Electronics, Tallinn Technical University, Kopli St. 82, 10412 Tallinn, ESTONIA)