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.
Introduction
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 payoff time. Another aim of the measurements was acquisition of data for new converter design.
Measurements
To determine power distribution, the following automated measurements have been carried out:
The simplified measuring system and tram power circuit is shown in Fig. 1. The voltage dividers (PJ) of the multimeters DT 830 E (UNIT) were used for adapting the voltages (DC net voltage:
600 V ) with the data logger.
The nonintrusive 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 datalogger. 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 datalogger and offloaded 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.
Calculations
The following formulae were used for calculations:
The speed of the tram, m / s
, (1)
where
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
(3)
The voltage coefficient of the DC motor with seriesexcitation, 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
(5)
The motor resistance is
(6)
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
(9)
The power of the start rheostat is
(10)
where is the voltage drop on the rheostat.
The energy consumed by the motor is
(11)
where n the number of measurements,
i the counter (i = 1 n),
t measuring interval, sec.
The energy consumed by the rheostat is
(12)
Total consumed energy is
(13)
Results
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)
Maximum starting current 

Maximum braking current 

Average power (per motor)
Maximum power (per motor) duration 1 s duration 10 s 
92.5 kW

Average speed 

Consumed energy 

Losses in rheostat, total 

at acceleration 

at electrical breaking 

Reduction of energy
consumption using IGBT converter without regenerative braking with regenerative braking, average in real conditions theoretical maximum 
26.3 % 46 %

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 stepdown converter is demonstrated in Fig. 4.
Fig. 4. Energy balance of tram KT4 with IGBT stepdown converter.
Conclusion
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 stepdown (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 payoff time of the IGBT stepdown 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 stepdown converters with existing DC motors of trams KT4. The starting rheostat must be replaced with the IGBT stepdown converter.
References

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) 