Aina

Question 1

Describe why it is preferable to design a traction motor for as high rotational speed as possible.

Answer 1

High rpm is desirably to increase power density and thereby decrease motor size and ease the budget.
Breakdown torque however decrees too, also decreasing top-speed.

Question 2

Adhesion

Answer 2

α=F/(mα*g)

Question 3

Gravitational resistance/acceleraton

Answer 3

Dg=mg*S/1000

S in per mille (2%=20)

Question 4

Power

Answer 4

Pm=Fv and Pe=VI

Question 5

In phase
Capacitive
Inductive

Answer 5

U and I in phase
U lagging I
I lagging U

Question 6

Power factor

Answer 6

is the relationship between active power and total apparrent power.

λ = P_lo/S

Question 7

AC/DC traction. pros and cons

Answer 7

Pros:
DC: Simple control
AC: Smaller motor due to high speed, sinusoidal line current, unit power factor (controllable).

Cons:
DC: Bigger motor due to low speed, wear of commutator, Consumes reactive power, generates harmonics
AC: More complex converters (at least historically)

Question 8

Assuming an AC railway power supply system with booster transformers connected between the contact line and return conductor (BT system). Propose three methods that would improve the power capacity of the
system.

Answer 8

• Larger cross sectional area of the conductor or return wire
• Additional AC feeder in parallel to the track
• Use a system with Auto transformers (AT) that means a doubling of the transmission voltage.

Question 9

In the early days of AC railway electrifications a low

frequency was chosen like 162/3 Hz. Please explain why?

Answer 9

The single-phase AC motor required a low frequency to ensure an acceptable commutation (too large induced emf in the commutating coil).

Question 10

Railway power supplies for 50 or 60 Hz are normally fed

directly from the public grid through transformers. Describe the pros and cons of such a solution

Answer 10

A higher voltage can be used that improves power capacity and 50 Hz is a standard frequency for the public grid. The main drawbacks are asymmetric loading of the three-phase supply and sectioning of the catenary that reduces the capacity for regenerative braking.

Question 11

Compared to operation below base speed. Increases or decreases cosϕ when a motor is operating in the constant power region of the tractive effort diagram? Please motivate your answer

Answer 11

cosϕ is increasing since the flux of the motor is reduced during field weakening (constant power region). A decreasing flux results in lower magnetizing current and thus a reduced consumption of reactive power.

Question 12

In parallel with the Swedish catenary system, there is a single-phase 130 kV line that is connected to the contact wire via equidistant transformers. Describe the purpose and function of the 130 kV line.

Answer 12

The purpose is to increase the power capacity of the system by lowering the impedances of the catenary system. The impedance of a parallel connection is substantially lower than that of the overhead wire solely.

Question 13

In the early days of AC railway electrifications a low frequency was chosen like 16 2/3 Hz.
Why?

Answer 13

High frequencies were difficult to handle. 16 2/3 is a third of the 50 hz national grid. Easy to convert.