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Vol: CXLVII, No: 3827 – May 19th 1939

The Variable Speed Gear of London

Hydraulic variable speed transmission

Engineering Blog
All about Engineering

Advert from *Engineering.*

Vol: CXLVII, No: 3827 – May 19th 1939

The Variable Speed Gear of London

Hydraulic variable speed transmission

Advert from *Engineering.*

Vol: 211, No: 1 – April 1971

Teleflex Morse of Basildon

Motion, power, signal transfer consultancy and equipment

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Vol: CXLVII, No: 3827 – May 19th 1939

The Morse Chain Co of Letchworth

Chain drives

We’ve been talking about step-down transformers

that can convert between a high line voltage and more moderate local

voltages. You might be asking yourself why we need to do such a thing in

the first place. Why can’t we just transmit utility power at 120 V or

240 V?

All right, let’s try it. Say we need to transmit 20 MW of

power over a 100 km. If we do it at 240 Vrms, we’ll have a current of

83.3 kArms flowing through the conductor.

Some

of the 20 MW we’re transmitting is going to be lost – that is,

dissipated as heat. We’d like to minimize losses, so let’s say we’re

aiming for 97% efficiency – no more than 3% of that 20 MW lost. Since

power dissipated is a function of current and resistance, we know we’ll

need a conductor with an overall resistance of 8.64 x 10^-5 Ohms over a

distance of 100 km.

The resistance of a conductor

is a function of its length, l, its cross-sectional area, A, and the

conductor’s resistivity,

ρ, which is a property of the material it’s made of.

We’ll assume a resistivity here of 8 x 10^-8 Ohm-meters. From here, we

can figure out what kind of cross-sectional area our conductor needs –

in other words, how large the cable’s diameter has to be.

To make this work, you’d need a conductor with a radius of 5.4 m. Clearly, this isn’t going to happen.

If

you want a conductor that’s actually practical, that means you need to

transmit power at high voltage – since a higher voltage at the same

power will result in a lower current, this means your losses will be

lower, allowing you to use a much smaller conductor. Here are the same

calculations done with a transmission voltage of 240 kVrms instead of

240 Vrms.

In this case, you need a conductor with a radius of about 0.5 cm, which is much more reasonable.