Why do older electric trains like the 1972 stock in London have a continuously rising electric noise while newer trains like the 1996 stock sound like it’s shifting gears?
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Correction: rising electric noise upon acceleration and braking
The older trains used a very simple analogue control which produces no sound. The sound you hear is from the motor bearings and gears. As the motor speeds up, the noise made by its rotation changes pitch.
The newer trains use an electronic control called a variable frequency drive. The electronics vibrate whenever they switch. The motor runs at roughly the speed of the switching. To make the motor run faster the electronics must switch faster. You can’t just run the electronics at full speed with a slow motor because it will jerk and overload the electronics or the motor. So the electronic noise goes at roughly the same speed as the motor.
Older electronic controls didn’t have great performance at extremes of speed (both too slow or too fast). So, in the 1996 stock, an additional trick was used, called pole changing.
A motor works by rotating a magnetic field which is made by one or more electromagnets. If you invert the polarity of the electromagnets, you can rotate the magnetic field by a certain amount.
Let’s take a simple electromagnet with 2 poles- North at the top, South at the bottom. If you flip the polarity, North goes to the bottom, and South to the top. If you had a rotating magnet aligned South at Top, North at bottom – then flipping the polarity of the electromagnet flips the rotor magnet by 180 degrees. So, a full electrical cycle – positive, negative and back to positive rotates the magnet 360 degrees.
Now imagine a motor with 4 poles on its electromagnet. North at 12 o’clock. South at 3 o’clock. North at 6, and South at 9. Now, when you flip the polarity of the electromagnets, the rotor gets dragged 90 degrees. So a complete cycle rotates the rotor 180 degrees in a 4 pole motor. If you add more pole pairs, you reduce the angle that a complete electrical cycle rotates the rotor.
Now imagine what happens if you have a 4 pole motor, but you swap the polarity of the 3-9 o’clock coils. You convert the magnetic field from N-S-N-S to N-N-S-S (or in simple forms N-S). You can switch a motor from 4 to 2 pole just by swapping two wires with a switch.
So, in order to improve the performance of the electronic variable frequency drive at high speeds and low speeds, it is possible to switch the number poles. 4 poles for initial acceleration and 2 poles for cruising. When the poles switch, there is a sudden change in the drive frequency – which sounds like changing gear – and in a way, that is exactly what is happening. The motor is switching from a low speed, high torque mode to a high speed, low torque mode, and the electrical driver has to adjust to that.
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