Wim Platschorre about the permanent magnet motor:
The new metro vehicle will be fitted with a so-called permanent magnet motor, a variant of the
squirrel-cage induction motor, a brilliant late 19th-century design by Nikola Tesla. Basically,
you’re making a rotating magnetic field of electric currents. This is achieved by putting
alternating current on three lines – and consistently shifting this voltage in phase. This creates
a rotating magnetic field. The revolving part of the motor – the rotor – will try to follow this
magnetic field. The motor will become even more compact than a regular squirre-cage motor
when magnets are embedded in the rotor unit – as is the case in a permanent magnet motor.
That is why this motor type was selected.
High frequency, limited vibration
Each of the three sinusoid currents required in this motor is made up of numerous smaller
blocks of current that are created by quickly switching between 750V of direct current and 0V
(zero). The average of all these pulses creates a sinusoid current. The frequency of this
current ranges from 0 hertz when idle to a few hundred hertz at high speeds.
The motor flows that are produced at this point show more or less rough lines and fluctuations
depending on the number of blocks that make up the sinusoid current. In order to limit noise
and vibrations, it soon comes down to switching between positive and negative at a frequency
between 3 and 5 kHz.
For sake of comparison: the previous metro vehicles switch at a frequency of a mere 300
hertz. This means that we will have to step the frequency up by a factor ten for the new
series!