The motor speed rises and falls on its own
Stop the machine and unplug it before reading the rest.
This symptom is misleading, which is why it deserves a page of its own. A motor rising and falling in speed looks like a power problem: people think of a bad socket, of the extension lead, of a dip on the household supply. They try another socket, it seems better for a while, and they carry on using it. That is the most common way of destroying an armature.
Why it isn't the socket
A contact really is giving way — but it is inside the motor, not in the wall.
The carbon brushes are two blocks of graphite resting on the armature and carrying current to it. A spring keeps them pressed down. When they are almost completely worn, the spring is at the end of its travel and the pressure becomes inconsistent: the brush loses contact for a fraction of a second, the motor drops, then picks up again. Heard from outside, that is exactly a voltage dip. Inside, it is graphite no longer making contact as it should.
Why people wait too long
Because at first it only does it now and then. One day yes and three days no, and on the three good days the machine seems fine. People telling us about it in the workshop nearly always say the same sentence: "it had been doing it for a while, but then it would pass."
In that "then it would pass" the metal holder of the brushes has already started touching the armature and scoring it. Those scores do not come out, and from there the only road left is a new motor.
How to tell it from a real supply problem
An electrical fault switches off: the machine stops dead and starts again when you move the cord. Worn brushes, by contrast, modulate: the machine never switches off completely, it drops in speed and rises again, often along with a new humming. If moving the cord changes nothing, it is not the cord.
What happens now
The motor is taken out in the workshop, new genuine brushes are fitted, the armature is cleaned and performance is checked on reassembly. If the armature is sound, we add two years to the warranty on the motor. If it is already scored we tell you with photographs before going any further, and a new motor costs 210 € instead of 360 €, because the 150 € of the overhaul comes off.
Leave the machine switched off and write to us: we'll tell you how to send it in.