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One thing to note about steppers, If you take them apart they will loose half their torque. In the factory they magnetize them after assembly. If you take

One thing to note about steppers, If you take them apart they will loose half their torque. In the factory they magnetize them after assembly. If you take them apart the lower permeability of air causes the rotor to somehow lose much of it's magnetic field. Physicists please chime in, IDK why this happens. We discovered this after machining some stepper backshells to accept optical encoders.

oakwhiz 4y ago
This is fascinating, I hope someone knows the answer. Is it possible that they magnetized the teeth of the rotor as a Halbach array?
- adrian_b 4y ago
  Any permanent magnet demagnetizes partially whenever it is taken out of a closed magnetic circuit. If the magnet is made from a material with very high coercivity, the demagnetization may be negligible, but it is always recommended to store permanent magnets only with a piece of soft iron in contact with their N and S poles. To reach the maximum remanence possible for a given material, a permanent magnet must always be magnetized after being assembled in the final magnetic circuit.
  - GeorgeTirebiter 4y ago
    What about, say, Neodymium magnets? https://www.kjmagnetics.com/neomaginfo.asp https://www.kjmagnetics.com/neomaginfo.asp What I'm asking is: are Neodymium magnets NOT used in these applications?
    - jbay808 4y ago
      They are used in these applications, typically very high grade, unless you're buying a legacy product line. In, say, a hybrid stepper the magnet is usually a wide thin round disc, sandwiched between two steel rotor lamination stacks. With the link below you can examine the BH curve and load line for a Ø20 mm N52 disc magnet, 2mm thick, at 20 degrees C. (It's not exact because this assumes a magnet in free space and neglects the steel of the rotor - but it's an illustration). https://www.kjmagnetics.com/bhcurves.asp https://www.kjmagnetics.com/bhcurves.asp You'll notice that the load line (which assumes the magnet is in free air) is already landing within the "knee" where the intrinsic magnetization starts dropping rapidly. That's working too far along the hysteresis curve, where the poles are already starting to flip and demagnetize. However, if that magnet were surrounded by the steel of the stator, the high permeability of the magnetic circuit would put the load line at a steeper angle, where the magnetic field through the magnet would be much higher. Small changes in permeability around that point would not damage the magnet, but allowing it to fall all the way down below the knee-point would. It would not be completely demagnetized by that, but it would lose some of its original strength.
jerzmacow 4y ago
Perhaps when you slide the rotor past the stator lamination, it demagnetises it a bit? Like those screwdriver demagnetizers
jbay808 4y ago
If you're familiar with electronics: Imagine you have a current source, driving 1 amp through a variable resistor set to a very low resistance. It only takes less than a volt to sustain that current, so your current source is quite happy to do this. Then, without turning off the current source, you increase the resistance: now the current source needs a lot more voltage to sustain that current. From the current source's point of view, the voltage across the resistor looks indistinguishable from a voltage source pushing back against it. Even though that voltage is coming from the resistive load, even though that voltage only exists across the resistor because of the 1 amp that the current source is itself driving, the load acts the same as a voltage source fighting to drive current backwards into the current source. Imagine that your current source is not ideal: it has voltage limits. If you want to squeeze the most power out of your current source, you'll set the resistance up such that the resulting output voltage is near the limit of what your current source can handle while still supplying 1 amp. If you increase the resistance further, you'll exceed the voltage rating and possibly damage your current source. Then even if you return the resistance to a low level after that, you might not get 1 amp anymore from that damaged source. All of this has been an analogy. Permanent magnets are a lot like non-ideal sources that cannot turn off. To squeeze the very most out of the magnet, you want to configure the load such that it's driving the magnet to its limits. When you remove the rotor from the stator, it now has to push magnetic field through air, rather than through steel. This increases the effective demagnetizing load working against the magnet (known as "reluctance", analagous to resistance). It's no different from the magnet's point of view than if you'd looped an electromagnet wire around it fighting against its magnetization. Permanent magnets are magnetized through a hysteresis process, and with a strong enough demagnetizing field, the internal domains can flip and the magnet gets demagnetized.

oakwhiz 4y ago

This is fascinating, I hope someone knows the answer. Is it possible that they magnetized the teeth of the rotor as a Halbach array?

adrian_b 4y ago
Any permanent magnet demagnetizes partially whenever it is taken out of a closed magnetic circuit. If the magnet is made from a material with very high coercivity, the demagnetization may be negligible, but it is always recommended to store permanent magnets only with a piece of soft iron in contact with their N and S poles. To reach the maximum remanence possible for a given material, a permanent magnet must always be magnetized after being assembled in the final magnetic circuit.
- GeorgeTirebiter 4y ago
  What about, say, Neodymium magnets? https://www.kjmagnetics.com/neomaginfo.asp https://www.kjmagnetics.com/neomaginfo.asp What I'm asking is: are Neodymium magnets NOT used in these applications?
  - jbay808 4y ago
    They are used in these applications, typically very high grade, unless you're buying a legacy product line. In, say, a hybrid stepper the magnet is usually a wide thin round disc, sandwiched between two steel rotor lamination stacks. With the link below you can examine the BH curve and load line for a Ø20 mm N52 disc magnet, 2mm thick, at 20 degrees C. (It's not exact because this assumes a magnet in free space and neglects the steel of the rotor - but it's an illustration). https://www.kjmagnetics.com/bhcurves.asp https://www.kjmagnetics.com/bhcurves.asp You'll notice that the load line (which assumes the magnet is in free air) is already landing within the "knee" where the intrinsic magnetization starts dropping rapidly. That's working too far along the hysteresis curve, where the poles are already starting to flip and demagnetize. However, if that magnet were surrounded by the steel of the stator, the high permeability of the magnetic circuit would put the load line at a steeper angle, where the magnetic field through the magnet would be much higher. Small changes in permeability around that point would not damage the magnet, but allowing it to fall all the way down below the knee-point would. It would not be completely demagnetized by that, but it would lose some of its original strength.

jerzmacow 4y ago

Perhaps when you slide the rotor past the stator lamination, it demagnetises it a bit? Like those screwdriver demagnetizers

jbay808 4y ago

If you're familiar with electronics: Imagine you have a current source, driving 1 amp through a variable resistor set to a very low resistance. It only takes less than a volt to sustain that current, so your current source is quite happy to do this. Then, without turning off the current source, you increase the resistance: now the current source needs a lot more voltage to sustain that current. From the current source's point of view, the voltage across the resistor looks indistinguishable from a voltage source pushing back against it. Even though that voltage is coming from the resistive load, even though that voltage only exists across the resistor because of the 1 amp that the current source is itself driving, the load acts the same as a voltage source fighting to drive current backwards into the current source. Imagine that your current source is not ideal: it has voltage limits. If you want to squeeze the most power out of your current source, you'll set the resistance up such that the resulting output voltage is near the limit of what your current source can handle while still supplying 1 amp. If you increase the resistance further, you'll exceed the voltage rating and possibly damage your current source. Then even if you return the resistance to a low level after that, you might not get 1 amp anymore from that damaged source. All of this has been an analogy. Permanent magnets are a lot like non-ideal sources that cannot turn off. To squeeze the very most out of the magnet, you want to configure the load such that it's driving the magnet to its limits. When you remove the rotor from the stator, it now has to push magnetic field through air, rather than through steel. This increases the effective demagnetizing load working against the magnet (known as "reluctance", analagous to resistance). It's no different from the magnet's point of view than if you'd looped an electromagnet wire around it fighting against its magnetization. Permanent magnets are magnetized through a hysteresis process, and with a strong enough demagnetizing field, the internal domains can flip and the magnet gets demagnetized.