Panic Level: 100-125 bpm

This week, CUER SunSpot takes you on a rather dull extremely fascinating journey inside the world of electrical and information engineering - a world where phases, magnetic fields, currents, voltages and, ultimately, sharp corners* cause a multitude of things to go nastily wrong. Or, alternatively, wonderfully right. But, this being problem-solving week, the probability that there is going to be a blog post about everything working absolutely fine is somewhere down there with you getting to see that live Michael Jackson concert next month…

Too soon? But seriously - today’s post is as a homage to that artist, who himself loved things that go round and round, such as ferris wheels, merry-go-rounds, vinyul records, CDs, occasionally himself and even questions about his sad and untimely demise. Today, dear reader, we learn about the Permanent Magnet Brushless DC Electric Motor, or PMBDCEM ‘motor’ for short.

Exploded view of motor. Please note that motor does not actually explode. Except in very unfortunate cases.

The motor casing is adapted from the Aurora design, with CSIRO electromagnetics. The basic principle (for the technically minded) is that the motor contains a toroidal coil of wire (the stator) and a ring of very strong permanent magnets (the rotor). The rotor and stator can move independently of one another. The electricity flows through the stator in three phases from the motor controller. The motor controller uses Hall effect sensors to track the position of the rotor and act accordingly. The resulting changing magnetic field induced by the flow of electricity in phases through the coil (remember school physics?) interacts with the permanent magnetic field and causes the rotor to spin. Thus the motor goes round and round.

The basic principle (for dummies) is that the electricity goes in, and the motor goes round and round.

The basic principle (for Harry Potter fans) is that the motor goes round and round by magic.

The basic principle (for the very religious/spiritual) is that the motor goes round and round by the power of faith and belief. Our current theory is that, when it worked first time, none of us could actually believe it. Then it broke down.  

The electrical team have been having some issues with the motor ever since the axle holding the stator sheared free of its holding plate and began to spin, taking the really-shouldn’t-spin-at-all stator with it. This caused the cables carrying the three phases into the stator to twist around each other as well. (By the way, the motor was running at the time. We’re not looking at spontaneous rotation or anything. Though that would have made a much more interesting post.)

The (by now) standard response to this incident was to go ‘oh no’, write it down, and then do nothing much about it. Until recently – when Jonathan finally put his foot down on Charlie’s skull and threatened to crush it and demanded politely requested that he be allowed to take it apart. This was duly done – yesterday, in fact – and CUER SunSpot was allowed a sneak peek inside…

The motor is a pretty delicate and heavy bit of kit. The magnets inside it are so strong that it needs a special separator tool just to prise the casing apart. After much careful bashing with pliers and screwdrivers, the bearing cover came off, only to fail to fit over the connectors on the ends of the cables. An inordinate amount of time was spent filing down said connectors before the cover could be removed. Many thanks goes to Michelin for this unorthodox use of their tyre soap.

Eventually it came apart and the extent of the damage was fully realised. It didn’t help that some idiot Jonathan had decided to stuff the axle (through which the cables passed) with plasticine to hold things in place. This had horribly backfired. Some green and gooey minutes later, the cables were finally untwisted, removed from the motor and could be examined more closely. As Jonathan had predicted (via some nifty electronics trick), Phase 1 was effectively grounded – the twisting had caused the heat shrink and about half the copper inside to be cut straight through by the edge of the axle.

As it turned out, Phases 2 and 3 were similarly damaged, although not to the same extent. As an interim measure, the damaged lengths of cable were cut off, some new lengths stripped and covered in heat shrink – although this is not enough to protect it in Australia. It will be properly wrapped in some abrasion-resistant stuff by then. The motor was then put back together and some torque tests run…

Unfortunately, the team then encountered further oddities within the motor controller itself, which will be elucidated in these pages at another time (i.e. when they’ve figured it out).

This was a useful learning experience, which hopefully will feed back into the third generation design. Current thinking is that, since the axle doesn’t need to turn, it’s really quite silly to make it circular, which then needs to be held in place via a friction fit. A friction fit, in fact, that requires more precision than we can currently provide. There are also several things inside the motor that can be tweaked so that the wiring is more resistant both to heat and to abrasion. SunSpot suggested that they should be made resistant to faliure, but got fobbed off with some excuse about being too vague…

Still – progress was made. Lessons were learnt. Snacks were had in abundance.

We leave you with this apt quote from Jonathan himself, which rather sums the situation up nicely:

“I wish I’d done aero instead.”

Sorry. This is the one we meant:

“I knew there was a possibility that it wouldn’t survive Australia; I didn’t expect that it wouldn’t survive Cambridge!”

Until next time, then, eco-fans!

*Well, very ultimately, it’s all electrons and quarks and gluons and the three fundamental forces that actually DO stuff to them, and so forth. At least, that’s what it is at the moment. We’re hoping to get an RSS feed from the LHC as soon as it actually works, which will keep us all up to date on how many elementary particles/dimensions there are currently making up our universe.