Car Alternators Make Great Electric Motors; Here’s How
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Car Alternators Make Great Electric Motors; Here’s How

May 31, 2024

The humble automotive alternator hides an interesting secret. Known as the part that converts power from internal combustion into the electricity needed to run everything else, they can also themselves be used as an electric motor.

These devices almost always take the form of a 3-phase alternator with the magnetic component supplied by an electromagnet on the rotor, and come with a rectifier and regulator pack to convert the higher AC voltage to 12V for the car electrical systems. Internally they have three connections to the stator coils which appear to be universally wired in a delta configuration, and a pair of connections to a set of brushes supplying the rotor coils through a set of slip rings. They have a surprisingly high capacity, and estimates put their capabilities as motors in the several horsepower. Best of all they are readily available second-hand and also surprisingly cheap, the Ford Focus unit shown here came from an eBay car breaker and cost only £15 (about $20).

We already hear you shouting “Why?!” at your magical internet device as you read this. Let’s jump into that.

One of the interesting facets of watching the UK Hacky Racer series grow from a bunch of friends making silly electric vehicles to something approaching a formal race series has been seeing the evolution of the art of building a Hacky Racer. As the slightly grubbier cousin of the US Power Racing series it has benefited somewhat from inheriting some of their evolutionary experience, but that hasn’t stopped the Hacky Racers coming up with their own vehicle developments. They’ve moved from salvaged mobility and golf buggy motors to Chinese electric bicycle and tricycle motors, and now the more adventurous constructors are starting to look further afield for motive power. One promising source for an inexpensive decently-powered motor comes in the form of the car alternator.

Searching for car alternator conversions reveals a variety of pages, HOWTOs, and guides, many of which can be extremely confusing and overcomplex. In particular there are suggestions concerning the three stator connections, with advice to break out the individual windings and apply special wiring configurations to them. Based upon the experience of converting quite a few alternators this appears surprising, as all the various models we’ve converted have had the same ready-to-go delta configuration that needed no rewiring at all. Perhaps it’s time to present a Hackaday guide with a real alternator, and explode any remaining myths while we’re at it.

So, fired up by the prospect of a cheap brushless motor by the passage above, you’ve got a Ford Focus alternator on the bench before you. How does one go about converting it?

On the back of a modern alternator is universally a plastic dust cover secured by a set of bolts. These devices are designed to be refurbished so (perhaps surprisingly for a modern automotive component) they are usually very easy indeed to dismantle. If you take off the dust cover you’ll see the regulator, rectifiers, and brushes, sometimes integrated into a single unit, but more usually as in the case of the Focus alternator with the regulator and brushes as a separate assembly to the rectifier.

There is often a copious quantity of silicone sealant which needs to be cut away, but any nuts or bolts that secure the regulator should be able to be undone, and with care not to damage the brushes themselves it can be lifted clear in one piece. Then the rectifier unit can be removed, a process in which it is sometimes simpler to attack it with side cutters rather than try to remove it in one piece.

You should be able to identify the three bundles of thick enameled copper wires coming from the stator coils, and detach the rectifier straps from them. In some alternators they’re soldered, but some other particularly annoying designs they’re spot-welded. At the end of the dismantling process you should have a bare alternator with three sets of stator wires protruding and a bare shaft with two slip rings, whatever remains of the rectifier pack, and the regulator/brush pack.

The next step is to remove the regulator circuitry while preserving the shape of the regulator/brush assembly, and to locate and preserve the brush connections where they meet the regulator. Yet again there will be copious quantities of silicone potting compound to hack away, but eventually the regulator should be exposed. These are universally some form of hybrid circuit on a ceramic or metal substrate, with connections emerging from the moulded plastic surrounding them being soldered to pads on their edges. It should be relatively straightforward to identify the pair of connections for the brushes, carefully unsolder them, and push out the regulator circuit.

Finally, you should have a bare alternator, a brush pack with a missing regulator circuit, and the plastic dust cover. Simply solder three suitably large-gauge wires to the three sets of stator wires and cover them in heat-shrink, solder a pair of lighter wires to the brush connections, and reassemble the brush pack to the alternator. You may need to put some form of strain relief on the wires to the brushes. The rectifier pack doesn’t need reassembling, so on some models you may need to make a spacer to replace it in supporting one side of the brush pack.

Holes can be made in the dust cover for all the various wires, and the dust cover fitted with all the wires poking through. At this point you’ve converted your alternator, and all that remains is to drive it with something. Fortunately that is a surprisingly simple process with off-the-shelf parts.

A so-called brushless DC motor is simply an AC motor with a bundle of electronics that turns a DC supply into an AC one to run it. They have the advantage over brushed DC motors in reliability, efficiency, and ease of speed control, but at the expense of more complexity.

The good news for people converting automotive alternators into electric motors is that a whole range of brushless motor controllers can be had for not a lot of money, in the form of electronic speed controllers (ESC) intended for those Chinese electric bicycles and tricycles. They take a battery DC supply and produce a three-phase AC suitable to drive a delta-connected motor, and they work well with converted alternators.

ESCs have two modes, one for motors with Hall-effect feedback sensors, and one for motors without such as our alternator. Usually a wire link needs to be made to enable this, consult the instructions for your controller. We’ve found that an alternator drives well as a motor from a 36V or a 48V supply, and as long as a controller with enough power is used then they do so reliably. A quick AliExpress search for “brushless motor controller 1500W” turns up plenty of choice.

Given a controller, there is one more requirement for our alternator to become a motor, it must have a DC supply to its rotor winding. It needs to have about 2 or 3A flowing through it, for which a current-limited PSU module performs the task admirably. Having to use that power makes the motor a bit less efficient than a permanent magnet one, but the cost of a scrap alternator is hard to beat.

The motor featured in our pictures is destined to be one of a pair providing traction in a new car for an assault on this year’s races. Personal experience with SMIDSY the Robot Wars robot would lead me to give them forced-air cooling, but unlike the electric tricycle motors these do seem to cope well with getting hot. An alternator motor might not be the one-stop solution to whatever your small-scale traction needs could be, but even so it’s worth being aware that they are an option without unexpected wiring rituals. If you convert one for a project, please make sure to write it up and send it to our tips line!