Blown Armature

The other day I was driving my daughter to a friends house in our Electric Car. I hammered it a bit taking off from the lights. Suddenly the engine seemed to shudder and loose power. It felt like a petrol motor that was “missing”. I stopped, pulled over and popped the bonnet. I could smell burning. Revving the motor I could see small sparks flying out near the brush end of the motor. Oh dear.

I managed to limp home at low speed and it was up on the ramps early the next day to drop the motor and take it down to my local electric motor shop. Eventually it was decided that the armature was kaput. Some dead windings and a 1 ohm connection to ground (normally both motor terminals are isolated from the chassis).

I phone up the good people at EV Motors Australia and they were surprised – the exact same motor has been running for 19 years in their EV with just a change of brushes. They said “the only thing that can hurt these motors is over heating”.

Oops. A suppressed memory came bursting forth!

You see just after we put our EV on the road 18 months ago the motor had a couple of “near death” experiences. At one point it popped out of gear at 80 km/hr. You can’t hear the motor at that speed over the road noise so we over revved and over heated it before we realised it was out of gear. After that the motor was making a sewing machine noise as the “commutator bars” had been raised. My friends at the local electric motor shop tut-tutted but machined the commutator which brought it back to life. However I was warned that I might be on borrowed time. About 18 months and 16,000km as it happens.

It popped out of gear because your friend and author of this post forgot to re-attach a small bracket between the gearbox and chassis. This bracket stops the gearbox twisting under torque. It was this twisting that threw my car out of gear during acceleration and started this sorry saga.

The second near-death experience was accidentally driving around in 5th gear. “What’s this red light on the dash?” asks Rosemary after the light in question had been lit for 20 minutes. That would be the over-temperature light, Rosemary. Please pull over quickly when you see that! Fifth gear at low speed (we normally drive in third) means lower revs and hence more current and heat. Electric motors have so much torque they happily pull 5th gear at 0 RPM. It’s just that they get hot after a while.

Here is the dud armature and a close up of the commutator. The carbon deposits on one bar indicate the dud armature winding (so I’m told, I’m not sure why).

A new armature was procured for $950 and fitted, and I put the motor back in the car. The photos below shows all the bits that spin, in the order they are assembled. When I started my EV conversion I had no idea what these bits looked like so I have labelled them for you.

The Adaptor Plate allows the electric motor to bolt onto the bell housing. The other parts in the picture come from the original car. Many EVs are clutchless, but I elected to keep the clutch. Although I would go clutchless next time – it’s just simpler. Without the flywheel it’s surprisingly easy to change gears without a clutch (I have tried this on other EVs). Gear changes are rare in an EV anyway, I just use 3rd for all forward driving.

The next picture shows the Adaptor Plate bolted to the motor and flywheel to the coupler.

The pressure plate is then bolted to the flywheel with the clutch plate held inside like a sandwich. The transmission spline pokes into the centre of clutch plate once the assembly below is bolted onto the transmission. Here is the whole assembly:

In this photo I have connected the power cables so I could give the motor a test run. Like unit testing software – it’s a lot easier to fix something now before it is “integrated” with the rest of the car. Important test – make sure the motor runs forward. The first time I ran my electric car it went backwards, and I had 5 reverse gears and one forward gear!

Then I hoisted the motor up into the car and bolted to the bell housing of the transmission. Here is an earlier photo from the EV’s life that shows the motor assembly secured to the transmission. That little motor accelerates our EV faster than the original 1.3 litre petrol engine.

Tip – design your battery racks so the motor can be easily pulled out. Mine just fits but I have to knock a rubber boot off a CV joint every time I drop the motor which is annoying. This time I did one silly thing – bolted the motor in with the hoist chain still wrapped around it. Here is the view of the bottom of the motor looking up:

There were some alignment problems when my adaptor plate was machined. I fixed it with a special jig and ground some alignment marks to help me re-assemble with correct alignment. Without correct alignment the gearbox shaft and motor shaft are not aligned and at low revs it makes a “creaking door” sound as the pressure plate and clutch orbit slightly different centres. At high revs such a mis-alignment would eventually destroy the gearbox or motor.

With our beloved EV off the road we had to resort to the petrol car. You know we had forgotten how painful visits to a petrol station are. And expensive! After a few years of driving an EV we have “unlearned” the habit of obtaining expensive energy from petrol stations. That method of refuelling seems so painful compared to just plugging in each night at home and walking away. How would you feel if you had to take your mobile phone to a special place twice a week to charge it? That’s what the petrol station model of energy distribution feels like to us now. A bad habit we are happy to be rid of.

Rosemary was very unhappy about being forced to use our “infernal combustion” car for two weeks. However the good news is that she is now happily back in her beloved EV:


David’s Electric Vehicle Page

One thought on “Blown Armature”

  1. As usual, there is always much to learn for your postings. I appreciate you willingness to share not only successes but also your problems. Makes me want to make my own EV…

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