Machine Design

How to hot rod an electric motor

Dennis Berube’s Current Eliminator site:

Lawless Industries Ltd.

MACHINE DESIGN covered Shawn Lawless’ electric
drag bike earlier this year:

EngineeringTV also did videos of the Lawless bike:

The 200-mph drag bike sitting in Shawn Lawless’ shop sports an electric motor that once was rated at 29 hp. Today, the bike’s power plant, made by GE and salvaged from an old forklift, puts out somewhere north of 800 hp.

Welcome to the world of electric drag racing, where racers modify their motors in ways analogous to how traditional hot-rodders boost engine displacement and crank stroke. But don’t expect to find many tips on electric motor hot rodding in online forums or magazines. Today, many of those tricks come close to being black arts practiced by a few gurus.

One of the most accomplished of these practitioners, at least in the eyes of drag-biker Lawless, is Dennis Berube. Berube souped-up the electric motor that sits in Lawless’ 200-mph drag bike. Berube also claims to be the fastest electric drag racer on four wheels, having piloted an electric dragster he built to 159.85 mph in a 7.956-sec run down the quarter mile.

Berube had his first inklings of how electric motors could be modified two decades ago when he got a job repairing and rewinding armatures. While learning his craft, he worked with a German engineer “who gave me a lot of good tips about dc motors,” he says. But most of his knowledge comes from 23 years of experimenting with his own race cars. “In evaluating an idea, you have to be diligent about documenting your setups. And you have to try the idea for one or two months, not change something after every run,” he says.” “That’s why I have over 4,000 time slips from quarter-mile runs with my electric dragsters.”

As you might expect, both Berube and Lawless are a bit cagey about discussing the details of their motor modifications. But they are willing to broadly outline how to turn an old motor from the scrap yard into something ready to challenge quarter-mile records.

All electric motors for drag racing are dc series-wound motors, rather than induction motors or something else more exotic, because “dc motors are torque monsters. They have extremely high-starting torques from 0 rpm,” notes Lawless.

Modification typically starts with installing low-friction, lowdrag bearings on the motor shaft. Next, parts within the motor that are nonconductive get extra insulation, such as a layer of powder coating, to minimize the potential for arcing. This step is not taken lightly. “After all, we are taking a motor designed for 200 A and putting 2,000 A through it,” says Lawless.

Meanwhile, all the motor’s conductive parts are typically replaced with beefed-up versions. The commutator gets special attention. “You can run direct leads into each commutator mounting point — in other words, get rid of parallel links within the commutator itself — or just make them a lot bigger,” explains Lawless.

Racers sometimes modify the motor windings as well. “Some people change the field windings depending on the specific rpm and torque levels they are trying to get. They may also shim the field windings to put them closer to the armature, but that depends on whether you are optimizing for torque or rpm. You are effectively altering the motor gearing when you tweak design parameters like that,” explains Lawless.

Finally, drag racers know when to shut things down. “You can’t run the motor long because it only has so many windings,” says Berube. “You are eventually going to overheat.”

© 2012 Penton Media, Inc.

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