Why is an electromagnetic driveshaft retarder not the same as regenerative braking?
April 21, 2009 8:37 PM   Subscribe

Why is an electromagnetic driveshaft retarder not the same as regenerative braking?

Background: Some heavy vehicles use (as a secondary braking system) what's called a "Telma" electromagnetic driveshaft retarder. It's basically a ring of permanent magnets affixed to the driveshaft, surrounded by a larger ring of electromagnets affixed to the vehicle chassis; applying current to the electromagnets creates a magnetic "drag" which slows the driveshaft.

Drivers who use a Telma say that it draws significant current when activated, to the degree that it can overtax a lower capacity battery/alternator. That makes sense to me abstractly in terms of conservation of energy: electrical energy must be expended in order to "do work" on the driveshaft.

In contrast, electrical regenerative braking (as in modern hybrid vehicles) also makes sense to me abstractly: kinetic energy is converted to electrical energy which recharges the battery.

What I can't figure out is how these two systems which seem so alike in concept (using permanent magnets + electric wire coils to attenuate kinetic energy) end up having such opposite effects: draining the battery versus charging it.

I'm convinced there's something simple and obvious I'm missing here, and hopefully this is an easy question for someone with a halfway decent understanding of the dynamics involved. Please enlighten me!
posted by night kitchen to Science & Nature (8 answers total)
You're right, they are essentially the same - but with one difference. When current is applied to the electromagnet, extra force (from the electricity supplied) is added to cause drag.

Imagine the 3 other scenarios:
1) No current supplied, electromagnets left open circuit - no energy used, no braking.

2) No current is supplied, but the electromagnets are shorted - the permanent magnets induce a current in the electromagnets, which in turn is used to create a magnetic field, which causes drag. This is roughly what happens with regenerative braking when the batteries are charged - no charging current is required, so the electromagnets are shunted.

3) No current is supplied, but the electricity generated in #2 above is used to charge the battery. There's still some drag, but less - some of the power generated is used to charge the battery, so the energy available to slow the driveshaft/hub down is reduced.

So, it comes down to the primary purpose of the device... Real braking, lotsa drag required, extra electricity required. Regenerative braking, the braking effect is just a bonus, the prime aim is to generate electricity for top-up.
posted by Pinback at 8:59 PM on April 21, 2009

Best answer: I just read about the Telma system, and I think the problem you're having is that there actually aren't any permanent magnets in the Telma system - only electromagnets. Basically, the driveshaft is connected to two metal disks that spin with the driveshaft. The two metal disks are separated by a bunch of solenoids. When you run current through the solenoids, eddy currents are generated in the metal, which results in resistive loss (heating of the disks), which results in braking.

In this case, current draw from the battery for the Telma coils is incidental - it doesn't figure in to the energy balance for braking; the energy expended by the coils goes to zero in the limit of superconducting coils. Thus the energy balance here is

Kinetic energy lost = heat energy generated

In regenerative braking, where there are indeed permanent magnets turning within electromagnets (or vice versa), the permanent magnet is indeed generating currents within the solenoid, which can be used to charge a battery, in which case the energy balance is

Kinetic energy lost = energy delivered to battery
posted by Salvor Hardin at 9:02 PM on April 21, 2009

You're not missing anything. The drivers are wrong. They know how to drive, but they don't understand electromagnetism.

An electromagnetic retarder is, effectively, a generator. Exactly as in hybrid vehicles. But because hybrid technology is still in its infancy, the generated energy has so far just been burned off (literally, via radiating circuits) rather than harnessed to charge a battery.

Regenerative braking is most beneficial in stop-n-go traffic, so we are starting to to see hybrid buses and urban delivery trucks.

The main issue with hybrids though is that they are complicated -- with electric motors at each wheel as well as the main engine, a very heavy battery pack, and sophisticated electronics to control everything. The other issue is that long-haul trucks spend most of their time being driven at moderate, steady speeds on highways, so in the big picture hybrid propulsion wouldn't have such a big impact.
posted by randomstriker at 9:04 PM on April 21, 2009

To clarify, in the Telma system, the kinetic energy goes to waste, and the mechanism for braking happens to use extra energy. In a regenerative braking system, the kinetic energy goes to the battery, and there is no extra energy required.

The analogous system for regenerative braking would have two electromagnets instead of one permanent magnet and one electromagnet, and you would drive current through one of the electromagnets.
posted by Salvor Hardin at 9:05 PM on April 21, 2009

I'm pretty sure Telma is *not* regenerative braking, but rather resistive braking
posted by Salvor Hardin at 9:06 PM on April 21, 2009

Best answer: Wow.. I'm getting confused.

1. Telma retarder -> using eddy currents induced by a rotating magenetic field to generate heat. Magnetic field is set up using electromagnets for easy on/off switching. (So it's not always running).

2. Regenerative braking in a hybrid. There is an electric motor somewhere in the system (I'm not sure if it's at the wheels or on the drive shaft, doesn't really matter. When you rotate the rotor on an electric motor, you will generate a current if there is a circuit. This circuit is hooked into the battery, so you have just turned your electric motor into a generator (crappy efficiency, but hey, a little bit of recovered energy!).

The telma retarder could in theory be set up to do the same thing, recover energy and send it to the truck battery. It wouldn't do much good though, because the truck isn't in a position to really use that energy, you could reduce the load on the alternator a bit, but you're not in a position to use that power to drive the truck (because you don't have an electric motor to drive it), so it's a lot of money for a fancy energy recovery system, which doesn't do you much good. The point of the telma is to save the mechanical brakes for when you really need them, since there are no real wear parts on the telma, as it converts kinetic energy -> heat very well.
posted by defcom1 at 9:36 PM on April 21, 2009

Best answer: I'm kind of late to the party here, but to me it's pretty simple:

- Magnetic braking and regenerative braking both convert mechanical energy into electrical energy, via conductive things turning inside magnetic fields (or fields turning inside conductive things).

- Magnetic braking creates this electrical energy in a solid metal rotor, which is basically a short circuit that turns all the energy into heat. The electrical energy cannot be recovered from this type of brake.

- Regenerative braking creates this electrical energy in a coil of wire, so it can be sent to a battery, capacitor, back into the power line, etc.

- Electromagnetic braking requires power from the vehicle because it uses electromagnets to create the magnetic field for the brake rotor to spin inside. It's done this way to make it easy to control. Magnetic braking can also be done with permanent magnets and controlled by moving them closer or farther away from the rotor. This requires no external power at all.

- Regenerative braking might use permanent magnets (using no power to create the field), or electromagnets, but in this latter case the power required to create the magnetic field is less than that produced by the regenerative brake, so it's still producing a net surplus of power.
posted by FishBike at 5:53 AM on April 22, 2009

Response by poster: Thanks, Salvor Hardin, for best answering the question (explaining why these two systems are actually very different -- the 'simple and obvious' something that I missed).

And thanks, defcom1 and FishBike, for summing up nicely and expanding on it.

Yes, Telma is not intended to be regenerative (its purpose is to assist and take some of the load off the mechanical brakes) and, as defcom1 points out, the vehicles that use it are typically combustion-engine vehicles and so couldn't readily store and use generated electricity for propulsion anyway.

Still, it seems like a waste of recoverable energy. The main reason I started puzzling over this was my problem-solver interest getting piqued after seeing an older truck with a Telma fail to run some of its more power-hungry electrical accessories (sirens, strobes, light bar) following about 5 minutes of downhill braking. Some regenerative braking could solve that. (Then again, it's really just time for a newer truck. And a Jake-brake.)
posted by night kitchen at 5:46 PM on April 22, 2009

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