OT – AC Motor Breaking

[i.am.ianParticipant @i-am-ian]

[i.am.ianParticipant @i-am-ian]

Hi,

I am playing around with the idea of using an AC motor to measure the power output from a exercise bike. At the same time I would also like to investigate if the same can be used to apply resistance to the bike flywheel.

My questions are, which I hope someone can help with:

1) I am assuming that when I run the AC motor from the bike, it will act as a kind of generator. Are there any dangers I should be aware of ? and do I need to dissipate the energy generated ?

2) How could I break the motor to add resistance to the exercise bike flywheel ? I have read a little about applying a DC voltage to the motor to break it, but no real details about how much. Any thoughts ?

Thanks for any suggestions,

Ian

[Neil WyattModerator @neilwyatt]

Welcome Ian.

A standard AC motor won't generate any electricity because it doesn't have any built in magnetic field. In use the field windings create a field that induces a reaction in the rotor. It's very different from a permanent magnet DC motor. you could use a 'universal' motor, but this would need an 'excitation current' in the field winding.

You can brake an AC motor by CD injection, but it's much simpler to brake a permanent magnet DC motor – all you have to is short it out (or use a suitable resistor to apply more modest braking).

Voltages generated by a DC motor will be comparable to the usual running voltage, but the current will be rather less. The heat generated in a shorted DC motor, assuming it is a good enough brake, will be exactly the same as in a friction brake doing the same work.

Hope that helps.

Neil

[Les Jones 1Participant @lesjones1]

Hi Ian,
I agree with all of Neil's comments. I think in DC injection braking on an AC motor (An induction motor either single phase or three phase.) The energy is disipated in the rotor so the motor would probably overheat if used for a long time as a brake. It is normally used to slow down motors more quickly when the power is removed. About 50 years ago I was involved in fitting DC injection braking to a three pase motor of about 20HP on a Lumsden grinding machine.I think we used a 24 volt DC supply which passed a current of 10 to 20 amps trough the windings for a few seconds. Is there a specific reason for using an AC motor ? (I am assuming like Neil that it is an induction motor.) On another job I worked on we used a DC shunt motor to measure mechanical power. The motor had a shaft coming out of both ends and we supported it on a bearing at each end so the frame was free too rotate. An arm was attached to the frame of the motor which had a spring balance at the end so the torque could be measured. This together with a tachometer gave the required information to calculate horse power. The braking torque provided was controlled by adjusting the field current and the value of load resistance. Other than losses in the motor (Working as a generator) the power was disipated as heat in the load resistors. If using an AC motor was not a requiment I would suggest using a car alternator with load resistors and controling the braking torque by adjusting the rotor current.

Les.

[John HaineParticipant @johnhaine32865]

Actually you can use an induction motor as a generator, as described here: http://www.redrok.com/cimtext.pdf for example.

As there are millions of surplus induction motors all around the world they are of interest to build simple renewable energy generators. But for your application either a dc motor or a car alternator would be a better bet.

[John HaineParticipant @johnhaine32865]

Of course another approach is a sledge hammer…

[Neil WyattModerator @neilwyatt]

That's interesting both from the fact it's using small amounts of residual magnetism to gradually build up a working voltage and capacitors to get a phase shift, but also from the very practical advice on checking the output voltage and working temperature!

Neil

[Russell EberhardtParticipant @russelleberhardt48058]

Posted by Neil Wyatt on 23/12/2014 10:35:19:

That's interesting both from the fact it's using small amounts of residual magnetism to gradually build up a working voltage

The same principle was used in the DC dynamos used in old cars. You could zap them to change from negative to positive earth.

Russell.

[i.am.ianParticipant @i-am-ian]

Thanks for all the answers. The AC motor was lying around, which is why I thought of using it, but it looks like a DC motor or a alternator is a better option.

As you have probably guessed, electronics is not my area. So thanks for the help.

If I understand correctly I would be looking for some kind of variable resistor to apply the load. Assuming I would want to provide a load up to say 500 watts, it does not look like that size of variable resistor is readiably available. Or am I just barking up the wrong tree again ?

Thanks

Ian

[Neil WyattModerator @neilwyatt]

You could use an array of light bulbs. The world record for sustained output power by a cyclist is about (an astonishing) 450 watts for an hour.

Neil

[Les Jones 1Participant @lesjones1]

Hi Ian,
You would only need a variable resistor if you use a permanent magnet DC motor as a generator. If you use a shunt wound DC motor or a car alternator you can use a fixed resistor as a load. You would control the braking effect by controling the current through the field winding of a shunt wound motor or the current through the rotor of a car alternator. Using pulse width modulation would be the best way to control the drive current. As Neil says light bulbs would be the simplest way to provide a load. Car headlight bulbs or 12 volt halogen lighting bulbs should be cheap to obtain. the only possible problem is that light bulbs have a much lower resistance when cold to their resistance when at working temperature. You could also make a resistor from the iron wire gardeners use to tie up plants. (My brother in law uses this type of wire to heat propergators powered from old 12 volt lighting transformers.)

Les.

[Neil WyattModerator @neilwyatt]

It seems RS stock 500W rheostats – but it might be cheaper to buy an new exercise bike…

Neil

[Michael GilliganParticipant @michaelgilligan61133]

Two more options:

1. re-use the element from an old radiant electric fire
2. buy shiny new nichrome wire from ebay

The elements are usually helical 'springs' wound on a ceramic support, and it's easy to shorten the original wire, or rewind with suitable resistance nichrome.

MichaelG.

[speelwerkParticipant @speelwerk]

Much nicer is to ride a real bike outside, and if you want more resistance you just ride hill up against the wind. Niko.

http://www.aviewfromthecyclepath.com/2009/06/cycling-as-medicine.html

Edited By speelwerk on 23/12/2014 15:59:34

[Neil WyattModerator @neilwyatt]

Well as this is the holiday season, a couple of my holiday pics for you Niko…

This is the top of Snowdon about 20 years ago:

And this (sadly bikeless) shot is Raven Crag above Thirlmere (I think) about thirty years ago. No idea who the hairy geezer is…

Neil

[Michael GilliganParticipant @michaelgilligan61133]

Neil,

To borrow from another thread:

He looks suspiciously like a 'mono-manical deep-green'

… Probably grew-up to be a Chartered Environmentalist

MichaelG.

[MuzzerParticipant @muzzer]

As you aren't going to be able to make an accurate measurement of the power directly from the electrical load, any measurement is going to be based on measuring the torque of the wheel (by measuring the reaction on an arm) and multiplying it by the speed. Just how you vary the load is up to you but the "turbo trainers" (wheel driven fans) are simplicity itself, so why reinvent the wheel(?!)

To make the measurement vaguely manageable, generally a dyno will run a constant speed or constant torque control loop. This then allows the other measurement (torque or speed) to be read off and measured directly as power. However, regardless of how you vary the load, you presumably need to consider how you would measure the power, which is the whole point of the exercise apparently. Going the sledgehammer route, perhaps an Arduino / R Pi etc could be used.

Murray

[Author]

Posts