treadmill permanent magnet DC motor as generator

[almac]

If the motor is brushed you don't even need the diode. If the motor is brushless you will need six diodes.

If you are getting the voltage meter to adjust the throttle by hand you WILL eventually destroy the batteries. If that's OK with you, go for it.

of course i dont want to ruin the batteries so i will need a charge controller after all. so do you really know what you are talking about? explain why the motor wont need a diode to stop the batteries feeding back jflorey2

[jflorey2]

cool, so all i need is a diode and a voltage meter , just a few bucks.

If the motor is brushed you don't even need the diode. If the motor is brushless you will need six diodes.

If you are getting the voltage meter to adjust the throttle by hand you WILL eventually destroy the batteries. If that's OK with you, go for it.

[almac]

thanks, i will try a 2:1 pulley ratio for starters, the battery load might drag the rpm down to 1000 at full throttle

[inetdog]

Probably something around 2000 RPM at full throttle.

[almac]

That's why you throttle back as load drops. All "simple" (i.e. non-inverter) generators use a throttle feedback mechanism to maintain frequency and voltage. Inverter-based generators do as well - it's just a bit more complicated since you no longer need to support a given frequency, and are thus free to throttle back significantly when load is low.

cool, so all i need is a diode and a voltage meter , just a few bucks. the calculations from the spec plate on th DC motor 3400 RPM 220 Vdc work out at 460 RPM for 30Vdc from this motor. i have no idea what RPM a 2 stroke lawnmower puts out

[sdold]

Serious question: why not use a reel mower? It seems to do exactly what you want, i.e. the mower gets its power from strenuous walking.

[jflorey2]

as the battery charges and the current falls the load on my victa lawn mower will also fall causing the RPM and voltage to increase then destroying my batteries in a smokey hot battery scenario , been there done that

That's why you throttle back as load drops. All "simple" (i.e. non-inverter) generators use a throttle feedback mechanism to maintain frequency and voltage. Inverter-based generators do as well - it's just a bit more complicated since you no longer need to support a given frequency, and are thus free to throttle back significantly when load is low.

[almac]

You don't use charge controllers with generators; you match the base speed to the generator RPM range and then choose your throttle setting to deliver the power you want.

ah thanks jflorey2, having just found the info about dc motor RPM and voltage that make good sense , but i can see another problem that might still require a charge controller. as the battery charges and the current falls the load on my victa lawn mower will also fall causing the RPM and voltage to increase then destroying my batteries in a smokey hot battery scenario , been there done that

[jflorey2]

thanks, at 1st i was thinking of wind turbine controllers due to the variable voltage but most are rectifiers for 3 phase ac generators. where would i find a dc wind turbine charge controller?

You don't use charge controllers with generators; you match the base speed to the generator RPM range and then choose your throttle setting to deliver the power you want.

[almac]

I'm not so sure that either an MPPT or PWM controller is a great choice. The mppt will believe it has control over the source, and attempts to find the maximum power point may vary the effective load in ways that lead to instability. In other words, the IV curve of the motor will not look like the IV curve of a solar panel, so the result may be unpredictable. Also, some mppt controllers switch to a PWM mode once bulk charging is complete, which would also be incompatible with this design.

A PWM controller is definitely not a good idea, since it can potentially open up the circuit between the generator and the load.

I think the best approach would be a shunt type controller with a fixed dump load. It should be able to run somewhat autonomously, and when the battery voltage hits the desired level the power from the generator can be shunted away until someone intervenes to shut it off (or fuel runs out). Blocking diodes or other reverse current protection are important. Commercially, many wind turbine charge controllers are designed this way, googling diversion controller or dump load controller also gives lots of hits. For those with some electrical skill, DIY designs are out there too.

thanks, at 1st i was thinking of wind turbine controllers due to the variable voltage but most are rectifiers for 3 phase ac generators. where would i find a dc wind turbine charge controller? also a quick search found this item about why solar charge controllers dont work for wind turbines / generators.

also found this useful piece about RPM and voltage calculations with permanent magnet dc motors.

Ametek and treadmill motors.
The best DC (Permanent Magnet)motor for use as a generator is the one that has the highest rated voltage at the lowest RPM figure (this applies to Ametek and treadmill motors).
There is some useful information on different Ametek motors which can be found at "tlgwindpower.com/ametek.htm". As treadmill motors are supplied by a multitude of vendors there is no one source of similar information.
You should be able to calculate the number of RPM required to generate one volt. This figure is useful for comparing DC generators. Simply divide the stated RPM on the rating plate by the voltage on the rating plate. DC motors are linear devices, voltage and speed form a straight line graph. Lets assume that one volt is produced for every 20RPM. As the controller requires about 17v to fully charge a 12v battery (see Introduction above) then the DC motor will have to turn at (17 x 20) = 340RPM. This is frequently called the "cut in speed" which is the lowest speed at which the generator will charge the battery. At speeds above 340RPM the quantity of current generated and delivered to the battery will increase. DC generators require a "blocking diode" to prevent the controller/battery from powering the generator as a motor.
You stand a much better chance of charging a 12v battery than a 24v one as the "cut in speed" is half for 12v compared with 24v systems.

http://www.ebay.com.au/gds/Wind-Turb...6308446/g.html

http://www.windbluepower.com/articles.asp?ID=127

[sensij]

I'm not so sure that either an MPPT or PWM controller is a great choice. The mppt will believe it has control over the source, and attempts to find the maximum power point may vary the effective load in ways that lead to instability. In other words, the IV curve of the motor will not look like the IV curve of a solar panel, so the result may be unpredictable. Also, some mppt controllers switch to a PWM mode once bulk charging is complete, which would also be incompatible with this design.

A PWM controller is definitely not a good idea, since it can potentially open up the circuit between the generator and the load.

I think the best approach would be a shunt type controller with a fixed dump load. It should be able to run somewhat autonomously, and when the battery voltage hits the desired level the power from the generator can be shunted away until someone intervenes to shut it off (or fuel runs out). Blocking diodes or other reverse current protection are important. Commercially, many wind turbine charge controllers are designed this way, googling diversion controller or dump load controller also gives lots of hits. For those with some electrical skill, DIY designs are out there too.

[Sunking]

thanks , will go with a mppt they start at about $100 for one that can take up to 100Vdc..

Sure about that? Way to cheap for MPPT. A real 15 amp MPPT controllers start at $200 and go up from there. Sounds like you are looking at Chi-Com knock-off frauds.

[almac]

Since you will have an essentially uncontrolled voltage, and a relatively stiff source which might be damaged by overcurrent I would recommend that you get an MPPT type CC.
Your ratio should be determined to produce a voltage of at least 32V, preferably 36, while at full load on the motor/generator. You will probably have to do some more extensive testing with controlled RPM and varying resistance loads to figure out what that RPM point will be.

You could also set there watching a voltmeter and ammeter and varying the throttle during the entire battery charge and maybe do without the CC. If you do that, be sure to put a high current diode or diode group between the motor and the battery bank.

thanks , will go with a mppt they start at about $100 for one that can take up to 100Vdc. would risk wrecking a PWM or the batteries with the diode method.

[inetdog]

ok i will have a go at this, the lawnmower and 2HP DC motor were both free of the kerb, so i will hook up this motor to the lawnmower and aim for a pulley ratio /RPM to deliver a voltage to comply with a yet to be determined solar charge controller. to charge my 24v battery on successive cloudy winters days

Since you will have an essentially uncontrolled voltage, and a relatively stiff source which might be damaged by overcurrent I would recommend that you get an MPPT type CC.
Your ratio should be determined to produce a voltage of at least 32V, preferably 36, while at full load on the motor/generator. You will probably have to do some more extensive testing with controlled RPM and varying resistance loads to figure out what that RPM point will be.

You could also set there watching a voltmeter and ammeter and varying the throttle during the entire battery charge and maybe do without the CC. If you do that, be sure to put a high current diode or diode group between the motor and the battery bank.

[almac]

ok i will have a go at this, the lawnmower and 2HP DC motor were both free of the kerb, so i will hook up this motor to the lawnmower and aim for a pulley ratio /RPM to deliver a voltage to comply with a yet to be determined solar charge controller. to charge my 24v battery on successive cloudy winters days