DC gearbox motor as a generator

[RVCCEngineer]

My project demands that I light 6 LEDs (20mA,3V ratings) for 30 minutes using capacitors.

I'm using dc motors as a generator to charge the capacitors. The project is demanding the smallest amount of charging time possible. This is highly stressed.

I am also limited by space, where my motor cannot be large than 4 inches by length (leads to shaft) although I may be able to lengthen this dimension if I find a serious solution.

I know I need 648 joules of energy for this to work, which requires a lot of capacitance. This isn't the problem though, as I have plenty of 1F supercaps at my disposal.

What I need is a motor that can output 30 to 45 Watts at max RPM under load. This means I am looking to store the stated energy in about 15 ~ 20 seconds.
I have motors to work with but let me give an example of the problems I'm facing.

I am currently using a Uxcell 30rpm 12V DC Gear(1:148) motor which turns relatively well with no load but requires intense torque (no values available but its damn hard) while under a six 1F 5.5V supercap load.

I need a motor that can give me the stated power output that is relatively manageable to turn under load.

I know the basic idea of how the motors work (I understand Faraday's Law), I want lots of wire rotations (N) and I want the biggest magnets to create the most magnetic flux. I'm still at a loss for where to look for motors since every motor specifies Voltage, torque, load current, power etc.

What are my limitations here? (current relies on torque - voltage relies on omega)

How can I quantitatively determine the torque required to get the rated power out?

What kind of specifications do I need to filter for when searching?

 

[itsmoked]

You know: HP = Torque x RPM ÷ 5252 where Torgue is in pound-feet.

HP = 746Watts

A J or Joule is one watt-second.

You know your joules (your number which seems way high to me).

You can work back to get the HP or Torque you need to make it happen. Then you have to comb thru possible solutions with the motive power you have match it's speed with an appropriate torque via whatever gear reduction to make it all work.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[3DDave]

Is it odd to use a motor to drive a motor as a generator using electricity to generate less electricity to charge some caps?

Also the caps act like a dead short, so the torque required of the motor will be very high. Adding a resistor should lower the torque requirement by limiting the current and hence the back-EMF of driving the motor.

 

[Compositepro]

A resistor would just waste energy. If the generator is hard to turn it must be charging the caps. By turning the generator as hard as you can you are generating as much as humanly possible. Turn less hard and you will generate less power. The OP seems to think that there is some way to generate more power than he is putting in. There is not.

There is probably significant friction losses in the gearbox at a 148:1 ratio. some gearboxes are not designed to be back-driven.

 

[3DDave]

Generators aren't typically driven into dead shorts. So the resistor wastes some energy. Big deal. It also allows balancing the torque available from the motor to the torque produced by the generator. As the caps charge the difference between the EMF of the generator will fall and so will the current, so the resistor losses will be lower.

 

[OperaHouse]

My solution to everything is electronics. With humans there is always perceived ideal speed for movement. I would be keeping generator voltage constant and trade voltage for current initially. 30W is a lot of human power.

 

[waross]

MS A debilitating condition that is responsible for the loss of untold hours of productive work annually.
It is also a disease.
I was putting the final touches on a post about basic motor/generator characteristics when and attack of MS intervened and high-jacked my computer with a gratuitous upgrade.
My...computer.... is .... now.... running...... very........ slowly.
Possibly the terminal stages of MS.

Back to motors.
The efficiency of a DC machine (motor or generator) is somewhat skewed by the constant loss of the field excitation.
Let's by pass that effect by considering permanent magnet DC machines.

With conventional DC motors and induction motors there is only a few RPM or a few Volts or cycles difference between a motor and a generator.

Imagine an electric vehicle. The vehicle is coasting, neither driving nor overhauling.
The motor, AC or DC is turning at 1800 RPM with either 120 VDC or 60 Hz applied.
Now the vehicle starts up a grade. The RPM of the motor starts to drop. When the RPMs have dropped to about 1750 or about 97.2% RPM, in this example, both motors will be fully loaded.
The vehicle tops the grade and starts down the other side. Now the motors are being over-driven. However they are no longer motors, they are now generators.
At about 1850 RPM or about 102.8% both machines will be fully loaded as generators.
The difference between a full load as a motor or full load as a generator;- about 5.6%. The difference between 10% load as a motor and 10% output as a generator;- about 0.56%
Similarly, when the operator wishes to accelerate, the applied DC voltage or the applied AC frequency is increased. at about 102.8% of rated voltage or frequency the motor will be accelerating at full rated load.
Drop the applied voltage or frequency to about 97.2% and the machine is a fully loaded generator.
Again 5.8 % difference between a fully loaded motor and a fully loaded generator and a much less percentage difference for light loads.

What does this mean to you?
Well start looking at toy permanent magnet motors. The output as a generator will often be close to the ratings as a motor.
The relationship between speed and voltage is linear.
Found a motor with the correct current rating but the rated voltage is too high by a factor of 10?
Just turn it slower by a factor of 10.
By the way, 1:148 seems like a very stiff ratio to back drive. You may want to consider a much lower gear ratio.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Compositepro]

"Is it odd to use a motor to drive a motor as a generator." The OP did not say that was what he was doing. He said he was using a DC gearmotor as a generator. The gearbox would be needed because he plans to hand-crank the motor used as a generator. Thus a resistor is totally unnecessary. Also, this thread is unnecessary because this is a hobby or student project.

 

[3DDave]

To be fair the OP didn't correct me either. He referred to motors so often that the context was lost.
"What I need is a motor that can output 30 to 45 Watts at max RPM under load" should have been "I need a generator that can output ..." but at least you got to correct someone on the internet.

Even hand cranked, the resistor can be used to tailor the output to the available effort. I assume you have never tried to backdrive a geared DC motor with shorted electrical connections. Or seen a gearbox detonated because the electrical control to it's DC motor was shorted. The resistor limits the rate at which energy can be extracted. Power = V^2/R; a capacitor has zero resistance, so generating any power results in trying to generate infinite power. Obviously not possible, but that's the tendency. Add even a 1 ohm resistor and now the power has a non-infinite value.

Ever notice the R in RC circuits? What happens to the time constant when R = 0? Infinitely fast charging as the limit condition.

 

[waross]

The resistance of the armature alone may suffice.

Bill
--------------------
"Why not the best?"
Jimmy Carter