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vahid66 (Computer) (OP)
10 Dec 06 22:07
This is a silly question. What is a DC motor that can also generate electricity?
Typically, how much electricity do these motors generate?
CJCPE (Electrical)
10 Dec 06 22:45
Pretty much any DC motor can generate if the field is energized and the shaft is rotated. Theoretically, DC motors and generators are the same machine. Electrical power is applied to the armature to obtain mechanical power from the shaft and mechanical power is applied to the shaft to obtain electrical power from the armature. Machine designs are optimized to suit the performance desired from the machine, but they can work pretty well either way. Many DC machines are routinely (with appropriate control equipment) operated as a motor part of the time to drive a load and as a generator to act as a brake. The same is true of AC motors, but the required control equipment is generally more complicated.
vahid66 (Computer) (OP)
10 Dec 06 23:02
Thank you for your explanation.
I am working on a prototype robot which will get around on wheels. Each wheel has its own motor. So, can I use any DC motor in my machine? Can I expect the batteries to receive a charge, even if it is a small amount, by cutting the power to the motors and let the momentum of the machine turn the wheels?
davidbeach (Electrical)
10 Dec 06 23:06
If you cut the power to the motor, what is the route by which anything would get back to the battery?  You'd need a full control system.
vahid66 (Computer) (OP)
10 Dec 06 23:16
The power to the motors are controled by a computer. Similar to a stability system in AWD carso nthe road, this system, in theory at least, RPM of all wheels and adjusts power to the motors. In theory, therefore, the same system can route ane power coming back from a motor. I have not though about how this 2-way power floww will need to be regulated yet.
Is there a type of motor which works better than others for this purpose?
CJCPE (Electrical)
10 Dec 06 23:17
The most simple control system provides for motoring only. Slightly more complicated is motoring plus braking with no energy recovery. Most complicated is motoring and regenerative braking. Recovering the energy from just the kinetic energy of the robot is probably not worth the effort. Unless the robot is carrying a heavy load downhill, very little energy would actually be recovered.
CJCPE (Electrical)
10 Dec 06 23:31
For two way power flow, you need a two way power conversion circuit controlling the armature voltage and current.

Whether or not you use two way power control probably doesn't influence the motor selection very much. I suspect that the motor voltage, speed and torque requirement will be in a range where a permanent magnet motor would be a good choice. Others here may be able to confirm that or recommend something else.
vahid66 (Computer) (OP)
10 Dec 06 23:40
How about battery technology? Is there a better type of battery (NiMH, NiCad, Li-ion, etc.)than others? How will the choice of battery and its charging characteristics will effect the choice of motor and/or control system?
The main goal for the robot is endurance. It is meant to roam around as long as possible under its own power. It will have as many solar panels to charge its batteries as physically possible.
MikeHalloran (Mechanical)
11 Dec 06 0:05
Here in sunny SoFla, solar power and/or recharging makes _some _sense, but regenerative braking would be silly, because there are no hills.

A robot large enough to carry enough solar cells to charge itself would be a serious impediment to our heavy traffic.

... Until it was stolen, probably in a matter of hours, less if it has visible copper or anything that looks like copper cable.


Sorry, it just seems like a waste of resources and time to make a robot that does nothing useful.





Mike Halloran
Pembroke Pines, FL, USA

waross (Electrical)
11 Dec 06 2:16
Tell us a little about the size and proposed speed of your robot.
For a robot weighing 20 or 50 pounds and travelling slowly, regeneration is probably pointless.
For a robot weighing over 700 or 1000 pounds, travelling over 20 MPH and making rapid starts and stops, regeneration is probably a good idea.
It depends.
BTW, regeneration is common on electric transit vehicles, whether AC or DC drive. (On diesel electric locomotives, with no way to store the regenerated energy it is disipated in resistor banks and provides braking.)
On some light rail transit systems, the linear induction motors regenerate down to very slow speeds.
Regeneration is great for heavy fast vehicles with a lot of start stop cycles, but not very effective for light slow vehicles.
respectfully
OperaHouse (Electrical)
11 Dec 06 9:47
The motor would have to spin faster than you were last driving it in order to charge the battery.  There are methods where you could use an inverter to boost that voltage to charge the battery in regenerative braking.  There have been some recent patents on DC BI-DIRECTIONAL TRANSFORMERS.  I thought it was a joke at first, but it is a rehashing of basic technology for legal purposes in the electric car industry.
sreid (Electrical)
11 Dec 06 13:42
As long as the drive is a 4 quadrant drive (as most are), current will automatically flow back to the battery if the motor is generating current.  Regeneration will not happen on AC powered drives as the power supply rectifiers block the current from flowing to the AC power (the generated current pumps up the power supply filter capacitor).
waross (Electrical)
11 Dec 06 19:21
Typically the field strength is increased for regenerating.
Also, with more than one motor, the motors can be reconnected in series for regeneration.
respectfully
cswilson (Electrical)
12 Dec 06 18:35
Vahid66 -- I'll come at this from a bit of a different angle than the commenters so far. If you ever want to decelerate faster than your electrical and mechanical losses would cause you to, you must have a place to "put" the kinetic energy you will be absorbing.

In a DC motor, if you apply current to create torque in the opposite direction from the velocity, you will automatically be generating. As sreid pointed out, a 4-quadrant drive will have an electrical path for the generated voltage and current. This path is through the "flyback diodes", and the net effect is to increase the DC bus voltage.

The important question is "now what?". Most industrial systems have pretty significant capacitor banks on the DC bus that can absorb quite a bit of this energy. Your battery bank could absorb it (if not fully charged), but the question is how fast -- what is the charge rate of the battery. Hybrid cars have "supercapacitors" to absorb this energy quickly, then bleed it off into the batteries. I suspect you may want some capacitance on the DC bus. You may even need a shunt resistor to dissipate this energy if the batteries can't take it or take it fast enough.

But as others have said, without knowing your particular situation, we really can't say what is needed in your case.

Curt Wilson
Delta Tau Data Systems
DaveScott (Electrical)
12 Dec 06 21:48
If you are looking for an efficient total system, then I would suggest a dc permanent magnet motor from Maxon (up to 90% efficiency). They supplied the motors which went to Mars.
4QD also make an efficient and robust speed controller, with regenerative capability.
powerjunx (Electrical)
13 Dec 06 0:06
Dave,
 I heard Maxon had attain that efficiency, but not supplying motors that went to Mars (this awesome) smile I should read that article over the net.
DaveScott (Electrical)
13 Dec 06 3:10
powerjunx (Electrical)
14 Dec 06 3:15
dave, thanks for the link. Maxon had a big break!

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