DC regen braking

[1984rangerDiesel]

Hi folks, trying to better understand regen braking with DC motors and controller. I understand that the DC motor is advanced, so there may be adding and brush damage.

1) to recharge the battery, one must run the controller below the required duty cycle to maintain speed. I.e. if 100% d.c. means 1000 rpm and the d.c. is adjusted to 50%, the speed wil change for 500rpm. If the motor rpm were somehow maintained @1000 rpm w/ 50% d.c., with an engine for example, would this scenario change the batteries?

Or

2) in order to charge the batteries, the controller must attempt to run the motor backwards, using the other half of the H bridge. This makes sense to me, as I understand that it is the back-EMF that will be charging the battery.

Or

3) something completely different.

Basically, to charge my batteries going down a hill in my golf cart do I #1 let off the accelerator slightly, #2 throw the cart in reverse, or #3 my golf cart will never charge its batteries, no matter what I do.

Hoping to make a hybrid assist for my engine, similar to the Buick BAS system.

Thanks for any clarification!

[herbivor]

I am no electrician but look take this example as an explanation. You wire up 2, let’s say 1.5v motors to each other direct. The motors have a black and red wire and these are just paired from one to the other motor, and that it! Now, if you turn one motor, the other one will also turn (albeit with 50% less speed due to losses). What this tells us is an electric motor is the same thing as a generator. So in your cart, as long as you are slowing down in gear, not only will you have the braking effect of the motor, the motor will be producing electricity which should be flowing through and recharging your battery.

[serialk11r]

I've spent a lot of time thinking about how to do this.

Brushed motor isn't a great idea. The brushes don't take kindly to high voltages and there's a regen vs. drive tradeoff. It's very easy to produce a high voltage in a car because the engine has such a large operating speed range.

Say you use a 48v motor, then you'd want it to have a back emf of 48v at 7000rpm or whatever your engine's rev limit is plus a little extra. Then at 700rpm, it's only putting out 4.8v which is not a lot to work with.

I think for this application it's best to directly drive the crank or somehow the transmission input shaft with a brushless DC motor. Outrunner is nice because you can reinforce the casing to let it take higher rpms.

The way to think about the power vs. voltage vs. current vs. torque vs. current of the motor is this:
-Back emf (voltage) increases linearly with speed. If you keep the motor spinning at 1000rpm and you measure 10v, then at 2000rpm it'll put out 20v.
-Current is given by the voltage applied minus the back emf, divided by resistance of the motor's windings. So if you apply 20V at 1000rpm for the example motor, the torque output is similar to applying 10V at 0rpm, the current is the same, but the electrical power is doubled.
-Torque output is approximately linear with respect to current.
-This all works in reverse. If you can hold the motor at 1000rpm, there will be 10V with no electrical load. Think of it as a 10V battery in series with a resistor that has the same resistance as the winding resistance. If the motor windings have a resistance of say 0.1 ohm, then if you keep the motor at 1000rpm, you can draw 10A current at 9V, 20A current at 8V, etc.

I know this sounds crazy, but I've come to the conclusion that a 120mm hobby aircraft outrunner is pretty well suited for this. At 7lbs and ~20hp output they're very powerful, and should be able to put out around 20Nm of torque. The Astro 3220 has 10Nm peak torque but it's a smaller diameter inrunner with similar length as the biggest 120mm outrunners, with samariam cobalt magnets which are a little weaker than neodymium iron boron. 20Nm is probably enough to start your car engine, and some of them are rated to 7500rpm which is similar to a car engine's typical speed range.

[1984rangerDiesel]

Originally Posted by serialk11r

I've spent a lot of time thinking about how to do this.

Brushed motor isn't a great idea. The brushes don't take kindly to high voltages and there's a regen vs. drive tradeoff. It's very easy to produce a high voltage in a car because the engine has such a large operating speed range.

Say you use a 48v motor, then you'd want it to have a back emf of 48v at 7000rpm or whatever your engine's rev limit is plus a little extra. Then at 700rpm, it's only putting out 4.8v which is not a lot to work with.

I think for this application it's best to directly drive the crank or somehow the transmission input shaft with a brushless DC motor. Outrunner is nice because you can reinforce the casing to let it take higher rpms.

The way to think about the power vs. voltage vs. current vs. torque vs. current of the motor is this:
-Back emf (voltage) increases linearly with speed. If you keep the motor spinning at 1000rpm and you measure 10v, then at 2000rpm it'll put out 20v.
-Current is given by the voltage applied minus the back emf, divided by resistance of the motor's windings. So if you apply 20V at 1000rpm for the example motor, the torque output is similar to applying 10V at 0rpm, the current is the same, but the electrical power is doubled.
-Torque output is approximately linear with respect to current.
-This all works in reverse. If you can hold the motor at 1000rpm, there will be 10V with no electrical load. Think of it as a 10V battery in series with a resistor that has the same resistance as the winding resistance. If the motor windings have a resistance of say 0.1 ohm, then if you keep the motor at 1000rpm, you can draw 10A current at 9V, 20A current at 8V, etc.

I know this sounds crazy, but I've come to the conclusion that a 120mm hobby aircraft outrunner is pretty well suited for this. At 7lbs and ~20hp output they're very powerful, and should be able to put out around 20Nm of torque. The Astro 3220 has 10Nm peak torque but it's a smaller diameter inrunner with similar length as the biggest 120mm outrunners, with samariam cobalt magnets which are a little weaker than neodymium iron boron. 20Nm is probably enough to start your car engine, and some of them are rated to 7500rpm which is similar to a car engine's typical speed range.

I appreciate the response, and I think we are on the same page. My application may be even more simple than what you are imagining. The truck has a 59hp diesel engine with a ~4500 rpm redline, so the rpm range of the motor may be smaller. The starting torque requirement is quite high though, due to the 22:1 compression ratio. I'll ignore auto start for now, I'm just focused on boosting power up hills and regen dowhill. Unfortunately, most of the bldc motors and controller s I've researched are less than 20hp continuous. (Kelly mainly)

If my understanding of bldc MCs are they must have a built in buck boost regulator to maintain the correct voltage to charge the batteries regardless of motor rpm. No idea how they handle lithium batts.... Do you know of any MCs capable of this that work well with the 120mm outrunners? I cannot imagine the hobbiest world cares too much about regen while their plane is falling from the sky. Kelly claims their kls series of controllers possess regen, and the joystick control is quite appealing. Small potentiometer on the shifter, forwards for a little boost uphill, backwards for regen downhills or coming up to a stop light. Any thoughts on a good motor / controller combo? I was hoping for ~22kw continuous, just lead acid batts.

[1984rangerDiesel]

Originally Posted by herbivor

I am no electrician but look take this example as an explanation. You wire up 2, let’s say 1.5v motors to each other direct. The motors have a black and red wire and these are just paired from one to the other motor, and that it! Now, if you turn one motor, the other one will also turn (albeit with 50% less speed due to losses). What this tells us is an electric motor is the same thing as a generator. So in your cart, as long as you are slowing down in gear, not only will you have the braking effect of the motor, the motor will be producing electricity which should be flowing through and recharging your battery.

I'm with you on the motor is a gen concept, but I'm thinking the regen may be more complex. If have batteries at 70%, lets say 11.7v, on a 12v sys just to make things familiar. and I let off the throttle at 10% motor max rpm, say 5mph or something, ill only be generating 1.2v. This will not recharge the batts, on draw 11.7-1.2v x current from regen less power. There would need to be some way of boosting the voltage to say 14v to charge the batts. Clear as mud?

[1984rangerDiesel]

chargedevs.com/features/a-closer-look-at-regenerative-braking/
Too young to post URLs....

[oil pan 4]

Originally Posted by serialk11r

.I think for this application it's best to directly drive the crank or somehow the transmission input shaft with a brushless DC motor. Outrunner is nice because you can reinforce the casing to let it take higher rpms.

GM tired this already and it was garbage.
Bolting a motor onto the engine to drive the crank lowered MPG on the cars they put it on.
Putting the electric motor between the gasoline engine and transmission like they did on hybrid trucks was so effective they eventually dropped the entire hybrid truck line after 3 or 4 years. In addition to the hybrid system the ecomodded* hybrid trucks got almost the same highway MPG as the non hybrid trucks. (they used a bed cover, grill shutters, bigger air dam, numerically lower rear gears, LRR tires to bring the highway MPGs lost because of the hybrid system back up to what the plane gas trucks were getting)

In this context ecomodded* means mostly simple dumb stuff you or I could do or have done to any vehicle. Not talking about the part where they designed and added all that hybrid stuff to the vehicle.

If you need more power, do what I did.
Roof top solar panels and MPPT charge controller.
You could have it done with in the next 2 weeks, do it for around $300 and it would actually work.

[freebeard]

Try posting the question in Fossil Fuel Free - Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com

The answer lies somewhere between AC and 3-phase DC. Field Oriented Control can instantly go from drive to regen; it's like riding a fixie bike.

[oldtamiyaphile]

Originally Posted by freebeard

The answer lies somewhere between AC and 3-phase DC. Field Oriented Control can instantly go from drive to regen; it's like riding a fixie bike.

I wish a fixie worked like that, I'd drive to the top of the biggest hill, get on the bike and be all regenerated at the bottom

[freebeard]

Your car would be like the picture of Dorian Gray.

thingstodo put a description of the complexities of regen in the Paul and Sabrina's Controller thread at Permalink 2483 (that's on page 249!).

When you regen down to 0mph, does the car then start backing up? Regen on the throttle to mimic back-throttle or on the brakes? &etc.