OK...noob q...where's the power go during hard coasting?

V

Voltron

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Looking it up too.... so I've been doing more hard hill climbing lately, and a lot of 30-40mph coasting back down. There's obviously power there, so where is it going if you're not tapping it for regen? Just chasing itself thru the windings and making heat?
 
If you have a DD hub motor, and you either set cruise control or hold the throttle open, then the motor puts energy back into the battery pack once it exceeds the no load speed of the motor. With a DD motor and CC set to a low speed, and you get the bike to a higher speed, it starts generating power back into the battery. But if the throttle is open and you are just coasting down the hill, nothing happens except you will coast a little faster than having the throttle engaged.
 
Voltron said:
coasting back down. There's obviously power there, so where is it going if you're not tapping it for regen?
Click to expand...

air and rolling resistance mostly.
 
I'm talking system on, but throttle off, just coasting. But it won't just start regening because I open the throttle a little will it?
 
Actually, if you coast down hills with no regen, and throttle off using a direct drive motor, something happens. It does make heat, and it does limit your speed some.

I saw this often, when I was riding with a temp sensor in my motor years ago. To not heat the motor any more that it is, coast down the hill with the throttle on to make your motor match rpms.

Depending on the situation, you may prize the cooling more than the saved power. Since it only takes about 50w, if your motor is all overheated after a long hill coast down the other side with the throttle on. It won't matter if it's just a crack, or full on, if you are coasting fast. The power used will be the same. Only about 50w.

When you do this, you might coast faster too, since the motor drag will not hold you back any. If you have a low rpm dd motor, you might prefer to have that drag limit your speed. IF,,, you aren't about to overheat at the moment.
 
In order for a hub motor to generate power, you need to have a closed circuit. With the throttle off, you have an open circuit, so no power can be generated.
In theory.

In reality, there is some leakage on the FETs, so you get a slightly closed circuit. This is what we call "Cogging." You can feel it on some motors when you try to pedal without power. A 9C 2807 for example, will generate 23 watts at 20mph. All of that power is lost as heat.

Little known fact; according to Justin, you can find out a motor's cogging power by setting the throttle to 3%, (it errors out at 0-2%) then manually sliding over the speed line on his simulator.
 
Guess is time to do a run but wait around at the top long enough to cool down and see how warm it gets just on it's own...fun having a 9 mile downhill roll to test on.
 
Drunkskunk said:
In reality, there is some leakage on the FETs, so you get a slightly closed circuit. This is what we call "Cogging." You can feel it on some motors when you try to pedal without power. A 9C 2807 for example, will generate 23 watts at 20mph. All of that power is lost as heat.
Click to expand...

Not all is lost to heat. Down steep hills when the no-load speed of a direct-drive hub is exceeded the motor back emf starts to exceed the battery voltage. The controller will rectify the voltage and send it back to the battery. So you essentially get some regen even if the controller is not "regen" per se. To see this for yourself simply hook up a voltmeter to the controller DC input and spin the wheel.

-R
 
It all turns to heat...some in the motor if it's a DD in the form of iron core losses caused by the alternating magnetic poles passing the stator teeth...some in the form of rolling resistance of the tires...heat in the brakes if you use the brakes...and the rest as wind resistance. This notion of energy going back to the battery if coasting speed exceeds no-load speed sounds like hogwash to me. I would have seen it show up on the CA the same as regen current, but I never have seen it.
 
John in CR said:
This notion of energy going back to the battery if coasting speed exceeds no-load speed sounds like hogwash to me. I would have seen it show up on the CA the same as regen current, but I never have seen it.
Click to expand...

It's not hogwash at all. The motor can also be a generator. As I said just spin it with a voltmeter or wattmeter connected to the controller DC input to see. As the motor rotates faster the voltage increases. When rolling down hill and the speed is over the no-load speed the voltage will increase above the battery voltage which will result in some regen. I ran an experiment years ago with a wattmeter inserted backwards to record the regen power. I only had a couple of short hills where my speed was high enough so the regen power was rather small but it was there. Of course for more significant regen at any speed get a controller designed just for that purpose.

-R
 
I have a 9c 12 turn motor that goes about 26mph on 80v in a 26" wheel. When i move the bike with no battery connected it turns on my cycle analyst at slower than a walking pace. My lower turn count motors dont do that. So the drag of the unpowered low KV motor generates higher voltage at lower rpm than my high KV hubs.

I know voltage and power are not the same but it was a visual reminder that the motion of the motor gets fed back into the system as electricity. Whether it goes back to the battery or just goes away as resistive heat is beyond my level of technical knowledge. Its just a good way to visualize the motor as a generator that I wouldn't have noticed without the CA powering up

The fact that the drag is still there with no controller hooked to the motor makes me think some or most of the energy just escapes as heat without a regen controller metering it back into the battery.
 
John in CR said:
It all turns to heat...some in the motor if it's a DD in the form of iron core losses caused by the alternating magnetic poles passing the stator teeth...some in the form of rolling resistance of the tires...heat in the brakes if you use the brakes...and the rest as wind resistance. This notion of energy going back to the battery if coasting speed exceeds no-load speed sounds like hogwash to me. I would have seen it show up on the CA the same as regen current, but I never have seen it.
Click to expand...


It's not a hogwash because motor is connected to batteries via 3 phase mosfet bridge, and each mosfet has anti parallel diode. Those diodes make 3 phase rectifier, and if the motor turns with high enough speed, generates voltage higher than batteries', it's AC voltage will get rectified into DC voltage that is higher than batteries', and it will have to charge them. There's no way to avoid this event.
 
Looks like I'll have to keep going up the mountain just to test coming back down a bunch :lol:

[youtube]eVc7e-yb0p8[/youtube]

heres the ride up... and the ride back down is just about as fast but coasting. So how two way is it... i,e, if it is taking 80ish volts and 40 amps to get it spinning that fast, is it making 80ish and 40ish back on the rolling down?
 
On my Currie folder with a geared hub that doesn't freewheel, I can see the battery voltage lights indicate increasingly higher voltage as I gain downhill speed. I'd believe they designed it this way because speeds over 15 MPH seem too much for the chassis & long handlebar stem.
 
riba2233 said:
John in CR said:
It all turns to heat...some in the motor if it's a DD in the form of iron core losses caused by the alternating magnetic poles passing the stator teeth...some in the form of rolling resistance of the tires...heat in the brakes if you use the brakes...and the rest as wind resistance. This notion of energy going back to the battery if coasting speed exceeds no-load speed sounds like hogwash to me. I would have seen it show up on the CA the same as regen current, but I never have seen it.
Click to expand...


It's not a hogwash because motor is connected to batteries via 3 phase mosfet bridge, and each mosfet has anti parallel diode. Those diodes make 3 phase rectifier, and if the motor turns with high enough speed, generates voltage higher than batteries', it's AC voltage will get rectified into DC voltage that is higher than batteries', and it will have to charge them. There's no way to avoid this event.
Click to expand...

Except that the "bridge" is closed, because as Drunkskunk pointed out the switches are turned off.
 
Depends on the circuitry of the controller?

http://www.4qdtec.com/pwm-01.html
We have already seen that the MOSFET is a bi-directional switch which conducts resistively (when it is turned on) for both directions of current. So consider the situation when the current is zero and the controller's output is now reduced. The motor's back e.m.f. is now higher than the controller's output voltage - so the motor will try and feed current back into the controller. If it succeeds in so doing the motor will be braked - we will have regenerative braking.

This type of circuit (where hi-side is turned on when the loside is off) is capable of sourcing current or sinking it. The way this works is that the reversed motor current is now a forward current to the flywheel MOSFET so when this is on it shorts out the motor - whose braking current rises during this period (arrow B, reversed). The Flywheel MOSFET now turns off, but this current must keep flowing - because of the motor's inductance. So it flows as reverse current through the drive MOSFET, recharging the battery as is does so. The extra voltage for this is derived from the energy stored in the motor's inductance.The process of switching from drive to braking is entirely automatic. Moreover it is done entirely by the motor's speed exceeding the drive voltage and without any change of state or switching within the controller. The regen braking is, if you like, a by-product of the design of the controller and almost a complete accident.
Click to expand...
 
John in CR said:
Except that the "bridge" is closed, because as Drunkskunk pointed out the switches are turned off.
Click to expand...

As I have mentioned a couple of times already you can easily verify the presence of the "regen" voltage yourself experimentally with a voltmeter or inline wattmeter and a spin of the wheel. To actually provide any usable regen current the motor must be spinning above the no-load speed so that the motor voltage exceeds the battery voltage. The effect will therefore be seen to a greater extent on low voltage and/or with low KV DD motors. For example a 9x7 9C on a 36V/26" E-bike will generate meaningful regen current and motor braking at 28 mph and up without an actual regen controller.

-R
 
Russell said:
John in CR said:
Except that the "bridge" is closed, because as Drunkskunk pointed out the switches are turned off.
Click to expand...

As I have mentioned a couple of times already you can easily verify the presence of the "regen" voltage yourself experimentally with a voltmeter or inline wattmeter and a spin of the wheel. To actually provide any usable regen current the motor must be spinning above the no-load speed so that the motor voltage exceeds the battery voltage. The effect will therefore be seen to a greater extent on low voltage and/or with low KV DD motors. For example a 9x7 9C on a 36V/26" E-bike will generate meaningful regen current and motor braking at 28 mph and up without an actual regen controller.

-R
Click to expand...

If it doesn't show up as negative current in the CA then it doesn't exist unless it just ends up some form of plug braking dissipated in the wires. I understand exactly what you two are talking about, and in theory it's fine as long as there's completed circuits, though you never mention the risks of over-volting components, and I've seen no quantified results in real world use. Now the ball is in your court to take whatever DD hubbie you want on a hill and demonstrate the regen current possible by exceeding no-load rpm.

In the meantime I'll stick with the simple fact that most of the potential energy at the top of hill is given back as heat thru wind resistance and the brakes if you use them (true regen braking is nice if the alternative is to use mechanical brakes to keep speed under control). The next most significant is iron core losses that occur whether the motor is powered or unpowered, and are dissipated as heat from the motor.
 
If I start my bike at the top of a hill with say 90.0V on the volt meter, gethe the bike to ~8 mph and set my CC, by the time I get to the bottom of that hill the voltmeter will read more than 90.0V. The same will happen if you just keep the throttle open. If I;m coasting and then start applying the throttle slowly, I can feel the bike slow down a little before I take the throttle past no load speed. Simple conclusion. Once the motor exceeds no load speed, it starts generating electricity back into the battery as long as the controller is active.
 
John in CR said:
Russell said:
John in CR said:
Except that the "bridge" is closed, because as Drunkskunk pointed out the switches are turned off.
Click to expand...

As I have mentioned a couple of times already you can easily verify the presence of the "regen" voltage yourself experimentally with a voltmeter or inline wattmeter and a spin of the wheel. To actually provide any usable regen current the motor must be spinning above the no-load speed so that the motor voltage exceeds the battery voltage. The effect will therefore be seen to a greater extent on low voltage and/or with low KV DD motors. For example a 9x7 9C on a 36V/26" E-bike will generate meaningful regen current and motor braking at 28 mph and up without an actual regen controller.

-R
Click to expand...

If it doesn't show up as negative current in the CA then it doesn't exist unless it just ends up some form of plug braking dissipated in the wires. I understand exactly what you two are talking about, and in theory it's fine as long as there's completed circuits, though you never mention the risks of over-volting components, and I've seen no quantified results in real world use. Now the ball is in your court to take whatever DD hubbie you want on a hill and demonstrate the regen current possible by exceeding no-load rpm.

In the meantime I'll stick with the simple fact that most of the potential energy at the top of hill is given back as heat thru wind resistance and the brakes if you use them (true regen braking is nice if the alternative is to use mechanical brakes to keep speed under control). The next most significant is iron core losses that occur whether the motor is powered or unpowered, and are dissipated as heat from the motor.
Click to expand...

At risk of repeating myself time and time again, I already ran the experiment with a Watt's-Up meter installed backwards between the controller and the battery to measure the regen power...and yes it registered. And again if you aren't coasting/pedaling at a speed above no-load the motor back emf won't exceed the battery voltage and there will not be any reverse current flowing. So if you have a fast-wound motor and over 48V battery you may never see any reverse current down hills unless they are very steep.

-R
 
John in CR said:
riba2233 said:
John in CR said:
It all turns to heat...some in the motor if it's a DD in the form of iron core losses caused by the alternating magnetic poles passing the stator teeth...some in the form of rolling resistance of the tires...heat in the brakes if you use the brakes...and the rest as wind resistance. This notion of energy going back to the battery if coasting speed exceeds no-load speed sounds like hogwash to me. I would have seen it show up on the CA the same as regen current, but I never have seen it.
Click to expand...


It's not a hogwash because motor is connected to batteries via 3 phase mosfet bridge, and each mosfet has anti parallel diode. Those diodes make 3 phase rectifier, and if the motor turns with high enough speed, generates voltage higher than batteries', it's AC voltage will get rectified into DC voltage that is higher than batteries', and it will have to charge them. There's no way to avoid this event.
Click to expand...

Except that the "bridge" is closed, because as Drunkskunk pointed out the switches are turned off.
Click to expand...


You're missing it, swithes are closed (actually open), but each mosfet has diode in parallel that can't be controlled! And those diodes form a 3 phase rectifier that works all the time, whether you want it or not.
 
So am I understanding it correctly that the controller can use switching/logic to actively control the voltage during regen but if you are spinning faster than the no load speed of the motor at a given voltage then it will generate regen power above that voltage regardless of the controller regen being enabled or the throttle position?

Also trying to check my understanding of regen in general.

It seems to me that the controller can manipulate the regen voltage any way it needs to make it higher than the pack voltage. Since it works even at low speeds the controller has to be stepping it up right?

Also if the regen voltage max is set higher than the battery voltage max is it a problem all the time or only when the pack is already full. I have my regen max set to 90v via R12 hack. My pack is full at 83v. Should I be concerned or only when regen braking downhill with a completely full pack? At first I was thinking it would only be a problem if I was going faster than the no load speed of my given motor at 83v. Now I'm not sure.

I'm thinking the pack would pull the regen voltage down just like it does when its discharged and I plug in the charger. Charger is at 83v no load but drops to pack voltage when connected. So if that is the case, having the regen voltage set too high will only be a threat to the pack when its full?
 
I don't see any regen just coasting DH unless my controller was engaged either with me holding the throttle open, or having set my CC at a slower speed. Keep in mind, that the CC on my controller is not a speed controller, but a power setting type that maintains the throttle setting at a certain power level. It does not increase power to maintain a speed, just maintains the throttle setting when it was set. So going up hill the bike slows down and going dh it speeds up. To be honest, it not enough to make much difference anyway. I don't no why people are making a big deal of it. Even true regen braking doesn't put a lot back into the battery, but sure helps with braking until you get to ~5 mph.
 
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