Page 4 of 5

Re: Power use at a stop light

Posted: Sat Mar 10, 2012 9:26 pm
by gregoryjward
I doubt very much that power is wasted running the electric motor against the friction brakes to provide the "creep".

The system is a control loop and the "creep" is electronically added at low/no speed depending on the position of the brake peddle. At some point the mechanical friction brakes are engaged and all power is removed from the motor. I'm sure that Nissan spent a considerable amount of time blending these things together and providing the "creep" of a automatic transmission car.

I personally wish they wouldn't have did this but it seems to make sense if you want the car to feel like a gas car.
One of the interesting things about good quality DC motors is that they use very little power while stopped, even if they are energized. I'm not saying Nissan *doesn't* turn off the motor while sitting stopped with the brakes engaged, but it wouldn't make a lot of difference if they didn't. You can check this yourself by taking your foot off the accelerator while climbing a steep hill. If the hill is steep enough, the Leaf will eventually roll backwards. (Good idea to check that no one is behind you before you try this!) Watching the energy screen, you'll see the power meter go to zero as you reach 0 MPH/KPH, then go into regen mode as you roll backwards!

That's right, regen is essentially the same as throwing the Leaf into reverse while moving forwards, and you can regen while moving backwards in D or Eco mode.

I would be interested to know all the same if anyone has been able to measure the actual power draw while sitting at a full stop in D/Eco versus N or P. I don't think the "Other Systems" meter is accurate. I think they have fixed settings for it based on which accessories are turned on, and don't measure this at all. If they did measure the DC/DC converter output, it would be very high while charging the 12v battery, then drop dramatically when the battery was fully topped up. They probably decided against that sort of reading, because they didn't want a lot of confused customers inquiring about it.

I have measured the draw from the 12v battery while in "On" mode (as opposed to ACC or "Ready"). This is similar to being stopped but without the traction battery engaged, so it's probably pretty close to the idle draw and it was around 8 amps with the fan, rear defogger, and headlights off -- about 100 watts. So, less than what the "Other Systems" meter reports by a factor of 2 or so.

Re: Power use at a stop light

Posted: Sun Mar 11, 2012 4:24 am
by garsh
gregoryjward wrote:One of the interesting things about good quality DC motors is that they use very little power while stopped
IIRC, the Leaf uses an AC motor. I've been hoping that there aren't any brushes in the Leaf's motor that would need replacing.

Re: Power use at a stop light

Posted: Sun Mar 11, 2012 5:01 am
by TickTock
garsh wrote:
gregoryjward wrote:One of the interesting things about good quality DC motors is that they use very little power while stopped
IIRC, the Leaf uses an AC motor. I've been hoping that there aren't any brushes in the Leaf's motor that would need replacing.
The Leaf uses a brushless motor (coils are on the stator and energized in sequence by the motor controller). So no brushes, but some heavy duty FETs.

Re: Power use at a stop light

Posted: Sun Mar 11, 2012 9:13 am
by garygid
In D (or ECO) at a stop, your foot will (should) be on the brake to tell the LEAF to NOT create "creep", however the brake lights will then be ON.

In N (or P), the car also does not create "creep", but your foot COULD be off the brake pedal (unwise), so a slight bit (insignificant amount) of power COULD be saved by not powering the brake lights.

Re: Power use at a stop light

Posted: Sun Mar 11, 2012 9:30 am
by gregoryjward
IIRC, the Leaf uses an AC motor. I've been hoping that there aren't any brushes in the Leaf's motor that would need replacing.
You are right of course. I realized my mistake after posting. Anyway, the Leaf's is an AC synchronous motor with a permanent magnet as the rotor, therefore brushless. When stopped, it operates in DC mode in a sense, since the field in the stator is held constant. The AC frequency is in lockstep with the motor RPMs, hence "synchronous".

Re: Power use at a stop light

Posted: Mon Mar 12, 2012 4:06 am
by TimeHorse
garygid wrote:In D (or ECO) at a stop, your foot will (should) be on the brake to tell the LEAF to NOT create "creep", however the brake lights will then be ON.

In N (or P), the car also does not create "creep", but your foot COULD be off the brake pedal (unwise), so a slight bit (insignificant amount) of power COULD be saved by not powering the brake lights.
That's all well and good and Neutral is how I usually pull into my driveway but don't throw your car into Neutral at a light just to save energy. You be the first person at the light and get rear-ended with no foot on the break and you could be rolled out into the intersection with oncoming traffic.

Or do, because we need new Darwin awards since it's been a while since I've seen one. :)

Re: Power use at a stop light

Posted: Mon Mar 12, 2012 4:39 pm
by planet4ever
TimeHorse wrote:That's all well and good and Neutral is how I usually pull into my driveway but don't throw your car into Neutral at a light just to save energy. You be the first person at the light and get rear-ended with no foot on the break and you could be rolled out into the intersection with oncoming traffic.
Which is probably the best argument there is for building in an artificial creep.

Ray

Re: Power use at a stop light

Posted: Tue Mar 13, 2012 1:07 am
by Ingineer
Some facts to clear things up:

1. The Leaf's creep feature is disabled at a certain brake pedal pressure. This is easy to see by watching battery current. (Same as in the Prius)

2. The Leaf's inverter uses IGBT's (Insulated Gate Bipolar Transistors), not FET's. (6 large ones)

3. Creep is an intuitive indicator that you are "in gear" and which direction is enabled, which is the main reason for it.

4. The Leaf's parking brake is the same as most current ICE cars with the exception of an electric actuator motor instead of a manual handle or pedal. It mechanically activates only the rear brakes.

5. When placed in "park", There is a pawl actuator in the transaxle that puts a pin into a toothed plate that locks the driveline. This only affects the front wheels, and unfortunately only one wheel need slip in order to have the car move. Therefore it's a wise idea to use the parking brake when parking on a hill. (also like the Prius)

6. The Leaf's various pumps, fans, relays, systems, displays and computers consume 100-200 watts from the 12v system at "idle" when the car is ready (depending on several factors). This power ultimately comes from the ~400v traction pack, not the 12v aux battery. (The DC-DC converter provides this 12v power anytime the car is in ready mode or charging.)

-Phil

Re: Power use at a stop light

Posted: Tue Mar 13, 2012 11:22 am
by kovalb
Excellent answer from Phil.

One additional reason to incorporate creep is it is extremely helpful during parallel parking.

Re: Power use at a stop light

Posted: Tue Mar 13, 2012 12:35 pm
by TomT
Interesting... I've been parallel parking manual transmission cars with no creep for decades with no problems. I don't find a car with creep to be any easier (or harder) to parallel park...
kovalb wrote:One additional reason to incorporate creep is it is extremely helpful during parallel parking.