# Starting in Third Gear



## rwaudio (May 22, 2008)

green caveman said:


> This is from another list:
> 
> 
> 
> ...


"naming" a gear is useless because the ratio's will be different for almost every car on the planet.

I start in 2nd gear most of the time, but 3rd gear if I don't feel like shifting. It is more load on the motor starting in 3rd but it's hard to say how that translates to load on the transmission.
To add context my ratio's are:
1st - 11.81
2nd - 6.949
3rd - 4.725
4th - 3.490
5th - 2.798

I don't use 1st or 5th, the other ratio's are not bad for an 11HV with 1000A.
From my point of view a ratio of ~5.5:1 would be just about perfect as a starting gear for spirited driving with 1000 motor amps and a 3000lb car. Then a "2nd" gear of around 3 or 3.5:1 as a higher speed cruising gear.

With 1400 motor amps I'd be willing to bet my existing 3rd/4th gear would work really well.


----------



## mora (Nov 11, 2009)

It doesn't matter what gear you start on as long as you get motor spinning. Don't stall the motor and feed it amps at the same time. You can imagine a high resistance point between brush and commutator. 500A through that exact point will create serious heat to a very small area. Windings might get hot if motor rpm stays too low for too long time as there will be inadequate cooling. This should not matter much if you have forced air cooling (separate fan/motor blowing air through the motor).


----------



## njloof (Nov 21, 2011)

What about AC motors? Is it only heat you need to worry about at low RPM?


----------



## Ziggythewiz (May 16, 2010)

Don't really know why, but along with the added heat, mine uses 20% more power in 3rd than 2nd.


----------



## Siwastaja (Aug 1, 2012)

As of too much torque being a problem, a proper controller should limit the torque and its slew rate. I know this may be a problem for some DC controllers, but many [or most] do utilize current measurement and can control torque pretty well. Modern AC may have better control which translates into both wider good-efficiency sweet spot and gentler starting torque -- but naturally the algorithm and the motor parameters need to be spot-on to achieve this. 




Ziggythewiz said:


> Don't really know why, but along with the added heat, mine uses 20% more power in 3rd than 2nd.


... but have you measured that you accelerate at exactly the same rate, and drive at exactly the same speed, when doing the comparison between the gears?


----------



## DavidDymaxion (Dec 1, 2008)

Starting out in too high a gear can certainly destroy the motor. As an extreme example, take off from a stop up a steep hill. In 3rd gear you might not have enough wheel torque to climb the hill. Gobs of current goes through the stalled motor and burns it up. Had you used 1st gear, the motor would have started spinning quickly and kept itself cool.

It takes about 1/2 the current to get the same wheel torque in 1st gear as 3rd gear. Since heating goes as I^2, this means a takeoff in 3rd makes 4 times the heat as a takeoff in first, accelerating at the same rate. That also means lessened range (typically small for everyday driving, but can be significant during a steep hill climb).

On level ground with a light enough vehicle, you could well have enough cooling overhead to handle that. The other path (favored by NEDRA racers) is to use huge motor(s) and controller(s) to handle all the heat of direct drive (which is like taking off in 4th gear!).


green caveman said:


> This is from another list:
> 
> 
> 
> ...


----------



## major (Apr 4, 2008)

DavidDymaxion said:


> It takes about 1/2 the current to get the same wheel torque in 1st gear as 3rd gear. Since heating goes as I^2, this means a takeoff in 3rd makes 4 times the heat as a takeoff in first, accelerating at the same rate. *That also means lessened range*.


That is incorrect David. The increased current is in the motor loop, not battery. The same wheel torque is the same battery current in different gears if the vehicle speed is the same, within a few percent.

*I am speaking about your "lessen range" comment.


----------



## tomofreno (Mar 3, 2009)

major said:


> That is incorrect David. The increased current is in the motor loop, not battery. The same wheel torque is the same battery current in different gears if the vehicle speed is the same, within a few percent.


 It appears he is talking about current in the motor loop, and the effect it has on motor heating.


----------



## major (Apr 4, 2008)

tomofreno said:


> It appears he is talking about current in the motor loop, and the effect it has on motor heating.


So how does that "lessen range"?


----------



## tomofreno (Mar 3, 2009)

DavidDymaxion said:


> Starting out in too high a gear can certainly destroy the motor. As an extreme example, take off from a stop up a steep hill. In 3rd gear you might not have enough wheel torque to climb the hill. Gobs of current goes through the stalled motor and burns it up. Had you used 1st gear, the motor would have started spinning quickly and kept itself cool.
> 
> It takes about 1/2 the current to get the same wheel torque in 1st gear as 3rd gear. Since heating goes as I^2, this means a takeoff in 3rd makes 4 times the heat as a takeoff in first, accelerating at the same rate. That also means lessened range.
> 
> On level ground with a light enough vehicle, you could well have enough cooling overhead to handle that. The other path (favored by NEDRA racers) is to use huge motor(s) and controller(s) to handle all the heat of direct drive (which is like taking off in 4th gear!).


 Let's see, it requires the same power to move the vehicle up the hill at a given rate no matter the gear used, assuming drive train efficiency doesn't change significantly. Power is proportional to the product of motor torque and rpm, so less torque is required at higher motor rpm for a given power, so less motor current is required. Torque seems to increase a bit more than linearly with motor current for series DC motors, so if 3rd gear is say 2.5x that of 1st, a bit less than that much more motor current would be required in 3rd. And of course the lower motor rpm results in less cooling as everyone has said. The max current is limited by the controller though, so seems you wouldn't burn it up unless you kept it at that high current for quite a while - depending on motor thermal mass.


----------



## tomofreno (Mar 3, 2009)

major said:


> So how does that "lessen range"?


 Oh, missed that.


----------



## DavidDymaxion (Dec 1, 2008)

Agreed 100% Major, for a superconducting electric motor and controller. 

I did not state it explicitly, but I meant motor current. Let's do an example.

This post by Tesseract says he measured 23 mOhm for the motor loop: http://www.diyelectriccar.com/forums/showpost.php?p=236548&postcount=45
.

Let's say driver A goes up a steep hill in 1st gear, just where 100% PWM starts, and 300 Amps. His motor loop heat is:

0.023 Ohms * 300 A * 300 A = 2 kW

Now let's say driver B has a direct drive car (otherwise identical) and goes up the hill in effectively 4th gear (a 3:1 change in gear ratio). The motor is at 1/3 the rpm, but makes triple the torque via triple the current (900 A). Note the battery Amps are the same for both cases. Now the motor loop heating is:

0.023 Ohms * 900 A * 900 A = 19 kW

Suppose it takes about 100 V * 300 A = 30 kW to climb the hill. In the first case 7% of your power is lost to motor loop heating, in the 2nd case it is 62% of your power! The first case goes further.

A steep hill is an extreme case. What about creeping along in heavy traffic? Let's say 50A and 150A (motor loop Amps!) for the 1st vs. 4th gear cases, for 1 second at a time.

1st gear: 0.023 Ohms * 50 A * 50 A * 1 s = 58 J
4th gear: 0.023 Ohms * 150 A * 150 A * 1 s = 520 J

So let's say you have a nasty Los Angeles rush hour commute and do 1000 such starts. Here's your energy lost in motor loop resistance:

0.02 kWhr for 1st gear
0.14 kWhr for 4th gear

For a car with a 144 V of 180 Ahr lithiums (26 kWhr) using 4th gear costs you maybe 1/2 mile (800 m) of range in this case -- not really a worry in a 50 to 100 mile range car.

What if we had to climb a hill for 100 s at 300A in 1st gear vs. using 4th gear?

1st gear: 2 kW * 100 s / 3600 s/hr = 0.055 kWhr
4th gear: 19 kw * 100 s / 3600 s/hr = 0.53 kWhr

In this case you are giving up around 2 miles of range by climbing the hill in 4th gear at the same speed as doing it in 1st gear. Again, for a 50 to 100 mile range car, not all that bad (if your motor stays cool enough!).

So the moral of the story is it won't hurt you much in typical daily driving on flat ground, but watch out for hills and other high current situations! San Franciscans I suggest you keep the tranny!

Note I tried to show the worst case by doing a direct drive (effectively 4th gear) car. For using 3rd gear it's only about 1/2 as bad.



major said:


> That is incorrect David. The increased current is in the motor loop, not battery. The same wheel torque is the same battery current in different gears if the vehicle speed is the same, within a few percent.
> 
> *I am speaking about your "lessen range" comment.





major said:


> So how does that "lessen range"?


----------



## Duncan (Dec 8, 2008)

Hi David

So you lose 0.5% - 2% of range by going direct drive

But you save ~ 50kg (by not having a gearbox) which is 5% of a 1000Kg machine

Sounds like a good deal


----------



## DavidDymaxion (Dec 1, 2008)

That's a good point, direct drive does offer slightly less weight and one fewer gear mesh. if you are building a rock crawler, keep the tranny. If you are doing steady cruising, the direct drive car can win for range.

For the OP, it was the same car, just the choice of gears. Typically there'll be a small range benefit to using the lower gears.


Duncan said:


> Hi David
> 
> So you lose 0.5% - 2% of range by going direct drive
> 
> ...


----------



## MalcolmB (Jun 10, 2008)

Interesting discussion, it brings up a question I've been wondering about. Is it more efficient to use two motors than one in a direct drive setup?
For example, using the Warp 9 table attached:

For a single motor: it takes ~500A to produce 100 ft lbs of torque. I squared is ~250,000. Motor power at this point is 42 hp.

For two motors: it takes ~310A (to each motor) to produce 100 ft lbs. I squared x 2 = ~192,000. Combined motor power is 52.6 hp.

This obviously ignores the fact that you have twice the commutator losses and the added weight of the second motor, but is it a valid comparison otherwise? Would you get a similar reduction in waste heat using a single, larger motor?


----------



## major (Apr 4, 2008)

Duncan said:


> So you lose 0.5% - 2% of range by going direct drive
> 
> But you save ~ 50kg (by not having a gearbox) which is 5% of a 1000Kg machine


Thank you Dunc. And David, notice I used a qualifier *within a few percent* in my statement. I realize that extra heat developed in the motor is energy unavailable for range.

You seem to enjoy calculating small amounts of energy to prove a point so might be interested in calculating the potential energy required to elevate that 50kg transmission up the grade. Or the increased energy required for the change in reflected or effective moment of inertia of the armature with the lower gear when accelerating the system. Both of which will offset energy savings due to reduction in resistive loss.

Another clue is to look at what the OEMs are doing in this regard. A few percent is difficult for DIYers to really identify and verify. But the engineering design departments for the car companies spend a lot of effort accounting for every last Joule in their EVs. Do they use multi-ratio transmissions?


----------



## Ziggythewiz (May 16, 2010)

Siwastaja said:


> ... but have you measured that you accelerate at exactly the same rate, and drive at exactly the same speed, when doing the comparison between the gears?


I accelerate a lot slower in 3rd for the 1st 30 mph, after that it's faster than 2nd. Cruise at same speed mostly. Power consumption is much higher in both cases.


----------



## dougingraham (Jul 26, 2011)

green caveman said:


> I could imagine destroying the transmission, but not the motor.


With advance timing any brushed motor will be in distress when operated in reverse. What happens is you have high current across the face of the brush as the comm segments pass over the face of the brush. This erodes the face of the brush and causes the leading edge of the comm segments to become rough. This will quickly consume one pole of the brush. If this is happening you will see unequal brush wear. Half the brushes will wear faster but brush wear and damage to the comm will occur. This can occur even at fairly low power settings because the arcing comes from the energy stored in the inductance of the motor windings.

This is why brushed motors that spend about half their time running both directions are neutrally timed.


----------



## dougingraham (Jul 26, 2011)

Ziggythewiz said:


> Don't really know why, but along with the added heat, mine uses 20% more power in 3rd than 2nd.


The added heat is the 20% more power. At higher motor currents the motor efficiency goes down. The I^2 R losses are greater. Theoretically the power is the same but the motor has to work harder in third and the motor efficiency goes down. With a motor like the Warp 9 the efficiency at modest currents (say 200 amps) could approach 90% but at 1000 amps could drop into the 60-70% range.

But all things considered I would not expect it to be a 20% difference.


----------



## rwaudio (May 22, 2008)

Ziggythewiz said:


> I accelerate a lot slower in 3rd for the 1st 30 mph, after that it's faster than 2nd. Cruise at same speed mostly. Power consumption is much higher in both cases.


Those are surprising results, in my 944 I can cruise in 2nd/3rd/4th on level ground at the same speed and draw the same battery current (within a few percent anyway). I haven't really noticed as much going up hills, I can say battery current is much higher going up a hill in 4th vs 3rd, but total Ah used on a trip doesn't really vary much unless I'm launching at full throttle from stop signs/lights, then I can significantly use more energy, but when caught in traffic and forced to accelerate/drive at a certain speed my energy use is VERY consistent regardless of gear.


----------



## DavidDymaxion (Dec 1, 2008)

My apologies Major if I somehow offended you, that was not my intent. Now, back to the technical discussion! Comments interspersed below.


major said:


> ... David, notice I used a qualifier *within a few percent* in my statement. I realize that extra heat developed in the motor is energy unavailable for range.


Not to worry Major, I know you know this stuff. I was making a point to the broader audience that for typical driving gearing doesn't matter much, but it can make a huge difference during the course of climbing a hill. I should have said "lessens range typically just a little" to make it clear I was talking about a typically smaller effect. I edited the post to reflect this.


major said:


> You seem to enjoy calculating small amounts of energy to prove a point so might be interested in calculating the potential energy required to elevate that 50kg transmission up the grade. Or the increased energy required for the change in reflected or effective moment of inertia of the armature with the lower gear when accelerating the system. Both of which will offset energy savings due to reduction in resistive loss.


Can't do everything in one post.  I was curious how big the effect was, and now I know, and hopefully others appreciated learning along with me. I think it is obvious that lightening a car 5% is a small effect compared to gearing for climbing a hill (kind of like riding a bike, would you rather take away 5% of the weight and climb a steep hill in the highest gear, or use the lowest gear?).

I have included rotational inertia effects (flywheel, armature, and wheels) for some of my simulations. It doesn't affect your range very much, but if you are looking for a tenth in the 1/4 mile a lighter flywheel could be a good place to look.


major said:


> Another clue is to look at what the OEMs are doing in this regard. A few percent is difficult for DIYers to really identify and verify. But the engineering design departments for the car companies spend a lot of effort accounting for every last Joule in their EVs. Do they use multi-ratio transmissions?


The real evil is current multiplication. My understanding is AC systems don't do current multiplication, so they get around this particular problem. The modern OEM cars use AC. As you are an AC expert I would love to hear, if that's right, why AC doesn't do current multiplication in the constant power mode but DC series systems do.

As a side comment, apart from the EV-1 it doesn't look like the car companies account for every Joule to me! Their EVs tend to be heavy and nowhere near as aerodynamic as the EV-1.


----------



## major (Apr 4, 2008)

DavidDymaxion said:


> The real evil is current multiplication. My understanding is AC systems don't do current multiplication, so they get around this particular problem. The modern OEM cars use AC. As you are an AC expert I would love to hear, if that's right, why AC doesn't do current multiplication in the constant power mode but DC series systems do.


You are misinformed. The inverter in the AC drive does do current multiplication. I can rarely find an instance where DC current exceeds the RMS phase current. And at low frequencies, the DC current is substantially lower. The 3 phase full bridge is like 6 buck converters put together one each on each half phase. And current multiplication isn't evil; it's the best thing since sliced bread


----------



## Siwastaja (Aug 1, 2012)

AC and DC are surprisingly similar in almost everything, at least when both are done correctly with proper control.

Just think DC motor as an AC motor with a electromechanical inverter attached and you are close.


----------



## dtbaker (Jan 5, 2008)

mora said:


> It doesn't matter what gear you start on as long as you get motor spinning. Don't stall the motor and feed it amps at the same time.



I'll second this as the key point to be made. you do NOT want to use the motor at a stop for 'hill holding'! this will damage DC motor rapidly with uneven heating of commutator bars that happen to be in contact w brushes combined with no cooling effect from the motor fan turning.

that being said, it doesn't matter *much* what gear you start in as long as you get moving. I wouldn't recommend too low an rpm for very long as you'll have cooling issues. If your vehicle has enough torque (like my Miata with 156v/Zilla/Warp9) starting in 3rd or even 4th is not bad at all as I am up to speed in a few seconds. My Swift (120v/Curtis 1221/ADC8) I start in 1st as it doesn't have enough torque to accelerate reasonably in 2rd or 3rd in most situations.

I do think there is a 'sweet spot' of efficiency in DC motors that is wider than ICE for sure, probably between 2500-3500rpm for most, so picking the right gear for continuous speed efficiency can make a little difference.


----------



## green caveman (Oct 2, 2009)

dtbaker said:


> I do think there is a 'sweet spot' of efficiency in DC motors that is wider than ICE for sure, probably between 2500-3500rpm for most, so picking the right gear for continuous speed efficiency can make a little difference.


I did find the attached here

It's for a small motor, but if those numbers scale, then the efficiency is higher at lower torque values. Somewhere I saw a suggestion that about 1/7 of stall torque was the max efficiency.

This would suggest that starting in a lower gear is advantageous.


----------



## major (Apr 4, 2008)

green caveman said:


> I did find the attached here
> 
> It's for a small motor, but if those numbers scale, then the efficiency is higher at lower torque values. Somewhere I saw a suggestion that about 1/7 of stall torque was the max efficiency.


That is a nice article on speed torque curves. And yes, a small motor, 9V, 1W. It is a PM motor. It would be difficult to use that method on series wound machines due to the behavior at no-load.



green caveman said:


> This would suggest that starting in a lower gear is advantageous.


Realize that the curve is for the motor with a constant supply voltage without a controller having current limit. That makes it difficult to look at that speed torque curve and relate to the motor operational conditions that would actually occur with an EV motor during a launch.

If you were to add the effects of a current limiting controller to your motor graph, it would cut off the right portion of the curve at about 0.13 oz.in. Remember that is a 1W motor and the EV would use a 100,000W motor. But the effect would be the same; cutting off the right side of the graph where the efficiency declines to zero. That is a reason looking at such a motor curve is deceptive when contemplating motor behavior in starting an EV from zero speed. The zero RPM condition on that curve does not relate to real condition of an EV motor application.

The other fallacy from using that motor graph is constant applied voltage. So at stall, which is zero RPM, on the curve, where you might imagine an EV starting to launch, the motor voltage is the same as it is at no load (9V in this case). In an EV using a 100V battery and a Warp9, this would be like saying you're starting at 5000 Amps. 

The EV motor controller takes care of these problems. It has a current limit which automatically reduces the applied voltage to the motor at start-up. By doing so, it multiplies the battery current to the motor. So high motor starting current requires only moderate battery current. This again essentially eliminates the right hand side of your motor curve. Zero RPM is achieved at lower motor voltage, lower motor current and even lower battery current.

While it is true the motor efficiency is always zero at zero RPM, the condition always encountered when starting from standstill, it is a condition which lasts for a very short time, so little energy is actually wasted. And it is true regardless of the gear ratio. The climb from this zero efficient starting point to reasonable motor efficiency is quite rapid. Lower gears (higher numerical ratio) will increase that rate. But the use of multiple ratios and shifting during accelerations introduces some downsides which counteract efficiency benefits from using those lower gears. The normal EV acceleration from standstill will not realize reduced energy consumption using a shifting transmission more than a few percent. And that percent gain, if actually there, will likely be eroded by the effort to carry the mass of the transmission and the losses associated with the power flow through the transmission and effort to shift it.

A shifting transmission will benefit an EV in certain applications, like those needing increased acceleration rates and those seeing extended low speed overload duty. But even in those cases, the transmission will not significantly increase range. We've been through this all before and things haven't changed. Use a transmission on your EV build if you want to. There are reasons to do so. Increasing range is not a valid reason. That's all I'm saying.


----------



## PStechPaul (May 1, 2012)

Perhaps it is better to look at losses rather than efficiency. Since efficiency is computed as (power out) / (power in), and the mechanical power out is proportional to speed, efficiency will be zero for locked rotor and will be very low at low shaft RPM and high torque.

For almost any motor, at low speed, losses will be predominately due to conduction losses (current^2 * R). So it makes sense that a motor running at a higher RPM and lower torque will have less losses, so a high ratio transmission will provide this. However, since the vehicle will mostly be running at highway speeds and not accelerating, the losses contribute only a small portion of the total energy and thus range.

A series wound motor can generate 5x to 10x rated torque for a very short period of time, as during take-off, and thus even a fairly small motor can work for direct drive. Thus the added weight of a transmission is a major factor. A PM DC or BLDC or ACIM has a maximum torque of about 2x to 4x nominal, so if you want to get enough torque to the wheels you must either use a larger motor or a transmission. Either way adds weight, and affects the overall energy requirement and range.


----------

