# Direct drive is too much waste of electricity?



## Genius Pooh (Dec 23, 2011)

Direct drive is too much waste of electricity? 

I think more motor's rpm need more voltage . so It car run faster..

Voltage is grower and total watts is higher..

Without transmission.. Is it too much waste of electricity?

thanks...

I really curious about this..

and Why benz and other people keep claim in wheel motor is best??? it's structurally good but too much waste of electricity ( of course in direct drive case )


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## PStechPaul (May 1, 2012)

Wasted electricity is simply related to the overall efficiency, which really should be calculated as Watt-hours delivered / Wh consumed (by the charger). Direct drive requires high torque at low speed which is limited by the peak breakdown or locked rotor torque of AC motors (usually about 3x rated), and possibly as high as 10x for series wound DC motors (at which point they may be only 10% efficient). The overall efficiency depends on how the vehicle is used.


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## Genius Pooh (Dec 23, 2011)

PStechPaul said:


> Wasted electricity is simply related to the overall efficiency, which really should be calculated as Watt-hours delivered / Wh consumed (by the charger). Direct drive requires high torque at low speed which is limited by the peak breakdown or locked rotor torque of AC motors (usually about 3x rated), and possibly as high as 10x for series wound DC motors (at which point they may be only 10% efficient). The overall efficiency depends on how the vehicle is used.


 
hmm...it make word ..

but more rpm need more voltage. so. higher speed need more voltage. and same torque need same current.. Am I right? If so it need more watt..

so Direct drive will need more watt at high speed.??

can you expain about this?


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## subcooledheatpump (Mar 5, 2012)

High torque is required for direct drive, which means high currents. Using a gearbox (transmission) will require less torque, but more speed (RPM) from the motor. 

The high torque requirement of direct drive means you would need a controller/inverter to supply a large current, which means more losses from the semiconductor switches. A multipole motor could also be used, but they have a high reactive current requirement, which can stress the battery

Using a gearbox adds gearbox losses like friction losses from the gears and their lubricating oil. The motor in this case can turn at a high speed without the same high current, but the faster it turns the more voltage it needs. This means selecting semiconductor switches with higher voltage ratings, which also cause more losses. 

The more pole pairs a motor has, the more it weights, so usually a gearbox and a high speed motor are chosen for electric vehicles to keep the weight down. Usually however, a direct drive system is more efficient for constant speed driving, and a gearbox/transmission system is better suited for heavy traffic


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## EVfun (Mar 14, 2010)

You need so much power to move a given car a given speed. If you do this at a low motor rpm then more current at a lower voltage will be required at the motor. If you do this at a higher motor rpm then less current at a higher voltage will be required. Either way, the watts required is almost identical. Generally direct drive is at a slight efficiency disadvantage except over a narrow rpm range that best suits the motor, where the efficiency is slighter higher (less gear losses.)


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## toyolla2 (Jun 21, 2010)

Look, it's very simple. *Each and every rotation* of the motor will yield a certain amount of mechanical energy for a specific amount of electrical energy input. Therefore for maximum performance you should allow the motor to rotate as quickly as possible.

In practice you want as many rpms per Km/Hr as the gearing will allow. But what gear ? Well, you need a gear ratio in the so called Goldilocks Zone. That is, Not too high and Not too low. A 10 :1 ratio will allow approximately 10,000rpm at 100Km/Hr and is close to optimal in my opinion.

Regarding motors. Two things. 

It is said, in the folk lore of EVs, that for motors running at the same speed, the power produced is proportional to the weight of the motor. That seems to be a reasonable statement so who would argue with that ? If we go further and energise any particular motor from an electronic inverter then we can further assume that for the same rpms the motor torque should be proportional to the weight of that motor.

Surprisingly, that particular assumption is entirely erroneous. Why ? Well, here is what you should know, and, incidentally, you won't be finding this information in any text book. The rule is that for any given motor weight then torque will be found to be proportional to the number of poles per phase that particular motor has. Who knew ? Well I for one, did not. Until last year, that is.

Accordingly you should avoid the use of a 2 pole motor unless mitigating reasons involving Volts/Herz are present. The use of a 4 pole will give twice the power for the same weight. The use of a 6 pole will give three times the power for about the same weight. Providing of course that an inverter is present to enable all these motors to run at the same speed for comparison purposes. Unfortunately the rule appears to be most prominent when motor sizes above 22Kw are examined. 

The rule does apply for smaller motors however it is partially overshadowed by the disproportionate weight of shafts and end frames on those sizes. I have not seen a mathematical proof of this or even a convincing argument. For the moment I accept it on faith alone. If you examine a manufacturer's catalog listings you will find it to be true.

I have to thank Weber and Coulomb over on the AEVA site for working with me and pointing this thing out.

To continue, the second thing to know is ....Perhaps I'll post that for tomorrow along with a tie-in to direct drive. I now have to terminate my computer time for health reasons.


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## PStechPaul (May 1, 2012)

No. A motor with more pole pairs will provide more torque for a given size, but not more power. If you overclock it to get the same RPM, then it may give more power, although at reduced efficiency. For small frame sizes, higher pole count often means lower power, probably because of winding pattern efficiency. 

So I have three motors of the same size. They are 3600 RPM 2 HP, 1800 RPM 1.5 HP, and 900 RPM 1 HP. For larger motors as may be suited to electric cars, in the 20-50 HP range, a four pole motor is usually the same size and weight as a two pole, and even a six pole (1200 RPM) may be about the same size. 

For electric cars, especially when using the original geartrain, it is best to have the motor run at a speed similar to that of the original ICE, which is about 3000-4000 RPM maximum continuous. A standard electric motor is generally capable of safe operation at such speeds based on its bearings and cooling fans and windage factors. So you can overclock a motor with 120-180 Hz to get 2-3 times rated speed and power, with some loss of efficiency (probably 2-3 times more losses). You can also get more torque at the same speed, but this increases current proportionately, and losses go up by the square of current. 

So a 90% efficient motor may have 5% losses from magnetics and 5% resistive. So it will have 20% resistive losses at 2x torque and 45% at 3x. Obviously this is only reasonable for very short durations. Also it shows how a 95% efficient motor is very desirable because the losses will be 10% and 22% for the same overloads. These magnetic losses are about the same until saturation is reached.

These figures are based on an ACIM and may not apply to series DC or BLDC types.


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## cts_casemod (Aug 23, 2012)

Direct drive involves having a motor capable of receiving a large ammount of current at lower revs.

The efficiency should be similar, since for a low REV you need low volts so the average current from the power pack is smaller that what you are putting into the motor, however a motor capable of such high torque/currents at low revs will be quite large and will add weight if you want a good aceleration, so you need to consider this. If you want to simply save weight I would think again.

I can do direct drive or over drive with 350V @ 50Amps but aceleration is slow. With 150Amps it would be perfect, but would need 700V to have a decent top speed. With gearbox i can keep voltage at 350V. This is for a small car with an AC motor. If you have a lower voltage you will need even more current.


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## toyolla2 (Jun 21, 2010)

@ CTS_casemod
Yes I basically agree. Direct drive = Heavy
= Slow accel
= Poor use of available material.

Using an industrial motor with its original Volts per Herz ratio at 8.0 is an unwise choice also. A rewind is always the best course.


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## toyolla2 (Jun 21, 2010)

PS you wrote "For electric cars, especially when using the original geartrain, it is best to have the motor run at a speed similar to that of the original ICE, which is about 3000-4000 RPM maximum continuous."

I am not sure it is the best of ideas to utilise the original multi-ratio geartrain without at least some modification. My experience has been that changing gear with an electric motor on the shaft is more difficult than with an ICE and requires skill by the owner. It practically guarantees a zero resale value for your vehicle down the road. It also has been a contributing factor causing both the motor and controller to be overheated and cause failure when the owner eventually wearies of downchanging to the higher ratio for hills and more so for the frequent starting in downtown traffic. 

The poor driveability primarily is that 1st gear at 14:1 is chosen to avoid clutch wear with the ICE and preventing stalling by inexperienced drivers. This first ratio is overly generous for an electric drive as well as being unacceptably noisy it will rapidly run the motor into its rpm limit. Secondly this poor driveability continues into the 2nd gear ratio at 8:1 which although it brings a more favourable wider speed range delivers it with a significantly reduced torque at the wheels. Some will say this is common knowledge. It's knee jerk obvious. Nothing to see here people. Time to move on. 

But that's not all, there is something else going on here. This is not merely a torque loss at the wheel type of event. It is in fact a power loss event. A predictable consequence of the accelerating vehicle requiring its motor to be clutched in to 2nd gear where it will of necessity be made to rotate at a temporarily lower rpm. Just the fact that the motor is throwing out the same torque but at lower rpms makes it a safe bet to say that it is developing less power. Then again, sometimes the accelerating vehicle will need 3rd gear providing yet another opportunity to temporarily reduce motor power. 

Choosing a single value reducer gear gets you away from that nonsense and at the same time the necessity to install a 150Hp motor to effectively supply 50Hp. 

But what value should that be ? Personally I prefer something in the Goldilocks Zone which is within plus or minus one of the 10:1 ratio. This discourages speeds much above 65 mph while still permitting good acceleration for starting in traffic. 

I've noticed that some constructors who have procured the Siemens motor from the AZD bankruptcy auction are now seeking out 2.36 :1 gear reducers to use with approx 4:1 rear ends in order to maximise their investment in these machines.

I think it would be good policy to explore the financial possibility of stripping a manual gearbox and having a local tool and die manufacturing shop fabricate the input shaft and the differential pinion shaft - for wont of a better name - mounting each with a single gear element to give the requisite 3:1 ratio that would yield an overal ratio falling within that aforementioned Zone.


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## Karter2 (Nov 17, 2011)

with a 10:1 overall ratio, the motor would need a rpm range of 10,000rpm in order to achieve a max road speed of 65mph ( approx figures)
..and at that max rpm , still be capable of providing sufficient power to both maintain speed ( rolling and wind resistance), and also have reserves to climb any likely hills, etc.
Most ICE gearboxes are not designed with that kind of input rpm in mind.
I suspect some may like a higher top speed potential, requiring an even higher max motor rpm ( 12-15k rpm ?)
This also assumes that the low speed ( 0-100rpm) torque is sufficient to deal with max payload starts on likely gradients ( steep driveways, car park ramps etc)
Are these motor speeds/performance requirements realistic and practical ?


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## Siwastaja (Aug 1, 2012)

You are making all of this so difficult .

Simply and shortly;

1) A car just happens to be a constant-torque load (linearly increasing power requirement), or even a increasing-torque load at high speeds, the opposite what transmissions are for. Electric motors happen to create constant torque. Therefore, transmission is only needed in the case of design error (for example, way too underpowered motor). A single reduction gear is always optimal.

2) Designing a motor for high RPM makes the motor physically smaller and lighter. That's why OEM's go to about 10 000 RPM with about 1:10 reduction gear. 5000 RPM and 1:5 works as well but the motor is a bit heavier. Probably better for a DIYer who wants to use the existing gearbox, not designed for 10 000 RPM.

3) "Direct drive" is a misleading term as it can mean a really direct drive (to the wheels), or drive through the existing reduction gear in the rear differential in RWD cars. In some cars, this reduction gear is high enough (it can be even 1:5) to be very usable.


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## Coulomb (Apr 22, 2009)

Siwastaja said:


> Electric motors happen to create constant torque.


It depends on the design and how it's driven. It can be made to produce approximately constant power over a wide speed range; that's how EV manufacturers make their motors. That way you get high torque at low RPMs where you expect it and need it, and about the same torque that you'd get through a gearbox for medium and higher speeds.

If a motor has adequate torque at low speed (enough to climb gutters and car park ramps) and can continue that torque to high speeds, then it would have amazing power at high speeds, and we don't expect to have the vehicle feel most powerful only at speeds over 60 mph. The motor to deliver that sort of torque with a small overall ratio would be huge. So you have a circa 10:1 overall ratio for decent torque at low speed, and design the motor so that it can deliver decent power at higher speeds. So you get the same sort of performance you'd get with a transmission, but without the cost, weight, efficiency, and space penalty of a transmission.


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## PStechPaul (May 1, 2012)

A car is not a constant torque load. The torque (of the wheels) depends on the diameter of the tire and the acceleration and weight (as well as rolling resistance and drag). The acceleration may be negative or positive, and may be due to the grade of the road or change in speed of the vehicle relative to the ground. The torque of the motor is related to the torque of the wheels by the drive train ratio, and this also determines the speed of the motor for any given vehicle speed.

Ideally, you want to be able to drive the vehicle up any slope you may encounter, including driveways, ramps, and even climbing out of a deep rut or over a curb. At worst, you may need enough torque for the equivalent of a 100% slope, which is 45 degrees. The force, or thrust, needed to do that is W/sqrt(2), so a 3000 lb car needs 2121 pounds of thrust. If the tires are 24" diameter, the wheel torque will be 2121 lb-ft.

Now, assume you want a top speed of *62 MPH*, and you want to use a normal motor which is limited to *5000 RPM*. The wheel at 62 MPH is *884 RPM*. So the drive train ratio is *5.65*. This means the motor must have a peak torque at 0 RPM (locked rotor torque) of *375 lb-ft*. Now assume that you have a 4 pole AC motor (nominal 1800 RPM) that you can overclock to 5000 RPM, and also assume you can get *3x rated torque* for a short time. You need an 1800 RPM motor rated at *125 lb-ft* nominal, which is *43 HP*. Many AC motors can only produce *2.5x* torque, and there are mechanical losses, so you really need a *50-60 HP motor for direct drive*. If you want a higher top speed you need to increase the size accordingly.

This is for AC motors. A *DC series wound motor* may be able to provide *6x* or even *10x* rated torque for very short durations, so you might get away with a *25-30 HP motor*.

You can play with the numbers using my http://enginuitysystems.com/EVCalculator.htm


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## tomofreno (Mar 3, 2009)

Hi Coulomb, generally agree with what you said except this:


Coulomb said:


> ...If a motor has adequate torque at low speed (enough to climb gutters and car park ramps) and can continue that torque to high speeds, then it would have amazing power at high speeds, and we don't expect to have the vehicle feel most powerful only at speeds over 60 mph.


 I think we feel acceleration rather than power. If the available torque remained constant over a wide motor rpm range then so would the available acceleration roughly (little lower at higher speeds due to increasing drag force). So the car would feel about as "powerful", or a little less, at higher speeds as at lower ones, not more I think. Torque is what counts for acceleration, power is what you need to maintain high torque at higher motor rpm. As you know, its difficult for a diyer to purchase an AC motor/controller with enough torque to give say 10 mph/sec acceleration and high enough base speed to continue that up to 8k or so rpm using a fixed single gear ratio. Please do visit this hemisphere more often, I enjoy seeing your voice of reason.


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## toyolla2 (Jun 21, 2010)

[
I have to respond that your third paragraph needs a little friendly criticism. First I don't think you should allow a 5000 rpm limit to constrain your designs. 5000rpm based on what ? I think it is better to experiment with each candidate motor before you come out with that sweeping generalisation. I personally have run a 3600rpm at 7200rpm with absolutely no undue vibration. Rotors can be balanced for higher speeds if necessary at the time of rewind when the rotor has to be removed anyway. Try to think positively. Automotive gearboxes will probably run quietly when all the extra gears have been removed. 

All through this you managed to avoid mention of the all important V/Hz. But I have read of your rewinding attempts elsewhere, so ?

You can't overclock a nominal 1800 rpm - you mean 60Hz operation I guess - to 5000 rpm without changing its V/Hz ratio appropriately.
otherwise its torque will trail off disastrously. Makes me question what you are trying to do, PS. Kill God ?

Look, over on the AEVA website we conclusively determined that efficiency improves with rpm. This infers that large increases in continuous rated power are permissible. From empirical methods, derived by measuring motor case temperature under full load, a curve fit was determined that equated mathematically to the 0.7th power exponent of the rpm ratio increase. In this case a 415Vac 375watt 1500rpm motor was run at 6000rpm with its four coils per phase, orginally series connected, but then snipped and reconnected in parallel. See "104volt winding" thread on AEVA. 

It is hard to predict what transient power even a 7.5Hp 4- pole could be made to supply continuously but the correct V/Hz will ensure that its nominal torque will be preserved even up to 12,000 rpm providing rotor balance is adequate and providing the headroom provided by the bus voltage is not compromised by the battery impedance.


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## Karter2 (Nov 17, 2011)

Somebody ( with more motor savy than me !) needs to clarify the relative pro's/con's for the different motors if using "single gear" drive.
IE currently available motors of either DC or AC ,
and custom designed motors of any specific type ( DC, AC, PM DC, BLDC, etc etc)
Some of these motors will be better suited to this type of drive.


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## cts_casemod (Aug 23, 2012)

toyolla2 said:


> @ CTS_casemod
> Yes I basically agree. Direct drive = Heavy
> [h1] Slow accel
> [/h1]
> ...


Yep, 100% right.
I have no problems in any of the gears but the aceleration is slow on higher gears. Also being a 4 pole motor theres not much point in using high revs, so with the gearbox I find that the 2nd gear is quite usefull for hard hills, 3rd for general city driving and 4th possibly for motorway (60MPH on my car = 3000RPM). Some motorrs like the siemens and many 2 poles can go up to 6000, so that would change things a bit.

Given my setup If I was to go back I would have used a smaller motor with the stock gearbox, for my use my motor is a bit overkill. Other than that I am quite happy it feels a bit like the old Diesel.


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## PStechPaul (May 1, 2012)

Of course a motor can be rewound and balanced and fitted with high speed bearings to get 3x overclocking from 60 Hz nominal, with 3x power. And with special laminations you may be able to design and build a full custom motor capable of at least 400 Hz or 7x. But I was illustrating what may be possible (and safe) without such special modifications. 

A three phase 208/230V 4 pole motor may be reconnected from wye to delta to make it nominally 120/140V, and then you may be able to use a 720 VDC battery pack and a 440/480 VAC VFD to achieve at least 3x overclocking to 5400 RPM and possibly 4x to 7200 RPM. The high pack voltage allows keeping the same V/Hz up to 3x. But generally it is not really safe to exceed about 5000 RPM, which is also a reasonable limit for standard drivetrain components.

There are also increased losses with overclocking above nominal design. But they may be less than proportional to the frequency, as you note with the 0.7 power. AIUI, this means that you may get twice the magnetic losses at 3x, so a 95% efficient motor may be 90%. But since you have 3x power, it may be more efficient. But I will have to take your word for that. Of course, rewinding for much lower voltages to avoid a 720V battery pack creates another source of inefficiency and/or expense and weight. The proportionately higher current requires heavier connections and larger components such as IGBTs and MOSFETs, and necessitates a custom controller or the use of a much larger standard VFD.

Another point is that, for direct drive, most of the maximum torque is needed at the low end. So a 60 HP standard motor may be needed for the low end torque, but much less is required as speed increases. So field weakening above 1x with a maximum of 3x overclocking still gives you 60 HP at top speed which is more than enough. The real "hit" is the extra weight of a 60 HP motor compared to a rewound 20 HP. But a standard 60 HP motor is likely to be much less expensive than a custom wound 20 HP motor, and the standard VFD and lighter gauge wiring also drive down cost and weight. 

I gave up on my rewinding idea because I saw that it seemed to be impractical, although I was looking at a more drastic rewind for 12 VDC nominal and 60 VDC for 5x overclocking. Reality hit when I realized that even a 5 HP motor would draw over 300 amps at 12V, and the wiring would be cumbersome. 

If you could provide some actual figures for the efficiency, size, weight, and ultimate cost of various overclocked AC motors, it would be very helpful. I've been on the AEVA forum and saw some threads about this with mixed reviews, and it's difficult to wade through the plenitude of posts to find exact figures. Thanks.

BTW, I think the thread you are referring to is: http://forums.aeva.asn.au/richos-ac-handi-sports_topic2910_post37931.html?KW=104v+winding#37931


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