# Overkill or under shot?



## Ozwald (Dec 8, 2012)

I have a 98 Honda civic i would like to convert to a hybrid with a single electric drive on the rear wheels. (Fabrication already in progress)

The specs:
Curb Weight: 2412 lbs
Fully Loaded apx: 3000 lbs
Feet in Mile: 5280 ft.
Cursing speed: 70 mph
Feet per hour: 369600 fph
Feet per minute: 6160 fpm
Tire circumference: 6.1 Ft.
Tire rotations of tire per minute: 1010 rpm
Gear Reduction: 2.53:1*
Apx. motor rotation required: 2556 rpm at 70 mph

*This is the rear differential out of a CRV that i will be using. I can add a chain reduction if necessary.

My question is:
Will a 24 hp 3 phase AC motor with a 3600 rpm be too big or too small for my application.


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## frodus (Apr 12, 2008)

depends on what motor. Is that peak? continuous?


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## Ozwald (Dec 8, 2012)

The motor says its rated at 24 hp and 3600 rpm so im guessing that is continuous.


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## PStechPaul (May 1, 2012)

The 24 HP 3600 RPM motor will give 35 lb-ft torque nominal and 2-3 times that as peak. The 2.53 gear reduction gives a maximum of 262 lb-ft on the wheels, which is also 262 lb thrust for 24" dia tires. For a 3000 lb vehicle this will only allow operating on a slope of 8.75% or less. For operation at 70 MPH on a flat road you need 104 lb-ft torque and continuous 20 HP. You may do better with an 1800 RPM motor which gives twice the torque at the low end, and you can overclock it to get the 2500 RPM you need or the top end. You can safely take it to about 3600 RPM with some loss of power unless you can run it at higher than nameplate voltage, which is generally OK for a 230/460 VAC motor.

You can use my http://enginuitysystems.com/EVCalculator.htm to play around with the figures, and there are others as well.

I think a 40-60 HP motor is much better or direct drive, if it will fit. 15-30 HP is about minimum for a car with a gearbox.

You really want enough torque to be able to negotiate the equivalent of a 25% slope, which is also 0.25G acceleration, or 5.6 MPH/sec, or 0-56 MPH in 10 seconds.


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## Siwastaja (Aug 1, 2012)

The concept of using industrial AC induction motor is usually based on overclocking and increasing cooling to get 3x or more rated power at higher RPM. This is done using a 4 pole motor -- 1800 rpm overclocked by 3x equals 5400 rpm. Then you of course need higher reduction than your 2.53.

Underlying idea is that high frequency and high rpm increases available power within certain limits without increasing the motor weight and size, compared to low frequency and low rpm. At 60 Hz, you end up having a pretty heavy motor.

The downside in overclocking is that you practically need to modify or rewind the motor for lower voltage.

To answer whether the motor is large enough, start by looking at the torque. If it is not given, calculate it from rpm and power. Then, you can safely assume that you can run the motor continuously at 150%, or peak (for 10 seconds or so) at 300% rated torque. Compare that with the original ICE torque rating; as you are going to direct drive (something like 4th gear), you will need comparably more torque than the ICE had. OTOH, the ICE torque was measured at some high rpm and it gave a lot less torque in normal driving.

You will probably find that the motor needs to weigh at least 100 kg for alu frame or 150 kg for cast iron frame, to give even barely usable performance unless you are ready to rewind or reconfigure it for lower voltage. OTOH, the reconfiguration may be very easy with some 4-pole motors. Try to find something like 230V in delta, 15 kW, 4 pole (1800 rpm) motor and change the pole pairs from series to parallel, and then use something like 400V battery pack.


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## Ozwald (Dec 8, 2012)

The ICE that comes stock for my car is only 103 Ft.lb @ 4600 rpm.
At 70 mph it runs at 3000 rpm on the manual transmission and has no problem climbing hills when loaded down.

Are you saying that i should match this amount or supersede it? If so by how much?

A 30 hp motor at 3600 rpm would give me 110 ft.lb of torque through my 2.53 reduction.

The 24 hp motor at 3600 rpm only gives 88.5 ft.lb of torque with the reduction. I could add in another 1.4:1 reduction to get up to 124.6 ft.lb.

Or should i scrap that and just find a forklift motor?

Which would be more coast effective?

Also, i am not familiar with overclocking a motor, guess i will have to look into that. Is it just running a higher voltage through it?


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## rwaudio (May 22, 2008)

The other thing to consider is that this is a hybrid and not an EV. What do you want the EV powertrain to do? 

1) Is the goal more power?

2) Is the goal better fuel economy?

3) Is the goal to run on batteries 90% of the time but have the gas engine available for longer trips?

1 & 2 don't need to be able to replace the original power completely and would only require a small battery pack, but possibly complex controls to make the system work smoothly.

3 requires a large enough pack to give the desired day to day range as well as enough power to handle those daily driving requirements.


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## mizlplix (May 1, 2011)

> Also, i am not familiar with overclocking a motor, guess i will have to look into that. Is it just running a higher voltage through it?


No, varying the frequency (HZ) is overclocking an AC motor. 




> Or should i scrap that and just find a forklift motor?


Most fork lift motors are DC. IMHO, You would be better using a DC system for your first EV. 

An off-the-shelf, commercial AC motor is set up for high voltage and low amperage. Most American EV controllers are lower voltage with high amperage, so will not work together without rewinding the motor, or just buying a purpose built EV AC motor.

PLUS: Most all local motor shops are not familiar on how to rewind a commercial AC motor to work in an EV application. They simply say "it can't be done."

So, my advice for your first vehicle conversion is to shop around and read a lot before deciding where your money is going to go. 

Miz


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## Ozwald (Dec 8, 2012)

Better economy, running on batteries 90% would be awesome but im not at the battery stage yet. i just need to find a motor that is strong enough to cruse the vehicle under its own power through a direct drive because i cannot mount a second transmission in the rear of the civic.


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## Ozwald (Dec 8, 2012)

and for the control i was thinking something simple like a tapped winding. for 30, 55, 70 mph on flat ground


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## PStechPaul (May 1, 2012)

If you are keeping the ICE and tranny for the front, that changes things considerably. But you still need a speed (or torque) control for the AC motor, and taps on the windings for various speeds won't work (even for a DC motor). It will be difficult to synchronize the ICE drivetrain and the electric motor, but you might be able to just put the tranny in neutral and switch to rear drive electric when desired. An EV really outshines an ICE for low speed stop and go driving, but a direct drive as you are considering does better at the high end. Also a "hybrid" as you envision will need to keep all of the ICE components, which provides little room for the electric motor, controller, and batteries, and the final weight may be excessive and poorly distributed which can create an unsafe handling situation.


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## Ozwald (Dec 8, 2012)

Thank you for explaining your toughts about the hybrid. I do plan to drop the transmission into neutral when the electric is on.

However, control of the motor and how it will work with the ICE is not what i am asking. I have already made room for a battery pack. I think i am only looking at an additional 300 lbs with motor and batteries witch required a little stiffer springs in the rear of the car.

The motor is what i am unsure about. Because the ICE puts out 103 ft.lb @ 4600 and can cruse loaded up to apx. 3000 lbs at 3000 rpm with no problem, do i need to match the 103 ft.lb or have more?


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## PStechPaul (May 1, 2012)

Usually you can get by with an electric motor of 1/2 to 1/3 the ICE rating, since those torque and HP figures are generally peak values. You will be OK if you match the 103 lb-ft torque of the ICE, but that's a 70 HP 3600 RPM motor.

According to my calculations, you need about 195 lb force to maintain 70 MPH on a 3% grade. Even at 40 MPH you need 166 lb force. This equates to about 66-78 lbft torque from the motor at 2.53 ratio. You need about a 50 HP 3600 RPM motor to get this as a continuous output. OTOH, a 25 HP 4 pole (1800 RPM) motor will also provide this torque, although it may be reduced at higher RPM without higher voltage.

You will still need an AC drive for the motor, and enough batteries to provide its minimum DC link voltage, generally about 250 VDC. That's typically several hundred pounds of batteries.


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## Duncan (Dec 8, 2008)

Ozwald said:


> Better economy, running on batteries 90% would be awesome but im not at the battery stage yet. i just need to find a motor that is strong enough to cruse the vehicle under its own power through a direct drive because i cannot mount a second transmission in the rear of the civic.


_because i cannot mount a second transmission in the rear of the civic.

_Why not?


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## Siwastaja (Aug 1, 2012)

Ozwald said:


> The motor is what i am unsure about. Because the ICE puts out 103 ft.lb @ 4600 and can cruse loaded up to apx. 3000 lbs at 3000 rpm with no problem, do i need to match the 103 ft.lb or have more?


As an example (sorry, I prefer Nm as a unit), a 50 Nm nominal electric motor (this is about 7.5 kW for a 4-pole motor and weights about 50 kg with alu frame) can put out 150 Nm for some time, especially with increased cooling. This matches many typical ICEs, so you will get the same torque; but you get that torque from the ICE only by flooring the pedal, accelerating near the red line. No one does that unless racing. This is why you don't need to _match_ the ICE to get the same _real world_ performance in normal traffic; with proper controller, you _can_ run AC motor near the limits and it will still be quiet and relatively efficient, totally unlike modern ICE which you never will get near the limits in a normal driving situation.

So, in general, I would say something like 1/2 of the original ICE peak torque is surely more than enough as the emotor peak torque. Translating that into nameplate value would be 1/4 to 1/6, depending on how long you need to sustain the peak torque (e.g., climbing hills vs. just accelerating).

*HOWEVER*, that holds true only if you have the same gearing as with the ICE. Do not forget that you are losing transmission here. Try to get going with 4th gear on your ICE and then do the same comparison...

If the ratio of 1st gear is something around 4, so you would need 4x more torque from the electric motor to get the same performance for direct drive. However, as said before, you are not probably getting the max torque out of the ICE, but only half of it, so factor of 2 would be enough instead of 4.

So, trying to compare apples to apples, if you have PEAK ICE TORQUE of 100 ft.lb @ 4600 rpm, get an electric motor with PEAK torque of 200 ft.lb. I'm positive it will be enough. And translating that peak to continuous (the nameplate value for industrial motors) would be 60...120 ft.lb, depending on how long you need to sustain the peak and if you can increase cooling. A 70 ft.lb (100 Nm) nominal 4-pole industrial motor would be around 15 kW in nominal power and weight about 100-110 kg in alu frame.

The overall "problem" here is that your final reduction gearing is too low so you will need a lot of torque, and hence, a large motor. *Physical motor weight pretty much defines the torque, regardless of rpm.* High rpm is the way to get more power out of small motor "for free", but you can't do it with your direct drive. You can consider leaving the gearbox and fixing it at, say, 2nd gear. Then you could still ditch the gear selector. Or then again, just use large enough motor to get the torque, but it will be in the range of 100 kg even with alu frame.

Now as for overclocking, the important part here is indeed that you increase the voltage too. This is what drives more power into the system. But, this is for top speed. For acceleration, and for direct drive which requires low-rpm torque, it is not that important. And, you can "overclock" with any VFD _without_ increasing voltage, you just hit constant power and lose torque. What I'm referring to when speaking about overclocking, it is increasing voltage too to increase *power* at high rpm, hence moving the point where constant power (dropping torque) starts farther. This basically gives more power out of the same motor weight, but this needs to happen at high rpm. But, *overclocking for more power does not give you more torque at low speeds*.


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## Yabert (Feb 7, 2010)

Ozwald said:


> find a motor that is strong enough to cruse the vehicle under its own power through a direct drive because i cannot mount a second transmission in the rear of the civic.


Like Duncan ask, why not.

A 70 lbs transmission + a 60-100 lbs motor (ac or dc) will be a better choise than a huge electric motor alone.


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## mizlplix (May 1, 2011)

Unfortunately, at present, I am not in a situation to give a meaningful answer to the "direct drive or not" question.

But, in a month or so, I will have completed a full cycle of driving the same car with a transmission and then with a larger motor/direct drive. I am looking forward to sharing the results, (good or not-so-good).

But, the sad "real world" answer is that direct drive is not suitable for 90% of the EV's on the road. (Maybe more).....

Most of the same arguments concerning ICE vehicles and transmissions still apply to an EV. 

Do you have many hills or long uphill grades?....Strike one.
Do you have a car over 2,000Lbs?..................Strike two.
Do you have a low voltage pack.....................Strike three.

The big manufacturers make it work by using high pack voltages, high motor

Give me a month and I can give a concrete 0-60MPH and a miles-per-charge comparison.

Miz


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