# Understanding Motor Specs and Vehicle Gearing



## major (Apr 4, 2008)

Hi aldaeron,

You're asking a bunch. Got a few minutes, so I'll give it a go....



aldaeron said:


> 1) It appears that most DC motor manufacturers test their motors at a constant voltage and list torque, RPM, amperage, efficiency etc. Is the motor linear (i.e. if my pack voltage is double the test voltage, will it use half the test amperage?).


Not exactly. If you double the pack voltage, then for the same power out of the motor, the battery current would be half. But that would include the motor controller or a change of gear ratio to be realized. For the DC motors used for EVs, the torque per amp is consistent regardless of the applied motor voltage. So, if you doubled the motor voltage, at a given load (torque), the current would essentially remain the same and the RPM would double giving you twice the power out.



> 2) Are there any downsides to running a higher pack voltage for a DC motor?


Yes. Depends on what you mean by higher. Say 48 to 96 volts. Probably a good thing all around. From 200 to 400 volts. Probably a lot of issues will bite you. Higher pack voltage means more cells in series. More links in the chain, so to speak. Only as strong as the weakest link. So high voltage pack management can become an issue. High voltage chargers, fuses, contactors, controllers and on and on can be problematic or costly. You just have to design your system using compromises.



> 3) Can a controller step up/down the voltage to do the same as what is suggested above (i.e. 72V @ 100 A in from batteries, 144V @ 50 A to the DC motor)


Theoretically, yes. But available controllers only step down.



> 4) What is a realistic controller efficiency?


Depends on the controller and system voltage. Generally they are quite efficient. Maybe in the 95% range.



> 5) Where can I find starting torque for popular motors (Warp, ADC)?


Most have published performance curves. Starting torque is dependent on the current limit of the controller. Find that current on the curve and read the torque.



> 6) What is realistic Drivetrain Efficiency (for a transmission, transfer case, differential and all the U-Joints in the drivetrain)?


Again, depends on the particulars, but I'd use something like 90% for starters.



> 7) Is it better to take the motor power and input it into the transfer case or use 2 motors (one powering front and rear differentials)?


Better? Who knows? Just different. So many other factors enter into the system design, there is no blanket "better" answer. 



> If I use two motors do I also need 2 controllers?


Depends on the motors and the application. Some guys run dual motors from a single controller. Dual motors each with its own controller are common in the lift truck industry.



> Do the motors or controllers need to be synchronized in any way (i.e to control front/rear power distribution)?


Probably not.



> 8) On that note, what is meant by series and parallel motors? I read a post where there was discussion about switching between two motors in series and parallel. I think what is meant is that the two motor shafts are attached. When wired in series the torque of both motors adds to give lots of torque at low speeds. When wired in parallel, the speed of the motors adds to give a higher top speed.


These motors have the shafts coupled, so they both turn at the same RPM, always. And the torque from each adds for the total output torque for the pair. When in series, the same current runs thru both motors, giving double the torque per amp, but the voltage is divided between the two motors. When in parallel, each motor gets full voltage but half the current.



> 10) Is there a good rule of thumb for choosing gearing? Obvisouly there is an RPM limit for the motor, but what else should be considered in this decision?


Look at wheel torque in relationship to the maximum grade and acceleration needed.



> 11) Can a transfer case be shifted from LO to HI while moving? What is the typical RPM limit for LO gearing in 2WD or 4WD?


Not like you would want to do it. This is a bad idea.


About all the time I have. Good luck.

major


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## roadrailer (Jun 10, 2008)

Hey aldaeron,

Sounds like you might be converting an SUV 4WD. I'm working on a Jeep Cherokee 4WD. I would be interested in hearing how your design/selection process is going.

Best regards.


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## aldaeron (Jan 29, 2009)

Major - Thanks for the quick reply. Couple of follow ups

How do I correctly read a motor specs graph? For example, the Warp 9 spec at: http://www.go-ev.com/images/003_15_WarP_9_Graph.jpg

1) If torque is related to current and speed is related to voltage (found the right article on wikipedia finally) then why is RPM plotted vs Torque?

2) What is relationship between motor speed and motor voltage? If the test voltage was 72V and I run the motor at 144V do I just double the RPM of the data taken @ 72V? The test data says it was taken at a constant 72V, but since motor speed is related to voltage and the voltage didn't change, how could the motor speed change? This is very confusing and/or I am missing a key point here.

3) It seems like for a given pack size, say 36kWh, a 144V battery voltage is better than a 72V battery voltage, right?

4) For calculating starting torque, which amperage rating from the controller should I use? MAX Amps, MAX 1 minute Amps or Continuous Amps?

Roadrailer - I am looking at a variety of options, one of which is the Jeep Cherokee. I live in Colorado and love to ski and backpack so my requirements/goals are to build an EV that can make it to the mountains and back with ease. This means I need an EV that can: 

- Climb 6% grades and sustain freeway speeds
- Climb 10% grades at a slow speed
- Operate in weather down to -20F
- Drive in snowy and icy conditions (4WD or AWD is a must)
- Be able to carry 4 passengers and some amount of gear (snowboards, backpacking gear, etc)
- Be able to make the 120-140 mile round trip in one charge (yes, it will take a lot of batteries! And this will be the toughest goal to meet. One backup option I am looking into is taking a large propane generator out of a home electric backup system and putting it on a 2 wheel trailer I can tow)


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## major (Apr 4, 2008)

aldaeron said:


> How do I correctly read a motor specs graph? For example, the Warp 9 spec at: http://www.go-ev.com/images/003_15_WarP_9_Graph.jpg


Hi aldaeron,

The motor specs graph is called the motor characteristic performance curve. Or performance curve. It is used to give the designer information about the motor output at a defined input. It is particularly useful for series wound DC motors because motor behavior cannot be characterized by simple RPM/volt and Torque constants as are PM motors.

Let's use your example motor performance curve as an example. First notice than in the lower left corner is "72 Volts". This is very important. It means that all values shown are good for the case where the motor terminal voltage is 72 volts.

Next, notice that the horizontal or X axis is torque in Lb.Ft. Torque is the load and therefore the independent variable. All other characteristic values (Amps, HP, Rpm and Eff) depend on the independent variable, the load or Torque, at the specified voltage.

So to use the performance curve, one finds the load for the application (torque) and at that specified voltage, the plot will indicate the resulting motor output (RPM, HP, Eff) and the resulting current draw (Amps). The load or torque is either measured or calculated from the vehicle design and conditions (rolling resistance, wheel diameter, gear ratio, aero drag, grade, etc.). Also, if the vehicle is accelerating, then there is a load component attributed to the force and mass, F=ma.

There is a one to one relationship between the torque and current. So, occasionally, the performance curve will be plotted with current as the X axis. Torque is normally used. But you can see, if you know the current draw for the motor, you can easily find the torque point. This is handy because the current is easier to measure than torque. So, if you have the motor in an application and know the current for a particular load, the performance curve can be used to find the torque load and resulting RPM, HP and Eff if the applied voltage is as specified for the curve (ie. 72 V).



> 1) If torque is related to current and speed is related to voltage (found the right article on wikipedia finally) then why is RPM plotted vs Torque?


See above.



> 2) What is relationship between motor speed and motor voltage?


A direct ratio is a very good approximation at a given load for series wound motors. So, in the example, at 40 Lb.Ft., at 72 volts, the RPM = 2600. If you had 96 volts at the motor and 40 Lb.Ft. load, RPM = 2600 * 96/72 = 3467. Again, this is a good approximation, not exact. Most useful in the mid-range. Error increases at very light loads and heavier loads.



> If the test voltage was 72V and I run the motor at 144V do I just double the RPM of the data taken @ 72V?


Yes. But the higher the deviation from the tested curve, the larger the error in the approximation. But even at this doubling of voltage, the ratio will give you a decent approximation.



> The test data says it was taken at a constant 72V, but since motor speed is related to voltage and the voltage didn't change, how could the motor speed change?


Well, first off, the motor speed changes with regard to load even at a constant applied voltage. This is clearly shown on the curve. The motor slows with increasing load. The RPM of the series DC is not just dependent of voltage. As the load increases, the current increases, which increases the motor flux. RPM is inversely proportional to flux. And as the current increases, the resistive drops in the motor windings and brushes increase which cause less generated voltage, meaning lower RPM.



> This is very confusing and/or I am missing a key point here.


Just learning, I hope.



> 3) It seems like for a given pack size, say 36kWh, a 144V battery voltage is better than a 72V battery voltage, right?


Better? Just different. Depends on the application, design, components, etc. I guess if you can get the right motor, controller, battery, charger and stuff, I'd be inclined to choose 144.



> 4) For calculating starting torque, which amperage rating from the controller should I use? MAX Amps, MAX 1 minute Amps or Continuous Amps?


It is called the current limit. Which is the maximum current for the controller. A good controller should be able to handle this current limit for long enough for a normal acceleration in a decent designed vehicle. It should also be self protected so if you remain in current limit too long, it will automatically cut back. This would normally come into play on steep grade with a heavy load or when the controller is running at elevated temperature.

Hope that helps,

major


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## major (Apr 4, 2008)

Message deleted. Sorry.


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## aldaeron (Jan 29, 2009)

Major - thanks again! A few more ... and if you're ever in the Denver area, I will buy you a beer!

1) Also, what happens to voltage at starting speeds? For argument's sake let's say my controller can only handle 300A which translates to 58 ft-lb of torque at 2300 RPM per the performance curve. With combined gearing in the transmission and differential at, say, 5:1 and the vehicle moving 5 MPH with a 0.7 m diameter tire, that puts the wheels at 60 RPM and the motor at 300 RPM. Does the voltage therefore drop to 9.4V to get the speed down to (9.4V/72V)*2300 RPM = 300 RPM?

2) The Warp motor only gives data to ~350 A, but controllers can handle much more that that (I have seen some Logisystems controllers that can handle 1000 A). Does this mean that the current limit is 350A for this motor? Or would I need to request the current limit info from Warp?

3) Can the controller adjust the voltage on the fly? For example if I have a 144V battery back and the controller is outputting 72V at 241 A, giving an RPM of 2600, can I set the controller to up the voltage to 96V on the fly, increasing the RPM to ~3467?

The reason I ask is that I have been looking at starting and top speed. It seems like it would be nice to decrease the voltage at starting speed and increase it at high speed to increase the overall top speed.

Thanks again!!!


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## TheSGC (Nov 15, 2007)

That's what the controller does. A controller uses Duty Cycles to limit current AND to adjust voltages, using Pulse Width Modulation. For instance, if your Throttle is at 50%, your Duty Cycle is at 50% and the average voltage the motor "see's" is 50% of the pack voltage. 

I suggest you read up on Pulse Width Modulation to get a good understanding on how it works.


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## racunniff (Jan 14, 2009)

roadrailer said:


> Hey aldaeron,
> 
> Sounds like you might be converting an SUV 4WD. I'm working on a Jeep Cherokee 4WD. I would be interested in hearing how your design/selection process is going.
> 
> Best regards.


Hey - me too. Mine's nearly done - it's taken a 100 foot test drive. It would be interesting to compare notes.


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## racunniff (Jan 14, 2009)

Oh, and on-topic for the thread - I've posted some comparative motor notes on my Electrojeep blog and Volt914 blog - might be some useful info for people to consider.


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## racunniff (Jan 14, 2009)

aldaeron said:


> I need an EV that can:
> 
> - Climb 6% grades and sustain freeway speeds
> - Climb 10% grades at a slow speed
> ...


You're going to have to go to exotic battery technology to achieve the "operate in weather down to -20F" and "make the 120-140 mile round trip in one charge" goals. Lead-acid won't cut it. One option though might be a pusher trailer.

PM me some time if you want to come up to Fort Collins to see the Electrojeep.


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## major (Apr 4, 2008)

aldaeron said:


> 1) Also, what happens to voltage at starting speeds? For argument's sake let's say my controller can only handle 300A which translates to 58 ft-lb of torque at 2300 RPM per the performance curve. With combined gearing in the transmission and differential at, say, 5:1 and the vehicle moving 5 MPH with a 0.7 m diameter tire, that puts the wheels at 60 RPM and the motor at 300 RPM. Does the voltage therefore drop to 9.4V to get the speed down to (9.4V/72V)*2300 RPM = 300 RPM?


Hi al,

Well, your example [(9.4V/72V)*2300 RPM = 300 RPM] is probably off by quite a bit. The direct ratio will not give good results for such a high deviation. But you really don't have to worry about it. The PWM controller takes care of it semi-automatically. It puts the driver's foot and brain in the feedback loop. So, he adjusts the throttle position to get the desired speed (or torque) from the motor and the controller takes care of getting the proper PWM and voltage to the motor.



> 2) The Warp motor only gives data to ~350 A, but controllers can handle much more that that (I have seen some Logisystems controllers that can handle 1000 A). Does this mean that the current limit is 350A for this motor? Or would I need to request the current limit info from Warp?


No, the motor can handle more current than 350. Easily 500 or 600 amps for short times. You can get an approximation by extending the X-axis and amps on the performance curve. Just use a straight line extension. It won't be perfect, but not too far off.



> 3) Can the controller adjust the voltage on the fly? For example if I have a 144V battery back and the controller is outputting 72V at 241 A, giving an RPM of 2600, can I set the controller to up the voltage to 96V on the fly, increasing the RPM to ~3467?


Yep, that's what they're there for. Simply push down on the throttle. But realize that the speed will not change instantaneously. So, when you increase from 72 to 96 volts, RPM stays the same and the current and torque increase. Which will accelerate the vehicle and ultimately achieve a higher speed. But this will come at a cost. It will no longer be at 241 amps (as at 72V), but at a higher current because you will have increased the load due to aero drag. So the resulting RPM will be less than 3467.

Regards,

major


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## roadrailer (Jun 10, 2008)

Hey aldaeron,

I looked at your ElectroJeep site - good information, particularly regarding the motor comparisons. My '88 Cherokee had the 4.0L six and automatic transmission. I had always planned on using an induction motor and direct drive. A variation I have considered would be to drive each axle with a separate motor. For now, though, I have found a used AC55 and controller. Here are some motor drive issues I would like to see discussed further:

1. Is a transmission really needed? I think direct drive is viable with the proper motor selection. The AC55 has a base speed of 2000 rpm, which is approximately 47 mph on my Jeep. My understanding is that means I am torque limited (i.e., grade, acceleration) up to that point; over 47 mph I am HP limited (speed for a given load or grade). I am more interested in steady pulling at lower speeds than peppy starts, and I don't plan to do much freeway driving. This would not work with some of those motors I have seen on certain EV sites rated at 6000 or 8000 rpm. If you peel away the smokescreen around the AC55 and others, and talk in standard engineering terms, what you are doing is matching the motor design in terms of number of poles to the service. At a 60-Hz reference, 2 poles = 3600 rpm (too fast for me), 4 poles = 1800 rpm (about right), 6 poles = 1200 rpm (a good lugger but no high end). Final point: diesel electric locomotives have been direct drive since they were developed - 100 years ago for DC and 20-30 for AC. They have high starting tractive effort and can run up to 70 mph at the standard gear ratio of 62:15.

2. What are all these parameters Azure is talking about programming to match controller to motor? I asked the Electro-Auto guy and he said that their controller is designed to run scalar control (otherwise known as V/f ) where the voltage and frequency are increased linerarly. This requires none of the feedback parameters that advanced inverter control need to run, say flux vector control.

3. Why would precise inverter control be needed for an EV drive? Typical slip (actual rpm difference from synchronous rpm (1750 vs 1800)) is 3-5%. That's +/- 3 mph at 60 mph. A good driver of an IC powered vehicle would do well to match that over a full trip.

Anyway, I am going to pick up the motor this weekend. Hope it works out. Thanks for sharing your info.


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## racunniff (Jan 14, 2009)

roadrailer said:


> Hey aldaeron,


 I think you're talking to me...



roadrailer said:


> 1. Is a transmission really needed?


Define "need." Yes, the AC55 has enough torque to get the ElectroJeep started up even in 4th gear. But it does not produce as much *power* as starting in lower gears does. Examine this chart - if you ran at a fixed gear ratio equivalent to, say, third gear, starting up would feel kind of like starting in second with your gas engine. And your max speed would probably be 40-45MPH.



roadrailer said:


> 2. What are all these parameters Azure is talking about programming to match controller to motor?


In addition to various torque curves that are related to the motor, there are differences between the speed sensor feedback signals that come back from the various motors. If your DMOC445 was not originally matched to an AC55, you're probably best off to send it to Azure to have them reprogram it. They are very responsive.



roadrailer said:


> 3. Why would precise inverter control be needed for an EV drive?


In an AC motor you need to make sure the waveforms you are sending to the various windings match where the rotor is (there is a sensor to detect where the rotor is). Beyond that, I'm not an EE, so your question is somewhat over my head...


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