Help With Torque Assumptions 2

[CVLDGolf]

Hello All,

So I am fairly new to designing electric vehicles and had a few questions.

So I am looking at a motor that specifies peak torque and continuous torque. What determines how long I can operate at peak torque (no times specified by manufacturer)?

 

[3DDave]

Typically when the insulation burns off.

 

[CVLDGolf]

So basically however long it takes to reach max operating temp?

 

[RedSnake]

It is usually specified in S1-S8 and not easy to understand from just that, it is based om temperatur and cooling times.

BR A

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[CVLDGolf]

Unfortunately I’m looking at operating the vehicle at fairly low speed and limited room for large differentials so motor rpms will be in the 200-500 range where most motors don’t have data. I was trying to assume some terrible efficiency and calculate rough heat up times based on specific heat and mass.

 

[waross]

Unfortunately I’m looking at operating the vehicle at fairly low speed and limited room for large differentials so motor rpms will be in the 200-500 range where most motors don’t have data.

This may be a good time to re=evaluate your design.
By running your motor so far below the published performance curves you are giving away a huge amount of the motor's potential.
Running a 3000 RPM rated motor at 200 RPM may be throwing away over 90% of the motor's potential power.
Consider gear reduction or a different design motor.
Edward Cowern has a formula and description for RMS horsepower loading of a motor with a varying duty cycle.
See the Cowern Papers;
Cowern Papers Download

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[CVLDGolf]

Lets assume I have maxed out my differential. How would one go about achieving a higher motor speed? Since the wheel speed is just a function of shaft rpm and differential?

 

[waross]

Planetary gears, a high pole count motor, worm drive?

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[electricpete]

Maybe look for a motor that can work well over a wider range of speeds.
I don't know much about motors used in electric vehicles, but the one in the video is optimized for a wide range of speeds
(acts as a synchronous-field motor at low speed and synchronous reluctance motor at high speed)

Tesla Model 3 motor - The Brilliant Engineering Behind it


=====================================
(2B)+(2B)' ?

 

[waross]

Nice link, Pete. lps

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[FacEngrPE]

Motor size and weight is more related to torque than power. If you are operating at very low speeds you might need to consider gearing (might wind up lighter and lower cost than the low speed high torque motor). This is typically the approach used with conveyor drum motors. There are advantages and disadvantages to both approaches.

A cycloidal speed reducer can be used to put a vary large speed reduction into a compact space. There are also wheel hubs with speed reduction built in.

 

[Gr8blu]

CVLDGOlf:
The "continuous" torque rating is what the machine can produce over a speed range that runs from zero to the "base (slowest listed) nameplate speed. This is accomplished - in most cases - by using a constant volt/hz approach (for AC) or a volt/rpm for DC. Operating above this base speed point is referred to as "field weakened", because the way to accomplish this is to hold the voltage constant and weaken the magnetic field strength. In this weak field operation, the motor delivers constant power - and delivers MUCH less than full rated torque.

Peak torque is something that can occur either as a regular part of the duty cycle, or as a once-in-a-lifetime catastrophic event. "Occasional" peaks refer to those corresponding to those that happen as often as catastrophic events; "frequent" peaks are anything that happens more often. That being said, the amount of available peak torque is dependent on the design of the machine, which is sometimes driven by the designer's knowledge of the application. For these types of machines, it is not uncommon to have a frequent peak value of 1.5 pu and an occasional peak of 2.0 pu. Note that 1.0 pu refers to the torque available at the speed being investigated: thus for speeds up to base speed, 1.0 pu = base speed torque. At higher speeds, it will be lower since the available torque is lower. According to NEMA (and various IEC documents), the machine is designed to withstand a peak torque event with a duration not to exceed 60 seconds. Practical industrial applications often limit the peak duration to something less than 3 or 4 seconds. Repetitive peaks have to be taken into account in terms of the machine duty cycle: peak torque is proportional to peak current, but heating is proportional to current squared. Most machine windings use a rule of thumb that the RMS average loading on the machine should not exceed nameplate when taken over a rolling 5-minute window. This corresponds to the thermal time constant of the winding itself.

Depending on the specific motor design, it is possible to have 98+ percent efficiency at speeds down to just a few (below 3) rpm. Probably not in something you could use for vehicular motion, but they DO exist. Other options - such as high-speed ratings - require some form of gearing to get the final drive ratio down to something the wheel can handle. Some possible methods have been noted in previous posts; there may be others of which I am not aware. Keep in mind: most electric machines are controlled such that the vehicle application occurs in the "constant torque" range. This means if you're opting for a semi-direct drive approach and slowing the motor down to 10 percent of its nameplate speed rating, you'll only have - at most - 10 percent of the nameplate shaft power available at that operating point.

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