Torque and voltage constants on DC motors.

[Maesterbaevern]

Hi!

We're gonna use a pretty powerful dc motor from Bosch in our car. Anyone who has any tables of common torque constants and voltage constants for DC motors in the 80-90 Nm range?

BR

[PeterK]

IIRC these constants depend on the motor construction. There are tests to determine them, you can't just use values from some other motor.

[Maesterbaevern]

Ok, great. Just wanted to try to get a head start since the long delivery time.
Thanks for the help!

[murpia]

How did you select this motor, without a data sheet or characteristic curves?

In my experience these are essential in matching a motor to an application and would either contain Kt & Kv or allow you to determine them.

Regards, Ian

[Drew Price]

Sounds like it was probably on sale at Sears....

[Maesterbaevern]

I just looked at the voltage and torque.
I'm a computer engineer with a major in control theory. Sadly, I've got too much theory it seems. ^^ My electronic skills ain't too great so sadly I'm not an expert so please help out instead of whine @ me.

The DC motor in question: http://www.bosch-pt.co.uk/bopt...2-LI/16839/index.htm (http://www.bosch-pt.co.uk/boptocs2-uk/Trade+and+Industry/Tools/GB/en/gw/14.4/18+Volt+robustseries/125963/GSR+18+VE-2-LI/16839/index.htm)
Not exactly on sale on Sears..

[Drew Price]

We the tech specs on that catalog page give you max torque delivered in the high and low gears - if you knew the gear ratios (you can take it apart when it comes) you could figure it out at the motor spindle assuming you knew for sure the battery was actually able to supply peak current at 18v - for a commercial product like that you unfortunately might just have to test it when you get it.

Drew

[EHog]

May I ask what you plan to use the motor for?

[PeterK]

My vote is for power windows...

Oh wait, maybe a shifter?

[Maesterbaevern]

Thanks, Drew. They use the same motor for 14.4V and 18V so hopefully we won't get too much loss of torque.

We will use it to actuate the clutch, EHog.
(Klicktronic for shifting, PeterK)

[EHog]

sounds about right. I was hoping it was something more glamorous like... active aero http://fsae.com/groupee_common/emoticons/icon_razz.gif

[Maesterbaevern]

Hehe true, got a request for a variable length intake of the engine crew.
However, since it's the first year in FS for the almost the entire electronics group of our team we've got our hands full already. We're renewing the entire software base since we're switching to Cortex M3-based microcontrollers and making a new approach on our semiautomatic gearbox is quite enough right now.

BR
Niklas Hansson
Electronics Group 10-
LU Racing

[EHog]

if there is one thing i have learned in FSAE, its "don't bite off more than you can chew."

[murpia]

The original question was for data a 90Nm motor not a 90Nm power tool!

http:// w w w .robotmarketplace.com/products/ETK-ETEKR.html

Regards, Ian

[Maesterbaevern]

Thanks, Ian.

(The motor is from the power tool)

[murpia]

But that Bosch motor from the drill is not powerful at all, just geared down a lot to produce 90Nm...

The original question sounded like a traction motor choice for FSE, not a clutch actuator.

Regards, Ian

[Drew Price]

Originally posted by EHog:
I was hoping it was something more glamorous like... active aero http://fsae.com/groupee_common/emoticons/icon_razz.gif


Go-go-Gadget-Feedback-Loop!


Ditto on the gearbox in that drill, although there's not necessarily a reason not to use the gearbox in your design anyway - like some people mentioned about using it for a shifter the slipper clutch that comes on the drill might be useful, but as Murpia pointed out -

...The data sheet for that motor only shows ~35 nm for 'soft' applications, probably the high speed selection, which may or may not be 1:1 with the motor.

Drew


Drew

[Maesterbaevern]

Ok, then we will keep the gearbox.
I wonder if we can get some decent speed on the clutch rod.
Otherwise, it's off to the sales at the swedish "Sears" ^^

How's your experience with DC-motors as an electric clutch actuation?

[murpia]

Originally posted by Drew Price:
Ditto on the gearbox in that drill, although there's not necessarily a reason not to use the gearbox in your design anyway - like some people mentioned about using it for a shifter the slipper clutch that comes on the drill might be useful, but as Murpia pointed out -

...The data sheet for that motor only shows ~35 nm for 'soft' applications, probably the high speed selection, which may or may not be 1:1 with the motor.

Drew

Originally posted by Maesterbaevern:
Ok, then we will keep the gearbox.
I wonder if we can get some decent speed on the clutch rod.
OK we are back to the fundamental confusion between power and torque which seems to be at the heart of this thread...

Describing a motor (or motor plus gearbox combination) as 'powerful' because it is capable of a particular torque (90Nm in this case) is incorrect. That 90Nm might be delivered at such a slow speed that the power is small and therefore the motor is not capable of doing the required job.

It seems the application is a clutch actuator and that Maesterbaevern is concerned about actuation speed. Any actuated system will require a certain amount of energy (work) and the actuator (motor) needs to supply that energy within the required response time. Hence the key specification for the motor is its maximum power. Not its maximum torque.

So, a sensible plan of action would be:

Measure the required actuated system force / displacement relationship.
Decide an appropriate response time for the application.
Combine the two to generate a power requirement for the actuator.
Look for a motor that offers that power.
Calculate how best to gear that motor to match the motor speed to the actuated system.

Regards, Ian

[Maesterbaevern]

Correct, Ian.

Measure the required actuated system force / displacement relationship. - Check!
Decide an appropriate response time for the application. - Check!
Combine the two to generate a power requirement for the actuator. <- Doh!
Look for a motor that offers that power.
Calculate how best to gear that motor to match the motor speed to the actuated system.

I guess we will have a look at the drill motor when it arrives. Hopefully it won't be too slow. Fingers crossed!
Otherwise we have to keep on looking.

Thanks for the advice and checklist!