Window Motor question

[Grim Tuesday]

We are using one window motor to power the winch that pulls up our ladder. The ladder assembly being pulled up is about 10 lbs, more if you count the lever being our grabber assembly at the end. I don't know how to calculate that, but it's 7 inches long, and the grabber weighs ~6 lbs, making the total 10+grabber. If someone could explain how to calculate this to me, I would love it!

Anyways, we were testing it today, and it randomly stops when going up. The motor is getting very hot, and when we cool it down (with snow of course!), it starts working again. However, when we measure the current going through, it's only 10 amps. I thought that the stall current of a window motor was 21, or 18 (I dont remember if we're using right or left here).

What is causing this problem? Is it that we are simply pulling too much with a window motor? Or is it the thermal breaker in the motor breaking? Is something causing the motor to overheat? Or is it the infamous issue with Jags and Window motors? Would that cause it to overheat?

We are running it on a Black Jaguar, on the CAN bus, wired to the 40 amp breakers.

Any help would be helpful!

Picture of device, imagine that there is cable going off of those spools to pulleys:

[Mike Betts]

Grim,

The window motor has a positive coefficient temperature (PTC) resistor built in. When it gets hot, the resistance increases causing less current and voltage being delivered to the motor windings.

You are asking the motor to do too much. Unless you can identify frictional losses which can be minimized, a redesign is warranted...

Regards,

Mike

[Mike Martus]

Mike B. is 100% accurate. This motor is not very strong relative to the CIM and FP. The thermal device will kick out on severe load and current. A 40 amp circuit is a bit much for this motor as the protection will cut out well before this level. Use a 30 amp max.

[Grim Tuesday]

Do you guys think that putting another window motor on the other side, as the winch was designed for would work? Or do we need to completely redesign it?

Will an FP with a gearbox be enough?

EDIT: The only other motor we have available is the RS550 with a 26:1 gearbox, and from what i've heard, banebots gearboxes arent meant to be used in high torque applications.

[kevin.li.rit]

Originally Posted by Grim Tuesday

Do you guys think that putting another window motor on the other side, as the winch was designed for would work? Or do we need to completely redesign it?

Will an FP with a gearbox be enough?

EDIT: The only other motor we have available is the RS550 with a 26:1 gearbox, and from what i've heard, banebots gearboxes arent meant to be used in high torque applications.

Without knowing exactly what the load; it's hard to tell what will be enough.

A second window motor will help and might do it. A Fisher price is usually good for most arms and if it doesn't do it then a redesign maybe necessary or a higher reduction in the FP.

Edit: Guessing at the size of the drum on the winch and using the other weight it looks like adding a second window motor would do the trick. The 10 to 12 current you were measuring is not too far off from the torque curve with the given weight. You could put the RSS 550 onto a fisher price gearbox if you can get a pinion on it.

[Grim Tuesday]

Does anyone know how to calculate the force exuded down by:

A 6 lb object, cantilevered out ~8 inches.


What is the gear ratio of the KoP FP gearbox? I don't have one on hand.

To respond to an earlier comment, the motor is only drawing 10 amps at stall, which is is a bit weird. This happens ever after the motor is starting from a cool stop.

[kevin.li.rit]

Originally Posted by Grim Tuesday

Does anyone know how to calculate the force exuded down by:

A 6 lb object, cantilevered out ~8 inches.


What is the gear ratio of the KoP FP gearbox? I don't have one on hand.

To respond to an earlier comment, the motor is only drawing 10 amps at stall, which is is a bit weird. This happens ever after the motor is starting from a cool stop.

I think we need to know where the force is being applied, otherwise you can just calculate the torque.

And I'm not 100% sure but I think the FP gearbox is a 117 reduction. If I'm not right, I'm close.

[Ether]

Originally Posted by Grim Tuesday

the motor is only drawing 10 amps at stall, which is is a bit weird. This happens ever after the motor is starting from a cool stop.

how are you measuring the current?

power down the robot, disconnect the motor, and measure its resistance with an ohmmeter... post the answer here

[Ether]

Originally Posted by Grim Tuesday

the motor is only drawing 10 amps at stall, which is is a bit weird

Oh, and measure the voltage at the motor terminals when it is drawing the 10 amps.

[ipburbank]

So another thing (I'm also on this team). The arm works fine for a few times, but after some amount of time it stops once an extra load is added (the second stage starts lifting too). The normal raising runs at 7-8 amps, and the stall is 10-11. This makes me think heat. I wonder if this has nothing to do with the load, and more with the duty cycle. The jump in the load only after a few runs makes me think that it is heat and doesn't have to do with the added weight. Is this the case, or could there be another factor I'm missing?

[kevin.li.rit]

Originally Posted by ipburbank

So another thing (I'm also on this team). The arm works fine for a few times, but after some amount of time it stops once an extra load is added (the second stage starts lifting too). The normal raising runs at 7-8 amps, and the stall is 10-11. This makes me think heat. Is there any part of the motor that would react to heat, or cause the stall to lower while the motor is warm (not the internal sensor, possibly resistance increases?)

The motor has a PTC the stops the motor when it gets too hot.

Running a drum off the motor will cause a higher torque needed to move the load as the drum gets larger.

I think a second motor and some fans might get you by the match. If you are drawing 10-12 amps a second motor would drop you near or below peak efficiency for each individual motors. Coupled by fan or two and you may find that it runs well.

[ipburbank]

Sorry for the poor wording of that question the first time, What I mean to ask is: a second motor wouldn't decrease the duty cycle, and I wonder if the difference in load would be enough to decrease the heat generated. From previous discussion the 7-8 amps isn't an enormous load indication, therefore even half that with the long duty cycles might not make much of a difference?

[kevin.li.rit]

Originally Posted by ipburbank

Sorry for the poor wording of that question the first time, What I mean to ask is: a second motor wouldn't decrease the duty cycle, and I wonder if the difference in load would be enough to decrease the heat generated. From previous discussion the 7-8 amps isn't an enormous load indication, therefore even half that with the long duty cycles might not make much of a difference?

The heat generated in total would be cut down as you are running the motors more efficiently. With two motors ( and based on your current measurements) you'd be running a at close to 80% efficiency, with a single motor you'd be running closer to 60% efficiency. You'd also have twice the surface area to dissipate less than half the excess heat.

I don't have too much experience running these window motors to know under what loads the PTC likes to trip.

Edit: I've been reading the RPMs as %efficiency... so my figures for efficiency are off...

[Mr. B]

The torque is the force x distance So 8 lb x 6 inches = 48 inlb
If you take the Banebots 550 motor and look to adapt it to the window motor gearbox I think you will solve your problem. FIRST take the window motor off--by removing 3 screws. On the end of the window motor is a plastic driver--slowly remove it.Look at the window motor shaft, there is 2 flats back to back.
Now take the 550 motor and mask/tape up all the openings. With a belt sander carefully sand the 550 shaft to match the window motor shaft. Note you will have to shorten the shaft slightly. Next make an adapter plate out of 1/8 alum bar stk. The banebots will bolt to the adapter plate and then bolt the adapter plate to the window motor gearbox. Worth a try?

[Grim Tuesday]

Originally Posted by Mr. B

The torque is the force x distance So 8 lb x 6 inches = 48 inlb
If you take the Banebots 550 motor and look to adapt it to the window motor gearbox I think you will solve your problem. FIRST take the window motor off--by removing 3 screws. On the end of the window motor is a plastic driver--slowly remove it.Look at the window motor shaft, there is 2 flats back to back.
Now take the 550 motor and mask/tape up all the openings. With a belt sander carefully sand the 550 shaft to match the window motor shaft. Note you will have to shorten the shaft slightly. Next make an adapter plate out of 1/8 alum bar stk. The banebots will bolt to the adapter plate and then bolt the adapter plate to the window motor gearbox. Worth a try?

I wonder if that counts as modifying the window motor, and is therefore against the rules?