# Window Motor question

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.

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

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 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.

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.

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.

You might try lubricating any sliding surfaces. If you reduce the diameter of the drum used to take up the cable you will effectively increase your gear ratio. Also look into different ways to string your cable to give you mechanical advantage. You can ease the load on your motor by providing counterweight that works to pull the lift up at all times. This may be an actual weight, or preferably springs, penumatics, or other springy like device to help your motor out.

Also look into loosing some of the weight of the part that moves up and down, weight up high is nearly always a bad thing!

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.

Also, in this application a 6lb weight will always exert 6lb force in the down direction. However the added force you need to lift this thing is due to the friction of whatever is holding it out away from the upright portion. you calculate that by summing the torque on the mechanism at a convenient point and finding the resulting force at the point of contact. Then you can approximate the actual force if you have some values for coefficient of friction for the materials. For more info look into a subject called “Statics”

Also as an aside thought, If you dont already, try to place a bearing at each place that contact is made between sliding surfaces, or at least some slippery plastic, that may help your motor some as well

how are you measuring the current?

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

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Though we have lubricated everything already, the surgical tubing counterweight is a good idea. We already considered making our spindles smaller, but we have no easy way to do so, considering how they were made.

In fact, we currently have a surgical tubing counterweight pulling it down

This might be another contributer to why this isn’t working.

Like I said, unless we can design a brand new type grabber in the next three days, I think its going to stay that way. Any suggestions for quick, simple, light grabbers? Keep in mind that we would like to keep floor pickup.

EDIT: We are measuring current with a clamp on multimeter. I can’t do anything to the robot right now because it’s in the school machine shop, and im in my house.

DOUBLE EDIT: We already have delron sliders at most contact points, and nylon washers at some others.

since you only have the RS 550 to work with… Is it possible you can press the pinion gear from a fisher price motor and drop the RS 550 into the Fisher price gearbox? The added power should be enough.

No, no, we have an FP, from the KoP, I meant that the only additional motor to that was the RS550.

In relation to a (or two) window motors, how fast and/or powerful is an FP on the gearbox? What is the output shaft of said gearbox? It would be a major plus not to have to take our entire winch assembly apart to lengthen the shaft on it.

Actually, does anyone have a spec sheet, or CAD (inventor) of the FP gearbox?

Try looking at this document, which explains how to calculate forces and how to pick the right motors and gear ratios for your mechanism.

In short, if you’re pulling 10 amps, you’re running at about 20% efficiency (based on the motor curves), which means 10 A * 12 V * 80% = 96 Watts getting turned into heat with nowhere to go. Generally, you should run motors close to their maximum efficiency, which for the Denso window motor would be a torque of about 2 N-m and a current of 5A.

The fisher price is about 12 times more powerful than a single window motor, or 6 times more powerful than two window motors in tandem. With a max torque approximately 3500 oz inches at the output of the gearbox at 40 Amps There is no output shaft at the end of the gearbox. It has a weird drum that it drives. The off the gearbox free speed is faster than the window motor.

The JVN calculator has the actual FP gearbox reduction. While

Question, How long before it randomly stops? If it is not too often, could you possibly cool the motor with the muffin fans?

We are able to go up and down twice before it stops.

I wonder if the fans would do enough?

It’s unlikely that we would ever be jumping around like that at competition, but you never know. Maybe with two motors, and a fan, it would be better.

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

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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?

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.