1 Victor 2 Window Motors

[wiiking123]

Now that my Team's season is finished, It is time for us to build an air cannon. We are sort of limited on funds, so I was wondering if it is SAFE to operate 2 window motors off of 1 speed controller. Its likely not FIRST legal, but in general, would I blow up my victor easier by using two of the window motors per 1 victor.

[drakesword]

Now that my Team's season is finished, It is time for us to build an air cannon. We are sort of limited on funds, so I was wondering if it is SAFE to operate 2 window motors off of 1 speed controller. Its likely not FIRST legal, but in general, would I blow up my victor easier by using two of the window motors per 1 victor.

Is it more likely? - Yes

Will it happen? - Depends on the load you put on the 2 window motors. If you bring them to a stall hopefully you have a breaker in the mix that will trip. Seeing that 40A is the max and victors have been driving that load for years I wouldn't see it as much more of a problem then driving one CIM at stall.

I personally have not had experiences doing so but logic seems to support that it would be ok. You need to account for the initial current spike is going to be greater or even double depending on the situation.

EDIT
And no it is not FIRST legal but that cant stop you off season.

[apalrd]

We do it on our practice bot. It works fine.
The stall current of each motor is about 20 amps, so stalling both motors is still only ~40 amps or so.

[wiiking123]

Also, would it be better to put a 20 AMP or a 40 AMP breaker on the victor? Seems that 20 AMP would be better in the situation one stalls but the other does not for some reason.

[Mark McLeod]

Breakers protect the wiring from overheating, melting or setting fire to things. They do not protect the electrical components, like victors.
Be sure to use 12 AWG or better wire for a 40 amp circuit.

A 20 amp breaker is too low for a pair of window motors, since the combination will pull around 40 amps, so I'd use a 40amp breaker.
A pair of window motors will not stress a Victor, which is conservatively rated for 40 amps continuous.

The built-in thermal protection in the window motors themselves will contribute too if you overwork them.

[PAR_WIG1350]

Breakers protect the wiring from overheating, melting or setting fire to things. They do not protect the electrical components, like victors.
Be sure to use 12 AWG or better wire for a 40 amp circuit.

A 20 amp breaker is too low for a pair of window motors, since the combination will pull around 40 amps, so I'd use a 40amp breaker.
A pair of window motors will not stress a Victor, which is conservatively rated for 40 amps continuous.

The built-in thermal protection in the window motors themselves will contribute too if you overwork them.

I understand why one doesn't want wires to burn up, but why aren't breakers used to protect components as well?

[Mark McLeod]

I understand why one doesn't want wires to burn up, but why aren't breakers used to protect components as well?
Because they aren't designed to do that job.

Circuit breakers, like the ones in your home or on our FRC robots, are designed for wiring. They react somewhat slowly and actually allow over-current, that could fry electronic components, for a given period of time corresponding to how long your wiring will take to dangerously heat up. They react faster as the current surges higher, but still are only trying to avoid wire meltdown.
Home circuit breakers are designed to prevent fires inside your walls, not to protect your refrigerator. Breakers are designed knowing that multiple devices of unknown characteristics will use the wiring they protect over a period of many years. Many of the devices that they will eventually provide power to haven't even been thought of yet. Your home appliances all have their own individualized protection designed in. They all have to watch out for themselves, because they know what their design limitations are.

Electronics require custom protection with much faster response to protect delicate circuitry, and they don't like over-current at all. Window motors, on the other hand, only worry about heat buildup, so different devices require different protection solutions to be designed in.
The internal PTCs and special fuses in our electronics react very fast, at speeds required to save that particular circuit design and the more delicate electronic components.
Our window motors have thermal protection built-in to avoid meltdown in the motor, because it's a buildup of heat instead of high current that's the problem being solved, while a CIM solves the problem with more mass that absorbs much more heat and heavier gauge motor windings.

It all comes down to what each breaker, wire, electronic component is designed and rated to handle.
In this case, circuit breakers have amp ratings that are matched to wire gauges. Those amp ratings can be used to determine how many home appliances or robot devices can be supported on a single circuit, but they are not matched to protect each individual electrical component on that circuit.

With enough variety in fuse/breaker values to choose from we can approximate custom circuit protection. A good example this season was the Tetrix motor and the battery with it's 20a fuse. The 20a fuse protected the wiring from over-current drawn from the battery, not the smoking motors. We could compensate by substituting a much lower rated fuse, however, we'd have to change the fuse rating based on how many motors we had, essentially custom designing a fuse circuit to match our application, and it was a suboptimal solution because it would burn out over time anyway.

[Alan Anderson]

I understand why one doesn't want wires to burn up, but why aren't breakers used to protect components as well?

In general, circuit breakers can't protect components. If a component is drawing more power than it is expected to, it is probably already broken.

[Al Skierkiewicz]

While there are breakers that can provide the protection you would hope for, they are also very complex and expensive. I agree with Mike, a 40 amp breaker should do the trick. However, if the two motors are linked mechanically, feeding both from the same victor may prove to be too frustrating in the end. The production standards are not tight enough to allow any two motors to run at identical speeds with the same drive. Although linked together, the difference may just damage one or more motors.

[Ether]

if the two motors are linked mechanically, feeding both from the same victor may prove to be too frustrating in the end. The production standards are not tight enough to allow any two motors to run at identical speeds with the same drive. Although linked together, the difference may just damage one or more motors

If the motors are linked mechanically, then they will run at the same speed. But due to manufacturing tolerances, they will not share the load (torque) exactly equally. As long as you design with this in mind and allow enough margin so that the more-heavily-loaded motor is not overstressed, there should be no motor damage.

[Al Skierkiewicz]

Strange as it sounds I have had teams report twisting shafts and breaking worm gears with this configuration.

[Ether]

Strange as it sounds I have had teams report twisting shafts and breaking worm gears with this configuration.

That doesn't surprise me at all. The motors were probably substantially overloaded.

[Alan Anderson]

If the motors are linked mechanically, then they will run at the same speed.

Not if they break the mechanical linkage or one of the gearboxes...which I've seen happen more than once when teams try to put two window motors on the same shaft. I'm pretty sure the underlying cause was electrical issues in both cases I was asked to look at.

The motors were probably substantially overloaded.

If you count trying to backdrive a window motor as "substantially overloaded", then every case of connecting two window motors together has that potential.

[Ether]

There doesn't seem to be much disagreement here.

If the mechanical linkage breaks, the two motors will not run at the same speed. I thought that went without saying.

If one motor is underpowered due to electrical issues and the other is trying to backdrive it that could certainly overload the powered motor.

[FrankJ]

In the case of breaking linkages, I suspect one motor was broken & not turning, wired so the motors were turning the in opposite directions, or the linkage was under designed to begin with.

I would think you would be more likely to get balanced torque with one victor feeding the same voltage to both motors, rather two victors with the same PWM signal to different motors. Especially if one gets overloaded & trips the breaker.

[wiiking123]

Note that these 2 window motors WILL be linked by the same shaft. Although, they will not be turning more than 60 degrees in either direction (the intended use was to be able to move an air cannon barrel up and down). If they are only moving 60 degrees, would I still have the high risk of twisting the shaft? If so, could it possibly be beneficial to simply use 1 window motor to move the shaft?

[EricVanWyk]

To weigh in on the 1 victor 2 motors 1 shaft issue:

This is no more dangerous than a 2 victor 2 motors 1 shaft set-up (and might be slightly more forgiving in some circumstances). I'm not saying that running two window motors on 1 shaft is a good idea: I am saying it is an equally bad idea (but not a horrible one).

[Ether]

Note that these 2 window motors WILL be linked by the same shaft. Although, they will not be turning more than 60 degrees in either direction (the intended use was to be able to move an air cannon barrel up and down). If they are only moving 60 degrees, would I still have the high risk of twisting the shaft? If so, could it possibly be beneficial to simply use 1 window motor to move the shaft?

If you've done the math and determined that 1 window motor will not be overloaded then yes it would be beneficial to use just one motor.

If however 1 window motor would be overloaded, then using 2 motors driven by 1 Victor would be better - but as has been previously mentioned, that setup is not ideal either, because it is vulnerable to cascading failure modes should one of the motors fail.

[EricVanWyk]

Would removing the anti-backdrive pins help?

[Mark McLeod]

Removing the pins might defeat their purpose of holding the barrel in a particular position.

[Alan Anderson]

Would removing the anti-backdrive pins help?

Based on my very limited experience, that wouln't help much, if any. They seem designed to make it truly impossible to backdrive the window motor when they are present, but the worm-gear stage is still almost impossible to backdrive with them removed.

If one window motor isn't quite powerful enough for the job, you might be able to get away with using a friction-slip coupling to get enough assistance from a second motor to make things work while avoiding the potential of breaking things when the motors don't quite move in perfect unison.

[wiiking123]

It may seem very basic, but could using something like rubber tubing as a layer between the shaft and the coupling attached to the SLOWER motor work good enough to avoid twisting the shaft and burning out the motor but still have the motor provide some power in turning the shaft?

[WizenedEE]

If you removed the anti-backdrive pins from one of the motors, then it would slow down/speed up to match the other motor, while still providing torque. Theoretically, that is.

Why don't you just gear down the one motor instead of using a second one? Making gearboxes is a much more valuable skill than screwing together PVC. It would also be lots of fun.

[wiiking123]

If only I knew how to make gearboxes. If anybody has any resources they know of like how to design a gearbox, it would be much appreciated! I'm kinda semi alone on making this cannon due to the lack of post-season team members so there are limits one person can do. :D

[Al Skierkiewicz]

Guys,
Let's remember that many teams this year linked two motors together but the difference is that each was fed separately and could be trimmed for differences between the motors. When fed by only one controller, you are stuck if the motor performance is radically different. How critical that difference is unpredictable. It is something you may just have to experiment with to prove. In this application, I think leaving the locking pins might be indicated.

[Gdeaver]

With 2 window motors and 1 shaft there is a possible problem with the locking pins if they are subject to shock loads. The pins on 1 motor could lock with out the other locking. Locking pins and position control with PID also do not mix well. Adding a spring, gas shock to balance the arm would allow 1 motor to do the job.

[FrankJ]

At St Louis, National Instruments had a tee shirt shooter, that a team built, the plans are on https://decibel.ni.com/content/docs/DOC-15868#comment-15229. It used one window motor set up as a winch to raise and lower the arm.

I do not think using one or two window motors in a direct drive will give you enough torque to rotate a shooter unless it was well counter balanced.

[11douglash]

Safe, yes. Legal, no.

I've seen Victor 884's support waay over 100 Amps (300, to be exact) for short periods of time. For Large periods of time, such current loads tend to be damanging (molten wires, ect)

Now, that being said, as long as you arn't doing crazy loading, the window motors probably are pulling only 4-5 amps combined. Even a cim only draws 2 amps freeload. A victor can handle 10-20 amps all day long without too many difficulties.

Mechanically speaking, even if the motors run at slightly diffrent speeds at the same voltage/amperage, it'll be fine. My team hooked 2 windows together to make a winch for 2011 build... and it was very reliable. and powerful.

[Al Skierkiewicz]

Doug,
Window motors stall around 20 amps. There is a difference between the left and right hand motors. 18.6 vs 21 amps. What kind of load produced 300 amps?