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wiiking123 28-04-2011 22:56

1 Victor 2 Window Motors
 
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 28-04-2011 23:21

Re: 1 Victor 2 Window Motors
 
Quote:

Originally Posted by wiiking123 (Post 1057345)
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 28-04-2011 23:23

Re: 1 Victor 2 Window Motors
 
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 28-04-2011 23:29

Re: 1 Victor 2 Window Motors
 
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 29-04-2011 08:31

Re: 1 Victor 2 Window Motors
 
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 01-05-2011 13:49

Re: 1 Victor 2 Window Motors
 
Quote:

Originally Posted by Mark McLeod (Post 1057418)
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 01-05-2011 14:20

Re: 1 Victor 2 Window Motors
 
Quote:

Originally Posted by PAR_WIG1350 (Post 1057682)
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 01-05-2011 16:07

Re: 1 Victor 2 Window Motors
 
Quote:

Originally Posted by PAR_WIG1350 (Post 1057682)
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 02-05-2011 11:28

Re: 1 Victor 2 Window Motors
 
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 02-05-2011 12:16

Re: 1 Victor 2 Window Motors
 
Quote:

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 02-05-2011 12:22

Re: 1 Victor 2 Window Motors
 
Strange as it sounds I have had teams report twisting shafts and breaking worm gears with this configuration.

Ether 02-05-2011 12:31

Re: 1 Victor 2 Window Motors
 
Quote:

Originally Posted by Al Skierkiewicz (Post 1058189)
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 02-05-2011 12:35

Re: 1 Victor 2 Window Motors
 
Quote:

Originally Posted by Ether (Post 1058188)
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.

Quote:

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 02-05-2011 13:15

Re: 1 Victor 2 Window Motors
 
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 02-05-2011 13:48

Re: 1 Victor 2 Window Motors
 
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.


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