Electrically Isolating Wiper Motors

[Jim Meyer]

Our students are having a very difficult time finding a way to electrically isolate their Valeo wiper motor from the frame. Is this a requirement if they only want to use one of the wiper motors?

I do want the decision makers to know our students have designed a significant portion of the robot with the expectation that this year we were getting motors that did not need to be isolated, as announced at the end-effector seminar. At this point, being forced to isolate the motor or find a replacement would require an extensive redesign and rebuild of the machine.

We will happily comply with any decision, I just wanted to attempt to communicate the effect this decision could have on our team.

Thanks,

Jim Meyer

[Mike Martus]

I am referring this answer to jim, as he is the expert.

It is however, my understanding that there is a problem if you are using two motors at the same time as they use a case ground. This ground also extends to the output shaft, and thus into your drive train.

I have talked about this with Mike McIntyre about this today.

Jim, is the official answer.

[Mike McIntyre]

According to Jim Zondag, the problem with shorting will only happen: "If 2 Valeos are used on separate speed controller outputs. However, any Valeo motor that is not isolated will connect the robot's chassis to the electrical system. Depending on the polarity of the motor power, your robot chassis would be either grounded or energized to 12 volts. FIRST always insists on having robots with isolated chassis for a variety of safety reasons. I recommend we do the same and make teams isolate these motors. Hopefully, this is not too much trouble for teams. It is important to realize that the output shaft is electrically connected to the case of the motor, so connecting the output shaft to metal shafts, sprockets, bearings, and chains can create a path back to the chassis." As far as the other motors are concerned, we did not write them into the kit materials list or address them in the How-To manual. They were mentioned only because we were told by the donating company they would be coming. I still believe that to be true, but I cannot say when. In retrospect, I know it would have been better not to even mention them. All I can say is that you will get them within 24 hours of when we get them.

[Jim Meyer]

If we are going to be required to electrically isolate our wiper motors from the frame, (I'm still unclear whether it is illegal or not) may we purchase and use plastic chain from a supplier other than a local hardware store?

Thanks,

Jim Meyer

[Mike McIntyre]

Jim Zondag and Joe Johnson have advised us that teams using only one wiper motor should not have to worry about isolating the motor; however, if two are to be used, at least one of the wiper motors must be isolated. Since we created this dilemma, plastic chain will be added to the additional materials list. As Jim says: "Creative use of wood and plastics" is called for; let us know if you need additional information about how to accomplish this.

[Jeff Waegelin]

Would it be possible to run two Valeo wiper motors on a single speed controller? Also, what other motors can be run off a single speed controller?

[Jim Meyer]

I'm guessing that the OCCRA officials will not allow 2 wiper motors to be connected to a single speed controller. The reason being, is that your stall current would exceed the 60 amps that the speed controllers are rated for. (I know a single drill motor can exceed this 60 amp level, thats why this is a guess and not a statement.)

I'm hoping the officials will allow two of any motor, other than the wiper motor and drill motor, to be connected to a single speed controller because their stall currents are all less than 30 amps. (the sum of two would then be less than 60)

I know this probably doesn't help much since I am not an official, but if I were a betting man....

[Jeff Waegelin]

Yeah, that's probably right. My only concern is the 30 amp fuse the speed controllers are connected to. Two motors in tandem would together draw more than 30 amps at stall, so that would blow the fuse, even if the Victors can take more.

[Mike McIntyre]

You guys know the enemy: make sure the stall current won't exceed the fuse rating. Our students ran 2 Valeos on a single speed controller last year in OCCRA and had no trouble: they put the 2 in series!