Is it okay to run one motor in a two motor transmission?
 

More specifically, say I have this two motor transmission, and for some reason (not significant to this topic) I want to only run one motor and not pull out the other -- only running one of the two motors. Is this safe to do? Am I going to melt wires? victors? motors? the robot?

Or what if I didn't enable the back-EMF in the victors, or even put a spike in between the victors and motors, and had that spike set to off? will that make it resonable safe?

Thanks,
-Jonathan

Re: Is it okay to run one motor in a two motor transmission?
 

As long as the motor can back drive, you should have no problem. We do it all the time, we just pull out the breakers for the motors and it runs fine with only 1 motor in the transmission. Just alittle info: We use the CIMs and drill motors with the back drive prevention pin pulled out. So if u have a similar setup I see no problems.

Re: Is it okay to run one motor in a two motor transmission?
 

Jonathon,
There is one critical thing to consider, the Victor is still connected to the unused motor. If it is jumpered for "brake" it will provide a dynamic load for the voltage generated as the unused motor is back driven by the transmission. If the Victor is in "coast", the motor will drive back voltage into the output of the Victor. Under most cases this shouldn't be a problem, but it is possible (not probable) that the motor could develop a high enough voltage to take out some power FETs. Do not wire a spike in series with the Victor, although it acts like a relay it is most definitely not.

"drill motors with the back drive prevention pin pulled out."
Ken,
I am assuming you are talking about the roll pins in the drill transmission that locks the output shaft. Thanks for reminding everyone, rookies especially, that these pins should be removed before using the drill transmissions.

Re: Is it okay to run one motor in a two motor transmission?
 

Hopefully Al will stumbe along this thread and give us a precise detailed answer ;) I've always wondered what happens to the electricity generated by a spinning motor (essentially a generator I suppose) that is not connected to anything. Does the electricity (for lack of a better term) dissapate into the air?

EDIT: Looks like Al beat me to the thread :) But I'm still wondering about my question.

Re: Is it okay to run one motor in a two motor transmission?
 

Back in 2004 UCF regional, on thursday evening our robot was 138.5 pounds. we had to take off weight from that robot. we had 2 motor transmissions... we took the chipua motors out... it was fine. :)

Re: Is it okay to run one motor in a two motor transmission?
 

Thanks for you help guys...

In theory we're just going to run the motors straight, no drill gearboxes (we remove the back-drive pins anyways when we use them). I was worried about the current generated through the non-powered spinning motors. I know that we should keep the braking setting off on the Victors, but even so I was wondering (like what sanddrag said) where does all that electricity generated through the motors go?

And what's the potential risk of using a spike in between the victor and motor? I saw a thread or whitepaper saying that using that setup would be the safest setup to run a motor (we never do it, but it does sound like a good idea, aside from the 20amp fuse on the spikes).

Re: Is it okay to run one motor in a two motor transmission?
 

The motor develops a "voltage potential" i.e. a measurable voltage, that is proportional to the RPM (DC brush motors). However, there is nothing dissipated just like the voltage potential at your wall outlet doesn't go anywhere. There is a small electrical principle known as 'open circuit voltage" that occurs. Since the voltage generated is not causing a current to flow, no voltage is dropped across the internal resistance of the motor. (essentially the resistance of the miles of wire in each armature coil) Since no voltage is dropped across the internal resistance, the voltage at the motor terminals can be pretty high. There are a lot of variables involved in generating voltage so it is hard to predict what open circuit voltage each motor will develop.

Re: Is it okay to run one motor in a two motor transmission?
 

If you pull the breakers AND set the Victor jumpers into "coast" there should be no problems what so ever, as long as you're not using any worm gears (yuck).

Re: Is it okay to run one motor in a two motor transmission?
 

It looks like you received good advise from others already on most points so I'll address your last question...

Do not do this.

The Spike is not a mechanical relay, but (I believe) it is a solid state relay. The spike is intended to run at a full 12 volts. The Victor would put out a variable voltage which is, in fact, a chopped DC waveform. The electronics in the Spike would not likely survive. Moreover, the Victor can put out negative voltage (reverse). This would definitely fry the Spike.

Whatever thread or white-paper you saw gave you bad advice.

Re: Is it okay to run one motor in a two motor transmission?
 

Thanks for the explanation, Mike.

What if I wanted to shut down a motor during competition? I can't just walk onto the field and rip out a breaker... Would just setting the victors (victors set to "coast" mode) to give 0 volts work?

Re: Is it okay to run one motor in a two motor transmission?
 

Turning off the motor would be easy enough to do through programming. As long as the victor's brake function is set to coast, you won't have any back EMF problems (probably). Just hit a switch, and you're on half power.

Reminds me of those V8 engines that, when there is no great load on the engine, shuts down every other cylinder turning the engine into a v4, saving some gas.

-Andy A.

Re: Is it okay to run one motor in a two motor transmission?
 

Alright cool. Thanks for the help...

I remember that too, i think the new dodge (magnum?) hemi V8's had that function to cut-out fuel for four of eight cylinders when cruising on the highway.

Re: Is it okay to run one motor in a two motor transmission?
 

The breakers are irrevelant so long as the jumper is set to coast and you are giving the controller a neutral signal. You don't need any extra relays etc.

The spikes are definately mechanical. You can hear them click if you listen closely. I have even heard one make a machine-gun like noise when toggled very fast.... then it died after a few seconds...oops

Re: Is it okay to run one motor in a two motor transmission?
 

I stand corrected... However, I stand behind my feeling that the reverse voltage and severe brownout conditions you would get by putting a Spike in series with a Victor would permanently damage the Spike.

Re: Is it okay to run one motor in a two motor transmission?
 

Rick,
This from the Spike Blue Data sheet..."Spike is an H-Bridge relay module custom designed for Robotics applications." I believe the "H Bridge" implies in this particular device, a solid state device. There is simply not enough room for four relays in that little box. The clicking you heard was likely the components failing.
Using a Spike in series with a motor in our kit, is redundant since the Victors give such complete control over all motor functions. You may have seen an application where a Spike was used to turn "coast/brake" on and off. If you check the rule book you will find that spikes can only be used on certain motors.
"5.6.3 Relay Modules
For information about the Relay Modules, refer to the Spike Users Manual available on Innovation First’s
website.
Warning! Attempting to drive the Drill motors, Van Door motors, Globe motors or Fisher-Price motors
directly with the Relay Modules could damage the Relay Modules and is, therefore, prohibited.." Add the Chalupa to that list as well.
In addition to that, common sense practices dictate that you want a minimum of components between the controllers and motors, adding a relay module adds that much more wire, another failure point and a bunch of connectors to the circuit. All of which are bad.
It is not necessary to pull the breakers unless you are unsure of your software preventing drive to those motors you don't want to use. The breakers are there to protect the wiring and Victor from flaming during a catastrophic failure. Voltage from a turning motor backfeeding the Victor will not be protected by the circuit breaker upstream from the Victor.

Re: Is it okay to run one motor in a two motor transmission?
 

I'm not so sure about this. It may be all electronically operated but there is definitely something in there that makes a noise when operating (as far as I know) correctly. Everytime we've "triggered" a properly working spike on my team we've heard it click. One interesting thing is this relay is listed as solid state whereas the Spike is not. But I don't know anything about H-bridges so I have no clue.

Re: Is it okay to run one motor in a two motor transmission?
 

If I can remember, I will open one tonight. Are you still using one of the old Spike modules from a few years back? Those were relays as I remember. The "H" bridge is the same as is used in the Victor, effectively it is four switches only two of which are turned on at a time. Think of a capital H where the motor (or load) is the crossbar and the four switches are the lines above and below the cross. The top of the H is connected to battery + and the bottom is connected to battery -. If the upper left and lower right switches close, then the motor sees current flow in one direction and turns forward. If the lower left and upper right turn on then current flows through the motor in the opposite direction and the motor reverses. If both the bottom left and right switches turn on then there is a closed loop to battery - shorting out the motor for "brake". If all switches are open then no current flows and the motor "coasts".

Re: Is it okay to run one motor in a two motor transmission?
 

Al did a good job of explanation. Here is the spec sheet for an H bridge driver I was designing into a product some time ago: http://www.st.com/stonline/books/pdf/docs/10386.pdf

This device is made for automotive applications and can switch 30A of DC current. The auto industry uses them to control window motors, seat motors, et cetera.

Re: Is it okay to run one motor in a two motor transmission?
 

Cool!
Under reverse battery protection, many people use a diode reverse biased across the battery input. It will only turn on when reversed and have no forward voltage drop to affect the elctronics. Hook it up backwards and the diode blows the fuse.

Re: Is it okay to run one motor in a two motor transmission?
 

Al and Mike Betts from previous. I think the brake/coast jumper serves the function of the FET at the bottom of mike's schematic, providing a dissipation path for the "reverse emf" or back current of the off but driven motor. I did a quick review of the InnovationFirst docs on the 884/883 victor and the section on the brake/coast jumper talks about using limit switches to do in operation change from brake to coast if the hold feature of brake is desired. In final note. Wouldn't an open circuit present an infinite resistance therefore represent a total current drop. The current would be zero even though the armatuer coils are breaking the magnetic fields inducing electron flow against an infinite resistance. Nough for now. LRU

Re: Is it okay to run one motor in a two motor transmission?
 

This is getting a bit confusing because we are talking about both Spikes and Victors.

First of all the Spike does not have a brake.

A brake is usually an active control where both lower side FETs are turned on (LSa and LSb) and both uppers (HSa and HSb) are turned off. When EMF is produced by the motor (acting as a generator) one of the two lower FETs would conduct and the opposite FET's anti-parallel diode completes the circuit giving the short circuit.

This same effect can be made with a N.O. relay across the output. It has the benefit for FIRST robots of continuing the brake action after power is removed. I believe that IFI uses the relay method.

I believe the FET at the bottom on that H bridge diagram is to give an active clamp across Vcc is the input voltage is reversed. The FET is turned off if Vcc is normal or if Vcc = 0.

Re: Is it okay to run one motor in a two motor transmission?
 

Okay wait,

Putting aside the mechanical vs. solid state debate aside... I know it was stated that as long as the victors are jumpered for "coast", i could have the motor spin freely without having to worry about some catastrophic detonation somewere down the line between the motor and the battery. Or will I? I have some itchy feeling that if I had only one of two motors in a dual motor gearbox running, the other motor (non-powered) will somehow generate enough current to cause the victors to transform into a fried $100 dollar paper weight...

Re: Is it okay to run one motor in a two motor transmission?
 

No. That will not happen. As long as you have the coast jumpered and the backdrive pins defeated, you will be OK...

Re: Is it okay to run one motor in a two motor transmission?
 

Ok, thanks.

Re: Is it okay to run one motor in a two motor transmission?
 

Larry,
The limit switches mentioned in the IFI docs are simply a microswitch with contacts wired to the brake/coast input on the Victor. A servo is then used to energize the microswitch giving complete control over brake. We have used this method many times in the past. As to the infinite resistance question, you are correct, no current flows when the coast jumper is selected. With no current flow, there is no way for the current in the armature to develop back EMF (Electro Motive Force) and therefore no opposing force to movement coupled to the armature. Mike's schemo is a device he was working on, not the IFI device. The Victor does not have a relay in the output and no low side FET in the common lead.
As a side note to everyone, a reminder actually. Victor circuits are open to the outside world to allow cooling air to flow. Metal chips can and do fall into the Victor and permanent damage is the result. When working on your robots, make sure all metal chips are removed first and cover the electronics with a drop cloth when you drill on the robot.

Re: Is it okay to run one motor in a two motor transmission?
 

I opened one of the Spike Blues last night. The black box sticking up is where all the power is modified. It has several pins on it but not enough to be individual relays. It is a sealed box none the less. I think IFI will have to give the answer on this one. It could be a special relay or solid state device.

Re: Is it okay to run one motor in a two motor transmission?
 

For those interested, the following link has a good discussion
on H-Bridge circuits of both the relay and solid state type.

http://www.dprg.org/tutorials/1998-04a/

Re: Is it okay to run one motor in a two motor transmission?
 

Doc,
Very nice find! For those who will read the article, in the final drawings just substitute three power FETs for each of the transistors and you will have the Victor output circuit.