Interesting…so just for fun I connected that RS540 motor to a switch, then to the 12v (at 8 A) output of an old 200W computer switching power supply. I turned on the supply, and the motor jumped and started turning, and kept turning.
This motor is rated at 0.95 A no load current.
Would the current spike at startup of the Chalupa (CIM) motors be any less than 95 A if they were under control of a Victor, as in real life? (in the situation of no mechanical load)
Squirrel,
The stall current for any motor is the start current since a motor that is not moving is by definition in stall. With no load the current spike is rather short and for this motor, the stall current is only 42 amps even though the no load is 1 amp. (from the Banebot website) Even a motor with a transmission that is not connected to anything still has some loading so it is likely that the computer power supply would not be able to start a motor coupled to a transmission. The caps in the output of your supply are the part of the power supply that is delivering current during that critical startup. If the current demand was higher or the value of the output caps were lower, the result would not be the same. What would be interesting is looking at the power output on a scope when you start the motor. This should show a significant drop in the output during the start time of the motor. The Chalupa motors are much higher current having 129 and 133 amps stalled respectively, as I remember. Run current, no load, is a little higher but the big difference is the amount of mass at start compared to the Banebot RS540. The armature of the larger motors may weigh more than the whole Banebot motor.
That sounds about like how I thought it worked.
Can the stall current be estimated by applying Ohm’s law, using the resistance of the motor, and the nominal voltage?
And could one add a large capacitor to the power supply output and be able to get a CIM turning?
as for the torque to turn a transmission, if you wanted to test the electronics, there really is no need to have the motor installed…my understanding is that the original question is how to test just the electronics without the hassle of having to deal with a battery. If the robot is assembled, the battery is not a hassle to deal with, as it is mounted in the robot, and easy to connect.
I put the setup on a scope, and as you predicted, the power supply voltage does drop significantly, to about 6 volts as the motor is switched on. Also the filtering at the switching frequency gets pretty bad.
I don’t have a storage scope, so I can’t offer any pictures.
The series resistance of the motor is also complicated at startup with the inductance of the windings and the back EMF developed in the armature. The tough part about brush type motors is that generally, more than one winding is contacted by the brush assy. Due to this switching, the series resistance and inductance changes as the armature turns. I believe for testing the factory takes several readings and then takes the highest one for stall current. Series resistance is also affected by the shape and wear of the brushes and a variety of other factors. To measure the series resistance takes a device that will measure down into the fraction of an ohm range. A typical Fluke will not get you close to the actual resistance. At 100 amps at 12 volts, for example, the series resistance would be 0.12 ohms. (The contact resistance of the leads could affect a reading that low.)
It is possible to add capacitance to the output but there is a down side to that. The inrush current for the power supply goes up as you add to the output. That could pop the fuse or cause the supply to clamp for over current when you turn it on.
The best method of testing is a battery. You can breadboard a test setup using all of the electronics, breakers and the motors(sufficiently secured to the board) and check everything that way.
Well, if you were to create a rectifying circuit to invert the AC to DC, you would need quite a bit of power from the AC line if you are going to run the entire bot off the line.
If you can find components rated for the current that you are going to draw, you can build a simple inverter using a transformer, rectifier (Diode Bridge), capacitor, and voltage regulator. The last two aren’t absolutly nessecary, but they help to even the output voltage of the rectifier.
Personally, we use a car battery with a set of jumper cables to run motors before we wire the whole thing. This is convienient because we can just raise or lower the screw mech with out needing to install victors, an rc, breakers, and wire the it all.
To run our lights this year one of our mentors helped us rig up a system that had the battery on the charger as it delivered power. It served as an AC power supply for the light, but the battery protected the system from inconsistencies in the AC supply from the charger.
Ed,
That is a still a chancy situation. In most cases the battery provides enough stabilization to keep the charger from producing higher than normal voltages. Keep in mind, though, that chargers still must output a voltage higher than the battery in order for charge current to flow to the battery.
Raptor9,
Please be advised that a simple rectifier fed from the line voltage still has the ability to place the supply at 110 VAC above ground depending on the way the electrical power is wired. Transformers of some type are needed to isolate the power line from circuitry that humans can contact.
A DC battery charger will NOT run your robot. It uses low current to charge the battery and is not advised to try and run you bot off of it.
Unless you have superior knowledge of AC-DC PSU construction, don’t make your own.
Cough up the cash and buy a nice PSU.
For test boards, with little equipment
I mean, for a test board, running like 1-2 motors, use this! ( I doubt that the large motors use 40 amps)
http://myrcsupply.net/index.php?main_page=product_info&cPath=22_229&products_id=13383
or for 2-4 use this
http://myrcsupply.net/index.php?main_page=product_info&cPath=22_229&products_id=13008
They are equivalent to a car battery
Don’t worry about blowing a fuse on the PSU’s…
For the WHOLE robot
I like this one http://www.v-infinity.com/adtemplate_child.asp?c=1002843&p=1002840&catky=764537&subcatky1=46887&subcatky2=320934
It has what I believe to have sufficient current for the bot, and a lot of safety features built in, preventing damage top you, the bot, and the PSU.
Hook up a current meter to the bot and see how much current you draw with everything on. You’re going to want a PSU that can handle a little more then that amperage.