Motor Controller Help/Advice Needed

Using Arduino Motors, Mechanics, Power and CNC
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I have some electric wheelchair motors that I want to control, they are rated at 75A each, 24V. I have a few questions:

  1. Can I connect multiple motor controller to one motor to double the curent limits? I was specifically looking at http://www.ebay.com/itm/New-DC-15-24V-40A-Brush-Motor-Driver-Module-Drive-Board-80A-MAX-/251264559326?pt=LH_DefaultDomain_0&hash=item3a8088e8de#shId to have a continuous limit of 80A and peak limit of 160A.

  2. Does anyone have any suggestions as to any motor controllers to use or not to use?

  3. What would you recommended for continuous/peak current?

  4. Any other suggestions?

I am also considering making my own H-bridge. I have seen that it might be a challenge, but I am interested in learning electrical engineering, I am up for a challenge, and I might be able to have a heftier motor controller.

  1. Any circuit designs you would recommend?

  2. Any tips, websites, books, and/or such to help out with the process?

Here are some circuits and sites I found them on (I know that the exact part numbers they have wouldn't work). Would any of these work? Any improvements to make for high current? Any other circuits?

circuit 1:
http://www.mcmanis.com/chuck/robotics/tutorial/h-bridge/bjt-bridge.html

circuit 2:
http://www.nathandumont.com/node/249

Any help is greatly appreciated.

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Those circuits are the same (but pick suitable transistors because you're going to handle quite a lot of current), but the first one has optocouplers.
Those will protect your circuitry like the Arduino because that is then electrically isolated.

The !enable in the first can do the same as the PWM signal, and so can be used for PWM (speedcontrol) too.
The exclamation mark in front of a pin name or function means it is active when low.
Other ways of making this clear is a slash / or a line over the function name.
The person who made those schematics didn't know how to do this so he decided to call the function ENA* where the asterisk (and the way things are connected in the schematics) tells you it's active low.
Because it is active low, the PWM speed control will work backwards.
30% duty cycle will give you a high speed and 70 % will give you a low speed.
But that's no problem as you're going to control it with a program of your own so you can make it work like you like it to.

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So the first one would be safer for the arduino, thanks.

Another question:

  1. Can I have more then one transistors on each part of the H to have the load shared among them? Will the forward voltage double?
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I am also considering making my own H-bridge. I have seen that it might be a challenge, but I am interested in learning electrical engineering, I am up for a challenge, and I might be able to have a heftier motor controller.

Apparently MOSFETs can be paralleled and share a load, so you may be able to do the same with H-bridges using MOAFETs. Below is a discussion concerning DIY H-bridges that might be of interest (aka, not as easy as one might expect).

http://forum.arduino.cc/index.php?topic=53425.0

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The schematics you showed are basic ones. You will want to add capacitors to the power bus bars, and you will also need some form of current control, weather it is a simple fuse, Limiting resistor, or a current sensor and code. What ever you end up doing, make sure you use opto-isolation, gate to source and gate to driver resistors, and some form of current limiting control. You may also want to use a Mosfet Driver if your going to be paralleling Mosfets.

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By the gate to source and gate to driver, are you talking about the pull-up/pull-down 10k resistors, or is there another place you are talking about? I know that I need (if the motors don't have them) a capacitor between the motor leads, but where are you talking about? Between the the 24v line and where? For the fuse/current sensor/etc. would that go between the motor and the h-bridge or between the batteries and the h-bridge, or both?

I just thought of another question:

  1. There is a 30A circuit breaker in the battery wiring. Would that limit the current available to the motor so that a 30A-40A (continuous amp rating) h-bridge would be alright even though the motors can themselves can pull more than that?
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By the gate to source and gate to driver, are you talking about the pull-up/pull-down 10k resistors, or is there another place you are talking about?

I would place a 10k or 15k resistor across the source to the gate, or as in the first schematic you showed used a 10k. This keeps the gate pulled in the off position, and keep the mosfet from avalanching during the start up of the micro-controller or in the event that the driver fails. The gate to driver is the resistor that goes in series with the gate to the driver to reduce ringing of the mosfet, this is the 1k resistor in the first schematic. This value will change depending on how many mosfets you use and what you drive them with.

I know that I need (if the motors don't have them) a capacitor between the motor leads, but where are you talking about?

The caps on the leads to the motor reduce emf, you will also need caps on the input to the motor controller, preferably close to the mosfets. This is to maintain peak current.

For the fuse/current sensor/etc. would that go between the motor and the h-bridge or between the batteries and the h-bridge, or both?

Either or both, this all depends on what the intended use is. Most people place the current sensor after the controller on the negative. you can use a breaker or fuse, but for any reason if the current goes above the limit, the controller will be disabled and depending on the speed of the fuse or breaker, it may or may not be fast enough to protect the controller. I have built a custom motor controller that is for a dc brushed motor. It can handle 144v dc and 30 amps. This is the first prototype that will lead up to a motor controller for an electric car. I use a Lem Hal 300-s current sensor and monitor the current, reducing duty as needed. It will also use hardware to shut down the controller in the event of a over current event. For what you are doing, a simple current limiting resistor may be enough, though it will need to be big!

Hear is a link to some info about building a 24v controller that could handle about 120A if it had current control. The creator oversized the controller by about 5 time to keep it from needing current control.

http://zeva.com.au/Projects/Speedy/

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Thanks for all the help.

Any recommendations as to what continuous current mins. for a 75A motor? I have seen some places where the recommend twice or more of the motor's current rating (for me 150A or more) and other saying that motors usually don't get to the rated current and you only need like 10% more (82.5A). Which is more accurate to what is needed to safely run the motor? I figured I would probably get a current sensor to monitor/limit power available. If the current gets too high would I then lower the signal I am sending to it, or would I lower the voltage going to the motor controller?

  1. There is a 30A circuit breaker in the battery wiring. Would that limit the current available to the motor so that a 30A-40A (continuous amp rating) h-bridge would be alright even though the motors can themselves can pull more than that?

Adding on to that, do I need to monitor the current coming from the battery (if I leave the breaker in) to make sure I don't trip it and have my robot immobile?

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In dc brushed motors, current is directly proportional to torque. Voltage is directly related to the speed of the motor and will also induce the current. The amount of current that the controller needs to provide is the amount of power or torque you need out of the motor with in the limits of the motor. A dc motor can easly draw 8 times its rated amperage when stalled. If you are going to oversize the controller so that you do not need a current limiter, you will need a very large controller. If you control the current you need the amount of amperage to give your motor enough power to do what ever it is it needs to do.

If the current gets too high would I then lower the signal I am sending to it, or would I lower the voltage going to the motor controller?

With a Pwm controller, duty is what you would reduce to reduce the power output. By increasing the duty, you are effectively increasing the voltage to the motor witch induces more current or power to the motor. You will need a loop that monitors the current and reduces duty in order to maintain the current limit.

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tr24em:
Any recommendations as to what continuous current mins. for a 75A motor? I have seen some places where the recommend twice or more of the motor's current rating (for me 150A or more) and other saying that motors usually don't get to the rated current and you only need like 10% more (82.5A). Which is more accurate to what is needed to safely run the motor?

First off, for a motor this size, I wouldn't even bother trying to build an h-bridge to control it, unless I was an expert on h-bridge design (even then - probably not). If you're not, you'll likely end up spending more money blowing expensive mosfets than you would have had you simply purchased the controller outright.

You are mainly interested in one rating for the motor, and that's the stall rating. Stall occurs when the motor is idle (not turning) - when you apply voltage, the motor is technically stalled, so your motor controller will see (for an instant) the stall current. If this is higher than what the motor controller can supply, either the motor won't turn (because the controller will be in a fault condition), or the controller will burn out or be severely degraded over time (depends on how much you spent on your controller).

To measure the stall current on such a motor (assuming you can't find the specs from the manufacturer - try to look there, call them up, etc - first; it's just easier), remove the motor from the gearbox (you may need to find out how to do this first; some are easy, some are a PITA), then hook a multimeter across the terminals and set it for resistance reading. Slowly rotate the motor shaft, and take several coil measurements. Once you have your measurement, average them. That is the average coil resistance. Apply Ohm's law for the voltage you plan to run the motor at to get the current. That will be the approximate stall current. Add 15 to 20% more amps just to be safe.

Now - once you know that, then you can shop for a motor controller. Let me tell you this, though: For a motor that size, it won't be cheap.

Two recommendations:

Vantec - VANTEC Vantec RDFR, RSFR, RET, CDFR families

Roboteq - http://roboteq.com/brushed-dc-motor-controllers/nxtgen-controllers-selector

Again - expect to spend several hundred on such a controller for your size motors. Also again - that is cheap; you likely won't be able to build such a thing without extreme experience.

One last option - if you insist on building your own controller:

  1. Get yourself a DC-controlled DPDT contactor rated for the current needing to be switched.

  2. Connect the contactor in standard h-bridge fashion to control the motor direction.

  3. Get yourself some beefy logic-level n-channel mosfets; hook in parallel, double the current rating you need for the motor.

  4. Hook the mosfet bank on the low-side of the h-bridge (between it and ground). Hook it to a heatsink for insurance.

  5. Use the h-bridge contactor to control the direction of the motor, use PWM on the to control the mosfet bank to control motor speed.

  6. Be sure to set the PWM to 0 before switching direction of the h-bridge (unless you want your contact welded or worse).

Now note - I've left out more than a bit on the above, but that's the gist to make a cheap high-powered h-bridge that won't kill you in the wallet over time; the problem with trying to build your own h-bridge is that finding cheap and matching p-channel mosfets isn't easy, so most go for an all n-channel design; the problem there is, of course, the high-side drivers. So, you need some kind of mosfet bridge driver chip that can handle the everything your h-bridge needs, etc - and hope you don't blow much up along the way, wasting money, etc.

Honestly, if you want to do this right - you'll just take your motor measurements and purchase an h-bridge (unless you plan on learning h-bridge design and money isn't an object - if so and if not - go for it).

:smiley:

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Not to discourage you, but cr0sh is on to something. Building a big motor controller is no easy feat. What has been talked about in this post is just the beginning and unless you are well experienced in electronics and switching design, you will most likely spend more building your own vs buying one. That being said, if you must build your own because you just want to and money is not what is most important, I would suggest doing lots of reading, purchasing an oscilloscope and be prepared for many late nights and inevitably watching a bank of expensive mosfets and diodes going up in smoke, as most people at some point or another do so. There is so much involved, from switching time, mosfet drivers, emf, esr, ect...

I personally am doing the final tweeks and testing to my 1/10th scale motor controller prototype. When I finish I will build the 300 amp continues power-stage.
The real question is whether or not you are doing it to save money of for some other reason.

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I tested the motor how cr0sh said to (I connected a multimeter to it and slowly turned the motor untill it wanted to stop (just a short distance) and read the reading), it averaged on 20 tests 3.55 Omhs, I = V/R = 24/3.55 = 6.76056338, and with a little safety factor 10A. Does that sound right? I wasn't able to find anything about who the manufacturer is, the only thing to go by is www.cimmotor.com printed on the motor and some parts numbers that I can't find on the internet (CM808-075D). I sent an email to the wheelchair company asking what the stall current is, but they haven't replied yet, and I don't know if they even have that information. I found someone who said they have a CM808-075B that is rated at 75A so I assumed this one would be close, by bad. And now the 30A circuit breaker makes sense to me. I think I will just buy a motor controller. the biggest reasons for trying to make one was to learn how and get one hefty enough to do the job (if I did it right) Part of why I though building one would be cheaper was at first when I looked to see what the MOSFETs, diodes, and such would cost I didn't check the current ratings and such and looked at it and saw that they were $1-2 each and I figured I could make one for $10-$20, then I could take 10-20 tries before I lost any money over it and it would be a great learning experience, but at ~$5 a MOSFET...

Thanks very much to all who helped!!!

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It is possible if the motor is not really rated at 75A. I would think that it would be higher than that, but if you have a volt meter you could use the current meter which is uasualy about 10 amps and test the stall current at 6 volts. This will give you a good Idea what to expect at 24v as it will be several time greater.

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I think it runs at 0A!!! I found the new super motor, it will solve all of the problems of batteries wearing out quickly!!! I could make millions!!! The motor/battery pair that never runs out of power!!! all in a very sarcastic tone

How do you measure current with a multimeter?

I looked around online and tried a bunch of what they said, I tried connecting the prongs as close to the motor as possible (the wires go into the side of the motor, no exposed leads) (both + to + - to - and + to - - to +), at the battery, in between, having one side go through the multimeter rather then the wire, I tried both on the ground side and the positive side. On the last two tests the motor didn't turn, is there too much voltage drop in the multimeter and I need to run it at the full 24V? I was doing this with 12V just in case the math was way off it would be around 1/2 the amps. In all of these tests it told me it was at 0A every now and then in would say .1A, but then when it wasn't connected it would say .1A just as often as when connected. I did have the prongs connected to the 10A and ground on the multimeter.

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Using a high-tech testing method (connect a xA fuse and then try to stop the motor), it will blow a 10A fuse in about 10-20 seconds of grabbing it as hard as one can, but not at 15A, so I will guess that the continuous 40A/peak 80A motor controller will do the job just fine, along with the 30A circuit breaker and a current sensor to try to not trip the breaker or fry a controller.

Thanks again to all who helped me out!!!!!

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