Motor contoller max amperage
D Faust
Posts: 608
What happens if a motor is stalled and the motor controller can't supply the current the motor requires to the motor.· Does it loose torque? (amperage relates to torque and votage to speed right?)· Thanks
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D Faust
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D Faust
Comments
If your power source can deliver that stall current, then maybe your semiconductors will croak if they're not rated for that kind of action.· If your power source isn't up to it, but your semiconductors can handle it, then·"no harm, no foul" (sort of) --·if it's batteries they'll drain, of course, but if it's a power supply it may have some sort of current-limiting ("foldback", etc.) built in.
That's how I see it.
Update -- Of course, you don't want to·subject a motor to a stall current situation or dwell near there either.· At start-up you're overcoming inertia, a body at rest, and there's a lot of work that goes into that (more weight = more work) and so there's an initial current surge, but you want to avoid that stall current [noparse][[/noparse]too much weight or mechanical resistance] situation.
Post Edited (PJ Allen) : 7/2/2007 12:52:24 AM GMT
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D Faust
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D Faust
(but they're slow-acting)
...and you have to push the button (a thrill for button-pushers)
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D Faust
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D Faust
Since the starting current is generally more than the running current for most motors, time-delay (slow-blow) fuses are usually used. I wouldn't use fast acting fuses on any motor, period. Fast acting fuses are generally used to protect sensitive electronic components, and specifically semiconductors. They are also often used in the telecommunications industry.
If you decide to go with low voltage circuit breakers, make sure they are suitable for motor use. If not, they may not have the desirable time-delay characteristics.
Regards,
Bruce Bates
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P.S.· As of my original question: Will the motor run, at all or a reduced speed/torque,·if its amperage demands are not met (at stall torque)?
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D Faust
Some motor controllers are made from solid state components and some are not. Remember that you are ONLY protecting what's DOWN-STREAM of the fuse or circuit breaker. The solid state components are UP-STREAM of the circuit protection device if the circuit protective device is between the motor and the motor controller.
If a fuse or circuit breaker is not labelled it is NEITHER slow-blow nor fast-blow. It has no particular time curve characteristics. It is just a "standard" fuse or circuit breaker.
Please also be aware that some circuit breakers may be labelled for AC ONLY or DC ONLY. If they are suitable for AC or DC, they should be so labelled.
If the amperage demands of a stalled motor are not met, the chances are better than even that it will either destroy the power supply, cook the motor windings, or burn up the wires feeding it, unless there is a circuit protective device included in the circuit. Therein, it won't run at all, after a fashion.
Regards,
Bruce Bates
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D Faust
The PROPER way to do it is to have a motor controller which is sufficiently sized for the motor. I suppose you could use 5 amp slow-blow fuses at the motor, and they should blow within a second or two of the amperage reaching 6.7 amps or above. Hopefully the motor controller can withstand a temporary overload, but I make no guarantees.
What is the normal running current of the motor?
Regards,
Bruce Bates
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0.2 A amps is the rating for no load. Here is a link (http://www.banebots.com/pc/MS-25XXX-180/MS-25010-180)
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D Faust
Scan down this page for the motor in question, and I think you'll see that the stall amperage is .63 amps, NOT 6.7 amps. The 6.7 refers to the stall TORQUE!
http://www.dswbrand.com/electromotor.php
BIG difference!
Regards,
Bruce Bates
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Post Edited (Bruce Bates) : 7/2/2007 9:05:06 PM GMT
and normally runs at .2A.· Generally, is the stall current about twice the unloaded requirement?· Again I say, THANKS.
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D Faust
motor current = (supply voltage - back emf) / motor resistance
where back emf is proportional to motor speed.
When the motor is stalled, back emf = 0 and the
motor current is limited only by the motor winding
resistance.
There isn't a general rule for relating stall current to
no load current. The stall current can be many times the
no load current, depending on motor and bearing design.
phil
If I have no idea what the stall currrent is, but I do know the running current, I often use a 10 X multiplier just as a starting point. So, if the running current is .2 amps, I would estimate the stall current to be no less than 2.0 amps.
Without other information, sometimes you can look at a current vs. torque curve graph and fuse the motor just beyond the "hump" of the maximum torque curve. If it exceeds that, you can be pretty sure you have a problem and ought to shut the motor down.
I would guess the incorrect data was just a matter of grabbing the information from the wrong column on the manufacturer's specification sheet, since it's rather obvious from the data that I showed you.
Regards,
Bruce Bates
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Wouldn't it be no more than 2.0 amps becasue the motor in question runs at .2 amps and stalls at .63.
Phil,
Thanks for the info about the equation.· That should help me in the future for determining currents.
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D Faust
If you would like to quote me, please do so in its entiretly. Here is what I said by way of an EXAMPLE if I DIDN'T know the stall current:
"If I have no idea what the stall currrent is, but I do know the running current, I often use a 10 X multiplier just as a starting point. So, if the running current is .2 amps, I would estimate the stall current to be no less than 2.0 amps."
Thus, I would fuse it at NO LESS than 2.0 amperes in this EXAMPLE. If the running current for the motor you have chosen to use is .67 amps, then you would need to fuse it at NO LESS THAN 6.7 amperes. Since 6.7 amperes is neither a standard fuse or circuit breaker size, I would round it up to 7.0 amperes and protect it there.
Alternatively, if you choose to round it down to the next nearest standard fuse size 6.25 amperes is a standard fuse size. Half an amp or so either way isn't going to make much difference since this is just an estimating technique.
In your case you KNOW the stall current and can fuse it at that value (or close to it) if you wish to. In your case you can check to see what the closest standard fuse size is to .67 amperes. Personally, I generally fuse motors at their maximum starting current and not at their stall current.
Regards,
Bruce Bates
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Post Edited (Bruce Bates) : 7/3/2007 2:50:50 PM GMT
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D Faust
If you think about the practical reality of such an equation, you might come to the conclusion that there could be no such thing. The starting current (other than no-load) is dependent on way too many factors for any equation to be practical. Here are just some of the consideration:
Environmental -
temperature, humidity, and barometric pressure
Mechanical -
Present load on the motor (which may vary from start to start).
Age of the bearings or bushings on the shaft(s).
Condition of the brushes, commutator, etc.
Presense or absense of approprate lubrication.
Electrical -
Starting voltage, available current, possibly the frequency of the line (usually synchronous motors), possibly power factor, and the condition of the circuits supplying the motor.
There are probably dozens of more considerations that are not mentioned above. A motor manufacturer could offer a no-load starting current, but unless you're using a clutch-driven system, that's hardly a real world factor.
Empirical testing is your best asset. Always err on the low side, and no magic smoke will be released. If this is for a hobby application, then I wouldn't worry nearly as much as you seem to be. If it's an industrial or commercial application then you need to dive in there with an ammeter to see what the real parameters are. Then you have something realistic with which to work.
Lastly, there are motor control chips which will allow you to interrogate the current at any given point in time. That way you can interrogate the amperage draw of the motor and make dynamic decisions based on that. Even short of that, you could include a "sense" resistor in line with the positive lead to the motor, measure the voltage drop across the resistor and derive the instantaneous current draw from that. The dual full bridge driver (L298) is just such a motor control chip, with "sense" outputs. A copy of the datasheet is attached.
If you want to know more, just ask, as this is quite a vast subject, and not one that can be covered in just a few questions. If the truth were to be known, we are really dealing with SAFETY ISSUES here, which includes but is not limited to fire prevention. A stalled motor given sufficient voltage and amperage can easily burn your hand if you touch it, and may burn other things around it, before the windings turn from TOASTER-mode to POOF-mode!
Regards,
Bruce Bates
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Post Edited (Bruce Bates) : 7/3/2007 6:47:00 PM GMT
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D Faust