Measuring motor current (anyone tried this?)

[chuckmerja]

Any progress on this? We built a rough terrain bot with CIMS and we know we are at or beyond the design limits for our CIMS and Victors, but that is what we had, and so that is where we started. We have 4 motors being driven by 4 Victors and a 55Ahr deep cycle battery. We ran a few hundred yards and measured the temps on the motors and discovered 3 of them at roughly 105 deg F (on 82 ish degree day) but one motor was 160 to 180 deg F! That's definitely an outlier, and we need to look into this further. I really think we need to monitor and record amps on each of the 4 motors - simultaneously. I think that hall effect current sensors is where we are headed http://www.allegromicro.com/en/Produ...sors/index.asp any help would be great.

Chuck

[kaliken]

In 2010/2011 we used the CAN interface with jaguars to get a simple current draw measurement. We actually tied the current measurement to a simple cut off for our ball handler.

Perhaps it may not be exactly what you want but it should be a zero cost, quick test approach

[ratdude747]

I had an Idea for future use.

I wonder if you could use an infrared thermometer to measure temperature? the extra amps being drawn equates to extra energy being used, and the energy has to be going somewhere (conservation of energy), which if the kinetic output is the same, the only other place would be heat. if it is bad enough, you might get burned by the motor heat (I did by accident a long time ago).

It doesn't give you amps but it would tell you which motor is drawing more than it should.

[Ether]

Originally Posted by ratdude747

I had an Idea for future use.

I wonder if you could use an infrared thermometer to measure temperature?

Just tape or glue a thermistor to the motor case and run the signal to an analog input.

[philso]

At work,we use larger versions of the LEM Hall-effect current sensors shown in the link below in the 3-phase motor controllers we manufacture. They will work down to DC and will have sufficient bandwidth to detect the PWM current into your motor. The one shown needs a +-15V supply. You may have to add a simple R-C filter on its output before you connect to your DVM. Many DVMs, especially the less expensive ones, will give wildly inaccurate readings if connected to a PWM signal.

http://search.digikey.com/scripts/Dk...me=398-1049-ND


I have seen clamp-on type DVM's by Kein Tools at Home Depot that claim to work down to DC but I have never tried them.

Phil

[roystur44]

Take a look at this product.

http://www.batteryspace.com/compactd...upto50amp.aspx

We used this to tune our minibot motors and it worked out great. Reading the specs I think the analyzer is within your test range. We used this with a variable DC power supply.

Here are some uses:
Set speed control cutoff voltages and currents
Tune performance by comparing different setups of propeller, motor, gearing, speed control and battery
Measure a full battery charge and compare to spec when charger claims done
Check peak currents are safe for battery, speed control, motor, wiring and connectors
Predict airplane flight time based on ACTUAL conditions in your model
Check battery capacity and health and whether battery should be retired
Verify that the minimum voltage under load is within specifications
Check for wiring and connector power losses.



Roy

[Dale(294engr]]

Don hope I'm not too late with this for you to use else good for future ref.!

Contrary to what many here may have reasoned
120A CB is a great poorman's 1 milliohm shunt!! (actually ~.96milliohm)

consider if Rcb was not tightly controlled trip, current would vary likewise!

(caveate: defective breakers internal R varies significantly =poor candidate)

I've been using 120A CB shunt yrs, include it in my FIRST wkshops & demo's
(previously 60A =bit over a milliohm so calibration factor req'd incr absolute accuracy - but relative current stability is quite good enough, in practice)

Using any DMM on 200mV scale place it across the 120A CB terminals

(best = install jacks to accept DMM pointed probes for competition ease use)

(Harbor Freight DMM works fine w/ 1Meg Rint on 200mV scale)

(when CB is open DMM reads overrange but csuses no harm [i sprotected])

when closed read the DMM +_199.9 A / mV full scale
polarity useful for drain/ batt charge current monitor if batt charged in robot

resolution is .1A!!

convenient to keep track of Robot quiescent current draw ~2A

useful to run each of 4 CIMs noload off floor as gauge of overall drive friction
.. motor, motor bearings& alignment, brush/commutator, gearing, chains, wheel bearing alignment, keeping in mind at zero friction current draw is Zero!

.. so keep log ea motor out of kit I-noload 12.6v reg pwr supply best cntrl
burn in to stable lowest full speed current draw. Future comparison will indicate: worn brush/commutator, bearings and shorted/open rotor windings

In competition = good quick test of left Vs right tank drive (or ea of 4 if 4WD)
do one at a time, subtracting quiescent draw, log result

It is remarkable just how sensitive this test is!!

NL current increase is measurable predicts a problems developing well before driver becomes aware of it!

and all Robot loads.. simply run each alone and subtract quiescent constant

using on-board A/D, a dynamic history may be recorded!!

with a little ingenuity ext. variable mechanical load constitutes handy dyno!

or with locked shaft and careful pre-set control of Victor / Jaguar
can test at any desired constant current up to 40A CB cycling which occurs at ~50A for each motor (for 3 motors tank cfg ~150A max ea side (~300A total) may be attained for a few seconds! - wise to monitor motor temp in this mode!)

.. great for finding resistive connections in wire, connectors, CB's, aging SLA's

also as dynamic 18AH SLA constant load battery characterization test:
use 50A, expect 9 minutes recording Vbatt every 10 sec
use Excel to produce Vbatt-at-terminals Vs Time for true batt discharge characteristic enlightenment!

[DonRotolo]

Originally Posted by chuckmerja

Any progress on this?

See my Post #28 for what we actually did.

Originally Posted by Dale(294engr]

Don hope I'm not too late with this for you to use else good for future ref.!

Well, yes it is far too late, but that IS excellent info for future reference, I think we'll incorporate dedicated test sockets across the 120A CB in future designs. Thanks!

[Al Skierkiewicz]

As a caveat, the resistance across the 120 amp breaker varies with temperature and manufacture. A one foot length of #10 wire is very close to 1 milliohm. Placed in the power return at the input of a speed controller allows one to measure input current, of constant polarity, at 1mV/A. Sample at 500 Hz or higher and throw away measurements that are less than 20mV and you will get a pretty good idea what the current is.

[Mike AA]

Originally Posted by Al Skierkiewicz

As a caveat, the resistance across the 120 amp breaker varies with temperature and manufacture. A one foot length of #10 wire is very close to 1 milliohm. Placed in the power return at the input of a speed controller allows one to measure input current, of constant polarity, at 1mV/A. Sample at 500 Hz or higher and throw away measurements that are less than 20mV and you will get a pretty good idea what the current is.

Al,

I have seen you say this a few times and I am now wondering, do I just leave the 1 foot of wire straight, should I wrap it around something like a screwdriver handle to take up space? I am thinking about setting up something for 2012 where we can monitor our motors a little better as I have gotten better with programming.

-Mike

[Al Skierkiewicz]

Mike,
A sense wire from each end of the #10 wire is all you need. If you search StangSense here on CD you will find a solution using a Maxim battery charging chip that we used many years ago. We used the same one foot piece for each channel.
Al

[WizenedEE]

Why not just get a 40-60 amp ammeter?

[Al Skierkiewicz]

Originally Posted by WizenedEE

Why not just get a 40-60 amp ammeter?

CIM stall is 129 amps and it is hard to chase the robot with an ammeter in your hand. I did it once (long ago) so I am speaking from experience.

[DonRotolo]

Originally Posted by WizenedEE

Why not just get a 40-60 amp ammeter?

...at a cost of...

"Inexpensive" and "40-60 A ammeter" are polar opposites. Sure, I'd do that at work, or mount a 200A shunt... but in the original post, I was off to St Louis with carry-on only.