Is this the correct way to measure the Back EMF Voltage?

[fishandchips]

Thanks. I am a bit confused. Before I connected the motor to the power supply, I plugged in a multi-meter to the outputs of the power supply to get a measured voltage of 11.1V. In the without gearhead case, the measured voltage across the terminals of the DC motor was 10.18V. In the with gearhead installed case, the measured voltage across the terminals was 10.2V. So, about 1V was dropped mysteriously perhaps due to my use of the very thin wires. Why you mentioned that I have several volts lost? Am I missing something?

Do you think the following kind of large alligator clip test lead with 22 gauge heavy insulation copper wires would make the voltage drop less?

https://www.adafruit.com/product/321


So, from your post, it seems that the method is correct but the wires and instruments are not good enough to get accurate measurements.

Do Ke=0.04 V-s/rad and Kt = 0.04 Nm/A look reasonable?

[rstofer]

Thanks. I am a bit confused. Before I connected the motor to the power supply, I plugged in a multi-meter to the outputs of the power supply to get a measured voltage of 11.1V. In the without gearhead case, the measured voltage across the terminals of the DC motor was 10.18V. In the with gearhead installed case, the measured voltage across the terminals was 10.2V. So, about 1V was dropped mysteriously perhaps due to my use of the very thin wires. Why you mentioned that I have several volts lost? Am I missing something?

See your reply #18 where you did some locked rotor tests.

Do you think the following kind of large alligator clip test lead with 22 gauge heavy insulation copper wires would make the voltage drop less?

https://www.adafruit.com/product/321

The resistance of #22 AWG is 0.05 Ohms per meter so if you had 2 19" leads, the resistance of the wire would be about half that of the motor.  You can work out the voltage drops versus running current.  0.05 Ohms times 2 Amps will only drop 0.1V in the leads.

So, from your post, it seems that the method is correct but the wires and instruments are not good enough to get accurate measurements.

It depends on how accurate your simulation needs to be.  I would probably settle for 10% accuracy for the overall project.

Do Ke=0.04 V-s/rad and Kt = 0.04 Nm/A look reasonable?

I really don't know.  I haven't thought about DC motors since I took the course back around '72.

[fishandchips]

The very small measured voltages in that experiment is indeed very strange. Maybe by stalling the motor, the system tried to get more current but the thin wires prevented it from happening. Then, strange thing happened. Let me get those thicker wires and retest.

Usually do people use those #22 AWG wires with alligator clips in this kind of experiments? Given that I can choose small, medium or large alligator clips, which one do you recommend?

[Ian.M]

If you are messing around with high current circuits, you shouldn't be using croc clips and  22AWG cable.   The leads used need to be made of wire with an ampacity rating sufficient for the maximum output of the bench PSU.  22AWG is only good for 7A unless it has high temperature insulation.   You should probably be using 18AWG or even heavier with crimp spade terminals at the PSU end that can be securely clamped by the PSU terminal binding posts, and at the motor end, either soldered, or crimped and bolted connections.  If you use 4mm plugs anywhere (e.g. to insert an ammeter) they need to be good quality brand name ones with sufficient current rating.

[fishandchips]

Thanks. In the real circuit, I use thick wires and solder the wires directly to the motor driver. However, in the test, I need a temporary solution. As I only got two hands, I used clips cables. What good quality brand name with sufficient current rating do you have in mind? In some stores, the products have no name nor specifications provided. In general, is the larger the croc clips the better? If I have no helping hands, how to I connect the two multi-meters to the circuit?

When I repeat the Back EMF experiments, I guess I better adjust the power supplying output so that the to-be-measured voltage across the motor, rather than the power supply, is 11.1V. Am I right?

I recall reading that in the resistance calculation experiment, one should set the current output from the power supply to 25% of the rated current. Why 25%? I am characterizing the motor at a higher voltage than the recommended one. I can directly adjust only the voltage output of my power supply. Changing the voltage output changes the current though.

[Ian.M]

The problem with clip leads is the small contact area and limited contact pressure.  You aren't going to find heavy duty 20A copper clips like Mueller BU-51C (http://www.alliedelec.com/mueller-bu-51c/70188674/ on any affordable off-the-shelf lead.   Get something similar with copper contacts not plated steel and an actual current rating and make up your own leads.   You'll also need some 20A rated 4mm plugs, or if you are cheap-skating it, use the crimp spades I previously mentioned for any end that goes onto a binding post.

[fishandchips]

Thanks. rstofer recommended two multi-meters in Post #21. The HP/Keysight is not in production and both units are quite expensive for me. For the Mastercraft 052-0052-2 and Sanwa meters I mentioned in Post #8, are they generally sufficient for most uses? I chose the Mastercraft 052-0052-2 as it was the only one in nearby stores that could measure 20A at affordable price. If it is not a good meter, may be I should return it to the store.

[Ian.M]

I wouldn't trust the Mastercraft DMM for high voltage or high current work, its perfectly adequate for general electronics use and limited line voltage work - fused plug-in appliances only.   Its 20A current rating is fairly dubious - see https://www.eevblog.com/forum/testgear/canadian-tire-mastercraft-dmm-new-and-old-revision-teardown/

If you are measuring high currents you should probably be using an external precalibrated current shunt.  Typically current shunts are designed to have a burden voltage of 50mV, 75mV or 100mV full scale, so if you want direct read-out on a cheap DMM's 1999mV range, with only a power of ten scaling to do in your head, you'll need to add a precision amplifier.

[Zero999]

Come to think of it. You don't need over-rated, chunky cable or a clamp meter. The cable just needs to be able to carry the required current, without overheating.

All you need is a separate amp and volt meter. Put the amp meter anywhere in series with the power supply and the motor. Measure the voltage, as near as the motor's terminals, as you possibly can. Adjust the power supply voltage, until the desired voltage appears across the motor and stabilises. The calculations can then be performed, using the actual motor voltage and current.

[fishandchips]

I recall borrowing two other multi-meters to do the resistance-related, stall motor experiment. Both gave me zero amp. Then, I bought the Mastercraft DMM and it gave me some non-zero amp values.

[rstofer]

The issue with the meters boils down to trying to compare measurements that vary by less than the inherent accuracy of the meter.

These shunts are 5 times better than the meter on DCV
https://www.digikey.com/product-detail/en/riedon/RSA-10-100/696-1598-ND/4967075

Maybe the 10 mOhm would work since 1A would produce 10 mV which would read 10.00 mV on the EEVblog Brymen BM235.

It would be nice to have a meter that has averaging because the readings are going to be bouncing around quite a bit.  This would be a great time to look into high side current sense amplifiers and scale the voltage up to something that can be handled with an Arduino.

http://cds.linear.com/docs/en/product-selector-card/2PB_currentsense_RevA.pdf

[fishandchips]

Thanks. Are shunts some kind of connection hub?

What is "EEVblog Brymen BM235"? Is it invented by a member of EEVblog?
http://www.eevblog.com/product/bm235-multimeter/

[Ian.M]

Q1. No.  See  http://www.rc-electronics-usa.com/current-shunt.html

Q2. See https://www.eevblog.com/forum/testgear/eevblog-bm235-multimeter-dilemma/msg856498/#msg856498.  TLDR: Dave Jones (EEVblog founder/owner) saw a gap in the Australian market for an affordable high quality multimeter.  Brymen make multimeters for resellers under many different brands and sell some under their own brand.  The BM235 was developed by Brymen in consultation with Dave to meet his exacting specs and the Brymen/EEVblog joint branded edition of it showcases that collaboration while helping Dave fund EEVblog and put bread on his own table.

[rstofer]

Current shunts are just high wattage low value precision resistors.  On larger units the current through the shunt could be as much as 1000A and would result in a 50 mV drop - 50 microohms.  Not knowing the rated current of this motor, it's hard to say which range is best but a 10 millohm shunt would drop 50 mV at 5A.  If the motor full load current is 20A, the shunt should probably be smaller - like 0.05V / 20A or 2.5 milliohms.

There's nothing magic about the 50 mV spec except that it is commonly used in industrial systems.  The meter manufacturers just produce one meter range (0-50 mV) and various scales for the application.  1000A, 100A, 1A - doesn't matter.  It's the same meter with a different scale.  Only the shunt resistor is different.

Using a DMM, there is truly no magic in 50 mV.  It doesn't matter what the drop is as long as it is insignificant to the circuit.  But the voltage drop must be high enough that it can be measured and the comment about the Brymen meter is that it will measure to 0.01 mV.  Some meters have better resolution, some have worse.  A bench meter might measure to 0.001 mV and my HP will measure to 0.0001 mV or 0.1 uV.

When using high side current sense amplifiers, the maximum mV range will be in the datasheet and then it's simply a matter of picking the right resistance.  The range should probably include stalled rotor such that the amplifier isn't overloaded during motor startup.

The Brymen has MIN-MAX-AVG recording and AVG might be just the thing to smooth out the readings during an experiment.   Many meters have this feature, it's not uncommon although it is often overlooked.  MAX may also be useful.

The BM237 is just a 6000 count meter.  It has decent accuracy, great safety ratings, and Dave's required uA scale.  Other than that, it's nothing particularly special.  I bought a couple of them simply for the branding and to support Dave's efforts.

It is currently unavailable at Amazon:
https://www.amazon.com/EEVblog-BM235-Brymen-Multimeter/dp/B01JZ1ADCO

Shorter answer:  You need a way to measure the current to a satisfactory resolution such that you can see the difference in current between having the gearbox installed and not having it.  The measurements need to make sense.

[fishandchips]

A local store has: 16 gauge and 14/2 with ground 300volts which have three wires inside. Which one should I buy?

[rstofer]

First of all, you want stranded wire.  It appears from your photo that the motor uses #14 AWG but that is just a guess.  Knowing the full load current would be helpful.

I sometimes cut up 2 wire extension cords.  These zip cords are neatly identified.  If you feel the edge, one will have a couple of ridges and the other will be just rounded.  The wire with the ridges is the 'identified conductor' and usually connects to the neutral of the AC system.  I use if for the ground wire on electronics related projects.

Cutting up 3 wire extension cords gets pricey.

[fishandchips]

That 14/2 is difficult to bend. Each wire seems to be made of 1 solid wire.

[fishandchips]

Q1. No.  See  http://www.rc-electronics-usa.com/current-shunt.html

Do I connect the Shunt in series with the motor of in parallel just like the voltmeter?

At the recommended voltage, the no load current is 1.1A and lock current is 19A. However, I am driving the motor at a few voltages higher.

[Zero999]

Q1. No.  See  http://www.rc-electronics-usa.com/current-shunt.html

Do I connect the Shunt in series with the motor of in parallel just like the voltmeter?

What do you think?

Consider what would happen if:

1) A low value resistor is connected in parallel with the motor? Work out the current, using Ohm's law. Would it achieve your goal? Will the current through the resistor be any different, if the motor wasn't there?

2) The same low value resistor is connected in series with the motor. Hopefully you'll see that the resistor's value is much less than that of the motor. The voltage across the resistor will be small, compared to the motor and dependent on the current.

[rstofer]

That 14/2 is difficult to bend. Each wire seems to be made of 1 solid wire.

Probably a better source is a 3 wire extension cord.  Cut off the molded plug, cut back whatever length you want for test leads and put a new plug on the cord.

There's a lot to know about wire and stranding.  The stiff stuff tends to have fewer strands and the nice stuff has a lot of strands.

#16 AWG has more resistance than #14 but it's a whole lot less than #22.  For short leads, #16 is probably adequate.  You would be looking at about 4 milliohms per foot of wire versus 3 milliohms for #14 or 18 milliohms for #22.  If you knew the exact resistance, you could use a foot of wire as the shunt resistor.

http://www.calmont.com/pdf/calmont-eng-wire-gauge.pdf

[rstofer]

If you decided to use a foot of wire (more or less) as the shunt, all you need to do is put a voltmeter across the 1 foot of wire and compare the reading with an ammeter placed in series with the wire (temporarily).

It would be better if you had a fixed load.  Automotive tail lights draw about 1A at 12V.  You would expect to see somewhere around 4 mV across 1 foot of #16 AWG.  Whether that is useful depends on the resolution of the meter.  If you can measure 0.01 mV then it probably works.

The other way to do is would be to use a high side current sense amplifier across the wire and multiply the voltage.

Linear Technology has a demo board using their LT1999 HSCSA but it's kind of pricey at $50.
http://www.linear.com/solutions/3867

For the LT1999-10, you would need a shunt resistor of around 0.008 Ohms or 2 feet of wire (#16 AWG).  The maximum input sense voltage of the -10 device is 0.35V so 0.35 / 0.008 => 43.75 Amps - more than adequate but not so high that resolution is lost.

For the LT1999-20 with a maximum sense of 0.2V, 1 foot of wire at 0.004 Ohms/foot would give a maximum range of 50A.

http://cds.linear.com/docs/en/datasheet/1999fd.pdf

The point of all this is that DMMs measure voltage with pretty good accuracy.  Current measurements are a little more iffy.

[Zero999]

One thing to bear in mind with a wire shunt is the positive temperature coefficient: as the temperature rise, the resistance increases. If you can keep the power dissipation low, then you should be able to ignore it. Otherwise you'll need to measure the shunt, as it heats up and take it into consideration.

[fishandchips]

I changed the wires from the power supply to the motor into a thicker ones. I also eliminated those "thin wires with clips". This time, I set the power supply voltage so that the voltage across the terminals across the DC motor was 4V. I stalled the motor to calculate the resistance. I did 10 runs.

When I used the thin wires in the past, the averaged measured voltage was 0.55V while the averaged measured current was 5.83A. In this revised experiment, the averaged measured voltage was 1.95V while the averaged measured current was 21.23A. The calculated resistance is still around 0.09 Ohm.

Given a 50% drop in measured voltage across the motor during stall, does that mean the setup is still not good enough to measure the Back EMF and resistance correctly?

[Ian.M]

I'd say its good enough as long as the worst case RUNNING drop stays under a few percent. Its giving you the same calculated resistance which increases confidence in your previous measurements.  Do check how much the voltage drops at the PSU output terminals when stalled as if the PSU's regulation isn't 'stiff' enough you could rewire with weldong cable and not see a significant improvement.

You may want to re-run your stall tests at an initial voltage of 3V as you are overloading the cheap multimeter, so the current measurements may be even less accurate than its poor specification.

As a matter of interest, what's the rated maximum output current of your PSU?

[fishandchips]

Thanks. This time, the motor did get hotter even at 4V. The most drop was 1.8V (23A) during stall.

As I only have two multi-meter, I took away the MasterCraft meter (used to measure the current in serial to the motor) and used it to measure the voltage across the output of the power supply while the motor was free running. The measured value was 4.05V while the voltage across the motor (without stall, no load turning) was 4V. Is this what you called acceptable? Few percent voltage drop while the motor was RUNNING rather than being STALLED?

Strangely speaking, when I tried to repeat the experiment at 3V, even I turned the dial to the minimum and without stalling the motor, the measured voltage across the motor was 3.8V. I could not make it lower. How come?


I am using the following PSU:

http://www.alinco.com/Products/ps/DM-330/

In the past, somebody mentioned that the big drop in voltage might be due to impedance of the PSU. I asked the manufacturer but they also do not want to give out the value. They wrote:

"The impedance may be calculated by the formula below just for your information in case you already have the device.

Impedance (ohm) = unloaded output voltage V1 - 30A loaded output voltage V2 / 30 (max current of 330MV)"

Know what that means? I forgot to mention that this time, I used the outputs at the back rather than at the front of the PS. The back outputs could provide 30A max.