Brushed DC motors and motor drivers

[pcprogrammer]

For my fischertechnik remote control project I'm looking for a motor driver that can handle the needed currents.

I found out that measuring on brushed DC motors is not that simple, but made a setup with a shunt resistor and a FET to switch the motor on and off. The FET is controlled with a function generator set to 100Hz pulse and adjustable pulse width. The 100Hz is what I'm also using in the receiver for the PWM carrier frequency.

My scope is monitoring all the signals against the common ground and I noticed that a lot of noise is picked up when the motor is running despite the fly back diode and 100nf capacitor across the motor. Winding the ground lead around the probe tip helps a bit, but still.

At first I tested with my Hameg power supply but either the test leads are not big enough or the supply is to slow, the voltage drop when pulsing the motor is huge, not giving a proper reading for what the current would be on a source that can really deliver. Then I switch over to the fischertechnik battery packs, but again either the wires are to resistive or the batteries are to old, again showing a big voltage drop.

Finally I connected a PC power supply but with it's 5V being lower than the planned 6V it still is not perfect. The voltage remains more stable though. I ordered a big buck converter on aliexpress, but that will take its time to arrive.

Attached are photos of the test setup, and screen captures of the scope at 20%, 50% and 100% pulse width settings, with zoom in on start and running stable. The yellow trace is the output of the pulse generator, the green trace is the voltage over the 0,12 ohm shunt (4 * 0,47 ohm in parallel) and the purple trace is the supply voltage.

The maximum current seen is ~9A and on average when running stable ~1A.

A friend of mine found this TA6586 motor driver and I ordered it from aliexpress for more testing. It has a bigger brother which is the RZ7886 that is pin compatible.

One thing is certain the MX1508 is not suited for the job.

Any advise or thoughts on this?

[Andy Chee]

Are you able to conduct a "locked rotor" stall current test?  Basically, lock up the motor to stop it rotating, and apply current.  This will be the maximum current your motor driver will need to provide.

Of course when you conduct this procedure, be quick with your measurements, otherwise you'll overheat and burn out the motor winding!

[pcprogrammer]

Are you able to conduct a "locked rotor" stall current test?  Basically, lock up the motor to stop it rotating, and apply current.  This will be the maximum current your motor driver will need to provide.

Of course when you conduct this procedure, be quick with your measurements, otherwise you'll overheat and burn out the motor winding!

I can certainly try. Maybe I can stop it in it's high gear setting. On low it is very strong. That won't matter I guess, stopped is stopped.

Would this not be the same current as when pulsed for a very short time where the motor won't startup in the first place. It needed at least 8% pulse width when I powered it from my Hameg supply on 6V and the current went up to about the given 9A before it started turning. Have not tried this with the 5V supply yet.

Edit: Tried to stop the motor, but it sounded like it started to grind teeth on the high gear, and on low it is way to strong. I will open up one of them and maybe I can stop the motor without the gearing on it.

[Benta]

No reason to be that brutal.
Just measure the DC resistance of the motor, that will tell you the worst case locked motor current.

[pcprogrammer]

No reason to be that brutal.
Just measure the DC resistance of the motor, that will tell you the worst case locked motor current.

That was what slowly came to my mind too, but with my multi meters I see a resistance of ~2 Ohms, and with my cheap component tester it shows ~12 Ohms. Both to high considering the measurements done with the scope. I expected something in the range of 0.5 to 1 Ohms. I did take into account the resistance of the test leads of my multi meters.

Unfortunately I don't own a good bench multi meter with the option of 4 wire measurements.

[Benta]

You'll need to turn the armature (rotor) back and forth a bit to get the minimum DC resistance. It depends somewhat on the commutator position.

[Zero999]

The Ohms setting on a multimeter might not pass enough current to wet the contacts enough to achieve as lower resistance as when the motor is stalled and connected to the full supply voltage. Try measuring the current, with a locked rotor at a much lower voltage, say ¹/₁₀ of the rated voltage. That should give an accurate enough reading, without overheating the motor, so long as it's not left like that for too long. Note to actually measure the voltage across the motor, whilst the current meter is in series, to take account for the volt drop over the shunt.

[pcprogrammer]

You'll need to turn the armature (rotor) back and forth a bit to get the minimum DC resistance. It depends somewhat on the commutator position.

I did this and indeed the resistance went down to at some points even a short. I eventually saw the ~0.4 Ohm I calculated from the measurements done. The shorts are caused by the brushes making simultaneous contact with the different pads of the commutator. By the looks of it, it is quite coarse and has only three windings and I guess 6 pads on the commutator. Can't see any carbon blocks on the brushes. Will try to make some close ups.

The Ohms setting on a multimeter might not pass enough current to wet the contacts enough to achieve as lower resistance as when the motor is stalled and connected to the full supply voltage. Try measuring the current, with a locked rotor at a much lower voltage, say ¹/₁₀ of the rated voltage. That should give an accurate enough reading, without overheating the motor, so long as it's not left like that for too long. Note to actually measure the voltage across the motor, whilst the current meter is in series, to take account for the volt drop over the shunt.

This explains why I sometimes saw measurements of several hundreds of ohms on the meter.

Probably safe to assume that the maximum current I measured is what it is, but I will try measuring it at the lower voltage with a blocked motor, now that I have it out of the case.

[pcprogrammer]

Some pictures of the motor and gearing.

In the first one you can see the "finger" on the commutator. I looked at it a bit better and noticed it only has three pads on it for the three windings.

[pcprogrammer]

Did all type of measurements on it and the results vary a bit. With a low voltage and blocked motor I did two measurements. My multi meters might not be very precise due to age or cheapness. For the voltage I used my Agilent U1242A and for the current a holdpeak HP-770HC

1V over the motor showed ~1.6A    ==> ~0,625 Ohm
0.4V over the motor showed ~0.7A ==> ~0,57 Ohm

The measurements with the scope show ~0,44 Ohm.

With a perfect switch this would mean 13.7A at 6V, but with loss over the H bridge it will be less.

Free running current at 6V over the motor is ~0.55A,  where the scope measurements showed ball park 1A on the complete motor assembly, but not driving a vehicle for which the motor is intended.

The question now is which H bridge to choose. There are so many out there.

Will the TA6586 cut it with 9A peak and built in over current limiting according to the database, or is it wise to go for something that can handle more current. The RZ7886 can handle a peak current of 13A.

Or maybe some less obscure brand with a better datasheet?

[Benta]

There's no reason to design for maximum stall current. It's normal to limit start-up/locked-rotor current for DC motors.
The main suppliers for DC motor ICs are Toshiba and TI:
https://toshiba.semicon-storage.com/ap-en/semiconductor/product/motor-driver-ics/brushed-dc-motor-driver-ics.html
https://www.ti.com/motor-drivers/brushed-dc-bdc-drivers/overview.html

Never heard of the TA6586.

[pcprogrammer]

Received the TA6586 a while back, but had other things on my mind that needed attention first.

It looks promising. Have to do some more tests and modify my receiver PCB to test it on that to make sure it will do.

By the looks of it the ic has build in current limiting and acts on it. Need to test it with a smaller motor to see if this is actually the case. One thing is for sure, it does need the decoupling capacitor. For my first simple test I omitted it and I noticed hefty oscillation on the power supply line.