General > General Technical Chat
Brushed DC motors and motor drivers
(1/3) > >>
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:

--- Quote from: Andy Chee on February 06, 2024, 12:15:27 pm ---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!

--- End quote ---

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:

--- Quote from: Benta on February 06, 2024, 07:06:37 pm ---No reason to be that brutal.
Just measure the DC resistance of the motor, that will tell you the worst case locked motor current.

--- End quote ---

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.
Navigation
Message Index
Next page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod