PWM based simple DC motor controller - torque is lost

[eimix]

Hi

i build some sort of DC motor speed controller:

**broken link removed**

It works fine,
soft-start works, reverse works, forward and turbo modes also works
but problem is that torque is lost (somehow) no more wheel spinning in toy car. Stall torque is low
But top speed looks the same.

U can say that's because of soft-start, but even if i let wheels spin (not touching ground at all) and after that i put it on ground - it stops after short time.

Earlier it was with IRF540N mosfet in use, but i changed it to IRLZ34N (lower turn-on Vgs), but almost nothing changed.

As R1 resistor (potentiometer) is variable i tried to make duty cycle of 555 timer 100% (upper side 10K oms, lower 0 oms) and in that case torque becomes as it was before adding new controller. (wheel spinning, almost good for drifting)

But as soon i set R1 to about 95% duty cycle (upper side 9.5K oms, lower - 0.5K oms) torque is lost It looks like half (50% or more) of torque is away

I tried C1 1uF and 100nF, almost no difference in torque - only sound of motor changes about 100Hz and 1Khz, sounds funny, but it does not matters, or should i go for higher frequencies? 10Khz? 20Khz 100Khz? (motor is similar to Speed 400 6V)

I hope that my description is clear, if not please let me know i'll try to clarify it.

Thanks for ideas

 

[eimix]

in other forum i was given a tip to add two diode across relay contacts.
I do not really understand the purpose of that, would it solve my problems?

 

[Analog]

Read this article, especially on the motor heating segment, I think it will help you:

https://www.4qdtec.com/pwm-01.html

 

[Ubergeek63]

in other forum i was given a tip to add two diode across relay contacts.
I do not really understand the purpose of that, would it solve my problems?

a schottky from the drain to the rail. someone else mentions the motor heating up.

I think it is a combination of things... running open loop you will have IR drop going up as the motor temperature does and at low speed it goes up faster since the torque is related to current and the speed is related to the voltage that the IDEAL winding sees, in other words minus the voltage drop across the winding resistance.

and get rid of the cap across the motor.

 

[eimix]

ok, i will try diode.
is capacitor in paralel with motor a problem? it was in motor original package and in motor manual it was said to use it.

 

[Ubergeek63]

ok, i will try diode.
is capacitor in paralel with motor a problem? it was in motor original package and in motor manual it was said to use it.

Then it is probably a small enough value not to effect the circuit, just enough to cut down the arcing at the brushes.

 

[picbits]

10k to the gate of the Mosfet is way way too high

chances are that you are not fully charging the gate with your PWM before discharging it again causing it to run in its linear region.

Try a 10-20 ohm (not K ohm !)

 

[eimix]

10k to the gate of the Mosfet is way way too high

chances are that you are not fully charging the gate with your PWM before discharging it again causing it to run in its linear region.

Try a 10-20 ohm (not K ohm !)

there was no resistor before, near the gate of the Mosfet, and i burned 3 of my timers they are cheap (0.4$) but i was tired of replacing them so i put that 10K, it solved problems. And you are right, 10K is too big (MOSFET now is always charging) But i do not want to reduce it very much to avoid timer overheating. will be 10-20ohm enough?

 

[picbits]

Try a 100 ohm resistor and see if it helps. If it does then possibly look at using a transistor to buffer the 555 and drive the mosfet from that.

 

[eimix]

Last modified circuit (running in reality)

**broken link removed**

It still works
But no significant increase in torque

MOSFET gate resistor reduced (R3), and grounded with (R4)

D2 diode added.

What else should i try?
Increasing frequency?

 

[picbits]

If you increase the frequency you need to drive the gate of the Mosfet a lot harder to overcome the capacitance of the gate.

Best to decrease frequency and see what happens.

 

[picbits]

Just as another thought (saves me looking up datasheets), what is the gate voltage for full turn on on your mosfet ? If you're not driving it on fully then that could be your problem.

If your motor is a low current one, have you tried a power transistor in place of the Mosfet ?

 

[eimix]

here is datasheet **broken link removed**

It is "logic level" mosfet. ~2V threshold, and almost fully turn on @6V
motor has 6A stall current, and MOSFET is cheaper.