6V/6A DC motor on a 12V line

Thread Starter

Jony130

Joined Feb 17, 2009
5,487
Can I run a 6V 6A DC motor with PWM and power it with 12V power supply?
Is this safe for the motor ?
 

crutschow

Joined Mar 14, 2008
34,281
Can I run a 6V 6A DC motor with PWM and power it with 12V power supply?
Is this safe for the motor ?
It is if you limit the PWM to no more than about 50% duty-cycle steady-state, or limit the motor current to 6A (if 6A is the maximum steady-state motor current rating).
 

ian field

Joined Oct 27, 2012
6,536
I would also use a fuse to protect the motor in case the PWM circuit fails continuously on giving a 100% duty cycle.
The impression I get is; the TS is wondering about applying 12V to a 6V motor at no more than 50% duty cycle - if the pulse repetition rate is high enough, the motor's inductance will protect it.

With a "proper" PWM PSU, you'd have an inductor feeding a filter/storage capacitor and you could design the regulation so the voltage out of the filter never exceeded 6V.
 

Thread Starter

Jony130

Joined Feb 17, 2009
5,487
In fact I was planning to use this circuit

pwm.png

Where the PWM frequency is around 36kHz. But I'm bit worried that the engine will not withstand this 12V at 50% duty cycle and will overheat and burn.
 

MaxHeadRoom

Joined Jul 18, 2013
28,617
In fact I was planning to use this circuit

View attachment 90324

Where the PWM frequency is around 36kHz. But I'm bit worried that the engine will not withstand this 12V at 50% duty cycle and will overheat and burn.
The 'Overheat' will not be as a result of voltage, but torque/load/current, 50% duty cycle on its own does not mean much.
In drives where the voltage is quite a bit higher than the rated, some kind of current monitor/ limit is implemented.
Max.
 

crutschow

Joined Mar 14, 2008
34,281
At 36kHz, there may even be too much inductive reactance in the motor to get its full rated torque at 12V on high duty cycle.
The motor inductance acts like a filter to the PWM frequency, giving a DC current proportional to the average DC voltage from the PWM. A high inductance does not affect that fact.
36kHz is fine although you could use a frequency in the low kHz region.

................
Where the PWM frequency is around 36kHz. But I'm bit worried that the engine will not withstand this 12V at 50% duty cycle and will overheat and burn.
Because of the motor inductance the 12V peak of the PWM will not cause high peak currents, so the motor won't overheat.
 

Thread Starter

Jony130

Joined Feb 17, 2009
5,487
Hi, today I test if 12V DC motor can survive the 24V DC. And yes, the motor survive 15 minute run without overheating too much.
I also build a op amp (LM358) based PWM for test purpose. And by accident the output frequency was 4.3kHz. And If I rememberer correct when I change the cap to lower the frequency (980Hz) the motor speed up for the same duty cycle.
As for 36kHz and ATtiny13 - this frequency is way to big because I have a lot of trouble with it. Even if I connect a PC fan. The fan and my PSU start behavior very strange. Maybe because I build it on the breadboard.
 

alfacliff

Joined Dec 13, 2013
2,458
the pc fan isnt a real dc motor, it has a small inverter circuit inside, its called a brushless dc motor. the frequency of the pwm and the frequency of the internal inverter might fighbt each other.
 
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