Running 6V DC motor at 12V with 50% duty cycle - Page 1
Forum section: electronics / beginners

author: ivan747
author_rank: Super Contributor
timestamp: June 05, 2023, 10:22:15 pm
content: Hi guys,Quick question. Do you think there might be any reliability issues if I run a 6V DC brushed motor at 12V at 50% duty cycle? I have not chosen a frequency for it yet but I'd guess it's in between a few hundred Hz to the tens of kHz. Doing this because the motor is on an H-bridge, runs on batteries and I don't want to spend time designing custom hardware if I can avoid it for this project. It's for personal use.Best,Ivan

author: DavidAlfa
author_rank: Super Contributor
timestamp: June 05, 2023, 10:26:40 pm
content: 25% duty cycle! Power increases by 4 as voltage doubles.Else than that, it should work fine.

author: ivan747
author_rank: Super Contributor
timestamp: June 05, 2023, 10:28:44 pm
content: Quote from: DavidAlfa on June 05, 2023, 10:26:40 pmI was thinking about the current 30 seconds ago... Good thing I asked!

author: Benta
author_rank: Super Contributor
timestamp: June 05, 2023, 10:39:38 pm
content: Forget the current, that's a different issue.50% duty cycle will give you 6 V for the motor.Is this one of the small metal-can permanent magnet motors? In that case, do not go over 3 kHz, losses tend to mount around there. You'll need to test it in practice, but that's my experience.

author: jpanhalt
author_rank: Super Contributor
timestamp: June 05, 2023, 10:40:59 pm
content: Continuous or intermittent duty? Ford made a tractor/car "long-shaft" starter motor in both 6V and 12V versions. We in the aeromodeling community ran the 6V version at 12V to get the speed and power we needed to launch sailplanes up to 4M or so (up to 10#) to an altitude of about 100M or higher. No problems.1) It depends on the motor.2) It depends not only on the duty cycle during power (we were at almost 100% during launch), but the duty cycle as in minutes on vs. minutes off. Launch was fast (< 1minute).More details of your application are needed.

author: Benta
author_rank: Super Contributor
timestamp: June 05, 2023, 10:45:28 pm
content: @jpanhalt. Your super-duper specific application is perhaps not very helpful here.The question was: will chopping 12 V at 50% duty cycle result in 6 V for a PMDC motor? Yes.

author: jpanhalt
author_rank: Super Contributor
timestamp: June 05, 2023, 11:27:45 pm
content: Quote from: Benta on June 05, 2023, 10:45:28 pmAgeed. The motor is a relatively huge inductor. However intended service, not PWM duty, cycle can help.

author: james_s
author_rank: Super Contributor
timestamp: June 05, 2023, 11:48:58 pm
content: In practice it will probably be fine, brushed DC motors are typically extremely tolerant to over (and under) voltage, most 120V brushed motors will run on 6V or less, and for low duty cycles 200% of rated voltage shouldn't be a problem. There are several limiting factors, one is the bearings and other mechanical parts, if you spin the motor too fast it can literally come apart. The next is thermal, the more power you push through it the warmer the windings will get and at some point they will burn up. The remaining and usually biggest limitation is the brushes, if you push too much current through those they can fail quickly, even catestrophically or just overheat.

author: IanB
author_rank: Super Contributor
timestamp: June 06, 2023, 02:06:07 am
content: Watch out for increased sparks and arcing on the commutator. Although reducing the voltage below design should be OK, increasing the voltage above design may not be, due to increased arcing, if the brushes and commutator are not designed for this. Some low voltage motors may not have carbon brushes, but may just have brass spring wire brushes instead. It depends on whether it is a miniature motor or a bigger motor.

author: Zero999
author_rank: Super Contributor
timestamp: June 06, 2023, 07:39:54 am
content: Quote from: DavidAlfa on June 05, 2023, 10:26:40 pmThat's only for a resistive load. Assuming the comutator can take the extra voltage, you just need to ensure the current within the maximum rating of the windings and the shaft speed isn't above the maximum rating, oherwise it can damage the comutator and bearings.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 06, 2023, 07:41:42 am
content: Quote from: DavidAlfa on June 05, 2023, 10:26:40 pmThis is completely incorrect. A motor is not a resistive load. 25% duty cycle leads to approximately 25% rated speed and 25% rated power, in steady state, but 50% peak current during locked rotor current condition - which is why you really would like to have actual current control.For OP, the really best answer is to add current sensing and active current limitation. If the motor is small and the mechanical load is quite predictable and never slows the motor down too much, then fixed duty cycle operation at 50% duty at Vin=12V is pretty much equivalent to running motor at 100% duty cycle at Vin=6V. PWM frequency should be high enough so that current ripple in motor inductance becomes negligible - so a few kHz typically.

author: Zero999
author_rank: Super Contributor
timestamp: June 06, 2023, 10:24:57 pm
content: Quote from: Siwastaja on June 06, 2023, 07:41:42 amOr ramp up the PWM slowly.