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I'm a little new to this, so bear with me:

I am working on a hobby-related project that involves the operation of two DC motors in parallel, powered by a NiMH battery. (Crappy circuit diagram included)

Part specs:

-Battery- NiMH 4000mAH @ 8.4 V DC

-Microswitch - Rated 15A SPDT to toggle motor operation

-Motors - Rated ~6V operation, 11A stall and 0.9A idle

-Wire 16AWG (if relevant)

Circuit: enter image description here Problem:

The battery I have for this project is rated @ 8.4V, whereas the motors are rated for 6 V. I've been looking into finding a way to reduce the voltage to prevent motor damage and remembered PWM from a class I took (I'm an EE undergrad). I figure I could use a PWM circuit to vary the duty cycle in order to prevent motor damage, while conveniently allowing me to alter motor speed with a dial (altering the duty cycle %) without the massive power dissipation a potentiometer would create.

However, I am at a loss as to how to implement such a circuit; the only background on pulse width modulation I have had involve the creation of a basic PWM controller, not on selection and designing one around a circuit.

The circuits I have found online seem like they would work voltage-wise, however when I look at the current ratings they always fall short of the motor stall current. (Example linked)

PWM 1

PWM 2

PWM 3

My question to you is either:

a) How can I make a PWM that is suitable for such a build, or

b) How can I select the right PWM controller to add to the circuit?

Thanks for any insight you can give, please let me know if any more information is needed.

EDIT: It is worth noting that stall current is transient, as the motors reach max rpm, and thus the idle current, fairly quickly. There is occasional slight load on the motors, however it's not enough to be an issue. My only concern would be the hypothetical PWM motor controller having issues with the initial stall current.

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  • \$\begingroup\$ PWM 3 is probably OK, the others do not have a high enough current rating. The on-board fuse could be used to protect against excessive current at stall. \$\endgroup\$ – Bruce Abbott Oct 12 '18 at 8:54
  • \$\begingroup\$ @BruceAbbott So the controller rating has to exceed the max stall current of the motor to prevent damage? It simply wouldn't just prevent the motors from reaching max torque? \$\endgroup\$ – ReThink Oct 14 '18 at 20:28
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In order to properly select a motor for a load and select a controller for the motor, you need to know the rated motor current, the current that the motor can safely draw continuously. The stall current is only tolerable for a very short time. Some larger motors can not tolerate stall current at all. A good controller should limit the current. Larger industrial motors controllers commonly limit overload current to 150% of rated current for one minute and have an instantaneous shut-off at 200% to 250% of rated current. That is probably a reasonable objective for your situation also.

The linked controllers have over-current features to protect the controller, but I did not see anything that would protect the motor. Something like that would be ok for careful operation as a learning experience. Set the speed to zero and connect the motor before connecting power. Turn the speed up slowly and up carefully. That will keep the motor from drawing too much current. Check the current with an ammeter. Operating the motor at 25% of locked rotor current will probably prevent it from getting to hot quickly. With out a specification from the motor manufacturer, you will not be able to determine. How much the current it will tolerate without overheating or what temperature it can withstand.

Two motors are usually not connected to one controller, but it is ok to do that as a learning experience.

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  • \$\begingroup\$ You mention that stall current is not acceptable; is there a way to alleviate the current spike from a hypothetical controller so that the motors could start up without impacting the controller (basically a way to bypass the controller during stall current draw)? Additionally, could you explain a little as to why I couldn't drive both motors with one PWM controller? \$\endgroup\$ – ReThink Oct 12 '18 at 0:42
  • \$\begingroup\$ A good controller will protect both itself and the motor. Even a simple controller can start a DC motor without excessive current it the output current is increased gradually. It is not guaranteed, but is quite likely that you can connect a DC motor to a simple converter and start it without excessive current simply by turning the speed up gradually. \$\endgroup\$ – Charles Cowie Oct 12 '18 at 3:02
  • \$\begingroup\$ You can certainly drive two motors with one PWM controller. It may work fairly well depending on the load. Without understanding what is being driven, I won't speculate on what might indicate you should have two controllers. \$\endgroup\$ – Charles Cowie Oct 12 '18 at 3:15
  • \$\begingroup\$ I'm a little new to motor controllers, but if I were to add a PWM controller with a max rating of 10A, would the motors only draw a max of 10A, or would the motors draw as much as the battery could supply/what they need for max torque (11A) and blow the controller? \$\endgroup\$ – ReThink Oct 14 '18 at 20:24
  • \$\begingroup\$ A good controller should be able to limit the maximum current to a level that you can adjust. That will limit the accelerating torque when starting and slow the motor it is mechanically overloaded. If limiting the current causes the motor to stall when mechanically overloaded it should stall without drawing more than the set current. With two motors, on one controller, you would need to contend with most of the current going to one motor if one motor is overloaded. The controller would not protect the motor very well. \$\endgroup\$ – Charles Cowie Oct 14 '18 at 20:37
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You would be wiser to use two motors in series and choose a 14V battery such as 4S Li-Ion or equivalent NiMH. This way the torque is sharing the same current.

But if you expect the motors to see different loads then run in parallel with two 8.4V batteries with conservative changes in speed to not maintain the stall currents for long during starts uphill with heavy loads.

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  • \$\begingroup\$ The motors themselves would see slightly different loads, so parallel would be the ideal configuration. Would a PWM controller also limit the current to the max of the controller (say 10A, for example), or would the motors draw as much as they need from the battery (which the battery can supply, 11A stall current) and damage the controller? \$\endgroup\$ – ReThink Oct 14 '18 at 20:26
  • \$\begingroup\$ With a half-bridge the average voltage with PWM controls max current and then max RPM which you regulate in kRPM/V or to the Motor V rating. \$\endgroup\$ – Sunnyskyguy EE75 Oct 14 '18 at 21:51

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