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Stall Current & RPM

So I'm looking at what motors to buy for my next project, I'm struggling to find many retailers in the uk that sell them but one that I've found have some that I'm interested in. The specs they supply are 390mA stall and RPM of 250, but these figures are based on a 6v supply. I however will be using a 4.5v supply, is there any way to calculate how these numbers will be affected by a lower voltage supply? Also I will be running these motors in parallel (sets of 2 motors) will this also affect the RPM? Thanks :)

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You can use an L293D at 4.5V but the voltage drop from the chip will leave you with somewhere around 3V going into the motors. You can run motors off a 3.7V lipo if you hook the chip's logic supply directly to the battery.

Anyhow, the RPMs will change in approximately the same ratio as the supply voltage. So at 3V the motors will turn about half as fast as they would've at 6V.

As far as stall current goes, a fellow named Ohm can help you with that...

Find the resistance of the motors:

R = V/I

R(motors) = 6V/390mA

= 15.38 ohms

Plug it into Ohm's Law with the new supply voltage (I estimated around 3.5V):

I = V/R

I(stall) = 3.5V/15.38ohms

= 227.6mA

Fantastic, thanks for that response, this means I should be able to use my current hardware. Don't suppose you can point me in the direction of how to calculate how the torque and RPM will be affected by a smaller voltage? :)

Generally speaking torque is affected the same way RPMs are. Half of the rated voltage will get you about half the torque.

Answer to question1- You'll not be able to run it on 4.5V supply. You need atleast a 6V supply.

Answer to question2- Number of motors doesn't affect the RPM. However, more the motors, the faster your battery drains. If you are thinking of creating 4.5V, I assume you are using non rechargeable batteries (zinc chloride type). With non rechargeables, the voltage tends to drop with time. The more the current draw (1 motor= 390mA, 2 motors= 780mA), the faster the voltage drop as total capacity of the battery remains constant. In simpler words, your battery is not on steroids, you may need to change it quickly and as voltage drops below 6V, you'll not be able to get 250 RPM.

Solution to question 1- Use rechargeable batteries. 5 of those when connected in series (1.2V X 5= 6V) will give you the correct voltage. Also, by using rechargeables, you help mother nature as you are saving new batteries from being created.

Solution to question 2- Same as above. Use rechargeables. One good thing about rechargeables is that they maintain voltage till you just have 10% of your total capacity in your tank. So there will be no fluctuations and you'll get 250 RPM for longer time. Also, you can get rechargeables with high capacities. The best ones in my knowledge is 2100mAh which will give you atleast 3 hours before you have to charge your batteries again.

 


 

Question I raise- I hope the first problem is sorted. But have you figured out how you're going to control your motors? If you connect the motors directly to the power, they'll rotate like crazy and will not stop till you either pull the power out or the battery dies down. To control a motor so that it spins just at the right time, you'll need a H-Bridge or atleast a transistor. I'll suggest a H bridge because they can control the motor to run both in forward and reverse which a transistor can't do. I'll suggest you to use a L293D H bridge motor driver as-

  1. Its cheap
  2. Its max current (600mA) is greater than your stall/max (dunno for sure, you never mention it. It should be max current or you'll fry the L293D) current (390 mA).

Sorry forgot to mention, I'm using the L293D at the moment, and yeah you're right in that it's maximum is 600mA per channel. I can't find another controller anywhere that will fit a picaxe 28x1 with a higher tolerance though. I've looked at motor drivers like this: http://robotbits.co.uk/motor-drivers/sabertooth-2x-5amp-motor-driver/prod_58.html but can't figure out how i'd connect that to my board :( if i had a better controller than the L293D then I guess it's problem solved, and like you said, I'd just upgrade to a better power supply (although I believe my board will only handle 5v max which creates another issue). You can see why I'm running around in circles, it's confusing for a newbie :P

Look into the SN754410 dual h-bridge. It's a pin-for-pin match for the L293D. It can handle more current but it has a bigger forward voltage drop IIRC. There is also debate on whether the ESD diodes suffice for flyback protection but I've yet to have problems with the smaller motors I use.

Thanks for the advice there, I've looked at the chip and it seems like a good option. Am I right in saying the L298 is also compatible? And how would that compare in you opinion?

but it lacks the protection diodes for inductive loads. It's what makes the L293D very convenient.