Power for Multiple DC Motors

[Kiatu24]

Hi everybody. First I'd like to say I'm relatively new to the world of electronics. I'm an experienced programmer and am trying to venture into electronics. And I'd also like to say sorry if I posted this in the wrong spot or if there's something else similar to this I missed. I've looked many places but can't seem to find anything relevant. I might just not know the correct terminology for the problem I'm facing.

So my goal, is to have 4 DC motors being controlled by an Arduino. The motors I've got are 9-18V DC motors and I'm currently powering them with 8 AA batteries for a total of 12V. I'm using transistors to control them from the arduino. Everything coding and control wise works as I expected, but I'm having some difficulty with the power supply.

When I hook up all 4 motors, I notice a pretty steep voltage drop. It usually ends up around 6-8V instead of the full 12. I've tested the batteries with a multimeter and it says 12V by inself and with one motor hooked up to it. Then as I add each motor, it drops. Then I tried to hook up two 9V batteries in series to get 18V and it performed even worse than the 12V. I'm still pretty unclear on how amps affect everything. So I apologize if this is a stupid question. But any light anyone can shed would be greatly appreciated.

Thank you,
Kiatu

 

[Arouse1973]

Hello
You need to make sure your battery can supply that much current. It could be that the AA battery can't supply enough current to run 4 motors, obviously dependant on motor specification. What battery are you using is it NICD or NiMH or supermarket own Alkaline? Also what's the spec of the motors, how much current do they draw? You may need some higher capacity batteries which have a lower output impedance so the voltage does not drop significantly when powering the motors. Can you try a larger battery like an old car battery or something.
Adam

 

[Kiatu24]

Hi, thanks for your reply. Well these are the motors I'm using: http://www.radioshack.com/product/index.jsp?productId=2102827

These are the specs listed on the page. I'm not sure what they really mean by load and 15% speed.

• No load: 9VDC 380mA current max., 11000&plusnm; 15% speed, 150g.cm min. torque
• No load: 12VDC 400mA current max., 15200&plusnm; 15% speed, 190g.cm min. torque
• No load: 18VDC 430mA current max., 24000&plusnm; 15% speed, 290g.cm min. torque
• With load: 9VDC 1.1A current max., 9300&plusnm; 15% speed
• With load: 12VDC 1.5A current max., 11500&plusnm; 15% speed
• With load: 18VDC 2.4A current max., 18000&plusnm; 15% speed

And the batteries I'm using are just the normal store bought alkaline ones. I read somewhere that normal AA batteries have 2400 mAh/hour or something like that. What would you suggest? My end goal for this is a quadcopter, so weight is a consideration. I've wanted to get a NiMH battery so it would be rechargable but for now I was just trying to get it to fly lol. And unfortunately, no, I don't have any spare car batteries to use . Thanks again for your help.

 

[donkey]

have you got a diagram showing how these are hooked up? are they series or parallel?
are you using a Hbridge or just a transistor? do you want to control the speed of each motor individually?
where are you measuring the power from? measuring at the wrong point could give weird reading
have you measured the amps? are they drawing the amps they need? if not then the batteries could be heating up causing voltage drop
are you using the same power source for the motors and for the controller?

for testing getting a $30 SLA 12volt battery is a good idea, they are rechargeable and a lot of fun. no issue with it not giving you the juice you need.
when you convert to the mobile model you can figure out which batteries are better, as well as motors etc. the design process is all about testing what works

 

[KrisBlueNZ]

The current required by those motors adds up pretty quickly to several amps, which is more than you should be drawing from AA cells, especially primary cells (non-rechargeable). How heavily are you loading the motors? Under load, at 12V DC, at 1.5A per motor, the total is 6A which is way too high for primary cells, and problematic for small rechargeable cells too.

The figure of "2400 mA/hour" you mentioned is probably 2400 mAh, 2400 milliamp-hours. This means that if you draw 2400 mA (2.4 amps), they will last an hour. If you draw twice that much current, 4.8A, they will last half as long. Actually, somewhat less than half, because higher current = lower efficiency. If you draw half as much current, 1.2A, they will last twice as long. Actually, somewhat more than twice, for the same reason.

I agree with donkey that a 12V SLA battery is good for testing if you have high current requirements. Be careful not to short it out though; they can make your wires red hot!

You should be looking into Lithium-based battery technology - Lithium-ion or Lithium-polymer - for this application.

 

[Kiatu24]

Hi everybody, thank you for the replies. So I'm gathering that I just need a better power supply. I'll look into that. Here's a diagram I worked up for how my motors are hooked up. Sorry about the crappiness of the diagram, I just made it in a paint program I have. I also have some diodes and stuff hooked up but the picture was already crowded. I believe these are in series. Should I set this up differently? Thanks for your help guys.

 

[KrisBlueNZ]

That looks like a good setup. I can't comment on the diodes unless you give us a reference for the motor driver board. But if you've followed the instructions for connecting motors to the board, you should be OK.

Use a fuse in series with your battery! Say 6~8A slow blow.

 

[BobK]

If those are bipolar transistors you need a resistor between the Arduino and the base.

Bob

 

[Kiatu24]

KrisBlueNZ - Thanks for all your comments. That explanation of milliamp hours makes a lot of sense. And thanks for the tip about the fuse. You've been very helpful.

BobK - Yeah, sorry I didn't want to garble up the diagram too much but I do have resistors for the arduino to base. Thanks!

I appreciate all the help everyone. I'm going to try to get a better power supply. I'm going to do some looking around. I see Lithium would be a good option, does anyone have a favorite they can recommend? Thanks!

 

[KrisBlueNZ]

Sorry, I didn't realise that your "motor driver board" on the right is just a breadboard!

You should have a diode across each motor, e.g. 1N5404, with cathode connected to the positive side, to protect the transistor against back EMF from the motor, which is an inductive load. Although that will probably affect the motor's freedom to turn when undriven; if that might be a problem, we need more info.

Your motor driver transistors must be saturated. If they are passing current in the amps range, and you're driving them from Arduino I/O, you need to use either Darlington transistors or logic-level MOSFETs. A standard bipolar junction transistor rated for a few amps has a minimum guaranteed gain around 30~50 and needs more base current than the Arduino can supply directly.

A Darlington has a very high current gain and solves this problem, but it has a significant collector-emitter voltage drop - up to 2V - which reduces the voltage available to the motor, and wastes power as heat in the transistor.

An N-channel MOSFET with logic-level gate drive voltage threshold is a much better option. The ON Semiconductor NTD4906N is a cheap, amazingly small and efficient switch: http://www.digikey.com/product-detail/en/NTD4906N-35G/NTD4906N-35GOS-ND/2194521