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I have been putting together a small robot using an Arduino Motor Shield R3. The shield has two sockets for motors, however the wheel carriage I bought has four motors. I tried putting both right side motors in one socket set, and the left side motors in the other socket set.

When I turn on the Arduino, the left side motors turn together according to how I coded the board, however the right side behaves differently.

One of the motors does not start. If I push it a little it will start turning. Though not always. When I plugged in the misbehaving motor by itself, it runs correctly.

I am wondering about cause of the problem here. Is it likely the second motor on the right side is faulty? Or am I trying to use up too much power and disrupting the current?

  • What are the ratings of the motors you are using? – Majenko Jul 14 '15 at 12:29
  • This is the info I have from a website, the motors themselves did not come with this.Operating voltage: 3V~12VDC (recommended operating voltage of about 6 to 8V) 1. Maximum torque: 800gf cm min (3V) 2. No-load speed: 1:48 (3V time) 3. the load current: 70mA (250mA MAX) (3V) 4. This motor with EMC, anti-interference ability. The microcontroller without interference. – radiobrain77 Jul 14 '15 at 13:01
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    Sometimes two motors in parallel causes this problem - the stall current of one motor causes the overall voltage to drop under load and stops the other motor from starting. So that second motor is probably NOT faulty. You might need a higher current battery (but the same voltage), it also might help to make sure the power supply to the arduino and the shield are separate - that shield seems to have a jumper for that. Best, but expensive solution would be to have separate drivers for each motor... – Andy Jul 14 '15 at 13:19
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    Do you have a DMM (digital multi-meter) available? It would help to take the guesswork out of troubleshooting problems like this. Even a cheap one is better than no meter. – dlu Jul 14 '15 at 13:27
  • @radiobrain77, one other thing. What voltage battery are you using? (The motor driver you are using may drop a little bit of voltage before it gets to the motors.) Also is there a "Stall current" for your motors,and if so what is it? – Andy Jul 14 '15 at 13:38
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The drive capacity of the Motor Shield should match the motors – two of the motors. The shield was probably designed for a load of one motor, and you're giving two, so it is possible that neither of the motors on that side is getting enough current to get started. Here are somethings to check to get started:

  1. Find the drive capacity of the Motor Shield and compare it to the motors (if you don't have specs for the motors you could measure the current they draw if you have an DMM). It might be wise to measure the current anyway, just in case you have a defective motor or shield.
  2. If the capacity or the current draw is out of line, that's your problem. You can probably just add a power transistor to the circuit to drive the motors in this case.
  3. Try it with just one motor on each output and see if they run normally. If they do, that would suggest a loading problem or a defective motor or shield. You can sort out which is which by swapping sides – if one motor runs and the other doesn't, swap them. If the problem follows the motor, then blame the motor, otherwise blame the shield.

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