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Topic: Why does a blocked motor slow down other motors which are running in parallel? (Read 309 times) previous topic - next topic

robertpainsi

Hi, folks!

I connected four 5V Motors in parallel to a 5V adjustable power supply (AC→DC, 2V-15V, 1.5A). The motors don't drive anything. The total current is ~350mA.

In my project, each of these motors will be completely blocked for most of the time. Meaning, they are still connected and "on", however, won't turn at all. I can't turn them off because they should still apply force.

Here is the thing I don't get. If I block the first motor, all other motors will slow down. If I also block the second motor, the remaining motors will slow down again. If I block three motors, the remaining motor is still spinning, but the rpm is far far less than if all four (, three or two) motors are spinning.

When blocking all four motors, the total current is ~750mA. So the power supply should easily be able to handle this.
In a circuit with a single motor, blocking it uses 250mA. In theory, 4x 250mA = 1A but as mentioned above, only ~750mA are measured.

* So, why does blocking one motor effect the others?
* What can I do against that? (Just a note, I will use two LM293D drivers to drive the motors because I need to spin the motors in both direction)

thanks for you help!

Robin2

You have not posted your program so it is not easy to know what might be going on.

A clear wiring diagram would also be a good idea.

...R
Two or three hours spent thinking and reading documentation solves most programming problems.

groundFungus

Monitor the motor supply voltage while doing your tests.  Does the voltage remain steady as you block the motors?   Have you actually measured the motor stall current?

MAS3

Hi.

What does "blocking a motor" mean ?
You didn't specify that.

I read it as you mechanically blocking the shaft of the motor so that it cannot spin anymore.
You said the total current of 4 running motors is 350 mA, and that of 4 blocked motors is 750 mA, so more than double.
This is what is called stall current, a property of any motor and which you should familiarise yourself with when working with motors.

So ask your favourite search engine about "stalled electro motor", i guess you'll find a lot interesting stuff to learn.
Have a look at "blink without delay".
Did you connect the grounds ?
Je kunt hier ook in het Nederlands terecht: http://arduino.cc/forum/index.php/board,77.0.html

hammy

A stalled or " blocked " motor as you called it will draw much more current than when it runs . This is overloading your power supply .
Unless designed to operate in that manner stalled motors will overheat and burn out

MarkT

Typical stall currents are 5 to 10 times running current for small motors, going up to 20x or more
for large motors.  Most motors cook(*) if held stalled for any length of time with a supply capable of
providing that current.

(*) themally destroyed, winding insulation melts/chars/catches fire, brushes may be damaged too.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

robertpainsi

Thanks a lot to all of you for your helpful answers. I learned new stuff and new technical words.

Quote
A clear wiring diagram would also be a good idea.
Ah, of course. I'll always add a wiring diagram in the future.


Quote
What does "blocking a motor" mean ? I read it as you mechanically blocking the shaft of the motor so that it cannot spin anymore.
Yes.

Quote
Monitor the motor supply voltage while doing your tests. Does the voltage remain steady as you block the motors?
I got a new power supply yesterday (0-30V, 0-10A, https://www.amazon.com/Eventek-KPS3010D-Adjustable-Switching-Spectrophotometer/dp/B073TW8H2S)
I have also removed all motors except one and measured the voltage and current of the motor.

When the motor is spinning:
Vm: 5 V
Im: 0.03 A

When the motor is blocked:
Vm: 4.5 V
Im: 0.3 A

So there is a 0.5V voltage drop (through wires and internal components?). That's a lot imo.

Quote
Unless designed to operate in that manner stalled motors will overheat and burn out
Yes, that will definitely be a big problem... hmm... maybe I could change my design or(and) PWM the pins.

MAS3

Hi.

Did you notice that the stall current is ten times that of the free running current (as predicted) ?

How did you do your measurements ?
Did you rely on the displayed values of that power supply of yours ?
Or did you use an external DMM and put the test leads at the motor ?
Did you also use those free alligator leads that came with your supply ?
If you can notice those heating up, then that's where the 0.5 volts went.
The leads would then become resistors (well actually they always are, but now you can see what that means/does), limiting the current through the motor.

Your power supply can regulate voltage and limit current.
Whenever it starts limiting the current, you'll see the voltage go down.
This is what is displayed on the supply's front (CV for Current Voltage, and CC for Current Current).
So the voltage shown is only equal to the set or desired voltage as long as the current limiter doesn't need to cut in yet.
Can we assume you did your tests with the current limiter set to max, 10 Ampere (using both dials) ?
Have a look at "blink without delay".
Did you connect the grounds ?
Je kunt hier ook in het Nederlands terecht: http://arduino.cc/forum/index.php/board,77.0.html

MarkT

A lot of cheap, thin-wired croc-clip leads you find on eBay are no use whatsoever for any
significant current, I've measured over 1 ohm end-to-end on some of them, often varying
with movement/flexure.

My advise is make your own croc-clip leads from known decent wire and crimped and soldered
for reliable connections.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

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