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Topic: 4A door lock motor - Really? (Read 236 times) previous topic - next topic

dougp

Jul 13, 2017, 11:43 pm Last Edit: Jul 13, 2017, 11:52 pm by dougp
Attached is a photo of a door lock motor and my ammeter. The motor is stalled against the stops because power is applied continuously; just for test purposes. I wanted to get an idea of the current draw before I proceed to the motor driver.

The motor is just one of the bargain types - so no data sheet - seen all over the internet. I wanted it for a linear actuator.

My question: Is 4 amps actually a realistic number for a small DC motor like this? Am I just reading the meter wrong?


Daenerys

>Is 4 amps actually a realistic number for a small DC motor like this?

It is when the motor is stalled!


dougp

It is when the motor is stalled!


OK. Thank you.

So, a suitably sized driver should handle what, two, three, or more times that?

Daenerys

A L298 H-Bridge driver should work, with outputs paralleled.

You can get 'em cheap.  About $2 (on eBay), shipped, for a complete board with the L298.

Buy a few spares just in case. 


 


dougp

I'll check it out. Thanks again.

MarkT

#5
Jul 16, 2017, 01:34 pm Last Edit: Jul 16, 2017, 01:36 pm by MarkT
OK. Thank you.

So, a suitably sized driver should handle what, two, three, or more times that?
It needs to handle one times it for short periods without being damaged (otherwise you'll
constantly be going out to buy replacement motor drivers)
The driver's continuous rating (thermal rating) can be a lot less since you should never be
in stall condition for any length of time - the motor will not handle continuous stall for
long periods, it will simply cook.

You prevent such damage by either limiting the on-time or using end-stop switches to detect
when to stop.  However its really tedious if your motor driver pops just because you hit the endstop
by accident or design for 1/2 a second.

And yes, motors take lots of current at stall - typically 5 to 20 times the rated current.  Its a constant
theme on these forums that people under-estimate the current requirements of motors and servos.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

dougp

Its a constant
theme on these forums that people under-estimate the current requirements of motors and servos.
I didn't want to be one of those people. Door locks have become so trivial an item I never gave much thought to them (except when they don't work) 'til I thought to try to drive one.  I do have some experience with three phase industrial motors so I knew it would draw more when stalled, that's why I decided to test it.

Which reminds me - daenerys suggested paralleling the outputs. Does this mean current hogging is not an issue with the L298?

jremington

The ancient L298 is a poor choice for a motor with a 4 ampere stall current. This is a better one.

MarkT

Stalling of an industrial induction motor is rather different to stalling of a DC motor, but yes, lots
more current flows in both cases.  With a DC motor the torque doesn't drop though, its at its maximum
at stall.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

dougp

The ancient L298 is a poor choice for a motor with a 4 ampere stall current. This is a better one.
Already ordered a pair of L298s. Oh, well.

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