DC 775 Motor not working on 12v 5A

Thread Starter

prerakk

Joined May 21, 2015
48
Hi friends ,

I am trying to make a fan using dc 775 motor .but problem i am facing is motor runs fine when i conect it directly to 12v battery .but when i try to plug it to wall adaptor it rotates one round and then dont work . When i turn of adaptor and turn it on again , samething.

I tried couple of chargers with followin specs but none of them seems to be working :

-12v 5A
- 19.5 65watt laptop charger
-12v 2A


Please what can i be do to make it work on wall power
 

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Martin_R

Joined Aug 28, 2019
106
It's quite possible that the start current will be way in excess of 5A and the power supplies are shutting down. The laptop charger voltage is waaaay too high for that motor, and the current demands would cause that PSU to shut down too
 

Thread Starter

prerakk

Joined May 21, 2015
48
It's quite possible that the start current will be way in excess of 5A and the power supplies are shutting down. The laptop charger voltage is waaaay too high for that motor, and the current demands would cause that PSU to shut down too
So what can be done in this case. Do i need to buy higher current adaptor or any additional component can make it work
 

Alec_t

Joined Sep 17, 2013
12,803
Did you notice the motor specification? The typical 775 motor rating I see online is 12V 80W. That implies a full-speed running current of 80W/12V = 6.6A and a start-up current of probably at least five times that, i.e. around 30A. So your power supplies are way too small. They will go into self-protection mode and shut down temporarily, then try to start up again. Rinse and repeat.
A high power rated series resistor might solve the problem, by reducing the voltage reaching the motor, but it will get hot.
 
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Thread Starter

prerakk

Joined May 21, 2015
48
Did you notice the motor specification? The typical 775 motor rating I see online is 12V 80W. That implies a full-speed running current of 80W/12V = 6.6A and a start-up current of probably at least five times that, i.e. around 30A. So your power supplies are way too small. They will go into self-protection mode and shut down temporarily, then try to start up again. Rinse and repeat.
A high power rated series resistor might solve the problem, by reducing the voltage reaching the motor, but it will get hot.
Yes but i want to use the fan continuously for 8-10 hours or even more so heating resistor doesnt sound efficient.

Also can this , inrush current limiter would be of any help ?
Source : https://www.ametherm.com/inrush-current/ptc-thermistors-for-inrush-current-limiting
 

vu2nan

Joined Sep 11, 2014
232
Hi prerakk,

The problem is that your power supply shuts down as the motor draws starting current (stall current).

That's because the motor stall current would be far in excess of the power supply rated current.

DC 775 is a dimension standard only.

There are models from different manufacturers with varied power ratings up to 300 W.

Only the no-load currents are specified, not the stall current.

If it's possible to measure the motor armature resistance 'Ra', the stall current 'Is' may be calculated.

Is = V / Ra.

The power supply / motor driver may be sized, based on the motor stall current.

Nandu.
 
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MaxHeadRoom

Joined Jul 18, 2013
25,190
The only precise way of measuring a DC motor current is with a known L.V. DC voltage supply of sufficient Va and a DC ammeter, measure current with locked armature , take a reading in a few locations of rotation and take the highest current in order to calculate resistance..
You may have to use a variable control on the motor for soft-start.
 
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