DC Motor delay start problem

Thread Starter

dante_clericuzzio

Joined Mar 28, 2016
223
I have a 12 volt DC motor and when using a battery -- it kick start immediately without problem. However i don't want to waste the battery charge all the time considering that it need to be recharge all the time. So i directly using a step down DC adapter which is also rated at 12 volt 7 Amp. It does work but there is a problem...

When the switch is quickly press the motor will turn a while and then stop -- but when the switch is slowly press the motor will start slowly and eventually normal. I feel this is quite annoying i don't have the patience to switch slowly and then full...it would be good if it just turn the motor when the switch is on no matter how it is done.

Would anyone shed some light what could be the problem and how to remedy this?
 

MaxHeadRoom

Joined Jul 18, 2013
21,182
Sounds like the SMPS . is going into current limit.
You could either build a ramp up circuit.
But the much easier way is to just use a straight mains transformer followed by a bridge rectifier, that is all you need for a DC motor.
What is the current rating of the motor?
Max.
 

Thread Starter

dante_clericuzzio

Joined Mar 28, 2016
223
Sounds like the SMPS . is going into current limit.
You could either build a ramp up circuit.
But the much easier way is to just use a straight mains transformer followed by a bridge rectifier, that is all you need for a DC motor.
What is the current rating of the motor?
Max.
1.5 Amp the motor should run...even at 7 Amp DC adapter won't turn it on quickly...i was thinking if an electrolyte capacitor will solve the problem. Do you think? direct transformer to bridge rectifier need to have a precise step down to 12 volt..which it cannot do
 

Dodgydave

Joined Jun 22, 2012
9,304
A capacitor won't increase the supply current, it will hold the supply for a brief period, you can try a 1000uF for starters...
 

Thread Starter

dante_clericuzzio

Joined Mar 28, 2016
223
A capacitor won't increase the supply current, it will hold the supply for a brief period, you can try a 1000uF for starters...
The reason is i was thinking if the current hold a brief moment and accumulate the power into the capacitor and then release to the motor to kick start
 

Irving

Joined Jan 30, 2016
939
1.5 Amp the motor should run...even at 7 Amp DC adapter won't turn it on quickly...i was thinking if an electrolyte capacitor will solve the problem. Do you think? direct transformer to bridge rectifier need to have a precise step down to 12 volt..which it cannot do
The starting current of a motor is close to its stall current, that could be several amps. A battery can do that, but an SMPS's overcurrent protection is too fast.

A big capacitor would possibly help, but it got to big enough to supply those few amps long enough for the motor to start spinning up. A very big one also needs a large charge current, which may cause the same problem with the SMPS.

A linear supply is the correct solution, or you could try a simple DC motor controller to provide a slow start function and ramp the voltage up slow enough to get the motor spinning without tripping the SMPS's protection.
 
Last edited:

MisterBill2

Joined Jan 23, 2018
6,659
The problem may also be related to the size of the wires. If the wire is too small then there will be some voltage drop and that may also be a problem.
But also I suspect that the power supply 7 amp rating is not actual, and that the power supply is indeed not providing 12 volts.
So really we need more information, and what kind of switch works differently with a fast press or a slow one? I am not familiar with that type of switch.
 

Irving

Joined Jan 30, 2016
939
There isnt a special switch. If you catch it right the motor has shimmied to a position where the next attempt pings it between poles giving it just enough momentum to start.
 

MisterBill2

Joined Jan 23, 2018
6,659
OK, and how does Irving know that much about the motor? Irv is not the TS, is he?? And what sort of application uses a motor controlled by just a momentary push-button switch? There is a lot of information not given here.
If the way that the motor starts depends on the position of the rotor than the motor needs to either be repaired or replaced. One simple option would be to have the motor powered by the battery and use that DC adapter to keep the battery charged .
Or possibly that DC adapter is rated at 0.7 amps. That is a much more believable number. A 7 amp adapter is not very common.
 

Irving

Joined Jan 30, 2016
939
Irving doesn't... but Irving has years of experience... and teaches students that do this stuff all the time... its a common issue... its an odd year if I don't have at least one student project that hits this exact problem...

And the OP dosn't say its a momentary, just 'when he presses the button', its probably a rocker switch. I'm guessing English isn't his first language...
 
Last edited:

MaxHeadRoom

Joined Jul 18, 2013
21,182
It sounds to me as though the OP is hitting the switch repeatedly-momentarily and effectively jogging the motor up to speed to avoid the continuous inrush current. :confused:
Max.
 

MisterBill2

Joined Jan 23, 2018
6,659
Irving doesn't... but Irving has years of experience... and teaches students that do this stuff all the time... its a common issue... its an odd year if I don't have at least one student project that hits this exact problem...

And the OP dosn't say its a momentary, just 'when he presses the button', its probably a rocker switch. I'm guessing English isn't his first language...
If some have hit this exact problem before then Irving should be able to provide not only the correct answer but also a description of why the solution works. My guess, based on not much information, is that the power supply is not able to deliver adequate current to start the motor. That it is not really able to deliver 12 volts at a full 7amps. In that case a capacitor of adequate size across the supply will be one way to make it start.
 

Irving

Joined Jan 30, 2016
939
I did, a simple PWM DC brushed motor controller will limit the volts and therefore the amps while the motor gets up to speed... as soon as it starts generating back-emf the current requirements drop off quickly. Of course if the motor is connected to any significant mechanical load then all bets may be off as raw torque = amps is needed to start under those conditions...

When the motor is stationary the only thing limiting the current is the armature resistance and that will be quite small, typically a few ohms or less... the motor I have on the bench right now has a winding resistance of 0.05ohm. On 24v it takes only 3A free running, but it needs 40A to start... just to get enough torque to overcome the initial inertia of its own armature and bearing friction. Stalled it will easily take 24/0.05 = 480A if the controller didn't current limit at 110A. I can run that quite easily from my 1.5kW bench supply (Digimax 30v/50A)

A large capacitor MAY be of assistance, it all depends on the motor windings and their resistance - the RC time constant needs to be enough to get 1/2 a turn at least in - that could be a few 100mS. Lets say a 10A pulse for 150mS Q = It = CV so 10A * 0.15S/12 = 125000uF, yep, that's a big capacitor!
 

MisterBill2

Joined Jan 23, 2018
6,659
There are soft-start systems that will limit the starting current to the normal running current. But I am reluctant to suggest such to somebody impatient.
 

Thread Starter

dante_clericuzzio

Joined Mar 28, 2016
223
The problem may also be related to the size of the wires. If the wire is too small then there will be some voltage drop and that may also be a problem.
But also I suspect that the power supply 7 amp rating is not actual, and that the power supply is indeed not providing 12 volts.
So really we need more information, and what kind of switch works differently with a fast press or a slow one? I am not familiar with that type of switch.
12 volt i guaranteed it does ...but 7 amp that is doubtful but it will work when press slowly...and then go to full speed is fine...but won't work if it is quickly press direct to full speed.
 

Thread Starter

dante_clericuzzio

Joined Mar 28, 2016
223
I did, a simple PWM DC brushed motor controller will limit the volts and therefore the amps while the motor gets up to speed... as soon as it starts generating back-emf the current requirements drop off quickly. Of course if the motor is connected to any significant mechanical load then all bets may be off as raw torque = amps is needed to start under those conditions...

When the motor is stationary the only thing limiting the current is the armature resistance and that will be quite small, typically a few ohms or less... the motor I have on the bench right now has a winding resistance of 0.05ohm. On 24v it takes only 3A free running, but it needs 40A to start... just to get enough torque to overcome the initial inertia of its own armature and bearing friction. Stalled it will easily take 24/0.05 = 480A if the controller didn't current limit at 110A. I can run that quite easily from my 1.5kW bench supply (Digimax 30v/50A)

A large capacitor MAY be of assistance, it all depends on the motor windings and their resistance - the RC time constant needs to be enough to get 1/2 a turn at least in - that could be a few 100mS. Lets say a 10A pulse for 150mS Q = It = CV so 10A * 0.15S/12 = 125000uF, yep, that's a big capacitor!
I have tried 2200 uF it almost work but not quite...i will get a bigger capacitance if it will do trick
 

MisterBill2

Joined Jan 23, 2018
6,659
The description in post #17 tells me that there is a lot of information that we are not given about whatever is controlling that motor. It is obviously much more than a simple switch. Without an adequate explanation there is no point in offering guesses about what the cause of the problem might be.
 

Thread Starter

dante_clericuzzio

Joined Mar 28, 2016
223
This is consider solved now....after long search finally i tried computer power supply unit and connect it to the 12 volt 22 Amp out put cable and it works brilliantly...thx for all the responds
 
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