DC motor soft start

Thread Starter

Dave Newbie

Joined Jun 12, 2005
23
Hi, looking for ideas to DIY soft start a DC motor @ 12 volt and 300amps draws about 1300 amp inrush. Any assistance appreciate.
 

drc_567

Joined Dec 29, 2008
1,156
...
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... Something like this may be sufficient. The start arm is gradually moved to the right until full speed is obtained. It is called a 2 point starter. The selection of resistors should include a safe wattage rating.
 

LowQCab

Joined Nov 6, 2012
1,372
It’s a engine starter motor
Do You realize that it will only run for a maximum of about
30-Seconds before smoke starts coming off of the Windings ?????
That is, if you start out at room-temperature, and not when its already hot,
then you have to wait for a good ~20 to ~30 minutes before you can run it again.

Car Starters will not work as a Bicycle-Motor.
The are extremely inefficient.
They are designed to HIT an Engine hard for ~3 to ~5 Seconds, to get it started.
They're not good for anything else,
except maybe spinning-up a giant 200-pound Gyroscope to around a thousand RPM.
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drc_567

Joined Dec 29, 2008
1,156
... As far as any kind of resistor in series between the battery and the motor, about the only thing that would work is a length of graphite rod, maybe 12 in.long and 1/4 in. diameter. It would glow bright red at the current level mentioned, but would likely not be damaged.
 
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LowQCab

Joined Nov 6, 2012
1,372
You can NOT reduce the Current to the Starter-Motor when it is working.
If you reduce the Power to the Starter-Motor, the Engine will turn slower, and take longer to Start,
this would defeat your whole purpose for reducing the required Current.

If your Engine does not start within ~3-Seconds, something is wrong with the Engine, OR,
the Starter-Motor is not getting ENOUGH CURRENT, and so it is turning too slowly.
You may need a bigger Battery, and/or, Bigger-Battery-Cables.

If the Engine turns and then stops, while the Starter is operating,
the Current will spike-up very high.
This is not "Inrush-Current", this is "Locked-Rotor-Current", which is a different thing.

You could also have a bad Starter-Motor,
but I would suspect the Battery, and the Battery-Cables, first.
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shortbus

Joined Sep 30, 2009
8,971
Car Starters will not work as a Bicycle-Motor.
The are extremely inefficient.
They are designed to HIT an Engine hard for ~3 to ~5 Seconds, to get it started.
They're not good for anything else,
Gee you should have told the snow plow industry that and the winch makers back in the day. Both of them used starter motors to power the lift on the snow plow and the drum on the winches. The both use Ford starter motors with the only modification being the end of the shaft. The modification was a key way instead of a spline for the Bendix drive.

You can take a wound field starter motor out of a car and it doesn't draw much current at all to run. It's the turning of the engine that causes the high current draw in a starter motor not the motor its self.
 

MaxHeadRoom

Joined Jul 18, 2013
23,950
A starter (series field) motor exhibits very high torque (current) under load, reduce the load, you reduce the current.
The down side is, being a series field, they operate in a run away condition, i.e. RPM governed by load.
 
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