DC Motor: Slow Start

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Old 01-15-16, 08:19 AM
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DC Motor: Slow Start

Alright electrical gurus, I need some help.

I have a 12v DC pump used for oil. When it's cold outside it takes a couple seconds for the pump to start running. When I first setup the circuit I only had a 5 amp fuse and it would blow when it got cold enough. I have since added a 20 amp fuse and the pump starts everytime but it has a delay.

Is there a way to put a capacitor in the circuit to boost the starting current to eliminate this delay. I'm afraid that I'll eventually start blowing fuses again.

If so, do you know what the circuit would be called.

If not, is there another way to help this situation?
 
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Old 01-15-16, 08:26 AM
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Do you know what the specs are on the pump in terms of watts or horsepower? Is the oil liquid at the cold temp or has it gelled up?
 
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Old 01-15-16, 08:29 AM
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The pump may be slow starting because the oil is colder and thicker. Just increasing the fuse size is not a good idea unless you are sure the wiring can handle the current, AND the pump manufacturer doesn't specify a lower current protective device.

The only thing that would boost the starting current is boosting the voltage, which is not recommended unless the pump is rated for higher voltage. And boosting the starting current will make it more likely to blow the fuse, not less.

Where is the 12 volt power coming from? How far is it from the source to the pump? You want to make sure that wire is heavy gauge or you will drop a lot of voltage across the wire, limiting voltage to the pump motor. 12 or 14 gauge should be good.

Finally, if the pump motor has brushes, it may be time to change them.

Hope this helps.
 
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Old 01-15-16, 08:44 AM
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The pump is an oil scavenge pump I bought off ebay to pump oil back to my engine after lubricating the bearings on my rear mount turbocharger. The pump is from china and did not come with a spec sheet but here are the specs I can find.

Flow Rate: 3.7 GPM
Amps: 3.8 A 12 V

I do suspect that the higher oil viscosity when cold is causing a lot of the problem.

I was thinking that a start capacitor setup similar to what you find on HVAC fans could be utilized. What would the difference be. I'm not familiar with the hows and whys of those circuits.
 
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Old 01-15-16, 09:21 AM
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HVAC fans and compressor motors operate on A.C.
 
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Old 01-15-16, 09:42 AM
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As PATTBAA said, a starting cap can increase starting torque on an certain kinds of AC motors; but it's no help for a DC motor. 3.8 amps at 12 volts is only 40 watts which translates to less than 1/20 of a horsepower. In other words, it's not a very powerful pump. I think it just isn't quite up to the task of moving cold, thick, oil.

Assuming you have this wired to be on when the ignition is on, you probably have two choices: Turn the key to on and wait for the pump to get going before starting the engine, or replace the pump with one that has enough power to move the cold oil.

It is worth checking that you used heavy enough wire to the pump...14 gauge at least to make sure you don't have much voltage drop when the pump is starting. The starting current can be higher than the nominal 3.8 amps so you need heavy enough wiring. Voltage drop across the wire will reduce the power available from the pump.

You may be thinking of the big caps the car stereo folks add for their big audio amps. Those are there to supply large amounts of power but for very brief periods of time, just fractions of a second. Won't help in this situation unless your wiring is undersized and you put the cap right by the pump. Even then I don't think it will solve the basic problem of the pump being undersized. Besides, those caps cost more than a bigger pump would.
 
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