slow start dc motor

[jack mcleod]

request for slow start schematic (that inter. home hobbiest can build) for
a 12 V DC boat water pump. running draw 3-5 A.

 

[jkmadsci]

2 questions
Why do you want to do this?
How long should it take for the motor to reach full speed?

 

[john1]

Its probably blowing fuses starting under load.
Suggest stage start, simple relay drops out after three seconds,
leaving motor on full supply.

unless theres more to it ....

John

 

[audioguru]

Maybe the motor over-revs trying to prime itself.
A PWM motor speed controller could use a capacitor slowly charging up to have a slow start.

 

[Russlk]

First, use a slo-blo fuse. Second, choose a series resistor that will limit the current to the fuse value. If you are using a 10 amp fuse (recommended), use 1 ohm, the motor resistance will be enough to limit the current to less than 10 amps. Third, short out the resistor when the motor is up to speed. You could use a time delay relay or a manual switch if it is a one time deal. Alternativly, a circuit could monitor the motor voltage and switch the resistor out at a set voltage.

 

[jack mcleod]

A 400 watt inverter (12 V DC to 120V AC, rated 300 watts continuous) runs a video & TV until someone gets a glass of water. The tap(houseboat) turns on a 12 V water pump... on occassions the TV flickers then continues..But usually the inverter shuts off. Yes I know the obvious non electronic answers. Source = two 12V deep cycle bats. in parallel. Will try the 1 ohm resistor shortly. Considering relay fron AC side of inverter to Sw pump source on to motor battery while inverter is running.
My guess is it takes 1-3 sec. for the pump to get up to speed. Considered large caps to expensive. Still interested in a slow start circuit. Still open
to suggestions. THANKS GUYS. Jack Mcleod.

 

[audioguru]

Hi Jack,
I think the symptom is caused by a wiring problem.
It sounds like the inverter and pump are connected together then have a long run on thin wires to the batteries. The inverter needs very heavy wires by itself to run to the batteries.

 

[bryan1]

A 400 watt inverter (12 V DC to 120V AC, rated 300 watts continuous) runs a video & TV until someone gets a glass of water. The tap(houseboat) turns on a 12 V water pump... on occassions the TV flickers then continues..But usually the inverter shuts off. Yes I know the obvious non electronic answers. Source = two 12V deep cycle bats. in parallel. Will try the 1 ohm resistor shortly. Considering relay fron AC side of inverter to Sw pump source on to motor battery while inverter is running.
My guess is it takes 1-3 sec. for the pump to get up to speed. Considered large caps to expensive. Still interested in a slow start circuit. Still open
to suggestions. THANKS GUYS. Jack Mcleod.

Hiya Jack,
that inverter your using is probably running flat out just running the TV and video, besides @300 watt you can buy them for a $100.00 or cheaper. Now how many amp hours are the batteries rated at? In Australia you can get a 1500 watt inverter with a 3000 watt surge for just over $500.00. My recommendation would be get a bigger inverter. Well I'm using a 1700 watt inverter in my shed and it runs a 1 hp motor on a drill press no problems aswell as running all the lights.

Hope this helps

Cheers Bryan

 

[williB]

A 400 watt inverter (12 V DC to 120V AC, rated 300 watts continuous) runs a video & TV until someone gets a glass of water. The tap(houseboat) turns on a 12 V water pump... on occassions the TV flickers then continues..But usually the inverter shuts off. Yes I know the obvious non electronic answers. Source = two 12V deep cycle bats. in parallel. Will try the 1 ohm resistor shortly. Considering relay fron AC side of inverter to Sw pump source on to motor battery while inverter is running.
My guess is it takes 1-3 sec. for the pump to get up to speed. Considered large caps to expensive. Still interested in a slow start circuit. Still open
to suggestions. THANKS GUYS. Jack Mcleod.

Using caps is a good idea..
Have you measured the voltage (12 V side) drop on the system when the pump comes on??
you may not need a super cap or anything like that but a couple of large capacitance low voltage caps should solve the problem..

 

[Nigel Goodwin]

Hi Jack,
I think the symptom is caused by a wiring problem.
It sounds like the inverter and pump are connected together then have a long run on thin wires to the batteries. The inverter needs very heavy wires by itself to run to the batteries.

I've no idea what the wiring on the houseboat is like?, BUT wiring in caravans is usually completely useless, using wire far too thin, and far too long.

It sounds like the system is wired extremely badly?, move the inverter close to the batteries, and use THICK wires directly from the battery to the inverter. This way the long wires are at mains voltage, where the current is small, so the voltage drop will be small as well.

 

[Russlk]

If you run the water pump direct to the battery, it should solve the problem. Heavier wire won't hurt either.

 

[audioguru]

If you run the water pump direct to the battery, it should solve the problem. Heavier wire won't hurt either.

It doesn't matter if the pump runs a little slower on its existing wire.
It is the inverter that needs all the power it can get.

 

[williB]

nevermind

 

[Russlk]

But the water pump is what is causing the problem, the inverter runs OK by itself. Running either the inverter or the water pump (or both) direct to the battery should solve the problem.

 

[audioguru]

If the inverter works fine at 10V with the existing voltage drop in its wiring, then no matter how the pump is connected the battery voltage drops low enough to kill the inverter.
Get 12V or more to the inverter and the problem is solved.

I wonder how old are the batteries. My last failed car battery dimmed my car's lights for about 1 week before it wouldn't start the car anymore. It was about 6 years old. Sealed and "maintennance-free". It probably dried-out.

 

[john1]

I generally open up and check out those sealed types.
Ive ended up with good batteries that have been discarded
and only needed a bit of TLC.

John