12 Vdc motor blow fuse

[weeb0]

Hi,

I have a small circuit :


12Vdc--fuse--SSR---motor---gnd

The parameter of the fuse : little green lookalike resistor 5A.
relay : 3.5A, 5A peak (1sec)
motor : 3.3A, 12V. When stopped, the resistance of the motor is 1ohm.

The problem I can see:
When the motor start, it consume about 12A for a small amount of time. My fuse is 5A, so she blow. But what about my relay ?

Do you have any suggestion to help me with this issue ? I don't know much about motors and how to control them.

thank you

 

[MikeHalloran]

For a shorter time, the motor draws a lot more than 12A of current. You need to get the data sheet for the SSR and see if it's rated for motor starting, or if it has some way to protect itself from excessive currents.





Mike Halloran
Pembroke Pines, FL, USA

 

[weeb0]

Thank you Mike,

I'll look for the datasheets of my SSR, I'm pretty sure he's not intended for motor starting. But we have a lot of unit shipped and I did'nt get any problem with the SSR (maybe the fuse is protecting it and if I try a bigger fuse I'll get problem with the relay).

what is the correct way to size the fuse ? Do I have to use a slow blow at about 4A ? or I need a fast blow at about 20A ?

do you know any SSR I could use ?

Thank you

 

[MikeHalloran]

Some people here are much more expert than I at sizing fuses. I hope they show up soon.

They'll probably ask you what kind of motor it is, and what load it's trying to accelerate.



Mike Halloran
Pembroke Pines, FL, USA

 

[itsmoked]

The normal way you would size a fuse for that application is to run tests. You pick what you think will work then try it. If it works then reduce it until it doesn't work. Then make sure you increase from that value a reasonable amount and then test it by rapidly turning on the motor many times in a row. If the fuse will live in a hot environment you need to make sure it works well in that temperature also.

Motors generally require slow blows because they almost always look like a dreadful short when first turned on, especially if loaded some. I would get several different slow blow values that are around 4A and try them. Settle on the lowest value that won't nuisance blow in a warm temperature.

Make sure you really test your solution! Nothing is worse then sending out a "solution" to the field that causes another problem...

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[weeb0]

Thank you for your help,

I'll add some more information

The motor is in fact a piston pump. The load is always the same. There is not a big pressure and not a big flow.

Thanx

 

[itsmoked]

A piston pump is about the worst thing you could be looking at. Because at certain piston positions the starting load could be much greater than in other positions and a bunch of tests may not even show you that worst case. Ouch.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

The starting time/current is also very dependant on back pressure. There will be times when the motor will try to start against almost full back pressure. This won't affect the maximum current but it will affect the time that the high current persists. That in turn will affect the fuse selection.
A fuse that works well with a low inertia fan load may not be able to start the same motor driving a piston pump against back pressure.
Consider dual element or "Slo-Blo" fuses. Code for AC motors is 125% of motor full load current rating for time delay fuses. That's probably a good starting place for the tests recommended by itsmoked. Also, do you want to protect the motor from overloads, the relay in the event of motor failure or the just protect the circuit board in the event that either the motor or the relay short out? Your protection philosophy will influence the fuse selection.
respectfully

 

[blacksea]

You need any "soft starter" device (instead SSR) for current limiting during motor's acceleration - Dc motor amp for example.

 

[weeb0]

Thank you all for your advice.

Do somebody could give me any part number ?

It`s for a 3.3A 12VDC motor.

Thank you

 

[weeb0]

What is the best practice to protect the equipment ?

In my mind, I'd like to limit the current so the motor will not blow my power supply. I think the fuse I want is to protect the relay, the pump and the board.

Thank you

 

[blacksea]

Look for something cheap - from Sprint Electric for example.

 

[sreid]

The most current the motor can pull at start up is 12 amps (12 V/ 1 Ohm). Small motors like this are designed to start with full voltage applied. The current will quickly decrease as the motor accelerates. I would think that a 5 amp slow blow fuse should work. To stop smoke and fire from the motor, you must keep the fuse value below 12 amps so the fuse will blow under stalled motor conditions.

Most DC relays are really FETs and I'll bet the 5amp for 1 second is a thermal rating and the FET will handle 12 amps for a short period of time (clearly you have a number in the field that continue to work). When you determine a fuse that will work, stall the motor and see if the relay blows before the fuse does.