115 volt solenoid

[sparkymike]

Looking for a new solenoid similar to that in photo below. (Or a good second hand one.
115volt.
Mike.

[sparkymike]

I forgot to add.
Bobbin size is around an inch long.
To protect this in future, what size fuse could be fitted in line ?
Mike.

[Andrew B]

A 100 degree thermal fuse?

[vidjoman]

What is it fitted to? What pulling power would it need or does it just latch something.

[broadgage]

Originally Posted by Andrew B

A 100 degree thermal fuse?

Agree, attempts to protect solenoids with an overcurrent fuse are unlikely to succeed.

[PJL]

I think we need more info. What's it from and how has it failed? If its failed electrically I would have thought rewinding is your best option.

[emeritus]

In the 1970's I rewound the mains solenoid that provided the door interlock on mum's spin dryer. I made an accurate measurement of the wire diameter and simply rewound the bobbin with the same diameter wire to the same depth. Unlike transformer windings, the absolute number of turns didn't seem to be critical, and there seemed to be little point in counting the number of turns when unwinding. But the bobbin of that solenoid was readily accessible and easy to remove from its frame.

I used the take-up spool arm of my 8mm cine projector to do the rewinding, as it has a slipping clutch that provided suitable tension for the wire, which was fed and guided using gentle pressure between finger and thumb to control feed rate. I mounted the bobbin using the centre of a spare 50' spool, cut to size and secured with sticky tape. Being designed for vertically-oriented spools, cine projectors have locks to retain a spool in position. A reel to reel tape recorder designed for vertical operation with spool retention provision would probably have done the job but I didn't have one.

[sparkymike]

The solenoid operates a 'kicker' arm on the payout drum on a fruit machine.
The mechanical links had jammed up hence burnt out solenoid.
I have found a second hand one on line, so keeping fingers crossed it will be ok.
I was contemplating putting a three amp fuse in line with it but would need to check the operating current first before I fit it.
Mike.

[ex 2 Base]

If you can locate a 240 one you may follow what I have done in the past, it may save the day. I have altered quite few 415 volt contactor coils to 240 volt operation for my friends, by simply carefully removing turns. I measure the original resistance and removed turns until I had approximate 57% of the original resistance. (240 dived by 415 = 57. Then I reassemble and tested, ensuring that they all worked OK. I did simple temperature tests using one of those £4.90 guns from PRC So in your case about 48% of the original resistance to remain. I also found things are not critical turns/resistance wise. Ted

[Lucien Nunes]

The force produced by a solenoid depends on the flux density in its core, which depends (albeit in a non-linear way) on the number of amp turns produced by its coil. To change the voltage of a solenoid without changing its strength, the coil needs to be altered to provide the same number of amp turns at the new voltage.

The current in an AC solenoid is determined by its impedance, part of which is resistive and part inductive. The resistance is proportional to the number of turns but the inductance to the square of the number of turns. Reducing the turn count or resistance of an existing winding in proportion to a reduced operating voltage will cause an decrease in MMF because the impedance change will lie somewhere between the ratio and its square, and not provide an increase in current at the lower voltage sufficient to offset the loss of turns. The current density will however increase, possibly leading to a higher working temperature for the remaining part of the winding.

The ideal solution is to remove the old winding and re-wind with larger wire, to maintain the correct current density and deliver the correct number of amp turns when the bobbin is rewound with approximately the same weight of wire.

As for protection, it's hard to protect wirewound magnetic devices reliably against damage with a fuse, especially intermittently-rated ones where burnout can occur even with the normal operating current if accidentally stuck on. At 115V, a 3A fuse will allow the solenoid to take up to 500W without blowing (strictly 500VA, heat dissipation depends on power factor) which is sufficient to cook it vigorously if left on. The actual current will probably be much lower. A thermal fuse built into the winding is a much more effective form of protection, although some success might be had with one of lower operating temperature strapped to the outside.

In the case of electric motors, either a thermal cutout or fuse is used, or the starter contains a thermal overload relay that to an extent models the heat / time characteristics of the most vulnerable parts of the winding and trips the starter off if the total heat input is excessive. It is much slower but much more sensitive to overcurrent than a wire fuse or circuit breaker sensing only instantaneous overcurrent.

[G6Tanuki]

I wonder if there may be a way to protect the solenoid against inrush currents by using one of the same sort of PTC devices as used in the degauss circuit of Trinitron [and other] CRT monitors?

A low 'cold' resistance - so allowing plenty of volts/amps to hit the solenoid to pull it in, but then the resistance rises as the PTC device heats up, reducing the current safely to something lower but still adequate to keep the solenoid pulled-in.?.

[In times-past I remember various circuits for use with DC relays that allowed full power for pull-in but reduced the power to a holding current after a few seconds. Electrolytic-capacitors-shunted-by-resistors were often used, sometimes with included 'steering' diodes to change the charge- and discharge-profiles of the capacitor]

Of course the thermistor-idea wouldn't work too well in an application where the solenoid was being rapidly turned on and off.

[Leon Crampin]

You could reduce the standing current in the solenoid (assuming it can remain energised for some time) by putting a bridge rectifier - AC terminals - in series with the solenoid, and a large value electrolytic capacitor suitably rated, shunted with a resistor connected to the DC terminals. The resistor could be chosen to provide sufficient current to hold the solenoid plunger once energised, and the capacitor chosen so as to provide pull - in force for as long as is needed.

This scheme can fall down if repetitive action is needed and the resulting time constant doesn't match the other constraints.

I have also achieved a similar result more simply by wiring a 24V 21W automotive stop lamp bulb in series with a solenoid. Lamps need to be carefully chosen according to application...

Leon.

[McMurdo]

If you can measure the current it takes when actuated normally and then the current when it is stuck 'out', and see if you can get a polyswitch to suit. It will be slow enough to ignore the inrush but being a thermal device should hopefully trip before the coil gets hot enough to burn out.

I'm assuming here its an AC coil.

[sparkymike]

Hi Kevin, yes AC coil.
Re. fuses in motors, in past experience I have found than the on-board protection in large motors (and small) hardly ever saves the motor from a stall and burnt out windings.
Re. fuses I could start with a very small amp fuse and if that blows in use up it by a small amount at a time.
I could put an ammeter in the circuit to see what the current drain is when operating and then work on that.
Mike.