British composition casters and super casters (and probably Monophoto filmsetters as well) often used motor switches made by Allen West of Brighton.
The unit is known as SCF starter, was designed in 1940s and is normally very reliable, can work a few decades undisturbed. Of course it's no longer manufactured and while used units pop up here and there on eBay, new old stock is very hard to get. If it fails, you have to either repair it or replace it altogether.
The good thing about this type of switch is that after a power loss, the motor is turned off and doesn't start automatically on power return.
As it turns out, there is also a possibility of remote control by connecting two buttons to certain contacts. Makes it possible to add an emergency stop button for safety.
Some of the starters I have:
The left one is lying around. I had the enclosure in one of our parts boxes, and got the innards from John. Actually it's the first starter I repaired, with an additional benefit of learning how it works.
The right one is on our super caster, there were some problems with nuisance tripping, but after cleaning and bending some contacts I got it right.
Sometimes these starters were rebranded as Meech (sold by "Meech Electric Drives Ltd." - this company is still around, but deals in some other industrial automation products: http://www.meech.com
- see their little old logo too
). They were still manufactured by Allen West, which is also still around: http://www.allenwestbrighton.com
- who knows if they have spare parts?
Here is the Meech-branded switch on a large composition caster I'm rewiring:
Two additional ones are on a composition caster and the other super caster. I didn't take photos, but they look like the first one.
The switch works as follows:
1. After a "start" button is pressed, a momentary contact (normally open - NO) is made, turning the main coil on.
2. The coil closes three pairs of main contacts and an auxiliary contact connected parallel with the push button.
3. A button goes off, but it's shunted by the auxiliary contact. The coil is still energized.
4. There is also a normally-closed (NC) contact wired in series with the main coil. It breaks the circuit temporarily after pushing the "stop/reset" button, and semi-permanently after the switch tripping (until the spring-loaded vertical bar in the middle is reset by pushing the "stop/reset" button). It stops the current flowing through the coil, opening the main contacts and auxiliary contact.
5. There are also two NC bimetallic contacts which open on overload (too much current flowing in a certain time, but not as large as a short circuit)...
6. ...and two coils visible right away when you open the switch's enclosure. They act as an electromagnetic trip in case of a short-circuit or current surge.
7. Under these coils we can see two knobs, called dashpots:
They are supposed to be filled with special oil from Allen West, supplied with the switch. I have no idea if this oil was offered by Monotype, and it's obviously no longer available.
These dashpots have plungers attached to steel rods. The coils (see 6.) pull up these rods in case of surge/short-circuit. The movement is retarded by the plunger-in-oil system, based on how high the rod reaches (it's adjustable by screwing the dashpots up or down) and the oil viscosity.
When going up, the rods beat up the rectangular hoop holding the bar in the middle, and the bar springs off (similar to the way a mouse trap is triggered), breaking contact of the NC switch (see 4.).
After tripping the bar springs off:
As for adjusting the switch, there is a scale on the plates:
Set both dashpots so that the upper edge denotes the current normally running through the motor when the machine is working.
The motors usually have 0.5 ... 1hp (0.375 ... 0.75kW). You need to divide the power (in watts) by your mains voltage (inter-phase voltage e.g. 400V for 2 or 3-phase mains) multiplied by the motor's power factor (usually 0.
So, if I have a 1hp motor (i.e. 750W) and 400V 3-phase mains, the calculation is as follows:
I = P / (U * cos phi)
I = 750W / ( 400V * 0.8 ) = 2.34A
where P means power, I - current, U - voltage, and cos phi is the power factor.
It's meaningful if, and only if, you have the proper oil in the dashpots!
With no oil the switch may trip easier on starting the motor (especially under heavy load, like Varigear set to fast or clutch still active). If the oil is too thick, the switch may not trip at all, leading to damaging the motor windings, or tripping a circuit breaker/blowing a fuse on the mains.
CAUTION: All metallic parts of the switch which are not in contact with the enclosure are live! European 3-phase machines have 400VAC on the control circuit, as it's wired phase-to-phase rather than phase-to-neutral. It is absolutely necessary to isolate the switch (pulling the power plug if the machine is not hardwired, otherwise using an isolation switch or fuses) before opening.
And how to wire the thing? It may not be as simple as you think...
Most modern contactors have the supply and load contacts located opposite of each other. Here it's not the case, but there are labels (markings punched in cardboard) for the screw terminals you need.
L1, L2, L3 is the input i.e. supply phase 1, 2 and 3; A, B, C is the output i.e. motor phase 1, 2 and 3.
L1 and L2 are located on the left side of the switch, L3 and B - on the right side, A and C - on the front (visible above the coils).
There are also two additional terminals: 1 and 2. The former is between A and C; the latter is a bit deeper and looks a bit different depending on when the starter was made (Allen West modified the mechanical design at some point).
They are meant to connect a pair of external buttons for remote control: NC (stop button) between terminals "L1" or "L3" and 2; NO (start button) between terminals "1" and "2".
Like I wrote, the whole on-off circuit is wired across the phases, so use extra caution and buttons rated for higher voltage!
Better still, use relays and extra low-voltage control circuitry. I'm considering adding those to my composition caster and coupling them with my Raspberry Pi-based controller. It's pretty easy to do, both in hardware and software, so I'll give it a try
This is how the older design looks:
And a newer model of the switch, seen from the top:
The "2" terminal looks different (it's moved farther to the back), and all markings on L1/L2/L3/B terminals are visible. Unmarked terminals on the bottom: A, 1 and C.
Okay, but what if shit happens and your switch stops working?
If you figured out (with my help or not) how it works, you can use an ohmmeter to check it for short-circuits or lack of current path where it should be. It may be a matter of cleaning the contacts or bending metal parts.
If the main coil is damaged though, you can look for a replacement from a different type contactor. The coil voltage and type of current (alternating/direct) must match, unless you know what you're doing*. The inner window must be large enough for the core to fit in, and the coil can't be too large - otherwise the core won't go all the way (the thing won't switch on at all).
I successfully used a 400V AC coil from a type SLA-7 or SLA-12 contactor I had lying around. Those contactors were licenced from Brown Boveri Corp. and manufactured in my city by a factory called Ema-Elester, they were rather popular a few decades ago and spare parts are still available. The coil fits in rather nicely, but the screw terminals must be broken off and wires need to be soldered to the remaining part, where the solenoid wire is attached.
Two brackets/rails (together with their screws), holding the coil and core in place, after taking off:
Replacement coil, with terminals facing inwards, and the original square plate between the coil and the fixed (top) part of the core:
And it works too.
*It is possible to use a 24V AC or DC coil and power the whole control circuit from an additional extra low voltage supply. It's also possible to use a 230VAC coil wired between one phase and the mains neutral wire.