Control Of 1/2hp Motor Using 20a Sp Switch

[Lucien Nunes]

This is an existing setup and I am wondering whether to modify it. It's not easy to get at so if I don't need to, I won't. I'm not sure if it will become part of the fixed wiring or whether it will be connected via a 13A plug, probably the latter, but basically we have a 370W (1/2hp) cap-start single-phase induction motor driving a pump, and a small DC power supply, controlled together by a 20A DP switch with indicator about 20 feet away. The supply arrives near the motor and feeds a double 13A socket outlet fixed to the chassis, but with a switch-cable in series. The motor and power supply plug into the two sockets. The switch cable is 3-core H07 with line, neutral and SL, the neutral being for the neon in the switch. Obviously, only one pole of the switch is used, so turning it off is akin to turning off the SP switches on the sockets themselves.

Doing it from scratch, I would put a normal contactor starter near the motor and run ELV control to a button station. However this is in situ and working, and for various reasons it's difficult to alter. The motor ought to have a motor circuit breaker / manual motor starter which can easily be added adjacent to the motor. No moving parts are accessible without the use of tools and there is no specific hazard from unexpected restart so I don't think it needs no-volt release. I wonder whether the switch is officially rated for very light AC3 duty - maybe one start per day? What about SP vs. DP?

Any thoughts?

 

[marconi]

My reply is in two parts; from experience and from thinking.

From experience: When I was a teenager my next door neighbour (-who taught electrical engineering and electricians courses at the local college) had a 1/2hp single phase capacitor start induction motor driving his woodworking lathe which was controlled trouble free by a round crabtree single pole dolly switch for many years. This switch was replaced later by a an mk 5Amp light switch only because he dropped a hammer on the crabtree and cracked the cover - this too operated without problems.

From thinking: For Ac3 duty - 5-6 times flc on closure and breaking the flc I reckon a 20Amp DP switch for this application (induction motor and dc power supply of type unspecified) has ample contact area, speed of making and breaking and width of contact separation to handle the inrush current on closure and draw the arc on opening. If you wanted to introduce some precautionary measures you could: a) wire a double pole so that its two sets of contacts were in series and b) fit a varistor across line and neutral inside the double socket. The varistor has the benefit of providing some protection to the neon indicator from transient over-voltage. From your description the switch is for functional control and not isolation purposes and so it is only necessary to switch the line. For varistor supply see:https://www.rapidonline.com/epcos-b...rd-series-metal-oxide-leaded-varistor-51-4628 or http://www.littelfuse.com/~/media/e...tors/littelfuse_varistor_la_datasheet.pdf.pdf

 

[Darkwood]

The use of switching devices and control for motors requires a little bit of knowledge of operation and duty of said motor as well as the starting methods and motor rating, it seems you have provided the majority of what is needed here to give somekind of response but as a designer of such systems, I tend to give the correct recommended solutions as oppose to what might suffice and last the duration.

Take a standard 20amp DP switch, these as far as I know are designed for resistive loads and light inductive if used to switch the loads on and off, because you are switching what is classed as a high inductive load then using the switch to operate the motor directly is not recommended, a few variables come in to play here from quality of brand of switch to whether it is actually used to directly control the motor.
The AC3 rating is a tad too broad and usually just applies to general inductive loads, what you would require is a switching device that can safely open and close on load and handle the demand and duty of the load, the rating you'll be looking for is an AC 22a for frequent operation or AC 22b for infrequent operation of high inductive loads.

Even when picking say a rotary isolator you would need to check whether it is designed for on load switching or not as many are just for isolation and are not rated for on load switching - (the wholesalers like to stock the cheaper brands to woo the customers).

Whether you can get away with a domestic 20amp switch is another question and not one I would like to say yes or no to given the variables, its not just about the rating but the damage the inductive arc does to the contact surfaces as the switch is opened, if the switches are not rated for high inductive loads then it can lead to poor switching contact, welded contacts and/or damage to the load.

You mention control circuits but as the reg's around that area are quite indepth then that would be for another discussion, although if your pump does not have more than 2 actuators then you can use the up to 400v control although other factors about the installation may over-rule this and limit your options. As for protection, at 0.37Kw it is not required to afford motor O/L protection but the practical solution is to fit protection to reduce maintenance and to highlight issues.

 

[Darkwood]

Just an additional here, most of us have a plug in domestic motor in our vac's although you tend to plug in off load and use the product switch to turn the vac on, handling the load is not an issue for sockets in this case as they are normally switched off load and in your case the 20amp switch is several times the rating of the FLC of the motor so off the cuff it will probably do, on paper it is a no no.

 

[Lucien Nunes]

Agree that it's not correct on paper, usual problem that it has been working for years so people think it must be OK. I don't know the brand of the control switch to ask the manufacturers what rating applies to it (although I could swap it to any brand for which I can get a rating that approves of such use).

Better might be to fit a contactor near the motor but leave it with 230V control via the switch, as you say there is only one control point other than the means of isolation. I have a 230V-coil DP contactor to hand rated 8.5A AC7B, should be ample as the starts are so infrequent, it might only be used a dozen times a week. Does this seem reasonable? I can't see any requirement for pushbutton control, it would be difficult to swap the switch to a button station and if we did, we would lose the indicator as there aren't enough cores in the cable.

I do want to see some protection added as the motor is out of sight and hearing so in the event of a fault it could be rampantly overheating before anyone noticed a problem. It's also 40+ years old and definitely doesn't have internal thermal protection.

 

[Darkwood]

If its hard wired in then you need to consider maintenance and the safe isolation methods, as the motor is also a distance from the power point then you need to either be able to lock it off or ensure in other means it cannot be energised when it shouldn't be, as you are also altering or making additions to the system then you also have to comply with BS7671 3rd Ed' for the general installation and the BS60204 for any control designs, the last 230v sump motor I did required Rcd protection as a matter of manufacturers recommendation, although that aside my installation methods for power and control wouldn't have required it.

 

[Lucien Nunes]

Ah, it's now been confirmed it will be plugged into a 13A socket, either on a dedicated circuit or the RFC.

 

[Darkwood]

Well there you go for Isolation then, you need to test the Motor Ins' values if its as old as you say, that RCD may not like what it see's

 

[Lucien Nunes]

Yes, that will be the first test. It's a Crompton Parkinson in good clean condition, hasn't been damp or anything so I expect it to be fine. I did wonder about nuisance tripping of the RFC RCD if changing the switching to DP without a leading neutral contact such as you occasionally get with fluorescents and other inductive loads on DP FCUs. Probably not going to be an issue.

The final niggle is that the 3-core flex is going to have a green/yellow core sleeved up as a line conductor. I'm not keen on that usage although AFAIK nothing prohibits it in a flexible multicore. Unless it's not ordinary 3-core but that's what I've been told it is.

 

[Darkwood]

The sleeving of an identified earth is not acceptable again on paper, as a conductor in itself it will be sized the same and give the same insulation been a flex so again it comes down to good and bad practices, can you not pull in new using the existing as a drag if its ducted or something ?

The flex is ok as long as its suitable for environment or has additional protection where required as is any cable used.

 

[happysteve]

The sleeving of an identified earth is not acceptable again on paper...

You got a Reg number for that, darkwood? I've been struggling to find one.

514.4.2 says, "Single-core cables that are coloured green-and-yellow throughout their length shall only be used as a protective conductor and shall not be overmarked at their terminations, except as permitted by Regulation 514.4.3." This is not the case here. So I would have no issue with this, provided the core was suitably identified at both ends.

Unless you're referring to regulations or guidance other than BS7671?

~tin hat on~

 

[Darkwood]

I was refering to that reg' Steve, my wording wasn't the best but I did say in a flex it would be bad practice hence no exactly against any reg... Tin hat off

 

[happysteve]

Ah, fair enough Thanks for the clarification, always happy to learn.

 

[marconi]

Rather than placing the contactor adjacent to the motor what about co-locating a small solid state relay(SSR)
with the DP switch? The DP switch would need to make the neutral pole first and break it last for what I am about to suggest. The neutral pole is used to supply the SSR with the line feed. The line pole is used to switch the SSR. The line pole derives its supply from the output of the neutral pole. With this scheme all load current switching is done by the SSR.
For SSR see https://www.rapidonline.com/Catalogue/Search?filterSearchScope=1&filterCategoryPathROOT=Relays+&+Solenoids&filterDiscontinued=False&filterCategoryPathROOT/Relays+&+Solenoids=Solid+State+Relays&sourceRefKey=RAhiSQZjx&keywords=*
Randomly switched SSRs are recommended for inductive loads but one manufacturer at least says its zeroswitching range are satisfactory when load PF is better than 0.7.
For thermal overload protection affix one of 1,2 or 3 below to the motor in series with the line;
1. thermal cutout https://www.rapidonline.com/pdf/61-2078.pdf
2. thermal fuse https://www.rapidonline.com/pdf/61-2078.pdf
3. thermal switch https://www.rapidonline.com/Catalog...cs&sourceRefKey=_l0TBFqnG&filterSearchScope=1

 

[Lucien Nunes]

An ingenious scheme but not one I could use in practice due to the non-standard configuration and lack of space to fit the SSR. I would also have to switch the neutral to the SSR input as I need the neon indicator which is tied to the load neutral terminal and I doubt the existing switch has sequenced neutral pole operation (although new MKs for example do have). If I were going to make any significant changes I would simply replace the whole lot, but as I say it's difficult to access, difficult to put anything but a 1g accessory where the switch is and evidence is that the existing switch has worked for years or decades. Re. overheat prevention, it's not practical to attach anything to the motor, therefore detection must be based on line current via a motor circuit breaker or thermal overload relay.