Compressor 3hp motor DOL starter question

[styleruk]

Hi, I kinda think I know the answer to this but will ask opinion anyway. I am fitting a new compressor pump and motor to my receiver and have bought;
twin cylinder pump;
3hp motor (2.2kw)

Although, the motor looks like this

So, I upgraded the ring main that the compressor will use in my workshop to 32 amp, I'm told it can take up to 40amp with no issue, but thought I'd put in a 32amp fuse for now and try it.

Now the confusing part is, machine mart sell compressors with exactly this setup on but only require a 13amp plug. Here. or this.

Spoke to my brother who is proficient in wiring and he said to put in a separate switch, so I went for a cooker switch, the same as my lathe. (which is on a separate circuit), but he checked with a wiring expert he knows and was told I should use a direct on line starter with a motor that big on single phase. Well, machine mart sell them for £45 here
Been reading about owners of these compressors and they do not have a problem triping. Most use normal 13amp plug! which would concern me.
I'm not a heavy user, I'm a hobby user, so it will not be running every day, just the odd weekend. I guess, a professional electrician would immediately go the super safe route and insist on a starter fitted, and I don't blame them. But I can't help feel it unnecessary.
so I'm holding off for now and will take these steps as I build the part.

1) fit the 32 amp fuse and cooker switch and try it out when I first build it. Guessing the worse that could happen is that it trips out every time I use it. Then step 2.
2) Order the DOL starter (10-16amp) and fit that in place of the cooker switch I already have.

A third option could be to replace the 32 amp with a type of slower trip. My brother mentioned this is possible. I have bought this, but apparently there is a circuit breaker that won't trip immediately. Bearing in mind this is from a separate board from my house.

So, anyone have any thoughts on this?

Regards
Simon

 

[davesparks]

A DOL starter provides precise overload protection for the motor and provides a no-volt release.

The wiring regulations require that a motor of this size must have a starter or device offering similar protections.

The 32A fuse will not provide adequate protection to the motor, under overload conditions the motor will burn out and be damaged before the fuse operates. The overload relay in a DOL starter however, if correctly set, will disconnect the motor before damage occurs from overload.

 

[styleruk]

OK, that's interesting. Why don't machine mart tell you this or explain or even sell a DOL with the compressors they sell? I understand their pro compressors but their standard compressors with relatively small tanks still have a 3 hp motor and you wheel them around and plug them into a 3 pin socket! That part of it makes no sense.

 

[davesparks]

OK, that's interesting. Why don't machine mart tell you this or explain or even sell a DOL with the compressors they sell? I understand their pro compressors but their standard compressors with relatively small tanks still have a 3 hp motor and you wheel them around and plug them into a 3 pin socket! That part of it makes no sense.

Because they are a shop selling things, they exist to take your money, they are not a wiring regulations advice service.

If it is sold with a 13A plug on it then it may well already incorporate the necessary protection, but since you have stated that yours connects to a 32A supply it is unlikely to be the same as those supplied with a 13A plug fitted

 

[styleruk]

Something I've just found when reading through the manuals of the Clarkes compressors.
Manual here
Firstly, on page 5 it states a 3pin plug.
Secondly, on page 11, it shows a reset switch on the motor. I have the same motor but I do not have that overload built in. There is instead, a blank plastic thread screw in place and nothing behind. Maybe, when they build their motors they fit an overload. Odd that they sell this as the same motor used on this compressor but there is no overload. That makes me think more that I should have the DOL starter.

 

[James]

the wheel round types if fitted with a uk plug as supplied, should have been designed and built with overload protection suitable for the motor and load connected.

this may in fact be the plug fuse.

because the compressor you have purchased is sold without a control system, you are responsible for designing and building something that meets requirements.

things to consider are
overload protection
over temperature protection
over pressure protection
behaviour in event of loss and restoration of power.

Edit:
Just re read the original post.
if I understand correctly,

you have bought a compressor (without motor)
a compressed air receiver
a single phase motor.

Putting all theses separate components together, makes you a machine builder and with that,
YOU ARE RESPONSABLE for the safe design and construction of the machine.

 

[styleruk]

the wheel round types if fitted with a uk plug as supplied, should have been designed and built with overload protection suitable for the motor and load connected.

this may in fact be the plug fuse.

because the compressor you have purchased is sold without a control system, you are responsible for designing and building something that meets requirements.

things to consider are
overload protection
over temperature protection
over pressure protection
behaviour in event of loss and restoration of power.

Edit:
Just re read the original post.
if I understand correctly,

you have bought a compressor (without motor)
a compressed air receiver
a single phase motor.

Putting all theses separate components together, makes you a machine builder and with that,
YOU ARE RESPONSABLE for the safe design and construction of the machine.

You are absolutely right. My situation is a working compressor with a small motor/pump. I am upgrading. I have the correct pressure release valves in the system to prevent over pressure, I have a pressure release also for when the motor starts to reduce load etc. It's the supply end that I am more concerned about. So a lot of the safety features I already have in place. My concerns are the effect of the motor starting and causing damage and why the compressors they sell do not need (or maybe they do), a DOL starter.
That said, the comment from 'davesparks', makes a lot of sense. I should be thinking about protecting my motor. So I think with that in mind, I might go immediately for a DOL starter from the off. I guess the short spike in amperage is enough to damage the motor over time. So I might go for this. I am assuming the rating 10-16amp, should match the motor, in this case 10/12 amp.

 

[Lucien Nunes]

The ready-built 3hp compressor will definitely have some kind of overload protection. It might be the thermal protector in the motor as you mention the button, or a direct-acting overload relay built into the pressure switch, but there will be something. It will have been tested as a unit and certified as compliant. When you buy a bare motor, it's your job to marry it up with suitable overload protection.

The main risk and reason for a thermal overload relay as used in a starter is that long, low overloads will set a motor on fire without blowing a fuse or tripping an MCB. Or, put another way, if you were to fit a fuse or circuit breaker that protects closely enough to prevent overheating, it would trip on starting. The thermal device has characteristics that match the thermal behaviour of a motor and allow for the fitting of a larger OCPD for short-circuit protection only, that will handle the starting current.

The 3hp 14cfm compressor that you link states that it requires a 30A supply, not 13A. However they are a bit wobbly on the motor specifics; there is a figure of 2250 watts but this is the mechanical output power, it's a direct conversion of 3hp. The electrical input, which you would expect to see given as a line current or (as it's a complete appliance rather than a bare motor) a power consumption, is not stated.

 

[styleruk]

The ready-built 3hp compressor will definitely have some kind of overload protection. It might be the thermal protector in the motor as you mention the button, or a direct-acting overload relay built into the pressure switch, but there will be something. It will have been tested as a unit and certified as compliant. When you buy a bare motor, it's your job to marry it up with suitable overload protection.

The main risk and reason for a thermal overload relay as used in a starter is that long, low overloads will set a motor on fire without blowing a fuse or tripping an MCB. Or, put another way, if you were to fit a fuse or circuit breaker that protects closely enough to prevent overheating, it would trip on starting. The thermal device has characteristics that match the thermal behaviour of a motor and allow for the fitting of a larger OCPD for short-circuit protection only, that will handle the starting current.

The 3hp 14cfm compressor that you link states that it requires a 30A supply, not 13A. However they are a bit wobbly on the motor specifics; there is a figure of 2250 watts but this is the mechanical output power, it's a direct conversion of 3hp. The electrical input, which you would expect to see given as a line current or (as it's a complete appliance rather than a bare motor) a power consumption, is not stated.

OK, so fitting a thermal overload into the box where there seems to be space for one is a good idea. Would you recomend a device?
I've seen this.
I've ordered the DoL starter, a thermal reset switch appears a relative low cost and simple item to fit, I assume I run the live through the switch and if it runs at a high amperage for a period of time it will trip, in the case of the one linked above, at 15amp, which is higher than the 10/12a usage but lower than the 32amp trip. Or will it trip on startup and act like a fuse? A little advice on this would be appreciated.
To reiterate the safety of my build

My build has;
One way valve into tank
0 load valve, so the motor will start with no compressed air pressure in the head
Auto switch off at preset pressure
Tank pressure tested and steel thickness measured
Overpressure valve on tank and in line with feed to tank from pump.

I'm adding;
32amp supply with isolator
Direct on line starter (ordered)

Things I have yet to add but working it out;
Guard for belt/fan/dangerous spinning bits
Thermal overload (when I figure out which one to buy)

Things I may not add.
Any device to deal with power cut as the motor will be protected by a trip, DOL starter and would not matter anyway as the motor switches off at pre set pressure....unless the DOL does that anyway.


Cheers for comments and help so far.

 

[David Prosser]

the direct on line contactor should come with or you should fit the correct sized overload for the motor it's controlling. You should not have any need for that that thermal overload switch as well.
I'm assuming your current set up would start the compressor automatically as the pressure drops bellow as cetain threshold? This should be run through the controls for the starter so a simple DOL starter will not work as you require it.

 

[styleruk]

the direct on line contactor should come with or you should fit the correct sized overload for the motor it's controlling. You should not have any need for that that thermal overload switch as well.
I'm assuming your current set up would start the compressor automatically as the pressure drops bellow as cetain threshold? This should be run through the controls for the starter so a simple DOL starter will not work as you require it.

I don't quite understand everything you say there.
You say the DOL Starter I'm getting should have a thermal overload switch?
this is the one I'm getting
So I could check that out.
The confusing bit you say is that the DOL will not work as I require. I run the switch for the motor from the pressure sensor, then that sensor is powered from the DOL. When the pressure drops, the pressure switch powers the motor. the pressure switch will be live as long as the DOL starter is switched on.

 

[David Prosser]

That starter has an overload relay inside which can be set between 10-16A.
With the DOL starter you have selected stop/start of the motor is controlled by pushing the buttons on the front of the starter. Your set up should have the pressure switch stopping and starting the motor via the DOL contactor and overload the pressure switch should not have the motor running current through it.

 

[styleruk]

That starter has an overload relay inside which can be set between 10-16A.
With the DOL starter you have selected stop/start of the motor is controlled by pushing the buttons on the front of the starter. Your set up should have the pressure switch stopping and starting the motor via the DOL contactor and overload the pressure switch should not have the motor running current through it.

So you are saying, I should have power going to the DOL, then to the pressure switch, back to the DOL then the motor. That really does not make any sense to me. Is that what you mean? In other words, the last thing before the motor should be the DOL?
If that is the case, then I should mount the DOL onto the frame of the compressor.
[automerge]1588251408[/automerge]

So you are saying, I should have power going to the DOL, then to the pressure switch, back to the DOL then the motor. That really does not make any sense to me. Is that what you mean? In other words, the last thing before the motor should be the DOL?
If that is the case, then I should mount the DOL onto the frame of the compressor.

Maybe if I put power through to the pressure switch, then to the starter, then the motor. But then, when the pressure switch wants to start the compressor again will the DOL reset to off? Or will it stay on?

 

[Lucien Nunes]

I can't stop to answer fully but there is confusion going on between a starter with a contactor i.e. relay, which is controlled by the pressure switch and takes the motor load itself, and a mechanically latching pushbutton switch with thermal overload which is what I think is in the link. The latter is normal for compressors of this size where the pressure switch is rated for the motor current, since the electrical latching of a contactor isn't needed and can't be used if under pressure switch control.

 

[David Prosser]

the starter is a big switch designed to take the motor current what operates this big switch is a little switch, in your case the pressure switch.

If you're not clear on how to connect them up correctly why not ask your brothers mate to have a look. Or just buy a prebuilt unit big enough for what you need. Try Atlas Copco they make good compressors with everything built in.