Compressor Woes

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Nourish
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Compressor Woes

Post by Nourish »

My 3hp Single Phase Compressor keeps popping the overload switch of the RCB.
It's wired in 2.5mm cable on a spur from its own RCB thru a metal clad 2 pole isolation switch and a 16amp(?) blue socket and plug.
The motor wire terminals (Lucar spades) to the 20amp overload switch have got hot and melted their sleeves. I have just replaced this switch with a new 25 amp one and it tripped the RCB I gather near full load.
Any ideas on what's going on?
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geoberni
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Re: Compressor Woes

Post by geoberni »

RCB, or RCD? :roll:
To a certain extent, there's too many acronyms and they tend to get used interchangeably, even though the devices are slightly different.
Residual Current Device (RCD) is also known as Residual Current Breaker (RCB) or Residual Current Circuit Breaker (RCCB). :roll:

An RCD works by comparing the current going into the device and the current coming out, i.e. the current in the Live and the current in the Neutral. More than, typically, 30mA difference, and it trips.
Of course this exact process depends on whether your protection device is actually an RCB, or a variant of.


Is yours also an actual Circuit Breaker, i.e. 13A Breaker with an built in Residual Current function, RCBO (Residual Circuit Breaker with Overload) or just a Residual Safety device?
I can't work it out from your description.

If it's tripping on current, then chances are the motor has a problem, perhaps brushes worn if it's a brushed motor (you haven't said how old or used it is).
if it's tripping on 'Residual Current', then there's an insulation fault.

The wiring of the Spur with the Isolation Sw and 16A Plug all sound correct.
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simmitc
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Re: Compressor Woes

Post by simmitc »

Is this a new installation, perhaps after a move, or a long term one that has been working but has developed a fault?

As above, too many acronyms, domestic installations used to have fuses, but now typically have Micro Circuit Breakers (MCBs) that trip a particular circuit when there is an excessive demand for current and RCDs as above. You mayl also hear the term "Earth leakage circuit breaker". RCDs can trip after varying amounts of timer, and might be rated at 30 ms. Post a photo of your consumer unit and details of your compressor for further advice.

For my 3HP single phase compressor I used a dedicated 6 sqmm cable from a 32 amp MCB with an RCD in the distribution board. Yes, it could just about run from less, but I prefer to have some margin for safety in these matters.
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geoberni
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Re: Compressor Woes

Post by geoberni »

Yep, a dedicated 32 Amp connector is ideal, but I guess the real issue may be the starting current of the particular installation, not the running current.
3HP is in the region of 2.3Kw, so you're around 10-11 Amps which should be fine on the Spur with the 16 Amp plug.
But starting current could be some 30-40% more depending on the characteristics of the motor.
That might be pushing the limits, depending on how long the starting current is drawn for...
Thing is, the data for individual machines will be written for different markets and phrased to get the sales, not particularly to inform the customer.
The starting current might draw for just a second or it might be for 10 sec......

As simmitc said, how long have you had it and how much use did it get before giving problems.
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Nourish
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Re: Compressor Woes

Post by Nourish »

Sorry but my info wasn't quite right, The supply is fed from a 16a MCB. But my concern really is that the wires connecting to the overload switch are overheating, It's this switch that more often trips than the MCB.
Its an older compressor but I recently bought a Grit blast cabinet and that's asking the most of it but I have been chasing problems with it over the years.
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simmitc
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Re: Compressor Woes

Post by simmitc »

You might just be running into capacity problems, grit blasting can have quite a high demand for air, You need to consider what the cabinet requires and what the compressor can deliver. Look at the Free Air Delivery and the capacity of the receiver, plus they dut cycle for which the compressor is rated. If the compressor has to run continuously to supply the cabinet, then it will probably overheat, which will cause the unit to trip - there is usually a heat related trip rather than an electrical one. Excessive overheating could melt the insulation.

Check that there is lubricating oil in the sump, that any air filters are clean, and that they cooling fins are free from debris.

Does the unit run OK when you are not using the cabinet?
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