Assuming 85% efficiency, 2.2Kw is likely to be about 2.6 Kw ,(About 11 Amps running on full load ) so start up current draw for a 240 Volt single phase machine is probably going to be close to 13 Amps from a normal 3 pin socket.

Whether a ring main, rather than a spur, could support that is beyond my knowledge.

Howard

The rating plate info given is 13A FLC at 240V (3.1kVA).

The rated power (output) is 2.2kW. The power factor and losses is already included in the FLC.

An electric motor however will always try to service the load and is selected to deal with short term overloads, starting conditions and the supply voltage variation allowed and repetitive start/stops.

In the case of the actual supply voltage being higher or lower the current at any specific load will decrease or increase. The worst supply condition is low voltage which inversely increases the current drawn. You can only operate a motor at lower voltage if you also reduce the frequency and hence speed. The operation of a VSD.

Motors are designed for particular applications with widely varying characteristics.

Pumps (unloaded at start) usually have low inertia and quick starts.

A large lathe however has a large inertia and the motor would be fitted with larger start windings and capacitors. A start capacitor also provides reactive power which partially offsets the inductive reactive power drawn by the motor.

My 2 kW rated lathe even starts successfully on a 5.5kW generator, even though it momentarily pulls the generator frequency down significantly.

My domestic supply is 80A 230V with a local incoming circuit breaker 60A fast curve and never a problem.

Please note:

1. The above simplifications only consider a domestic type use.
2. Any industrial equipment/plant supply design must only be done by appropriately qualified persons.

Willie.

The problem with the assumption is that most compressors aren't unloaded throughout the start run-up. Rather the opposite, because they're driving a cold reciprocating pump that immediately runs good and hard to charge the tank. The normal electrical current start spike is multiplied by the mechanical load, which is why the blurb usually warns static converters are unsuitable for running compressors; the capacitors don't store enough energy to start the motor. Very different from a lathe, where the motor isn't heavily loaded until the operator starts cutting.

I guess the reason questions are being asked is to determine the ratings of the installations protective arrangements, and there are many alternatives.

An initial high burst of starting current can be tolerated provided it doesn't last long enough to overheat cables or fuses. Fuses blow when they get hot enough to melt, and heat is the result of current and time. BS1362 specifies an ordinary domestic 13A fuse should run indefinitely with a 1.6x overload (nearly 21A), but a 1.9x overload (nearly 25A) must blow the fuse in less than 30 minutes. 13A fuses are tolerant of greedy start motors like vacuum cleaners.

Fast blow and slow blow fuses are available. Circuit breakers, which don't relying on heating, react much faster to overloads, but can be chosen to have slow blow characteristics. What's needed depends on the nature of the load.

Life gets 'quite interesting' when one crosses the boundary between home and industrial electrics. There's more to electrics than is found in a typical home installation. Homes assume domestic loads and are regulated to keep it simple. Industry, likely to be more complicated, collect facts and engineers a solution. If this is the reason they want to know, then I'd be inclined to over egg the estimate, to make sure the compressor doesn't pop breakers or whatever.

But I've no idea how big the start overload current of a compressor might be! As we all know the motor ratings in sales literature are notoriously unhelpful, tending to quote minima when buyer wants to save energy, and the 'just short of catching fire' rating, for the sort of buyer who thinks more power is always the answer. Bit of a minefield. The manufacturer would know, but it may be difficult to get sense out of the seller of a home compressor. Domestic compressors are supported by salesmen, only industrial compressors seem to come with proper specifications…

Dave

Just ran up my 3hp CompAir for you.

Motor plated at 15A for full load (it's 20years old) General Electric old skool moving coil clamp meter.

Inrush on the needle peeks at 25A, running current is 10A from Zero psi.

A brand new motor would be made today with a much higher efficiency, but the pump being new would be stiffer to turn.

SOD

Lathe power requirement varies according to design.

A 6inch chuck is a heavy lump to accelerate a 4inch is not

Gear drive has large losses, a belt drive has not

How long is your piece of string?

I have to disagree with SOD. Most compressor sets have an unloader valve to aid start up. This is a little widget that opens the compressor delivery to atmosphere for startin (no load), leaves the line open for several seconds and then shuts the vent to atmosphere to bring the machine under load. My own little 25 litre set has one, my friends twin cylinder 100 litre set has one. Most compressors simply wont start against pressure in the tan without one. If you haven't got one, FIT ONE.

That being so, you are simply swinging up the inertia of motor and compressor, so relatively modest inrush current is to be expected, I would go with the figures suggested by Dave Halford. Theoretical inrush will be higher but of very short duration.

Martin

Ideally the unloader valve should vent a decent size manifold or intermediate tank as well as the cylinders to prolong the period of working into less than tank pressure.

My old Atlas Copco KE2 Vee twin compressor, rated for 16 cfm at 100 psi and 1,000 rpm, had a large cylindrical manifold on the outlet side of about 0.1 cubic ft volume. Audibly obvious when the compressor started working against significant back pressure by which time the motor was pretty much up to speed.

Due to the (relative) difficulty of accelerating through the characteristic torque dip partway up the speed range induction motors driving compressors have a nasty potential gotcha where the motor sits around half speed and can go no faster. Which kills the motor fairly quickly due to seriously excessive current and low cooling airflow.

Single phase motors have a particularly horrible version of this where the motor oscillates between going just fast enough to open the start winding switch and just slow enough for it to close again. With predictable results on start switch and start winding life. True compressor duty single phase motors are designed to avoid this by having higher torque during run up and, often, a higher speed before the start winding kicks out. Usually at the cost of a percentage point or three in efficiency and somewhat larger start current. Twin capacitor motors where the second capacitor allows the start winding to stay in circuit during normal running have much less torque drop when the start switch kicks out. These are the preferred type for driving compressors. Generally good enough given modest over-rating to avoid the need for true compressor duty ones.

VFD driven three phase motors on compressor duties can play similar tricks. Especially if the weather is cold and the electricity supply not as good as you'd ideally like. Most VFDs have a torque boost setting to give a bit more oomph during run up but it may also be necessary to adjust acceleration rate too. Torque boost increases the stress on the internal capacitors reducing lifetime but folk like us probably won't notice. Oversizing the VFD gives more leeway to play parameter games. Last time I saw this issue was on a Hydrovane HV02 that didn't want to play below about 3°C using the parameters the vector drive VFD had chosen. Digging out the torque bood setting fixed it.

Clive