View Full Version : Motors, 3 phase and MCB's

03-10-2009, 05:30 PM
I'm installing a two post car lift.

I phoned the company up who supply spares for them and maintain them, I asked what amp MCB I would need in the ramps box ..as it's not there..he told me 16 amp type C ..

OK fare enough ...put the phone down then thought about it .

The motor on it is 3.7 kw

To my way of thinking ..that's the max the motor will draw ..OR IS IT

This works out

3700 / 415 = 8.9 amps

Is this guy wrong .. OR am I wrong

My phase converter only puts out a max of 4 kw with boost of 600 percent for 10 secs for starting.

SOMEHOW ...I don't think the converter is ever going to put out 16 amps on the three phase side of it ....well not for long enough to trip 16 amp c type MCB.

So what the general opinion on what is the most suitable amperage of the 3 pole MCB

The max lift it will ever lift is 1.5 tons .it's rated at 2.5 tons

And having had nothing to do with three phase MCB's till now ...how is a 3 pole MCB configured ...is it 16 amp per phase or all of them combined = 16 amp ?

All the best.markj

03-10-2009, 05:37 PM
Mark, that sounds like the difference is stall current versus continuous duty current.

High-quality industrial motors are rated by continuous duty current. Stall current is often quoted on consumer equipment and import machinery. Which is why the soup-can sized motor on my 7x10 mini-lathe is claimed to be 3/4 HP, but the 3/4 HP Baldor motor on my Clausing drill press weighs 70 lbs :)

So the 10 second overload you have on your phase converter should be fine.

Mark McGrath
03-10-2009, 06:33 PM
The guy has given you correct info.The fact that you`re not supplying it from the mains doesn`t come into it.The breaker will trip if the motor lines pulls more than 16 amps for X time.You will still need a starter and overload to protect the motor,the mcb is there to trip in the event of a short.You need to remember a three phase motor on British voltage will pull roughly four times full load current on start up if it is D.O.L.

03-10-2009, 06:38 PM
yes it has starter and thermal overload connected ..

the mcb ratings are so
plucked from the net this afternoon.

a type B will allow a 2-5 times surge,
a type C will allow a 5-10 times surge,
a type D will allow a 10-50 times surge.

so my mind this afternoon was thinking it should have 10 amp type c

all the best.markj

J Tiers
03-11-2009, 01:20 AM
Don't forget a few really important factors......

1) 3 phase works differently than you assumed. The line current would be (3.7kw/415)/1.73 = 5.15 amps per line, assuming perfect phase balance, which is unlikely with a converter.

2) motors are not perfect...... therefore they pull more power than they produce at the shaft... the difference is heat. A smallish motor like that may be 80% efficient, in which case the actual draw will be more like 5.15/0.8= 6.5A

3) motors have crummy power factors...... maybe up to 0.6 or so when loaded. therefore, if your motor would theoretically pull 6.5 amps, the actual ampere draw will be 6.5/0.6 = 10.7A. This does NOT equal more power, as it is drawn later, when the voltage is lower (current is phase shifted)

4) it is common to upsize the breaker to account for various factors such as start-up surge, and so a 16A breaker is actually reasonable.

You said "converter" and not "inverter" or "VFD", so I assume you have a rotary converter.

A rotary converter isn't really a significant factor, since two lines go through with no interruption. Only the third line is really "passed through" the converter.

The circuit protection before the converter will be a larger current rating, to account for the total current including the idle current of the converter, and also to account for the power the converter transfers into the 3rd line.

03-11-2009, 10:54 AM
I don't no

my rotary , i think, is incapable of producing the 16 amps required to trip 16 amp 3 phase mcb ..

think it would trip its own single phase mcb before it tripped 16 amp three phase one.

our single phase is ....is one phase and neutral ...so the phase converter makes the other two phases ..not the one, as in your stuff in the USA.

all the best.markj

J Tiers
03-11-2009, 10:05 PM
our single phase is ....is one phase and neutral ...so the phase converter makes the other two phases ..not the one, as in your stuff in the USA.

all the best.markj

I don't think that makes any difference. What you end up with is still just making one extra phase, in a configuration very much like the US "corner grounded delta" which used to be seen in some areas.

Our prior building had that, it is a delta 3 phase service with one line grounded, providing a sort of neutral to an inherently "no-neutral" setup (delta has no neutral).

You would still have three wires with approximately 120 deg phase difference in the currents and relative voltages. Still acts like (and is) 3 phase.

03-11-2009, 10:17 PM
I'm sure you're right ..
I was just clearing something up .

to remind you that I'm in the UK

seen adjustable 3 pole mcb's on ebay.......10-16 amp ...with a little pot on them ..might buy one of those .

spoke to the service agent again today ...he says actual amps he's measured on them when they been lifting cars was 8-9 amps...uk typical car is less than 1.5 tons....lift designed for 2.5 tons .....when lifting that ...may well be a lot more than 10 amps

he says lift is underrated purposely.........and 2.5 tons is half of what its capable of ..but don't go there.

all the best.mark