motor circuit

[jim23]

Hey guys, this is not really a DIY question but as there's some knowledgeable heads on here was hoping someone could assist.

got a motor circuit to select a starter/overload relay for.

The circuit consists of 3 (3 phase) motors on a radial circuit fed from a bs88 fuseboard. The starter/overload relay is currently missing hence the need to fit a new one. The motors are for extractor fans.

specs from the nameplate are voltage 380, amps 0.44, kw 0.245.

i need to select 1 starter and overload relay to protect all 3 motors as they cannot be put on seperate ones. im aware i do not have to give overload protection to the motors as they are under 0.37kw but i would prefer it anyway.

im having trouble selecting the overload i should use as im not sure of my total current. 0.44x3 =1.32A for all 3 motors. as my actual voltage is more likely to be 415v this will alter my current, perhaps outside the range of the 1.1-1.7 relay i was thinking of using.

any help on the calc or advice appreciatted as i must admit ive only ever worked on one motor per circuit.

regards.

 

[Spark123]

Why can't you have 3 separate contactors and overload units?
I don't think that having one overload to protect all 3 motors will be much use. Even if you have one main contactor then one overload per motor is a better setup.

 

[ColJack]

get an inverter and control with that..

built in overload, speed controll ( helps with noise reduction.. ) and much better efficiency.. ( better for the environment. )

 

[briwire]

Having one overload isn't likely to provide any protection for the motors. The impedance of each motor is proably so high that if a motor was almost stalled, it still wouldn't trip the overload and would burn out its windings.

If you can't provide individual protection for each motor, just rely on the fuses and be assured that you have complied with the regs.

 

[1 interceptor]

Why dont you just get 3 motor circuit breakers like the Telemecanique GV2 and add them to 3 contactors, thats the best way to do it..

http://www.telemecanique.com/en/products/index_fon10_fam15_motor_tesysgv2.htm

 

[ColJack]

no it's not at all...

that makes 6 din rail mounted things he needs to find room for..

the overload clips directly to the contactor and usually only adds 20-30mm to the bottom of it..

 

[bernardgreen]

This worries me. Motors driving extractor fans often have slow acceleration due to the inertia of the fan and may take start up current ( several times normal running current ) for several seconds.

To allow 3 motors to run up to speed at the same time without tripping a single overload feeding all three will require an overload setting high enough that a single stalled motor may not be protected.

If these are large motors sequenced starting may be necessary if individual overload protection is not provided.

Do you know what was in the "missing" starter unit ?. Are these motors being moved to a new application and the control box did not come with them ?

 

[briwire]

they are only 0.245kW

 

[jim23]

thanks for the interest so far guys.

the extractors/motors are in a factory roof(non critical/ essential), the starter enclosure is there but the actual contactor/overload relay is missing. all wiring is still there.

to be honest i would rather run seperate feeds to each motor with overload protection for each one but customer is on a tight budget and the boss wants to leave circuit as is (providing visual inspection passes. The circuit calcs out ok by the way, i.e vd, zs). at the very least a switch disconnector needs fitting between fuses and starter as it doesnt currently have one(or a starter incorparating a disconnector)

am sceptical about whether one overload for three motors would be any good in terms of protecting them. am aware i dont have to provide overload protection but does anyone really think this is a good idea? i ask because i always like to see motors individually protected by an appropriatte overload relay.