3 phase motor

mark_d_spark

hi, im wiring up a 3 phase 3Kw motor. can anyone tell me will a "c type" 6 amp mcb do, or do i need a 10 amp. And is it ok to use 5 core 1.5 cable. I am able to do the wiring at the board and stuff but need some help working out the cable size and protection, thanks for any help

hey_hey

Here is the formula :
line current (IL) = total Power / sq root of 3 (1.73) x voltage line (VL) x cos& (power factor)

presuming your 3 phase supply is 400 volts, and power factor is 0.85 (which is the norm) here is ur answer

IL = 3000
1.73 x 400 x 0.85


Line current = 5.1 amps

If you wire the motor in star the phase current and line current are the same, if wired in delta the phase current is sq root of 3 of the line current. What system r u thinking of using???

choosing ur cable size depends on
1. type of cable (swa, nymj etc)
2. length of run
3. method of cable run (cable tray, clipped etc.)
4. size of mcb used
5. load

clinchy

wats it for ?

arse..biscuits

hey_hey wrote: »

If you wire the motor in star the phase current and line current are the same, if wired in delta the phase current is sq root of 3 of the line current.

If you wire it in star, the power will be one third of that if it was wired in delta. I presume the 3kW is for a delta connection.

clinchy

theres not much point in considering star my opinion its a pretty small motor and you would want to use it to its full power but then again depends on wat you are use it to drive

brightspark

I wouldn't advise wiring the motor in delta without seeing the motor nameplate data.

Most small motors (up to about 3 or 4kW) are designed for 400V when wired in star and 230V when wired in delta.

But you would need to check the nameplate.

(I've wired small 3-phase motors in delta, but only when connected to a single phase invertor.)

2011

Most small motors (up to about 3 or 4kW) are designed for 400V when wired in star and 230V when wired in delta.

A 3 phase supply with a line voltage of 230 volts is not normally available in Ireland.

A 400 volt 3 phase supply means that the line voltage (voltage between phases) is 400 volts and the phase (voltage beteen any phase and neutral) voltage is:

400/(square root of 3) = 230 volts.

A 3 phase 400 volt supply is quite normal in Ireland.

If however you require (as suggested) a 230 volt 3 phase supply this means that the line voltage is 230 volts and the phase voltage is:

230/(square root of 3) = 130 volts

There is some good info here:

http://en.wikipedia.org/wiki/Induction_motor

I've wired small 3-phase motors in delta, but only when connected to a single phase invertor

Sorry I dont understand what you mean here. Are you using the invertor to reduce the starting currnet by gradually increasing the frequency and voltage??

brightspark

2011 wrote: »

Sorry I dont understand what you mean here. Are you using the invertor to reduce the starting currnet by gradually increasing the frequency and voltage??

No that's not what's happening, I was stating the only times I have wired small motors in delta was when I was supplying them from an invertor.

There are a lot of small inverters which are supplied with a 230V SINGLE phase supply and output a 230V Three phase supply (as only 230V line voltage can be derived from the 230V supply).

(I've used them a lot on conveyors etc)

2011

I see what you mean. You are correct a single phase invertor would have only a 230 volt supply.

From reading what the OP has said I see nothing that indicates what way the motor is connected (star or delta) or that the intention is to use an invertor.

I would be interested to know why you are using invertors like that though. I have often used 3 phase invertors (VSD) for large motors, but not single phase ones.

brightspark

You're right the OP said nothing about how the motor is to be wired.

Seems we have gone off-topic slightly.

arse..biscuits

mark_d_spark wrote: »

hi, im wiring up a 3 phase 3Kw motor. can anyone tell me will a "c type" 6 amp mcb do, or do i need a 10 amp. And is it ok to use 5 core 1.5 cable. I am able to do the wiring at the board and stuff but need some help working out the cable size and protection, thanks for any help

To get this back on topic, can tou tell us
(a) What is the lenght of the cable run?
(b) What type of cable are you using?
(c) What is the installation method? (Cable in conduit, in wall surrounded by insulation etc)

Bruthal

brightspark wrote: »

I wouldn't advise wiring the motor in delta without seeing the motor nameplate data.

Most small motors (up to about 3 or 4kW) are designed for 400V when wired in star and 230V when wired in delta.

But you would need to check the nameplate.

(I've wired small 3-phase motors in delta, but only when connected to a single phase invertor.)

Surely it would be better to get a 400 volt motor and connect to the 3 phase supply than go getting an inverter to power it from single phase, i never would of thought most small 3 phase motors were 230v in star and 400 in delta. The whole idea is they can be connected 400 v star to be 1/3 the starting current that it would take to start in 400 v delta.
Its true a 230 v line voltage rated motor would be able to take 400 in star, but 3 phase motors for use here are 400 volt rated for delta connection, why would they be 230 volt??

Also 400 volts can be derived from a 3 phase invertor fed from single phase 230 volts, it has a step up transformer in it as well as the 3 phase motor to generate the 2 extra phases.
But an inverter is hardly a good solution for running a 3 phase motor unless there is only single phase available.

Also if it is a 230v line voltage motor then connecting it in delta to 400 volt star would give you the same power as 230v delta, so again- why bother with invertor

Bruthal

mark_d_spark wrote: »

hi, im wiring up a 3 phase 3Kw motor. can anyone tell me will a "c type" 6 amp mcb do, or do i need a 10 amp. And is it ok to use 5 core 1.5 cable. I am able to do the wiring at the board and stuff but need some help working out the cable size and protection, thanks for any help

10 amp c or d type 3 phase mcb should do. Cable sizes depends how far to the motor from the board. The high starting current of motors show up as the motor being slow to reach full speed if its a long run with too small a cable. A star delta starter would help reduce the starting current.

This is assuming you are connecting it to a 3 phase supply

brightspark

Robbie (and anyone else interested)

Here is the first 3 phase 3KW motor I located on a search of the Radionics site,

http://radionics.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=4428193

Overview
2 Pole & 4 Pole, Foot Mounted Induction Motors, ATEX

Three phase motors available in foot mounting and flange mounting, ideal for a wide variety of applications including pumps, fans and process machinery drives.
Motors in frame sizes 63-100 may be connected in either STAR (Y) for supply voltages of 380-420/440-480V 50/60Hz or DELTA for 220-240/250-280 50/60Hz. Motors in frame sizes 112-132 are suitable DELTA connected 380-420/440-480V 50/60Hz and are also suitable for STAR/DELTA starting.

(The 3KW version has a frame size of 100)

As the OP has not yet replied with details etc, we cannot assume his motor is suitable for star-delta starting

Bruthal

Well if its rated 230 volt delta, why not connect 400 volt star, where does the need for an inverter come into it. 230 volt delta has 230 volts across each winding, connecting 400 volt star has 230 volt across each winding also. So i would of thought 230 delta is exact same performance as 400 delta on the same motor. Anyway i never bought a motor myself, just connected them.

Maybe your ones only had the 3 ends available??

Bruthal

Its a good point though. I would of thought most here had 400 volt windings. Something new every day