Thread: Very Basic 3 phase question

Hi all. I am a real novice at lots of metalwork/electrical things, so please bear with me.

I have recently acquired an old bulldozer mechanics workshop, with all the gear. I am sorting my way through the chaos slowly.

The shed seems wired for 3 phase, and most of the heavy equipment connects to the power supply with cylindrical multipoint connectors ( they remind me of the round trailer plugs)

Am I right in guessing that these are ~400 volt outlets ?

There are a few old but large electric motors plugged into some of the outlets - can they ever run on 240V ?

Thanks for any help.

Originally Posted by rwatson

Am I right in guessing that these are ~400 volt outlets ?

Yes they are most likely the 3 phase outlets (there are high amp single phase connectors that look similar [less pins], but you don't see them around a lot). The voltage phase to phase is 415 and the voltage from each of the phases to neutral is 240V.

Originally Posted by rwatson

There are a few old but large electric motors plugged into some of the outlets - can they ever run on 240V ?

Most likely, but it may, or may not be cheap and/or easy to do.

Do the sockets have 4 or 5 pins? If only four then its most likely 415V 3phase, if 5 pins you most likely have a netural so can run 240V stuff as well.

If the motors are over 5hp I'm told it's unlikely they will run on 240V. Below 5hp its likely that they will.

Stuart

The simplest three phase outlets have four contacts, one for each phase (L1,L2 and L3) and a protective earth. These supply three 415V nominal circuts between L1 and L2, L2 and L3 and L3 and L1. These circuits are referenced to the system Neutral connection at the supply point, but the neutral is not present on the outlet.

The five contact outlets provide the system Neutral on the extra contact and can supply 240V nominal from either phase (L1, L2, L3) to the Neutral, while maintaining the 415V nominal phase to phase that is present on a 4 contact outlet. The neutral is bought to the outlet to allow low consumpsion single phase equipment to operate in conjunction with high power 3 phase gear, e.g the computer system in a CNC machine. However care must be taken to avoid drawing large single phase currents as they can unbalance the 3 phase system and overload the Neutral cable.

With regard to the motors, read the data plates omn the motors for an idea of their requirements and capabilities. Older motors are generally larger and heavier than recent models. I have some 1950's 3/4HP single phase motors that are twice the size and weight of the 3HP on my 10 year old compressor.

Motors over 5 HP would most likely be 415V 3 phase only, smaller three phase motors may be able to be configured for 3 phase 240V or 3 phase 415V. There is little chance of getting high power (>5HP) motors to run on 240V single phase without using some form of phase converter to generate the extra phases.

Many thanks for all the info everyone - I will have to go and count the pins - my memory says 4, but it is a while since I looked.

Also, there is no dire need to convert the electric motors, it was just a passing thought. As Malb said, they are usually heavier and less efficient than newer ones. Hopefully they have more copper too - in case they end up in the re-cycle cart.

I will report back after I get down to the shed again.

I said that older motors are bigger and heavier, but did not say anything about efficiency.

Yes, they tend to have more iron and copper than newer models, but the data plates tend to give an indication of the nett power output (usable work) the motor can achieve on a continuous basis, rather than the gross power input indicated on a lot of current models.

The size difference is largely a consequence of thermal considerations, older winding wire insulation was typically rated for 70deg C, current synthetic coatings are rated for 105 deg. Allowing for operating at 35 deg C ambient temp, the older motors were designed for a safe temp rise of 40 degrees, whereas the newer stuff can cope with with a rise of 70 degrees. Hence the winding wire is much smaller, and the motors generally more inefficient (extra heat, less power) and more prone to fail under moderate overload due to insulation failure.