Balanced and Unbalanced Loads in 3 phase

pauldavis123

I just got a new unit and it has 3 phase power.

Looking up the basics of it so I have some idea of how it works.

Can anyone give real-life examples of how a load would be unbalanced?

2011

pauldavis123 wrote: »

Can anyone give real-life examples of how a load would be unbalanced?

Yes. Let’s aay you connect a three identical 4kW loads betwen L1 and neutral and nothing between L2 and neutral and nothing across L3 and neutral. Result is an unbalanced load resulting in a neutral current that is equal in magnitude to the L1 phase current.

Now if I were to take the same three 4kW loads and connect one to each phase and neutral the result would be a 3 phase balanced load. The neutral current would be zero.

There are a number of adavantage to balancing loads. Generally they can not be perfectly balanced but thy is not a huge issue.

Make sense??


Apologies for typos, it’s an iPhone post!!!

pauldavis123

OK I get that.

Would a 3 phase device be wired like that from a manufacturer?

Or would it be a case of the wiring up of the device being incorrect if it did not take the load balancing into account?

Thanks.

Bruthal

Balancing of loads is important at the grid generation end to ensure smooth running with little vibration. At that level it would be very balanced, which naturally happens by keeping all installations as reasonably balanced as possible over 3 phases.

A 3 phase load such as a motor will be balanced as that`s how it is manufactured. They need to be to run smoothly. As such, they require no neutral.

A 3 phase machine with individual switchable components such as heating elements could be unbalanced in certain modes. But they would simply be like having single phase loads connected to 3 phases supplying the machine. A neutral would be required.

Something like a 3 phase heat pump as an example, would be a 3 phase compressor making up most of the load, but might have a neutral for controls, indicator lights, solenoids and valves etc.

2011

pauldavis123 wrote: »

OK I get that.

Would a 3 phase device be wired like that from a manufacturer?

Or would it be a case of the wiring up of the device being incorrect if it did not take the load balancing into account?

Thanks.

As a general rule a 3 phase device will be designed to balance the load across the 3 phases as evenly as is practical. This will be achieved by the installer connecting the first phase to L1 of the unit, the second phase to L2, the 3rd phase to L3 and the neutral to N.

There are some units such as cookers where L1, L2 and L3 terminals can be lined out allowing it to be connected to a single phase supply. Obviously the instructions must state that this is an option.

What is the 3 phase device that you have in mind?

Tuco88

A broken neutal on a 3 phase system can have intresting effects to say the least.

Say L1 supplying a load, Phase and neutral (230v) are working as designed. This will stay fine.

But say the neutral has broken for both L2 and L3. Depending on the impedance of the loads, this can cause the phase voltage(230v) to creep up to almost line voltage 400v on single phase loads.

So if you check for voltage and get a measurement of 292v it may not be the meter thats goosed.

2011

Tuco88 wrote: »

A broken neutal on a 3 phase system can have intresting effects to say the least.

Known as “neutral displacement”.

Tuco88

2011 wrote: »

Tuco88 wrote: »

A broken neutal on a 3 phase system can have intresting effects to say the least.

Known as “neutral displacement”.

Floating neutral is probably another.

Neutral drop.

How unbalanced dya have to be to melt the neutral?

Bruthal

Sir Liamalot wrote: »

Neutral drop.

How unbalanced dya have to be to melt the neutral?

The neutral will have a max when only 1 phase is loaded. So the neutral doesn't really have a higher current than any phase, disregarding possible harmonic loading some might mention.

Way out of balance would be bad for generators. it would be like a spinning wheel with the wheel balance off. Very balanced and it's nice and smooth.

Bruthal

Tuco88 wrote: »

A broken neutal on a 3 phase system can have intresting effects to say the least.

Say L1 supplying a load, Phase and neutral (230v) are working as designed. This will stay fine.

But say the neutral has broken for both L2 and L3. Depending on the impedance of the loads, this can cause the phase voltage(230v) to creep up to almost line voltage 400v on single phase loads.

So if you check for voltage and get a measurement of 292v it may not be the meter thats goosed.

That 292v is going to be the voltage across phase to EXPECTED neutral. In reality you are now testing a load which is in series with another load which are now connected in series between 2 phases. The higher impedance load will now show more of the 400v than the lower impedance one.

So it's not really phase to earth that is showing an increasing voltage. It's phase to floating neutral point. Of course any earth rods connected to the floating neutral will be at it's potential along with ground immediately connected to it.

pauldavis123

Bruthal wrote: »

A 3 phase machine with individual switchable components such as heating elements could be unbalanced in certain modes. But they would simply be like having single phase loads connected to 3 phases supplying the machine.

Perfect, thanks

Bruthal

Bruthal wrote: »

The neutral will have a max when only 1 phase is loaded. So the neutral doesn't really have a higher current than any phase, disregarding possible harmonic loading some might mention.

That'tswhat I thought but I've been told it's possible, couldn't figure out how.

Bruthal wrote: »

Way out of balance would be bad for generators. it would be like a spinning wheel with the wheel balance off.

I've seen 80kVA genes dancing in time to the subs because of this. :pac:

Bruthal wrote: »

Very balanced and it's nice and smooth.

Uses less fuel for the load too.