3 phase power to supply 3 heaters

Question

I have been looking all over the web and finally found and joined this site to get an answer to this question.

Ok, I am working on an old plastic injection machine. It has been badly abused in its 40+ years of life so trying to figure out how it came from the factory is not possible.

It has 3 old heating elements, each controlled by a new digital controller and relay. Each comes on and off independently as needed.

One of the heaters shorted out and tripped the breaker, so I took a look at the wiring. All 3 units were being powered by a 2 tiny wires coming from one phase of the 3 phase power. How it even worked was a surprise. It did take a long time to heat up. (it is a 3 wire system)

I am replacing all 3 heating elements and want to power them efficiently.

Assume heaters are A - B and C

Heater A would be powered by phase 1-2 Heater B would be powered by phase 1-3 Heater C would be powered by phase 2-3

But I am afraid this will result in boosting voltage to 400+, because of the cross connection and ruin everything. Some posts and diagrams seem to indicate this would happen and fry the new heaters.

Option 2 would power heater A from power 1-2, and then power heaters B and C from power 1-3. This would leave one of the phases unused. ( 2-3)

I think I have to use option 2. I have a pro coming over, but I want to know what I am talking about when I try to explain it to him. Plus he is Russian and being able to explain things clearly is critical.

One other thing I found odd. With NO power, there is continuity between terminal 1-2 but not 1-3 or 2-3. With power turned on there is 220V between 1-2-3 in any combination.

Why is there continuity between 1-2 when power is off? Perhaps it has something to do with the rotary phase inverter?

Answer 1

I happen to have the manual for small injection molding machine at hand- an old Battenfeld 200kN machine. You should try to get the manual for yours.

In any case the heater zones are simply wired across different phases in delta configuration. L1-L2, L2-L3, L1-L3. They are single phase heaters and switched independently so this is about as balanced as you can get. The hydraulic power is about double the total heater consumption so momentary imbalances of 1/3 of that is not all that big a deal.

Try to find out the original heater power ratings- they will have been sized to compensate for the heat generated by shear in the plasticizing screw.

Using a rotary phase converter for an injection machine is somewhat brave- make sure the idler motor is rated a lot higher than the 3-phase motor it is operating.

Answer 2

I don't understand what Option 2 is supposed to give you - why leave one phase unused? There are two common ways to wire a 3-phase load: star and delta.

In a delta configuration, the heaters would be between the phases (L1-L2, L2-L3 and L3-L1).

In a star configuration, the heaters would be between each phase and neutral (L1-N, L2-N and L3-N). That assumes you have a neutral at the machine. You could use star wiring without a neutral, but only if all three heaters are turned on and off together. The point where the three heaters meet effectively creates a neutral.

On a 220/380V supply, delta would give you 380V across each heater. Star wiring would give you 220V across each heater. You need to check the rating of the heaters you are buying to work out which is right.