3 Phase 2 Leg Heat SCR Control

[snappy987]

Hey Guys,

I am working on a project where we are using a Watlow 3 Phase 2 Leg zero crossing SCR controllers to vary the power going through a set of heater in a delta load. Each leg of the delta load has the same resistance across it. The problem I am having is that the current on the two controlled legs is different than the current on the uncontrolled legs. Similarly, voltage from controlled leg to uncontrolled leg is higher than that of the voltage from controlled leg to controlled leg.

It is very important that I have equal power going to into each one of my heaters as I need a uniform heat flux. I have contacted Watlow and they assure me that everything is fine and it is an issue with the fact that measurements taken on the SCR controlled legs are unreliable because they are not full sine waves. I am not convinced and would like to understand this better though and was hoping for some assurance from the EE's that I am in fact getting equal power distribution. When purchasing the 3 phase 2 leg controls it seemed to be a common method of controlling power into resistive loads, so I am assumed I would have uniform power into each leg.

 

[itsmoked]

A couple of things..

One would be: how you're measuring the currents to make your 'uneven' statement.

The other would be: are you sure your power isn't a high leg supply?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[electricuwe]

Voltage drop of an SCR is in the range of 1.5 to 2 V. So, for operation at line voltage of 400 or 480 V this should cause any significant imbalance. However, if you supply is lower this might have noteable impact.

 

[zzzjim]

Could power levels in 3 loads be verified with low wattage incandescent loads in parallel , or is this too unprofessional ?

Worst case could be 2 controlled legs some how alternate so power
would never directly flow between them in a 3 phase load .
This is THEORY ,and would be a Sync issue .

Do take care in a Low wattage condition as the load terminals ,
will be Live at uncontrolled phase potential !

 

[sreid]

It seems to me that you have a valid concern. First, an RMS current meter shouldn't lie to you.

On general principles, equal phase currents in a three phase system depends on having symetrical three phase voltages. If the voltages to two phases are being controlled and the third is fixed, I would expect the phase currents to be unequal.

 

[snappy987]

For measurement purposes I am using a true-RMS current transducer designed to give accurate readings on distorted waveforms like SCR and VFD signals. This is not a high leg supply. I switched up which legs were controlled and ended up with the same results (current through uncontrolled leg is lower).

After talking to one of the EE's here we came to the conclusion that the power is not going to be the same across all legs. The amount of power that flows from controlled leg to controlled leg will have to be less than the power that flows from controlled leg to uncontrolled leg.

As a solution I have rewired all heaters across only two legs. One leg is controlled and one leg is uncontrolled. This has rectified the problem and the power going in to all heaters appears to be equal.