Control Transformer Capacity

What is the output voltage of transformer when at full load?

What kind of rectifier, are you using in you control Circuit.

If transformer is not overloaded then something is wrong with your electronics controls. May be your electronics card is designed for higher voltage. Some of the components may be operating at or below min operating voltage, lots of possibilities. But must be inside,

Just see what is current drawn in case of a working and non working transformer. If difference is too large means something inside is not getting triggered keeping some part of circuit non functional.

Quick method to verify is swap working and non working transformers. And see the results.

What I'd do is take each transformer and measure the voltage and current draw using the same load. You could rig up a couple of contactors and connect their coils to the transformers and make your measurements.

If you can see the magnet wires on the transformers you could also check if they have the same wire sizes. Since the ratings are supposed to be the same, the size of the wires should be the same.

It's a simple matter to put a sticker on a piece of equipment and say that it's certified or approved.

Practically speaking, you get the specified current draw of each device you're going to connect to your transformer and add them up. Then you choose a transformer that can handle the current.

One other thing, I had one technician once who had a similar problem when his solenoids refused to operate though everything else was functional. He replaced the solenoids but had the same result. He later discovered that the solenoids had a higher voltage rating than the rest of his circuits. It pays to check everything.

Whenever you energize an inductive load, such as an electromagnet coil on a contactor or relay, there is an initial inrush of current as the core of the armature magnetizes. The CPT must be capable of mainatining its output without a significant voltage drop when that happens, otherwise the voltage drop causes a current surge in all the existing loads. CPTs are unfoprtunately only rated by continuous output, so a 75VA rating has little to do with how well it will deal with a high inrush. CPT internal design however has a LOT to do with the inrush capacity of the transformer. The winding material and the way it's wound, the type of steel usedand even the transformer core shape all contribute to the ability of the transformer to handle brief overloads as a result of turning on coils. Cheap transformers are going to be worse at it, the best ones are not going to be cheap. When I was using a lot of CPTs at one time, I did my own testing by connecting a continuous load of 75% of the CPT capacity, then turning on a contactor with a coil that had a "sealed" VA rating equal to the remaining 25%. The only one that would stand up to that abuse was made by Sola / Hevi Duty SBE series. They use interleaved copper windings and high silicon steel laminations so that even though they rate them for continuous duty like everyone else, the transformers are capable of much higher inrush than the others.

http://www.solaheviduty.com/products/transformers/Industrial/SBESelection.html

If your CPT supplier provides the CPT's Inrush Rating, as Sola does, then that web page contains a couple of selection formulae you should use to pick out the VA size. If not, then use the old-fashioned method of

CPT VA = Total steady load (sealed coils and all other loads such as laps and power supplies) + the Inrush VA of your largest coil.

You are right, GE contactors R&D dept. told me that their contactor 65 AMP needs inrush power 104 VA although the normal consumption is only 17 VA so they recommend using bigger size transformer to withstand inrush value !!!

But I think the CT should be designed to withstand the inrush value same as what the motor contactor does with starting motor current or this is an additional feature to be separately specified by the CT manufacturer !!

Better CPT suppliers will provide you with an Inrush Capacity rating, cheaper ones may not (perhaps because they have something to hide).

By the way, you should probably get out of the habit of referring to Control Power Transformers as "CT"s, because that is the abbreviation for Current Transformer, which is an entirely different device. That's why we use CPT to differentiate.

I agree with response #3.

However, be sure to include the inrush current of all coils that have to be picked up simultaneously.

In my experience oversizing the Control Transformer was a small price to pay for having coils/contactors not pick up. Large contactors can have surprisingly high inrush.

Most manufacturers will give a percent impedance which is the open circuit to loaded voltage drop divided by rated voltage. High impedance means you suck the voltage down on inrush. Low impedance will hold up the voltage better on inrush.

Post 3 soundslike a definate possibility. In my HVAC units at my plant I've installed delay timers to prevent all of the control circuts from energizing simultaneously. I would also recommend isolating and testing each individual component to determine thier integrity and benchmark thier draw characteristics.

Can you use a DC contactors? If you can then just use a rectifier and a high ripple capacitor that will take care of the inrush current. 1000 uF/50V will do the job very well for your situation. For example for 1A, 24 V @ 1ms you are good with 100 uF/ 50V. I am not sure of the exact situation but I have used 24 volts 4A solenoid @ 100 ms working interval with a 50VA transformer, 8A bridge rectifier and 6800uF/63V. This design not only gives you the desired instantaneous power but do not overstress the capacitor. I don't think your situation is as critical as the example but it may help you.

Sincerely,

Enrique

One other thing. In one factory they had a machine that would shutdown at 12 midnight every night. No breakers were tripping and the system still had power. It was getting so that the operators were joking about calling in an exorcist.

I found out that the CRT based HMI they were using had an automatic degaussing feature that occurred at 12am. When the degaussing kicked in, the current on the control transformer shot up, dropping the voltage and causing the relays to release. Curiously, the breaker supplying the transformer was rated just above the maximum degaussing current ensuring that it would not trip off when the degaussing came on. Someone had specified the right breaker but the wrong transformer!