CT Connections

Well kinda...

http://www.fundamentalsltd.co.uk/sites/default/files/downloads/averaging_vt.pdf

http://www.electronics-tutorials.ws/transformer/current-transformer.html

I think i should clarify the problem a bit.

I have a 15hp 3phase 600v. delta connected motor.The load supplied to the motor is controlled by a PID controller, with a CT input from LINE 2 on the 15HP motor. (As the current through line 2 increases, the PID Controller decreases the load, to keep current to an acceptale level, thereby keeping the main 15HP Motor about 95% loaded at all times.

This works great about 90% of the time. However, durning peak demand times, the utility voltage is quite unbalanced resulting, resulting in huge differrences in line currents 1,2 &3.This causes the motor to be greatly over or underloaded, depending on which line CT is installed on. Therefore i would like to get an average line current to keep the motor loaded at all times. The PID only has one CT input.Any suggestions?

Could i hook up 3 differrent CTs in series to get an average value? (divide by 3)

I know i could hook up KVA meter with 0-10vdc output, but budgeting doesn't allow for that.

Thanks for any suggestions.

Why not tune your PID circuit to average the phase the CT is attached to?

Then add two more CT's to the control circuit input, and tune the PID to average these as well, then maybe you can see the entire picture, not the "hole" picture.

The thermal capacity of the motor is compromised if your line currents are unbalanced. The motor rotor heats up significantly with unbalanced currents, about 15% is a tripping point for a good overload relay.

Your current control scheme may actually be limiting the thermal damage by dropping load, where if you optimize the motor loading, you could end up with a failed motor.

Depending on the mechanical characteristic of the load you are optimizing, a PWM inverter might provide a solution with fewer moving parts/improved reliability, and you can then measure motor shaft torque or horsepower, and get a real mechanical value to control to, rather than motor current, which could be all over the place, under your described conditions.

It is better to identify the reason of unbalance voltage during peak time, be it in your electrical distribution system or at utility's side.If it is at utility's side,the same should be immediately take up with the authority, as they are bound to supply electrical power to the consumer at certain limit of unbalance as per local law. If it is at your electricnal distribution system, please check thoroughly for any loose connections, to identify under size conductors, bad cable joints,etc. Actually, unbalance voltage in electric motor causes damage to the motor,if it is run prolong with full capacity.Unbalance voltage at motor terminal generates negative sequence current in the motor and heating effects of negative sequence current is nine time higher than positive sequence current and there by increases temperature of the motor.It is estimated that per 8 to10 degrees celcius rise of motor insulation temperature over the rated insulation class, the life of insulation becomes half.That is why under unbalance voltage condition, the motor capacity is derated.In fact only about 2 percent unbalance voltage may be safely allowed without damage of motor.It is estimated that if there is 5 percent unbalance voltage, the motor capacity is derated about 25 percent.

Manindra

Good answer. Imbalance, whether speaking in terms of electric power or mental capacity, is its own reward?

That would be an ideal way of creating a current imbalance measuring device.

...however it would be simpler to put the CT around the neutral conductor instead.

you beat me to it!!

Only by a single dog hair, or the scuz on my teeth.

HHHave a nice day, OMG is that left over garlic feedback I am getting?

LOL

Practical lesson..

Take any old AC clamp on amp meter and place around all three conductors, not each individual conductor, but ALL three. The circuit has to be live and powered a load.

Report back with the value you get!

I got diddly squat here. But that might be because I have not done anything.

If you are doing a feed-forward loop control like this, using current is going to be the wrong way to do it, no matter how many phases you look at. That's because current fluctuates with voltage (as you have already discovered), which is not indicative of true load on the system.

What you really need is kW monitoring, which is actual shaft power regardless of any voltage swings. If you want to do it correctly, you need to change the budget. Spending money on other equally useless options is just a waste.

If you already have a motor starter on this motor, consider replacing the Overload Relay with a newer Solid state OL relay that has the ability to provide an analog output tied to motor kW. There are several on the market capable of this; The Allen Bradley E300 OL and the Siemens Simocode units are both capable of this. On both of them, you must add a voltage sensing module so that they can detect phase angles. As stand-alone units go, there is a company called Load Controls that makes one as an all-in-one unit.

Thanks. Now I know what I need.

If your 3phase circuit go through a single CT, this CT should be read the value of the sum of these three phases, and normally as three phase is 120' different to each other. So it becomes the other kind of Earth Fault CT. and it should be around 0 if there is no fault.

Did you mean 120°, and not 120' (120 ft.), or 120' (120 minutes)?

Surely you did....

Yes, it becomes sort of like an Earth Fault detector. If you have CT cores on three separated windings that each only pertain to one leg of the three phase, then you know the actual current per.

If i have CT cores on all three windings, BUT connect output of all CT's in series, what current will i get then?

You would get the instantaneous sum of transformed current between the two connections. OR, you might get a loud pop.