VFD

AlanR said:
You don't want to let a CT go open circuit, imagine one on a 120KV transmission line open circuit, not only magic smoke but magic arcs.
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I agree that you don't want to let a CT go open but the voltage it will develop should you do so will depend only on the voltage drop across the very small impedence of the one-turn primary. This voltage will be proportional to the primary current. The voltage level of the primary with respect to ground is irrelevant. With the secondary open the core will saturate and start heating up if the primary current is high enough.
 
John Hasler said:
I agree that you don't want to let a CT go open but the voltage it will develop should you do so will depend only on the voltage drop across the very small impedence of the one-turn primary. This voltage will be proportional to the primary current. The voltage level of the primary with respect to ground is irrelevant. With the secondary open the core will saturate and start heating up if the primary current is high enough.
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Ray, come back! You need to tell us that CT stands for conspiracy theory, then we can all go back to machining.

(Many, many years ago I used to work on power station and substation design. I thought enough about current transformers then to last me a lifetime, it was boring.)
 
AlanR said:
Ray, come back! You need to tell us that CT stands for conspiracy theory, then we can all go back to machining.

(Many, many years ago I used to work on power station and substation design. I thought enough about current transformers then to last me a lifetime, it was boring.)
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Connecticut -silly... VA is Virginia and CT is Connecticut
 
Well Ray, if it is any consolation to you, I have been trapped in the same position many a time. Try to simplify something so anyone can understand it then they zing you with a technical point that only really matters if you are in the lab or technical design. What can you do? You must eat it because their point is valid. If you are in the lab or technical design. Not a whole helluva lot of importance in the real world, but it does exist! :thinking:
 
not sure where thread is going but my answer to question of 110 or 220 volt drive, first the drive should be rated for single phase so it doesn't have to be derated. Buy which ever one is easiest for you to install.(probably 110 just plug it in anywhere) with small hp motors and drives it is a none issue.
 
sevansx said:
I just purchased a TECO vfd and 3phase motor from Dealer's. Works great. Great customer service.
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I bought a TECO VDF and 3 phase motor from Dealer's Electric. Two things I found. In the manual it was suggested they it has it's own breaker.
The other thing was with my combo the motor made a high pitched squeal that I could only hear with my hearing aids in. There is a setting that will eliminate it, I believe it is the input frequency setting (going by memory now - see hearing aid comment and factor in age) and Dealer's support put me right on that.

John
 
Ray C said:
Todd, the meter is reading both legs -I can guarantee that. What comes in for residential power is two hot wires and a ground/neutral (see note). The two hot wires are 180 degrees out of phase. If you touch the two hot wires together you better be wearing your welding helmet.

A properly functioning meter will read the current through both legs and keep track of power used. We have the new meters in our area that automatically send the usage data to the electric company. The new meters are smart. They even watch the ground for excess voltage. They also monitor balance. If it detects issues, we get a call or letter from the power company. I got a letter indicating a possible balance issue. If they can tell me that, I know for sure they're reading both legs. I can also login to my account and it shows the power usage on an hourly basis (and I'm not really crazy that they collect such information about me).


Ray

NOTE: Please, let's not re-start the discussion about ground vs neutral at the service entrance panel.
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The two hot wires form the circuit that the electrons flow through, they are single phase.

The term 'phase' is more properly phase angle and measures the time in relation to a complete cycle at which each of the three phases begin their travels in the positive and negative going parts of their own sine wave excursions in the land of electrical theory. Google sine wave to see how we represent this...

Each phase begins it positive going rise from the zero voltage line 120 degrees after each preceding one. 360 degrees begins at zero continues up to the maximum positive voltage, continues back to zero and them travels to it's maximum negative voltage and back to zero for the next round. Each of the three phases are often shown on one graph, but that is as close a connection as they ever get. Each phase is it's own independent circuit and has no electrical need of the others aside from the fact they share a common wire. This common wire is not the same as a nuetral.

Yes, there would be a great noshing of teeth and a shower of vaporized copper if these two leads are brought together, but the exact same thing would happen if you shorted out the positive and negative posts of a large lead acid battery... or any kind of battery. POOF The reuslt of your experiment would be no different if you shorted out one of the leads to the neutral or center tap or ground for that matter. The distinction between neutral and ground is more one of code and less of electrical results. POOF regardless.

The only distinction regarding 110 vs 220 input to a VFD for the same horsepower motor is that the current in the electronics will be half for a 220 volt VFD. Less current in a semiconductor is good. There is less voltage drop and less heat. But the effect in practice for the OP is negligible.


paul
 
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