Wiring up a 30HP rotary phase converter

I see what you mean. I'm willing to try it out. Talked to someone with a 30HP and it seems the in-rush is quite a bit lower than what the motor spec says, although his motor is different. That gives me some confidence.
 
So one thing that was brought to my attention is starting caps. I have ten 270μF caps, and it was mentioned to me that I can add as many as I want to help with the starting of the motor, thereby lessening the in-rush. I can easily get another 10 for $80 on Ebay. What sayeth thou?
 
I am no electrical engineer, but I think capacitors are essentially a energy storage device. It seems adding more would require more current to initially charge them, thus increasing inrush. But that is just my speculation.
 
Barncat said:
I am no electrical engineer, but I think capacitors are essentially a energy storage device. It seems adding more would require more current to initially charge them, thus increasing inrush. But that is just my speculation.
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Yes they are storage devices, yet RPC's include them as part of the start-assisting procedure. In my logic, they are not charging while the motor is already attempting to start, for it would make your second sentence true and be a contradiction to call them 'starting capacitors'. They must be fully charged prior to the motor actually attempting to start, and then either release their energy simultaneously or shortly before the motor windings are energized for it to get up to speed. That's how I see it. I don't know which of those two scenarios actually applies though.
 
Barncat said:
According to the example equation here, that would appear to be the case. If I am understanding it correctly. https://www.electricaltechnology.or...increases-capacitive-reactance-decreases.html
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yes that is the basic theory behind them. To charge up with energy, they must consume electricity in the first place. The real question that neither of us know is "when". Exactly when do they charge, and exactly when do they discharge to function in their intended purpose as motor-starting-assistance.
 
General Zod said:
yes that is the basic theory behind them. To charge up with energy, they must consume electricity in the first place. The real question that neither of us know is "when". Exactly when do they charge, and exactly when do they discharge to function in their intended purpose as motor-starting-assistance.
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I certainly do not know the answer to that.
 
In this type of application the capacitors create a phase shift which helps with starting the idler motor. The angular displacement of the three legs is what creates a rotating magnetic field in the motor stator windings. In a three phase ac induction motor that rotating magnetic field induces a voltage in the rotor, which produces a rotor field. These two fields then produce torque on the rotor. Very simplified explanation of that part.

In a generated 3 phase system the phases are 120 degrees apart. In a rpc application L1 and L2 are 180 degrees apart because they are two ends of a center tapped single phase transformer. A capacitor has a 90 degree phase shift (to keep things simple). This phase shift is what the starting caps are doing. This is enough to get the idler motor started and up to speed.

The simplified purpose of a capacitor is to oppose a change in voltage. This is done by a charge between two plates. When discharged, a capacitor opposes a change in voltage.. ie: for a brief moment it doesn't. The amount of time it takes to charge is a function of the circuit impedance and the capacitor's capacitance. The energy used to charge the capacitor is then "stored". When voltage is removed from the circuit the capacitor discharges, opposing that reduction in voltage. In this application the phase shift is what we are going for. The phase shift is caused by the capacitive reactance of the capacitor. This only happens in an ac circuit. An inductor is similar, but it opposes a change in current.
 
Thanks for the explanation. How does the amount of capacitance play a part in reducing the in-rush? If you please.
 
General Zod said:
Thanks for the explanation. How does the amount of capacitance play a part in reducing the in-rush? If you please.
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I haven't done the calculations for this converter of yours. I'm not saying adding additional start capacitors will do what you want.

My advice is to connect it up and see what it does on an ammeter. You'll need a clamp-on ammeter, appropriately sized conductors and breaker. This is what I did and it all went really well. And, it all jived with what the manufacturer stated.
 
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