1. Junior Member
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## L1 L2 Controls

How does the circuit operate when you receive power from 2 phases? Shouldn't the circuit trip because of a phase to phase fault?

2. gar
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170812-1025 EDT

What do you expect to trip, and why?

.

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Jay, the fuses F1 and F2 supply power to the primary of a control transformer. If your were to measure the resistance of any transformer primary it would read as a short circuit. However transformers are basically inductors and have an impedance ( AC resistance that depends on hertz) that prevents the them from being a short circuit.

I'll also point out that your schematic is missing the heater elements between M1 contacts and the motor and also the three overload relay contacts between M1 coil and neutral/ground.

4. Originally Posted by jaymiller
How does the circuit operate when you receive power from 2 phases? Shouldn't the circuit trip because of a phase to phase fault?
Fuses may need proper time delay characteristics to allow the impedance of the transformer primary coil to develop, but once there the impedance of that coil is what limits current flow in that circuit.

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Originally Posted by gar
170812-1025 EDT

What do you expect to trip, and why?

.
Perhaps from inexperience, but I would expect the OCPD from either L1 or L2 to trip because of the connection between the two seperate phases.

I'm not arguing the logic , simply trying to understand.
If I connect two wires from phase A to phase B, it would immediately trip, right?. How do we get power for the control circuit if we are connecting phase A to phase B? Shouldn't power be from a phase and neutral so the circuit has a path back to its origin? or even phase to same phase so we create the magnetic field required for the solenoid?

I'm fairly new to motor controls and have neglected it for years. I'm studying and maybe I asked the question prematurely and Mike Holts dvd set will explain?

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Originally Posted by jaymiller
Perhaps from inexperience, but I would expect the OCPD from either L1 or L2 to trip because of the connection between the two seperate phases.

I'm not arguing the logic , simply trying to understand.
If I connect two wires from phase A to phase B, it would immediately trip, right?
....
Please look at the motor. It is connected to L1 and L2 AND L3! It does not trip the breaker because it is a three phase motor. Your control transformer will not trip the breaker (or blow fuses) because it is a single phase transformer designed to run on the supplied voltage.

As to the lack of O/L, is that because the motor is a fractional HP motor?

7. gar
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170812-2354 EDT

jeremysterling:

What trips a breaker or fuse? Current thru the protective device. The trip point setting (for most breakers this is determined at the time of manufacture and interrelated with time and temperature) determines when trip occurs.

A delta source has no neutral.

Only a wye system or single phase center tapped secondary has a neutral.

It is not usual to have neutral current monitored for tripping purposes. But some times it is needed.

Exceeding the protective device trip point trips the device.

Will motor control circuitry trip the power source breaker to a motor? Highly unlikely. A motor control circuit will usually require much less current than does the real load, the motor.

You need to do a lot more study on circuitry theory.

.

8. Originally Posted by jaymiller
How does the circuit operate when you receive power from 2 phases? Shouldn't the circuit trip because of a phase to phase fault?
Power (to the motor) is three phase.
Control is single phase and usually negligible.

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What voltage would you expect to see between phase A and phase B? And for that matter between any two phases? What voltage would you expect to see from any phase to neutral or ground? Assume a 120/208 Y service.

This is the basics of three phase systems.

Originally Posted by jaymiller
How do we get power for the control circuit if we are connecting phase A to phase B?
Now think about a transformer with a 208 volt primary and a 24 volt secondary.

-Hal

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Originally Posted by hbiss
What voltage would you expect to see between phase A and phase B? And for that matter between any two phases? What voltage would you expect to see from any phase to neutral or ground? Assume a 120/208 Y service.

This is the basics of three phase systems.

Now think about a transformer with a 208 volt primary and a 24 volt secondary.

-Hal
Thanks, this helps.