gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
121125-1410 EST
Some basic theory:
Ohm's law was created at a time when virtually nothing was known about AC circuits, and it does not include a power or energy equation. With the evolution of AC circuits Ohm's law was extended via the concept of impedance to include analysis of AC circuits. But it still does not include power.
In an AC circuit at constant frequency, with components that do not change their characteristics as voltage or current changes, the component generally obeys Ohm's law. Thus, raise the voltage across the component and the current increases proportionally. This characteristic applies to normal resistors, inductors, and capacitors.
An AC induction motor with a locked rotor is nothing more than a transformer with a resistive load. However, with normal applied voltage under these conditions the core may be saturated and it would not be considered to have a constant inductance. If voltage levels were below saturation values, then current would be approximately proportional to voltage.
In a DC fixed field motor, permanent magnet field for example, with the rotor locked the armature current will be approximately proportional to applied voltage.
In an unloaded DC motor, again a fixed field, the motor speed will be proportional to armature voltage minus a loss factor. The current will increase as voltage is increased. The way in which the current varies with speed is a function of the dominate type of losses. Some losses to consider are friction, windage (a different type of friction), and I2*R.
Next put a constant torque load on the DC motor with fixed field and adjust voltage. The following are approximations. The speed changes, the torque and current do not change. The power input decreases with decreasing speed.
Change the load to a constant power load. Now as voltage goes down power input remains approximately constant. RPM decreases, and current goes up.
For an AC motor based on a rotating magnetic field, or pulsating field (induction and synchronous motors) for an approximation the motor can be considered a constant speed motor. For a constant power load or constant torque, because the speed is constant, the input power to the motor must be constant. Thus, as as voltage drops the current must increase.
Now to the original question. It has already been answered. But I will repeat.
The description of the problem indicated that the breaker was already overloaded before the wire size was changed. Only by luck the breaker was not being tripped. Note: all breakers and fuses have tolerances and have an actual operating point (tripping or blowing) that may not be at the nominal rating. Further there is a trip or blow time characteristic that may vary. Also ambient temperature effects.
When the wire size was increased the impedance of the wire was reduced producing a higher voltage at the load. The higher voltage at the load increased the load current. All of this was just enough to get over the trip point of the breaker.
Solution --- correctly wire the installation. Correct size wire and breaker, etc.
Web site problems. Spell checker activates, but no scroll capability.
.
Some basic theory:
Ohm's law was created at a time when virtually nothing was known about AC circuits, and it does not include a power or energy equation. With the evolution of AC circuits Ohm's law was extended via the concept of impedance to include analysis of AC circuits. But it still does not include power.
In an AC circuit at constant frequency, with components that do not change their characteristics as voltage or current changes, the component generally obeys Ohm's law. Thus, raise the voltage across the component and the current increases proportionally. This characteristic applies to normal resistors, inductors, and capacitors.
An AC induction motor with a locked rotor is nothing more than a transformer with a resistive load. However, with normal applied voltage under these conditions the core may be saturated and it would not be considered to have a constant inductance. If voltage levels were below saturation values, then current would be approximately proportional to voltage.
In a DC fixed field motor, permanent magnet field for example, with the rotor locked the armature current will be approximately proportional to applied voltage.
In an unloaded DC motor, again a fixed field, the motor speed will be proportional to armature voltage minus a loss factor. The current will increase as voltage is increased. The way in which the current varies with speed is a function of the dominate type of losses. Some losses to consider are friction, windage (a different type of friction), and I2*R.
Next put a constant torque load on the DC motor with fixed field and adjust voltage. The following are approximations. The speed changes, the torque and current do not change. The power input decreases with decreasing speed.
Change the load to a constant power load. Now as voltage goes down power input remains approximately constant. RPM decreases, and current goes up.
For an AC motor based on a rotating magnetic field, or pulsating field (induction and synchronous motors) for an approximation the motor can be considered a constant speed motor. For a constant power load or constant torque, because the speed is constant, the input power to the motor must be constant. Thus, as as voltage drops the current must increase.
Now to the original question. It has already been answered. But I will repeat.
The description of the problem indicated that the breaker was already overloaded before the wire size was changed. Only by luck the breaker was not being tripped. Note: all breakers and fuses have tolerances and have an actual operating point (tripping or blowing) that may not be at the nominal rating. Further there is a trip or blow time characteristic that may vary. Also ambient temperature effects.
When the wire size was increased the impedance of the wire was reduced producing a higher voltage at the load. The higher voltage at the load increased the load current. All of this was just enough to get over the trip point of the breaker.
Solution --- correctly wire the installation. Correct size wire and breaker, etc.
Web site problems. Spell checker activates, but no scroll capability.
.
Last edited:
