AC Motor Control Circuits

Working on a job site with an experienced technician, you are tasked with trying to determine whether the line currents going to a three-phase electric motor are balanced. If everything is okay with the motor and the power circuitry, of course, the three line currents should be precisely equal to each other.

The problem is, neither of you brought a clamp-on ammeter for measuring the line currents. Your multimeters are much too small to measure the large currents in this circuit, and connecting an ammeter in series with such a large motor could be dangerous anyway. So, the experienced technician decides to try something different - he uses his multimeter as an AC milli-voltmeter to measure the small voltage drop across each fuse, using the fuses as crude shunt resistors:

He obtains the following measurements:

Line Fuse voltage drop 1 24.3 mV 2 37.9 mV 3 15.4 mV

Do these voltage drop measurements suggest imbalanced motor line currents? Why or why not?

One method of achieving reduced-voltage starting for large electric motors is to insert series resistances into each of the motor’s power conductors. When starting, all power must go through the resistors. After the motor has had time to speed up, another set of “starter” contacts bypass line power around the resistors, directly to the motor windings.

Draw a diagram showing how this could be done for a single-phase electric motor, using two starter contacts: “R” for “run” and “S” for “start”. Hint: you only need two contacts and one resistor!