Circuit Solutions, Pump Motor Monitor by David Johnson P.E.


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Pump Motor Monitor: How do you know if an inaccessible pump motor is turned on, without hiring an electrician to wire in some special equipment?

This circuit (see schematic) solves this problem in a novel way. It clips on the outside of the pump motor’s power cable and provides a transistor switch closure whenever the circuit detects current flowing to the motor. The circuit draws so little current that a small lithium cell will power the circuit for years.

Background

Several times over the last few years I have received calls from people looking for a simple circuit that would tell them when a sump pump or water well pump motor was running. These people told me that the electrical circuits that turn on and off the motors were frequently inaccessible, often below ground, making it difficult to know if the motor was running or not. What they wanted was a simple way to let them know when the motor was on or off, without wiring something into the 120vac or 240vac power supply. At the time I didn’t have a ready circuit for them but I put the requirement into my subconscious and let my mind work on the problem for a while. After some thought, I came up with the solution described below.

How it works

The traditional method for determining if a motor is running or not is to measure the current flowing through one of the wires supplying power to the motor. When the motor is off, the current should be near zero and will jump to the running current when the motor turns on. The classic sensor for measuring AC current is a current transformer. In a typical AC current transformer configuration, one of the wires carrying current to the motor would be routed through the transformer. The one wire forming a single turn primary. The magnetic field produced by the current flowing through the wire induces a voltage in the secondary winding of the transformer, proportional to the current. The output voltage of the transformer would then be connected to a circuit, which would activate a switch, whenever current was drawn by the motor. However, this single wire connection is often difficult and can be dangerous for a novice to wire into an electrical system. The installer would have to cut into the power cable to expose one wire or find an exposed single wire somewhere else, perhaps inside the circuit breaker box. Clamping a typical current meter, using a current transformer probe, over the whole power cable will result in a zero current reading, since the field produced by the return wire, which is also inside the cable, will be canceled by the magnetic field of the supply wire. So, how do you detect the current? The solution is to rely on the fact that the two wires carrying current inside the cable are not perfectly in parallel with each other. A small coil, acting like a transformer, placed against one wire, will pick up more of its field and less from its neighbor. Although the signal produced by the coil will be small, enough can be collected to indicate if the motor is drawing current or not. When connected to a sensitive circuit, a simple monitoring device can be produced. The user just has to find a suitable place to clamp the box containing the monitoring circuit to the outside jacket of the power cable, supplying current to the motor.