A commercial customer calls. There is no air-conditioning in the upstairs office. This is a three-story building with an air handler on each floor and a chiller in the basement.
The service technician arrives and politely introduces himself to the customer. After handing the customer a busi-ness card and describing the service he can provide, the technician asks some pertinent questions using everyday language the customer can understand. The technician then carefully listens and makes the customer feel like part of the solution to the service call problem. Once the technician has permission to enter the building, he goes to the fan room where the complaint is. There is no need to go to the chiller or the other floors because there would be complaints if the chiller was not providing cooling to those locations.
The chilled water coil piping feels cold; the chiller is definitely running and furnishing cold water to the coil. The fan motor is not running.
SAFETY PRECAUTION: Because the motor may have electrical problems, the technician proceeds with caution by opening, locking, and tagging the electrical disconnect.
The technician pushes the reset button on the fan motor starter and hears the ratchet mechanism reset. The motor will not try to start while the technician is pushing the reset button because the disconnect switch is open. The unit must have been pulling too much current for the overload to trip.
Using a digital multimeter, the technician checks for a ground by touching one lead to a ground terminal and the other to one of the motor leads on the load side of the starter. The meter is set on ohms and will detect a fairly high resistance to ground. See Figure 20.23 for an example of a motor ground check.
Figure 20.23
SAFETY PRECAUTION: Any measurable resistance displayed when the meter's leads are connected between ground and the motor lead would indicate a ground—so caution is necessary. If the meter reads little to no resistance, the circuit should not be energized until the ground is cleared up or physical damage may occur.
The motor is not grounded. The resistance between each winding is the same, so the motor appears to be normal. The technician then turns the motor over by hand to see whether the bearings are too tight; the motor turns normally.
The technician shuts the fan compartment door and then fastens the amp-clamp on the multimeter to one of the motor leads at the load side of the starter. The electrical disconnect is then closed to start the motor. When it is closed, the motor tries, but will not start and pulls a high amperage. The motor seems to be normal from an electrical standpoint and turns freely, so the power supply is now suspected.
SAFETY PRECAUTION: The technician quickly pulls the disconnect to the off position and gets a digital multimeter.
The continuity through each fuse is checked. The fuse in L2 is open. The fuse is replaced and the motor is started again. The motor starts and runs normally with normal amperage on all three phases. The question is, why did the fuse blow?
What is the problem and the recommended solution?
