heater number, we must know the horsepower and voltage and if the motor is single or three-phase. Once we have that information, we look at table 7-4, view A, and find the full-load motor amperage. Using the chart from table 7-4, view B, we can find the heater number for this motor. For example, we want to know the number of a heater for a 5-horsepower, 230-volt ac, single-phase motor. Checking table 7-4, view A, we find that the motor draws 28 amps. Referring to table 7-4, view B, we find heater number 42227 has an amperage range from 26.0 to 28.3. This is the heater we should use. Also in the table you will find the maximum fuse size and the amperage at which the heater will open the control circuit. Remember that each manufacturer has its own heater table to be used with its across-the- line starters.
HEATER TROUBLESHOOTING. - A heater must be manually reset at the motor starter. If the magnetic starter fails to energize, the trouble is within the control circuit. However, if the coil should energize but the motor fails to run, the trouble must be within the load circuit or motor. The load circuit can be checked at terminals TI, T2, and T3. If the proper voltage requirements is there, the trouble is most likely in the motor.
An example of a push-button station with overload protection is shown in figure 7-52. In this case, the controller is connected to a 208-volt single-phase motor. This controller is a single-phase, double-contact device which connects or disconnects both undergrounded conductors to the motor. It has a start and stop button that mechanically opens or closes the contacts. Pressing the start button closes both contacts, and pressing the stop button opens both contacts. The control has two overload devices connected in series with the contacts. If an overload condition occurs, either overload device will open both sets of contacts. A typical application of this type control would be to control small machine tools.
Reversing magnetic controllers use two magnetic across-the-line starters whose power leads are electrically interconnected to reverse two of the three phases. The two motor starters are generally contained in one box and are mechanically interlocked so that one cannot close without the other opening. They are sometimes also electrically interlocked to help prevent closing both starters at the same time.
Figure 7-52. - Schematic for a single-phase manual controller with overload protection.Continue Reading