Because there are so many different kinds and makes of controllers, we will outline a genera! procedure for locating the source of trouble.
1. If the motor does not start when the main con- tacts close, the trouble may be as follows:
a. Open overload heater coil or poor connection.
b. Main contacts not making contact. It is not unusual for one or more contacts to wear to the degree that they will not make when closed. This fault will also occur if the contacts become dirty, gritty, or burned.
c. Broken, loose, or dirty terminal connection.
d. Loose or broken pigtail connection.
e. Open resistance units or open autotrans- former.
f. Obstruction of the magnet core, preventing the contacts from closing.
g. Mechanical trouble, such as mechanical interlocks, gummy pivots, and poor spring tension.
2. If the contacts do not close when the START button is pressed, the trouble may be as follows:
a. Open holding coil. This can be tested by connecting a voltmeter across the coil terminals when the START button is pressed. If there is voltage when the START button is pressed but the coil does not become energized, the coil is defective.
b. Dirty START button contacts or poor contact.
c. Open or dirty STOP button contacts. If more than one station is connected to the same controller, each station should be checked. If FORWARD-REVERSE stations are used and they are interlocked, check all contacts.
d. Loose or open terminal connections.
e. Open overload-relay contacts.
f. Low voltage.
g. Shorted coil.
h. Mechanical trouble.
3. If the contacts open when the START button is pressed, the trouble may be as follows:
a. Contacts that do not close completely or are dirty, pitted, or loose.
b. Wrong connection of station to the con- troller.
4. If a fuse blows when the START button is pressed, the trouble may be as follows:
a. Grounded circuits.
b. Shorted coil.
c. Shorted contacts.
5. If the magnet is noisy in operation, the trouble may be as follows:
a. Broken shaded pole causing chattering.
b. Dirty core face.
6. If the magnet coil trouble may be as follows:
a. Overvoltage. is burned or shorted, the
b. Excessive current due to a large magnetic gap caused by dirt, grit, or mechanical trouble.
c. Too frequent operation.
The example used here is a control that is operated by a remote switch, such as a float switch. It is assumed that the device being controlled (a three-phrase motor) is in good working order but is not receiving power. Figure 7-18 shows such a circuit. The first thing you should check is the line voltage. To do this check, remove the cover of the control box and test each line with a voltmeter. You should take the volt readings between L1 and L2, L2 and L3, and then between L3 and L1 . If full voltage is found, you should visually check the power circuit for loose connections. These terminals include L1, L2, L3, T1, T2, and T3. Look for signs of heating at these connections. When a connection becomes loose, the terminal becomes very hot; and the screw, wire, and terminal become discolored or charred. Check all terminals and tighten them if necessary. You should only do this checking and tightening with the power OFF.
Next, check the control circuitry within the controller. Do this check by looking at the control circuit shown in figure 7-18. The external controls, the magnetic holding coil, and the normally closed overload contacts are always located between line 1 and line 2. Unless the control has been altered, line 3 is not part of the control circuit. Check also that the externally located controlling switches, such as theContinue Reading