This chapter is strictly for those technicians that are servicing light commercial equipment. In most residential equipment, a contactor is used to start an electric motor, whether it is a compressor motor or a pump drive motor; however, the contactor offers no built-in protection for the motor it operates. The motor starters are designed to operate electric motors, but have an over-current protection by using overloads. The overload is similar to the circuit breaker in the way it acts upon the motor starter contact. Figure 8-1 shows a typical motor starter.
Fig. 8-1. Motor starter.
The control wire that causes the magnetic switch to close the contacts, is series-wired through bimetal auxiliary switches. As you have learned, bimetal will flex when heated or cooled. L1, L2, and L3 are wired from the line side through heaters to the load side. These heaters are designed with a specific resistance that causes them to produce heat when the designated limit is reached. The heaters can be designed for very close tolerances. They heat the bimetal very quickly and cause the control circuit to open. This in turn opens the contacts of the starter and the flow of current is stopped. The bimetal switch is a manual reset type. When the heater cools, the motor will not begin to run again until someone manually resets the switch. The advantage of this arrangement is two-fold. One, the tripping amperage is sized to the exact amount a motor can conduct before damage is done. Second, if a problem exists with the motor, it will not try to restart itself. A technician would check things before attempting to start the motor again. You should use an ammeter to confirm whether the heaters and bimetal switches are functioning properly. If they are, you might find a bad bearing, belt, or some other source causing the motor to drag and overheat.
This type of installation might be seen in a residence that uses a pool circulating pump, or perhaps a lawn sprinkling system with a pump large enough to be protected with a motor starter. This type of motor starter is considered to be automatic and can have a remote switch somewhere to turn it on and off. There are manual motor starters that have the same overload protection as the automatic ones, but they must be turned on and off manually at the switch. Figure 8-2 shows this manual type of starter. These switches have replaceable heaters that can be sized to any amperage load, whether it is a single-phase or three-phase application.
Fig. 8-2. Manual motor starter.
An auxiliary switch or switches will be found mounted on some of the automatic motor starters. These are usually n.o. switches, that close causing operation of a fan or other appliance or appendage in the system. If a water pump is located in a small room or closet, an exhaust or intake fan might be energized whenever the pump starter starts the pump. The n.o. auxiliary contacts close turning on the fan. Some starters can be acquired with more than one auxiliary set of contacts.
This is a fitting time to go into a little more detail about the wiring of three-phase motors. Three-phase motors have six windings. Wire leads from these windings are usually brought into a terminal box by the manufacturer. It is here in the terminal box that the branch circuit wiring is attached to the motor windings. Usually there are nine leads brought from the windings; they are marked with bands showing their numbers. Each wire lead having a different number from one through nine. In Fig. 8-3 you can see a wiring schematic of a typical electric motor circuit. Motor voltages vary with the location and application for which they are manufactured. Let me state again, this book is not a pertinent text for heavy commercial and industrial motors. Their voltages are different from residential and light commercial application; for this reason 120, 240, and 480 volts are discussed. If a motor is rated for two voltages, it will be up to you to wire it for the correct supply voltage. Make sure of the voltage; it doesn’t take long for a winding to burn, when it is designed for 120 volts and 240 volts is put through it.
Fig. 8-3. Wiring schematic of the average motor circuit.
In our industry, L1, L2, and L3 designate the line sides of a device. The line side is the conductor that has the voltage constantly ready to flow. The other side, designated by T1, T2, and T3, is the load side of the device. The load side is the conductor to which the device is wired. The T side doesn’t carry any voltage when the control switch is in the open position. Figure 8-4 shows the hookup of the windings and line side for dual voltages. This motor has a low voltage of 240 volts ac. Figures 8-5 through 8-7 show different wiring schematics with Y windings along with dual speed motors. This will be an easy reference on a job someday.
Fig. 8-4. Wiring schematic for dual-voltage six windings.
Fig. 8-5. Wiring schematic for a single-speed, delta-connected, dual-voltage electric motor.
Fig. 8-6. Wiring schematic for a two-speed, single-winding, variable-torque electric motor.
Fig. 8-7. Wiring schematic for a single-speed, Y-connected, dual-voltage electric motor.
Don’t forget the importance of rotation. It is very important that you remember this minor point. Fan blades, water pumps, some compressors, just to mention a few need to have their drive motors turning in the proper direction. This holds true for single- and three-phase motors. Some single-phase motors do not have reversing capabilities. These are fixed rotation motors. Reversible rotation single-phase motors accomplish this by electrically reversing two connections in the motor. As I stated in the beginning of the book, three-phase motors are easily reversed by reversing two lines. By reversing L1 and L2 with each other, the rotation of the motor will reverse. This procedure was discussed in Chapter 3 in the section about compressors with locked rotor.
