Single phase motor

Manual and Magnetic Starters Manual motor starters are operated by hand. The simplest type of manual starter is a snap switch with no overload protection, used only for motors of 1,492 kW (2 hp) and smaller, usually single-phase motors with integral overload protection. 

Some overload-protection schemes measure motor-winding temperature directly various methods are used. Small single-phase motors are available with built-in overload protection, A thermostat built into the motor senses motor-winding temperature directly. 

In I 888 Nikola Tesla (1856-1943) at Columbus, Ohio, USA, invented the first induction motor which has become the basic prime mover to run the wheels of industry today. Below, for simplicity, we first discuss a polyphase and then a single-phase motor. 

Since the kW developed by a 3-0 winding is 50% more than by a 2-0 winding for the same value of stator current /, the economics of this principle is employed in an induction motor for general and industrial use. As standard practice, therefore, in a multi-phase system, only 3-0 induction motors are manufactured and employed, except for household appliances and applications, where mostly single-phase motors are ttsed. 

Energy management guide for selection and use of single phase motors... 

If the motor is switched on, in a single phasing condition, it will not rotate in the absence of a rotating field, similar to a single-phase motor without a start winding. 

Single phase motors do exhibit operational problems in pony motor applications. The capacitors in these motors store energy and the motors resist external attempts to rotate them. Single phase motors can be made to work on pony motor applications by splicing into the capacitor circuit and connecting it in series with an open auxiliary switch in the active motor starter. 

Circuits that carry AC current employing two, three, or more sinusoidal potentials are C2 ed polyphase circuits. Polyphase circuits provide for more efficient generation and transmission of power than single-phase circuits. Power in a three- (or more) phase circuit is constant rather than pulsating like the single-phase circuit. As a result, three-phase motors operate more efficiently than single-phase motors. 

Single-phase Motors. Alternating-current single-phase motors are usually induction or series motors, although single-phase synchronous motors are available in the smaller ratings. 

Repulsion Motor. A repulsion motor is a single-phase motor that has a stator winding arranged h)r connection to a commutator. Brushes on the commutator are short circuited and are so placed that the magnetic axis of the stator winding. This type of motor has a varying-speed characteristic. 

Table 2-1 yields ballpark motor efficiencies. The table applies to three-phase induction motors only. For singlephase, multiply by about 0.8. Single-phase motors are not generally recommended above 10 HP. 

Single-phase motors may be started (a) by incorporating a commutator, (6) by hand and (c) by operating them as polyphase motors, a phase splitter being used to throw two or more circuits out of phase with each other. ... 

Other motors do not fall under the EPAct regulations, for example, footless designs, other horsepower ranges, 900 rpm and slower speeds, two-speed versions, 50 Hz, 200/400 and 500 V, NEMA design C and D types, U frame, single-phase motors, frame sizes 56 and smaller, and motors designed for inverter use only covered motors may be used with inverters and still fall under the provisions of the EPAct. Underwriters Laboratory listed motors may be handled on a case-by-case basis. 

Electrical Symbols AAVv— Fixed resistor Photo resistor — Variable r istor +1 -. .-N> h- j, l ode Ci acitor r 7 vm K <1 Meter Speaker Solar cell Single-phase motor... 

Fig. 3. Single-phase motor winding, cylindrical or spherical rotor.

Phase The number of circuits over which electric power is supplied. In a single-phase motor, power is supplied in a single circuit or winding. In a three-phase system, power is provided over three circuits, each circuit reaching corresponding cyclic values at 120° intervals. AC motors are typically qualified as single-phase or polyphase. 

Shaded-Pole Motors. Instead of a start winding, shaded-pole motors have a continuous solid-copper loop around a small portion of each salient pole (Fig. 5.107). This shading coil causes the reaction necessary to start the motor, but produces rather low starting and accelerating torque. Because of their low starting torque, shaded-pole motors are best suited to hght-duty apphcations such as direct-drive fans and blowers. Efficiency and power factor are also lower than that of other single-phase motors. 

AC Three-Phase Synchronous Motors. Like single-phase motors, three-phase synchronous motors cannot start by themselves. One of two starting methods is used in most motors dc excitation or reluctance. 

Single-phase motors also have NEMA design letters, but they do not specify the performance characteristics to the same extent that the three-phase design letter does. The letters N and O are used for FHP motors and L and M for IHP motors. 

Power supply will be either single- or three-phase. A single-phase motor of the proper voltage can be used on a three-phase system if properly connected. But a three-phase motor cannot be used on single-phase supply. Three-phase motors generally cost less, perform better, and last longer than single-phase motors of the same size. 

TABLE 5.31 Single-Phase Motors—Selection by Characteristics... 

Wound-rotor (repulsion) types Vf, to 10 Very hard Low Larger dian odier equivalent single-phase motors No... 

If single-phase motors are wound for multiple voltages, six or seven leads will be found in the junction box, as shown in Figure 21-12. With six leads and low-voltage operation, typically 125 V in the United States, three of the junction box leads are connected to each of the two line wires. With six leads and high-voltage operation, typically 230 V in the United States, three of the junction box leads are connected together and one or two of the other leads are connected to the two line wires. Details for the specific motor are often printed on the inside of the junction box lid. 

Figure 21-12 Typical wiring for single-phase motors. (Courtesy of Chemineer.)...

Single-voltage motors can have simplified wiring because the other connections are made internal to the motors. Portable mixers, especially those with single-phase motors, can be prewired with a suitable electric cord. 

A pump impeller will spin backward because the motor wiring is connected incorrectly. I m referring to a three-phase motor. For a single-phase motor, the wiring connections may be reversed without affecting the direction of rotation. 

It is possible to rewire a three-phase motor into a single-phase motor, and thus reduce its speed from 3600 to 1800 rpm. Then its power consumption would drop by a factor of 8. I ve done this once at an East Coast refinery, which had a hugely oversized sulfur plant incinerator air blower. For most projects, a more typical option would be to retrofit the motor for variable-speed operation, using frequency control. However, not all motors are amenable to frequency control, which is a relatively expensive option. Feedback from my clients that have retrofitted pump drivers with variable-speed AC motors is often unfavorable. 

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