AC synchronous motor

In the preceding discussion of multispeed ac motors note that only induction motors are considered. These have no discrete physical rotor poles, so that only the stator-pole configuration need be modified to change speed. To operate multispeed, a synchronous motor would require a distinct rotor structure for each speed. Thus multispeed is practical only for squirrel-cage induction motors. 

Inverter-AC Motor Drives. An adjustable-frequency control of AC motors provide efficient operation with the use of brushless, high-performance induction, and synchronous motors. A typical system is shown in Figure. 3-14. Such a system consists of a rectifier (which provides DC power from the AC line) and an inverter (which converts the DC power to acljustable-frequency AC power for the motor). Inverter cost per kilowatt is about twice that of controller rectifiers thus the power convertor for an AC drive can approach three times the cost of a DC drive. 

Figure 3-14. Typical invester AC motor drive consisting of rectifier-DC link, adjustable-frequency inverter, and induction of synchronous motor [10].

AC methods include standard squirrel-cage induction motors, wound rotor induction motors, synchronous motors and commutator motors. Speed variation is obtained by the control of applied voltage to the stator or the control of current and voltage in the rotor by external circuit connections. 

There are two basic types of high-power inverters that are used to supply AC induction or synchronous motors, see References 2 and 9. 

The hoist electrical ac motor is a dual stator winding synchronous motor with independent excitation, offering a speed range of 0-68 rpm, a frequency range of 0-11.333 Hz. The nominal voltage and current of each stator are 3150 V and 190 A respectively. The nominal motor torque is 280000 Nm and its inertia is 20500 Kgml... 

The hoist was supphed with a high voltage AC PWM drive, coupled to a 76.4 rpm overhung synchronous motor. A pony drive was provided to allow controlled movement of the hoist should personnel become stranded in the shaft on a conveyance during maintenance activities, due to a power outcige or fculme of the main drive system. 

The m-g UPS also can be built around a synchronous AC motor, as illustrated in Fig. 10.130. UtiHty AC energy is rectified and used to drive an inverter, which provides a regulated frequency source to power the synchronous motor. The output from the DC-to-AC inverter need not be a well-formed sine wave, nor a... 

Fig. 20.2 An ac induction motor (synchronous), with a solid armature.

Once it gets going, the rotor has to come around to the correct position for repulsion, just in time for the next ac cycle to occur. This is one of the principles of ac induction motors, and the speeds are usually "synchronous" with the 60 Hz ac. That is, the revolutions per minute (rpm) is 60 times per minute, divided by some factor that is dictated by the design, although there is a few percent of "slippage" behind an exactly synchronous speed. (Experiments with controlling the speed of an ac motor will be described in the next chapter.)... 

In order to have a synchronous motor run at slow speed, as in electric clocks, the rotor can have many poles, which is illustrated in Fig. 20.5. Clocks can have hundreds of tiny poles and turn at only a few rpm when driven by 60 Hz ac (3600 cycles per minute). 

Synchronous speed The synchronous speed of an ac motor is that speed at which the motor would operate if the rotor turned at the exact speed of the rotating magnetic field. However, in ac induction motors, the rotor actually turns slightly slower. This difference is the slip and is expressed in percent of synchronous speed. Most induction motors normally have a slip of 1 to 3 percent. 

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. 

Static controller A controller in which the major portion of all of the basic functions are performed through the control of electric or magnetic phenomena in solids such as transistors, etc. Synchronous motor coutroller A controller consisting of a three-pole starter for the ac stator circuit, a contactor for the dc field circuit, an automatic synchronizing device to control the dc field contactor, and a cage-winding protective relay to open the ac circuit without synchronizing, in order to start a synchronous motor, accelerate it to synchronous speed, and synchronize it to supply frequency. 

Synchronous Motor Starting. This method is used for power-factor correction of heavy concentrations of induction motors. It is also used for constant-speed, slow-speed industrial drive applications and for maximum efficiency on continuous heavy loads in excess of 75 hp (55 kW). Three-phase ac power is connected to the stator and dc to the rotor (which has both a field and a squirrel-cage winding). 

The Escape uses a permanent-magnet AC-synchronous electric motor with an output power rating equal to 94 HP. 

The brushless DC motor (BLDC motor) is really an AC motor The current through it alternates, as we shall see. It is called a brushless DC motor because the alternating current must be of variable frequency and so derived from a DC supply and because its speed/torque characteristics are very similar to the ordinary with brushes DC motor. As a result of the brushless DC not being an entirely satisfactory name, it is also, very confusingly, given different names by different manufacturers and users. The most common of these is self-synchronous AC motor , but others include variable frequency synchronous motor , permanent magnet synchronous motor , and electronically commutated motor (ECM). 

---