Skip to main content

Reluctance Motor - principle, Working, Advantages

Reluctance Motor

The stator has the main winding and an auxiliary (starting) winding. In general, the stator of a single-phase reluctance motor is similar to that of any one of the single-phase induction motors. The rotor of a reluctance motor is basically a squirrel cage with some rotor teeth removed at the appropriate places such as to provide the desired number of salient rotor poles. Figure 9.1 shows the 4- pole reluctance type synchronous motor. Her teeth have been removed in four locations to produce a 4-pole salient-pole structure. The rotor bars are kept intact even in the spaces from where teeth are removed. The two end rings short-circuit these bars as in a cage rotor.

Reluctance Motor

Working principle Of Reluctance Motor

When the stator is connected to a single-phase supply, the motor starts as a single-phase induction motor. At a speed, of about 75 percent of the synchronous speed, a centrifugal switch disconnects the auxiliary winding, and the motor continues to speed up as a single-phase motor with the main winding in operation. When the speed is close to the synchronous speed, a reluctance torque is produced due to the tendency of the rotor to align itself in the minimum reluctance position with respect to the synchronously rotating flux of the forward field. The rotor pulls into synchronism. For this to happen effectively, the load inertia must be within limits. After pulling into synchronism, the induction torque disappears but the rotor remains in synchronism due to the synchronous reluctance torque alone. 

Torque Speed characteristic of a reluctance motor

Figure 9.2 shows the typical torque-speed characteristic of the single-phase reluctance motor. The starting torque is dependent upon the rotor position because of the salient pole rotor. The value of the starting torque is between 300 to 400 percent of its full-load torque. At about 75% of the synchronous speed, a centrifugal switch disconnects the auxiliary winding and the motor continues to run with the main winding only. When the speed is close to synchronous speed, the reluctance torque developed as a synchronous motor pulls the rotor into synchronism and the rotor continues to rotate at synchronous speed.


Torque Speed characteristic of a reluctance motor

The motor operates at a constant speed up to a little over 200 % of its full-load torque. If the loading is increased beyond the value of the pull-out torque, the motor loses synchronism but continues to run as a single-phase induction motor up to over 500 percent of its rated torque. Reluctance motors are subject to cogging at the time of starting. This is due to the saliency of the rotor. The cogging is minimized by skewing the rotor bars and by having the rotor slots not exact
multiples of the number of poles In reluctance motors since the rotor is unexcited and has saliency, the power factor is lower than that of the equivalent induction motor. The maximum output of a reluctance motor is greatly reduced due to the absence of de field excitation. Therefore the size of a reluctance motor is larger than that of an equivalent synchronous motor. 

Advantages Reluctance motor

The main advantages of a reluctance motor are its simple construction (no slip rings, no brushes, node field winding), low cost, and easy maintenance. In spite of its shortcomings, the reluctance motor is widely used for many constant-speed applications such as electric clocks timers, signaling devices, recording instruments and photographs etc.

Popular posts from this blog

Limitations of Terzaghi Theory

Limitations of Terzaghi Theory The value of the coefficient of consolidation has been assumed to be constant.  The distance d of the drainage path cannot be measured accurately in the field. The thickness of the deposit is generally variable, and an average value has to be estimated.  There is sometimes difficulty 1n locating the drainage face, sometimes thin previous seams that can act as good drainage face are missed in the boring operations. The equation is based on the assumption that the consolidation is one-dimensional. In the field, the consolidation is generally 3-dimensional. The lateral drainage may have a significant effect on the time rate of consolidation. The initial consolidation and secondary consolidation have been neglected. Sometimes these form an important part of the total consolidation. In actual practice, the pressure distribution may be far from linear or uniform. Read More Muller-Breslau principle

Price Guard Wire Method

Price Guard Wire Method Some form of  Price Guard Wire Method  is generally used to eliminate the errors caused by leakage currents over insulation. Fig. 3.14 illustrates the operation of This Method. In fig 3.14(a), a high resistance mounted on a piece of insulating material is measured by the ammeter voltmeter method. The micro-ammeter measures the sum of the current through the resistor (IR) and the current through the leakage path around the resistor. The measured value of resistance computed from the readings indicated on the voltmeter and the microammeter, will not be a true value but will be in error.   Figure 3.14 Application of  guard  circuit for measurement of high resistance In fig, 3.14 (b), the  guard  terminal has been added to the resistance terminal block. The  guard  terminal surrounds the resistance terminal entirely and is connected to the battery side of the micro-ammeter. The leakage current IL now

Negative Booster

Negative booster A negative booster is employed to conform to the regulation that the potential difference between any two points of the rail return shall not exceed 7 V. Two boosters, positive and negative, are used which are mechanically coupled together and driven by a DC motor. The positive booster is connected to the trolley wire (near the generating station) and the negative booster (separately excited) is connected to the track rail.  The 'positive booster' adds voltage to the line while the 'negative booster lowers the potential of the point it is connected to. As we go along the trolley wire away from the generating station/sub-station, the potential drop increases, and the voltage of the trolley wire falls. Since the current returns via the track rail points away from the generating station acquire high potentials. This potential is brought down by the negative boost provided by the negative booster. When the load is sufficiently far aw