Single phase Synchronous Motors and Universal Motors.

Introduction:

The constant speed motor operated from a single phase ac supply is simply called as the single phase synchronous motor.They are suitable for applications such as clocks,timers,etc.

There are two types of single phase synchronous motor.They are described below in brief:

Reluctance motor:

A reluctance motor is a motor which depends on reluctance torque for its operation. Reluctance torque is the torque induced in an iron object (such as a pin) in the presence of an external magnetic field, which causes the object to line up with the external magnetic field. This torque occurs because the external field induces an internal magnetic field in the iron of the object, and a torque appears between the two fields, twisting the object around to line up with the external field. In order for a reluctance torque to be produced in an object, it must be elongated along axes at angles corresponding to the angles between adjacent poles of the external magnetic field.
A simple schematic of a reluctance motor is shown in Figure 3.4.1.

It can be shown that the torque applied to the rotor of this motor is proportional to sin 2δ, where δ is the electrical angle between the rotor and the stator magnetic fields. Therefore, the reluctance torque of a motor is maximum when the angle between the rotor and the stator magnetic fields is 45°. Like a normal synchronous motor, it has no starting torque and will not start by itself.A self-starting reluctance motor that will operate at synchronous speed until its maximum reluctance torque is exceeded can be built by modifying the rotor of an induction motor. In this figure, the rotor has salient poles for steady-state operation as a reluctance motor and also has cage windings for starting. Although no torque is developed due to induction motor action, however, torque due to salient pole structure of rotor structure of rotor will not be zero.The power developed by the rotor of 3-phase synchronous motor with salient pole structure is given by,

The second part of the above power is due to the difference between the value of Xd and Xq which is due to the saliency of the motor and is called reluctance power that varies with sin2δ.Because of this torque, the rotor continues to rotate, even the emf and current in the rotor conductor is zero and thus, it rotates at synchronous speed.

Hysteresis motor:

Hysteresis motor employs the phenomenon of hysteresis to produce a mechanical torque. The rotor of a hysteresis motor is a smooth cylinder of magnetic material with no teeth, protrusions, or windings. The stator of the motor can be either single- or three-phase; but if it is single-phase, a permanent capacitor should be used with an auxiliary winding to provide as smooth a magnetic field as possible, since this greatly reduces the losses of the motor. Figure 3.4.2 shows the basic operation of a hysteresis motor.

When a three phase (or single-phase with auxiliary winding) current is applied to the stator of the motor, a rotating magnetic field appears within the machine. This rotating magnetic field magnetizes the metal of the rotor and induces poles within it. When the motor is operating below synchronous speed, there are two sources of torque within it. Most of the torque is produced by hysteresis. When the magnetic field of the stator sweeps around the surface of the rotor, the rotor flux cannot follow it exactly, because the metal of the rotor has a large hysteresis loss. The greater the intrinsic hysteresis loss of the rotor material, the greater will be the angle by which the rotor magnetic field lags the stator magnetic field. Since the rotor and stator magnetic fields are at different angles, a finite torque will be produced in the motor. In addition, the stator magnetic field will produce eddy currents in the rotor, and these eddy currents produce a magnetic field of their own, further increasing the torque on the rotor. The greater the relative motion between the rotor and the stator magnetic field, the greater the eddy currents and eddy-current
torques. When the motor reaches synchronous speed, the stator flux ceases to sweep across the rotor, and the rotor acts like a permanent magnet. The induced torque in the motor is then proportional to the angle between the rotor and the stator magnetic field, up to a maximum angle set by the hysteresis in the rotor. Since the amount of hysteresis within a particular rotor is a function of only
the stator flux density and the material from which it is made, the hysteresis torque of the motor is approximately constant for any speed from zero to synchronous speed. The eddy-current torque is roughly proportional to the slip of the motor. These two facts taken together account for the shape of the hysteresis motor's torque-speed characteristics.

It is commonly used as the driving mechanism in electric clocks. An electric clock is therefore synchronized to the line frequency of the power system, and the resulting clock is just as accurate (or as inaccurate) as the frequency of the power system to which it is tied.

Universal motors:

A universal motor is a special type of motor which is designed to run on either DC or single phase AC supply. These motors are generally series wound (armature and field winding are in series), and hence produce high starting torque. Most of the universal motors are designed to operate at higher speeds, exceeding 3500 RPM. They run at lower speed on AC supply than they run on DC supply of same voltage, due to the reactance voltage drop which is present in AC and not in DC. The schematic construction of universal motor is shown fig 3.55 below with its torque speed characteristics.

A universal motor works on either DC or single phase AC supply.When the universal motor is fed with a DC supply, it works as a DC series motor. When current flows in the field winding, it produces an electromagnetic field. The same current also flows from the armature conductors. When a current carrying conductor is placed in an electromagnetic field, it experiences a mechanical force. Due to this mechanical force, or torque, the rotor starts to rotate. The direction of this force is given by Flemming's left-hand rule.

When fed with AC supply, it still produces unidirectional torque. Because, armature winding and field winding are connected in series, they are in the same phase. Hence, as the polarity of AC changes periodically, the direction of current in armature and field winding reverses at the same time.
Thus, the direction of magnetic field and the direction of armature current reverses in such a way that the direction of force experienced by armature conductors remains same. Thus, regardless of AC or DC supply, universal motor works on the same principle that DC series motor works.

The application of universal motors are given below:

• Universal motors find their use in various home appliances like vacuum cleaners, drink and food mixers, domestic sewing machine etc.
• The higher rating universal motors are used in portable drills, blenders etc.