Universal, Shaded Pole,and Fan Motors
The three types of motors discussed in this chapter are quite common and are used for a variety of applications- especially small appliances. All are single-phase motors, and most are designed to operate either on 120 or 240 V.
UNIVERSAL MOTORS
The universal motor is a special adaptation of the series-connected de motor, and it gets its name universal from the fact that it can be con nected on either ac or de and operate the same. All are single-phase motors for use on 120 or 240 V.
In general, the universal motor contains field windings on the stator within the frame, an armature with the ends of its windings brought out to a commutator at one end, and carbon brushes which are held in place by the motor’s end plate, allowing them to have a proper contact with the commutator.
When current is applied to a universal motor, either ac or de, the cur rent flows through the field coils and the armature windings in series. The magnetic field set up by the field coils in the stator reacts with the current-carrying wires on the armature to produce rotation.
You will find universal motors used on such household appliances as sewing machines, vacuum cleaners, electric fans, and the like.
The universal motor shown in Fig. 10-1 operates on the magnetic interaction between the armature and field poles and runs in the same direction whether the current flows in on line A or on line B, since revers ing the flow of current in the line wires changes the polarity of both the armature and field poles at the same instant. Therefore, if such a motor is supplied with alternating current, the torque developed will always be in the same direction. However, to operate successfully on ac, all parts of the magnetic circuit must be laminated to prevent undue heating from eddy currents, and element windings are usually desirable on the arma ture to ensure acceptable commutation. On the compensating windings of larger motors improvements have been made to provide more efficient operation and to reduce brush sparking.
The universal motor will produce about four times normal full-load torque with two times normal full-load current. The torque produced in creases very rapidly with an increase in current. The variation in speed from no load to full load is so great that complete removal of the load is dangerous in all motors of this type with the exception of fractional horsepower ones.
This type of motor is widely used in fractional hp sizes for fans, vacuum cleaners, kitchen mixers, and portable equipment of all types such as electric drills, sanders, saws, etc. Higher ratings are employed in traction work and for cranes, hoists, and so on. In general, universal motors are suitable for applications where high starting torque or univer sal operation is desired.
Principal troubles occur due to wear of the commutator, brushes, brush holders, or bearings; opens, shorts, or grounds occur in the arma ture, field, or associated apparatus; loose connections are another source of trouble. To reverse the direction of rotation, reverse the armature con nections or the field connections but not both.
SHADED POLE MOTOR
A shaded pole motor is a single-phase induction motor provided with an uninsulated and permanently short-circuited auxiliary winding displaced in magnetic position from the main winding. The auxiliary winding is known as the shading coil and usually surrounds from one third to one half of the pole. See Fig. 10-2. The main winding surrounds the entire pole and may consist of one or more coils per pole.
In the unshaded section of the pole the magnetic flux produced by the main winding is in phase with the main winding current, whereas the flux produced by the shading coil acts as a phase-splitting device to pro duce the rotating field that is essential to the self-starting of all straight induction motors. As the movement of the flux across the pole face is always from the unshaded to the shaded section of the pole, the direction of rotation can be determined on the normally nonreversible motor by noting the position of the shading coil with respect to the pole itself. This type can be reversed by removing the stator from the frame, turning it through 180°, and replacing it.
The starting torque will not exceed BOo/o of full-load torque at the in stant of starting, increases to 120o/o at 90o/o of full speed, and decreases to normal at normal speed. This type of motor operates at low efficiency and is constructed in sizes generally not exceeding 0 hp.
Applications for this motor include small fans, timing devices, relays, radio dials, or any constant-speed load not requiring high starting torque.
DISTRIBUTED-FIELD COMPENSATED MOTOR
The distributed-field compensated motor, a type of universal motor, has a stator coil similar to that of the split-phase motor, and two types are normally available: the single-field compensated motor and the two-field compensated motor. The former has one stator winding, while the latter has two.
The two-pole compensated motor has a stator winding like the main winding of a two-pole split-phase motor, with fields wound into the slots of the stator. Field poles, of course, must be of opposite polarity and con nected in series with the armature. This type of motor may be reversed by interchanging either the armature or field leads and then shifting the brushes against the direction in which the motor will rotate.
Two windings are used in the stator of the two-field compensated motor, similar to the starting and running windings of a split-phase motor; they are located 90 electrical deg from one another. The compen sating winding is used to reduce the reactance voltage present in the armature when it is operating on alternating current, caused by the alter nating flux.
The speed control of this type of motor may be regulated by several methods, some of which include a centrifugal switch, using a tapped field, or inserting resistance in series with the motor.
REVERSING SHADED POLE MOTORS
Some shaded pole motors are designed to be reversed by means of a built in control switch, but most motors of this type require internal changes before they can be reversed. One method is to disassemble the motor, reverse the stator end for end, and then reassemble.
On motors of this type that can be reversed externally, one main winding is present along with two separate shaded windings. The stator of this motor has slots into which the windings are placed. The main winding is usually distributed over several slots but may have only one coil per pole. To reverse the motor, the closed shaded winding is opened, while the other shaded winding is closed, changing the position of the shaded poles with reference to the main poles.