Squirrel cage motors
The majoritory of 3-phase AC induction motors are of the squirrel cage construction. They are constructed in two parts, a fixed outer portion, called the stator and an inner portion which rotates inside the stator, this is called the rotor. Between the two there is a carefully engineered air gap.
In a 3-phase induction motor a rotating magnetic field is created naturally within the stator, this is due to the nature of the 3-phase supply. Two sets of electromagnets are created within the motor, the first is formed in the stator, and this is due to the 3-phase AC supply to the stator windings. The alternating nature of the supply induces an EMF in the rotor by Lenz’s law, this action generates another electromagnet in the rotor. The interaction between these fields creates a twisting force or torque. This results in the rotating of the rotor in the direction of the torque.
In a single phase induction motor the rotating magnetic field in the stator in not naturally occurring, in this case the motor has to be started by inducing two supplies of differing phases from the single phase supply. When the motor is beginning to run at operating speeds a centrifugal switch cut out the starting circuit and will return the supply to single phase alone. Below shows one of many starting technique for a single phase AC induction motor, capacitor start capacitor run starting.
All about circuits. (2009) Single-phase induction motors.
http://www.allaboutcircuits.com/vol_2/chpt_13/9.html. Accessed 29/11/09.
The speed of the motor is dependent on the slip between the two rotating magnetic fields, if the stator magnetic rotation exceeds that of the rotor the motor will act as a motor, if this is reversed and rotor field exceeds the stator field then the motor will act as a generator. If there was to be zero slip i.e. the fields are rotating in unison then the windings become a transformer, this state is inadvisable and can cause damage to the motor. Speed is also a function of the frequency of the supply and by the number of poles contained within the motor, the speed if an induction motor can be shown as
Ns= The synchronous speed of the stator magnetic field in RPM
f= The frequency of the supply
P= Number of poles within the motor
The stator of an 3-phase induction motor is constructed as below
Balu, S. (2009) How Are Squirrel Cage Induction Motors Constructed?. http://www.brighthub.com/engineering/electrical/articles/43723.aspx#ixzz0Zmyhl8jY. Accessed 10/12/09.
Though not shown on the diagram above, the stator is commonly made up of thin insulated laminations of aluminium or cast iron; these thin laminations are preferred to a solid mass as they reduce eddy currents produced in the casing. They are either punched or clamped together forming a hollow ring, known as the stator core. It can be seen above that the stator is constructed with channels running through the inside of the core, the insulated stator windings are installed in these slots. Slotting is used in both the rotor and the stator this improves the motor by,
- Reduces the size of motors by giving the windings somewhere to be run, without increasing the overall bulkiness of the construction.
- It ‘focuses’ the lines of flux created in the machine, allowing less magnetic losses.
The slotting reduces the magnetic reluctance of the machine.
- The lamination of the core and increase in surface area by slotting also reduces the eddy current losses in the machine.
Each opposing phase winding creates a pair of poles (or an electromagnet) on the application of the AC supply. The above diagram shows a machine with six poles. The three phase windings are placed on the slots of laminated core and these windings are electrically spaced 120 degrees apart. These windings can be connected as either star or delta depending upon the motor requirements. The leads are taken out usually three in number, though more or less are possible, and brought to the terminal box on the external of the motor frame. The stator is constructed in this way for most of the AC induction motor family.
A squirrel cage rotor, is constructed as shown in the diagram below
Knowledge publications. (n.d.) No title. http://knowledgepublications.com/doe/images/DOE_Electrical_Science_Squirrel-Cage_Induction_Rotor.gif. Accessed 1/12/09.
A squirrel cage rotor consists of, as with the stator, a number of thin laminated cores (shown well in the top diagram) which when constructed gives a number of skewed slots which contain the conductors, which are not wires, in the conventional sense, but thick, heavy bars made of copper or aluminium or similar alloys. The conductors are evenly spaced, are solid with no joins and are connected mechanically and electrically by the copper end rings (as shown in the diagram). The end rings are commonly welded, electrically braced or bolted at both ends of the rotor, thus maintaining electrical continuity.
One important point to be noted is that the end rings and the rotor conducting bars are permanently short-circuited, thus it is not possible to add any external resistance in series with the rotor circuit for starting purpose.
The squirrel cage design is robust and reliable, this is the reason why they are so wide spread, the reason for these positive points are due to the skewed nature of the rotor, and this is an advantage because
- It makes the motor run in a less noisy way, by reducing the magnetic hum of the machine.
It decreases the slot harmonics
- It helps reduce the locking tendencies of the motor. Rotor teeth may become locked with the stator teeth when direct magnetic attraction occurs between the two. This only occurs when the number of stator teeth is equal to those in the rotor. With the skewed formation on the rotor this is many less likely.
- Increase in effective ratio of transformation between stator & rotor,
- Increased rotor resistance due to comparatively lengthier rotor conductor bars,
- Increased slip for a given torque.
The rotor is mounted on a shaft using bearings on either end to allow rotation occur. One end of the shaft is normally lengthier to allow for a load to be applied. Some motors may also have an ancillary shaft at the non load end allowing the mounting of speed or positioning devices. An air gap is still maintained allowing the induction between the two parts to occur. Regardless of the rotor employed the principle of rotation is the same.
Squirrel cage motors can have heating issues at low speeds, this is due to the fact that the fan is fitted to the end of the rotor and is only able to rotate at the same speed, the cooling capability of the motor is proportional to the rotor speed, thus at low speed the motor is unable to cool itself and overheating can occur.
Another common problem with cage motors occur when high inertia loads are being driven. The start up current is very high and only falls when the reverse EMF is created in the stator through inductive nature of the motor, as previouly mentioned. If the start up time is prolonged due to having tto start a high inertia load then the current will remain higher for a longer period, this causes greater heating in the windings, and thus increases the potential for damage to them, it also causes more mechanical stress on the machines parts and in turn lowers the motors life expectancy.
The wound rotor motor
Tpub. (2009) No title. http://www.tpub.com/content/doe/h1011v4/img/h1011v4_32_1.jpg. accessed 04/09/09.
The wound rotor motor is a variant of the squirrel cage motor arrangement. The stator of a wound rotor motor is constructed and operates in the same way as the squirrel cage motor. The rotor conductors, though, are not short circuited but are terminated to a set of slip rings (as in the diagram above), these slip rings do not have power applied to them, and they are there purely for application of resistances. This is a help in adding external resistors (which is impossible in a squirrel cage rotor) to reduce currents drawn in the starting of the motor.
The slip required to generate a maximum torque, is directly proportional to the resistance of the rotor. In the wound rotor motor, external resistors can be installed; this increases the rotor resistance, which in turn means it is possible to increase the slip of the motor. If the slip is increased then the maximum torque will be available at a lower speed. If a particularly high resistance is applied to the rotor then the maximum torque maybe attained at almost zero speed, this provides a high torque with a very small starting current.
As the motor is accelerated the resistance maybe adjusted to suit the load requirements. When the motor reaches the required speed then the resistance is removed completely and the motor operates as a standard induction motor.
The wound rotor motor is perfect for driving high inertia loading, in this situation it is required that maximum torque is applied at the lowest possible speed in order to start the load rotating as the speed increases the torque required is lowered and so the resistance is reduced and at full speed is removed fully.
Electrical Resource. (2009) wound rotor motor construction
http://www.electrical-res.com/wound-rotor-motor-construction/. Accessed 09/12/09.
The construction of the wound rotor differs to that of the squirrel cage motor because instead of the rotor bars it contains three windings similar to the stator. The windings, which are open, are connected to three slip rings mounted on the rotor shaft (as seen above). This rotor circuit is connected to three brushes to an external star connected variable resistance as mentioned before in the operation section.
The main disadvantage of the wound rotor motor is its requirement for maintenance to the slip rings and brushes, as squirrel cage does not require this. It has advantages in the ability to drive high inertia loads, controllable start up currents, speed can be controlled (by resistance) from 50-100% of full speed.