Did you know that the motor speed tells you almost everything about the rotor losses of an electric motor? Slip is responsible for this. It directly reflects the proportion of losses in the air gap power, i.e. the effective input power of the stator minus the stator copper and iron losses.
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The slip is easily calculated: Slip = field rotation speed minus nominal speed divided by field rotation speed.
Example: At a field rotation speed of 1,500 rpm and a nominal speed of 1,460 rpm, the slip S is 0.. That is 2.67% slip.
If the air gap power Pδ is already known from the stator losses and the input power, the remaining losses can be deduced: In the rotor iron these are low due to the minimal frequency and in the bearings they are usually only a few watts. The main proportion is due to ohmic losses of the rotor cage. If the air gap power Pδ – for example 3,140 W – is multiplied with calculated slip S, the rotor cage losses result in 83.73 W.
A good identifying feature of low loss motors is therefore low slip, because this means that the speed of the motor is very close to the field rotation speed. Thus, the 4-pole 3 kW motor (50 Hz) from NORD DRIVESYSTEMS in its IE1 version, upon which the example is based, with for example a speed of 1,415 rpm can easily increase to a speed of 1,460 rpm as an IE3 version and the slip reduces from 5.67% to only 2.67%.
However, in ventilation and pumping applications the driven machine must be modified for this! Otherwise, the saving of losses by the energy saving motor results in higher energy consumption due to higher speed of the fan characteristic curve because the air flow power increases.
The slip ring motor is generally referred to as phase wound motor which is quite commonly used in almost all sectors. They are designed in such a manner that they can be initiated with a full line voltage supply across. The full voltage has to be supplied across terminals which makes the entire process of transmission possible.
The wound rotor containing the rotor winding terminals are attached to a slip ring which is mounted on the shaft, but at the same time, they are insulated separately from the shaft. The lead connection fitted to the slip ring along with the brush pressing on the ring is exposed to external resistance.
Adjusting the flowing of current
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During the initial phase of operation, the flow of current in the motor is maintained by adding external resistance to the circuit which proves to be quite useful to avoid excess thermal heat with which usually caused by the rise of temperature.
Torque stabilization
Now after the initial phase of maintaining the electric current balance as the rotors speed up its operation the level of external resistance is brought down bit by bit. This step to reduce the external resistance influx is taken to stabilize the level of torque, especially, when the rotor enters the second clause where it begins to gain acceleration.
The final stage
In the last phase when a stable torque level has maintained the supply of external resistance is completely cut off. This is brought down to allow the rotor to gain full load torque with the reduced involvement of the slip ring.
One of the important points to note in this entire operation is that the slip which is evidently required to generate a maximum level of torque is for. a matter of fact directly proportional to the rotor resistance. Thus, as the external resistance passing on to the slip ring rotor is increased the level of torque consequently increases and vice versa.
The design pattern of a slip ring motor
During, the initial rotational phase the slip ring technology included in the system failed to increase the efficiency of the motors, but with time the slip ring manufacturers came up with innovative structural changes that made the slip ring motors as a while more effective. The commonly used slip ring induction motor or wound rotor motor now come with a variation of the stator that resembles as the squirrel cage induction motor. In this case, the application of slip ring is considerably different. Here, the slip rings which are closely associated with the rotor windings are not introduced to initiate the flow of current. In fact, they are brought in the system to induce resistance in the series as and when the rotor windings start its initial operation.
The entire conventional application of slip ring for allowing the flow of current and signals is put to rest in the slip ring based motors. This, we stress on the various functionality of aspect of the slip ring which is set have more new components as the technology progresses in near future.
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