Slip Ring induction motors starting; Slip ring induction motor starter

Starting Of Slip Ring Induction Motors

Slip ring Induction motors had external resistance connected in line. These motors are usually started with full line voltage applied across its terminals. During starting of slip ring induction motors the value of starting current is adjusted or kept minimum, by increasing the resistance of the rotor circuit.  The external resistance is connected in star and kept at maximum during starting so to minimize starting current. By increasing the rotor resistance it will not only reduces the rotor current but the stator current too.

This means that whenever a resistance is added in rotor circuit that will leads to reduced starting current. Thus because of this, the starting torque is increased due to the improvement in power factor.

Usually resistance is added during starting and slowly made out of circuit when motor attains the speed and this resistance is disconnected by using a contactor in line.  Slip ring can be taken in line by using manually also. The 3-phase supply to the stator has a switching contactor along with over-load and no or low-voltage protective devices. There might be also an interlock provided to ensure the proper sequential operation of the control gear and starting devices.



Slip ring motors circuit diagram is as shown above.



Torque curve of Slip ring motors is shown above


As per torque formula
Torque is directly proportional to resistance.
So as the resistance is high in slip ring motors torque is also high during starting.

As these motors have considerably high starting torque with low starting current, these motors can be started on load. The external resistance is used only for the starting purpose, after which the motor gradually picks up the speed, the resistance gradually cut-off. These rings are isolated after the motor reaches its rated speed. The carbon brushes are lifted and the rings are short circuited thus making them very similar to squirrel cage motors.


Applications of Slip Ring Induction Motors

These motors are used where the load is intermittent and comes on very sharply for brief periods, such as a punching machine. A heavy flywheel is fitted in the drive, preferably between the work and any speed-reduction gears. The flywheel shares the load with the motor, thus enabling a motor of lower rating to be employed. For load sharing to take place automatically, the motor speed should drop considerably as the load increases and this is ensured by using a motor having a high full-load slip, say for example 10%.

Characteristics Of Slip Ring Induction Motor

As other induction motors consists of Stator and Rotor circuits slip ring motors also have same Stator circuit there is only difference in rotor circuit. Rotor circuit consists of external resistance in the circuit. The stator circuit is slip ring motors is rated as same in the squirrel cage motor, but the rotor is rated in frame voltage or short circuit current. The frame voltage is the open circuit voltage when the rotor is not rotating and gives the measure of turns ratio between the stator and rotor. The short circuit current is the current flowing when the motor is operating at full speed, with the slip rings shorted and the full load applied to the motor shaft.

Advantages of Slip-Ring Motors:

1. These are used where there are high Inertia loads  as these motors have excellent starting torque.

2. These motors have low starting current then other induction motors.

3.  It is easy to control the speed of the motor from 50% to 100% of the full speed,

Disadvantages of Slip Ring Motors

1. Higher brush and slip ring maintenance required,
2. As the brush wears out, it may lead to intermittent contact, and thus heavy sparking.
3. Also speed control of motor comes along with increased losses as heat comes across resistance.



Comments

Post a Comment

Popular posts from this blog

A2XWY; A2XFY Cables meaning; XLPE cables advantages over PVC cables

Technical Specification of Aluminum Cables in terms of codes So many times you have seen coding while specifying cables technical such as A2XWY now what does this means. Now there is nomenclature for the same which is used while describing the same. Where A -  Aluminium 2X- XLPE Insulation W – Steel round wire armour F- Steel Strip Armour WW- Double Steel round Armour FF- Double Steel Strip Armour Y- PVC outer Sheath Wa- Non Magnetic round wire Armour Fa- Non Magnetic Strip Armour AW- Aluminum Wire Armour Now As per A2XWY – This describes that it is Aluminum cable, XLPE insulated, With Steel round wire armour and having PVC outer Sheath. This coding is as per IS 7098(Part1):1988. You have also seen that there is coding for copper so there is no coding required in case of copper cables. XLPE cables advantages over PVC cables/ Thermoplastic cables:- 1.       XLPE cables have higher Power rating about 1.2 times PVC cables 2.       XLP...
Read more

Digital Multimeter working principle; DMM working

Image
Digital multimeters are most widely used in every industry as it used for measurement of Voltage, Current, Resistance, Capacitance and various other parameters. DMMs have been available for very several years. Yet it is still challenging to locate advice about how a digital multimeter works. The operation of a digital multimeter is comparatively uncomplicated, although there are obviously differences in how digital mulimeters work if they're from different makers.Despite this, there are several similarities and some general principles of how digital multimeters work. Fundamentals of how a DMM functions The key process occurring within a digital multimeter for any measurement is that of voltage measurement. All other measurements are derived from this basic measurement. So the best technique for understanding how a digital multimeter functions is in understanding this procedure. There are many kinds to digital converter, ADC. Nonetheless the one that is most widely used DMMs in di...
Read more

Wireless voltmeter using Arduino and RF 433MHz receiver and transmitter

Image
In this project i showed you how to make a wireless voltmeter using arduino and RF 433MHz receiver and transmitter. You can use this code and concept for many other projects, any project which used sensor with analog voltage output to sense temperature, gas leakage or light , etc. For example you can put a LM35 temperature sensor rather than potantiometer, you just have to putting its formula which convert LM35 output pin voltage to exact temperature. You also can make a wireless AC or DC power meter by doing some change in code and hardware (using hall effect current sensor and single phase voltage sensor). This type of RF module has distance range up to 200 meters according to datasheet but when i bought this RF module from Aliexpress its distance rang was very low, but i set its adjustable RF coil very carefully and i get very higher distance rang.(Don't play with RF coil if your module works good), You also have to use a 6 inch antenna to increase distance range to max as possi...
Read more