Alternating current (AC) motors are electric motors driven by an alternating current. The current that supplies the power sockets of your home that enables you to charge your phone or watch television is the same power source that powers AC motors.
An induction motor is a type of alternating current (AC) motor that works on the principle of electromagnetic induction. It is mainly classified according to its rotor construction – squirrel cage type and slip ring type.
In this article, we will mainly focus on the benefits of using slip ring in AC motors.
Slip rings in AC motors
Slip rings are commonly used for transferring power or data signals, as in electrical slip rings. However, slip rings in AC induction motors are used for inserting resistance into the rotor windings in order to perform tasks such as speed control.
The slip ring induction motor utilizes three slip rings mounted to the shaft of the motor. The three phases of the rotor winding are connected to each slip rings. The slip ring brushes are then connected to a resistive device such as a rheostat.
Slip rings are typically made of copper, steel, or phosphorous bronze. Its material should be harder than the material of brushes to avoid any occurrence of wear on the slip rings.
Slip rings are used in induction motors
The slip rings rotate with the rotor while the brushes maintain its constant contact with the slip rings, effectively transferring the resistance to the rotor windings.
Now let us look at the benefits of this external resistance on our AC motors.
Benefit #1: Low starting current
The addition of resistance to the rotor windings of the slip ring induction motor makes the rotor current more in-phase with the stator current. This results in an increased torque production with relatively low current.
The starting current is approximately two to three times lesser than the full load current. This decrease in current then avoids the occurrence of overheating during start-up.
Slip ring induction motors are usually employed in printing presses, compressors, and large ventilating fans.
Benefit #2: High starting torque
A slip ring induction motor has a much higher starting torque as compared to squirrel cage induction motor, with approximately 200% – 250% of its full-load torque. It adds an external resistance in the rotor circuit during start-up to give higher starting torque at an improved power factor.
Slip rings also allow high braking torque and highest torque at any slip to be easily maintained. Common applications that demand high starting torque can be found in cement plants, mill drives, mines, utilities, and waterworks among others.
Benefit #3: Low rotor resistance at normal operating speed
After start-up, resistance has to be slowly decreased from the rotor circuit in order to avoid poor speed regulation, a decrease of full load speed, more rotor losses, and lesser frequency. The slip ring is shorted out to allow this lower rotor resistance in order to give fewer losses, lower slip, constant speed, and higher efficiency of the AC motor.
Benefit #4: Speed control
You can also use slip rings AC motor in applications needing a good speed control such as pumps and driving lifts. Speed control is achieved in two ways – by inserting varying resistance in the rotor circuit or using a thyristor circuit.
Thru insertion of varying external resistance
The slip ring induction motor’s resistance is dependent upon the slip and independent of the torque. Through variation of resistance after the motor attains its rated speed, torque value can be kept constant despite any changes in the slip.
The slip value is inversely proportional to speed. This means that any increase of slip value results in a decrease of motor speed while maintaining constant torque. However, do note that is only applicable in ring-type motors.
Thru the use of thyristor circuit
This method is based on the principle that the rotor current is directly dependent on its torque and motor speed. Speed is controlled by varying the current through the use of a thyristor ON/OFF circuit.
This type of speed control method is highly efficient since it minimizes the occurrence of operational losses.
Other benefits of slip ring
Slip ring motors also have smooth acceleration under heavy loads. They are beneficial for applications with high overloading capacity such as in heavy machinery, mechanical handling units, hoists, elevators, railway traction, and lifts.
Though the slip ring motor performs very useful tasks, it also brings some disadvantages. Countermeasures must be executed as it requires a higher degree of maintenance due to the presence of brush gears and slip rings. Slip ring motors also have a relatively higher purchase price than others.
However, by knowing the benefits of using the technology and by understanding how they function relative to your application, the benefits largely outweigh the drawbacks. Slip rings could be the solution to your problems that you do not know about until you have it.