Slip Ring Motors
The high starting torque and variable speed control of slip ring motors make them a popular choice for many industrial applications. However, these motors do have higher initial and maintenance costs than other motor types and are more sensitive to variations in supply voltage. This article will explore the key features of slip ring motors to help you select the right one for your specific application needs.
The basic principle of operation of a slip ring motor is that it uses a conductive slip ring to transfer electrical power from the stationary stator to the rotating rotor. During rotation, the rotor is powered by a combination of magnetic fields generated in the rotor and electric currents induced in the rotor conductive elements from contact with the stationary stator. This interaction between the rotor and the stationary stator generates a constant velocity with a high starting torque that is suitable for heavy loads.
Unlike standard cage type induction motors that require additional inertia load resistors to dissipate the energy of high-inertia loads, slip ring motors are designed with this inertia loading built into the design. This reduces the amount of inrush current that is required at start-up, and also prevents rotor damage by discharging the full power of the load into the secondary circuit.
As the name suggests, a slip ring motor has a rotating metal ring that is connected to the rotor via carbon brushes. These brushes, which are typically made of graphite or phosphor bronze, rub against the outside perimeter of the rotor to establish contact and convey the current/signal. Depending on the type of motor, slip rings can be configured as either single or multi-ring systems to transfer power and data at various rates.
Key Features of Slip Ring Motors
In addition to carrying current, data and signals, some slip ring types are designed to transport fluids like oil, gas and ethernet. There are also a variety of mechanical and electrical specifications available for slip rings to suit unique machinery and operating environments.
The core of a wound rotor slip ring motor is a laminated cylindrical core with slots that include three single phase windings, wired to form a star or delta. These are then insulated with slip rings and connected to the rotor shaft. The rotor is then wound with three insulated coils for the same number of poles as the windings in the stator. As the rotor rotates, a series of conductive bars or coils in the rotor are connected to the rotor shaft by sliding on and off the slip rings.
By increasing the rotor resistance, it is possible to alter the speed/torque characteristic of the motor by introducing external resistance through the slip ring connections. This allows the rotor to operate at its maximum efficiency under a range of load conditions. A slip ring motor is therefore an efficient and effective solution for driving large torque inertia fans, transportation machinery and rolling mills. In mining, slip ring motors are used for crushers, mills, hoists and conveyors to drive a wide range of materials through the process.