Source: http://www.brighthub.com/engineering/electrical/articles/43725.aspx

Slip Ring Induction Motors Basics

Introduction

We have seen the construction, operation, starting, speed control & classes of squirrel cage induction motors. In this article, we will discuss on slip ring induction motors. As discussed earlier, a slip ring induction motor is an asynchronous motor, as the rotor never runs in synchronous speed with the stator poles. Lets understand the construction and operation of slip ring induction motor.

Construction

Stator:

The stator construction is same for both squirrel cage & slip ring induction motor. The main difference in slip ring induction motor is on the rotor construction and usage. Some changes in the stator may be encountered when a slip ring motor is used in a cascaded system, as the supply for the slave motor is controlled by the supply from rotor of other slip ring motor with external resistance mounted on its rotor.

Rotor:

The slip ring induction motors usually have Phase-Wound rotor. This type of rotor is provided with a 3-phase, double-layer, distributed winding consisting of coils used in alternators. The rotor core is made up of steel laminations which has slots to accommodate formed 3-single phase windings. These windings are placed 120 degrees electrically apart.

The rotor is wound for as many poles as the number of poles in the stator and is always 3-phase, even though the stator is wound for 2-phase. These three windings are starred internally and other end of these three windings are brought out and connected to three insulated slip-rings mounted on the rotor shaft itself. The three terminal ends touch these three slip rings with the help of carbon brushes which are held against the rings with the help of spring assembly.

These three carbon brushes are further connected externally to a 3-phase start connected rheostat. Thus these slip ring and external

rheostat makes the slip ring induction motors possible to add external resistance to the rotor circuit, thus enabling them to have a higher resistance during starting and thus higher starting torque.

When running during normal condition, the slip rings are automatically short-circuited by means of a metal collar, which is pushed along the shaft, thus making the three rings touching each other. Also, the brushes are automatically lifted from the slip-rings to avoid frictional losses, wear and tear. Hence, under normal running conditions, the wound rotor is acting as same as the squirrel cage rotor.

What happens when external resistance is added?

In case of squirrel cage induction motor, the rotor resistance is very low, so that the current in the rotor is so high; which makes its starting torque poor. But adding external resistance in case of a slip ring induction motor, makes the rotor resistance high during starting, thus the rotor current is low, thus the starting torque is maximum. Also the slip necessary to generate maximum torque is directly proportional to the rotor resistance. In slip ring motors, the rotor resistance is increased by adding external resistance, thus the slip is increased. Since the rotor resistance is high, the slip is more, thus possible to achieve pull-out torque even at low speeds. As the motor reaches its base speed (full rated speed), after the removal of external resistance, under normal running conditions, it behaves in the same way as squirrel cage induction motor.

Thus these motors are best suited for very high inertia loads, which requires a pull-out torque at almost zero speed and accelerate to full speed with minimum current drawn in a very short time period.

Advantages of slip ring induction motors

The main advantage of slip ring induction motor is that its speed can be controlled easily.

"Pull-out torque" can be achieved even from zero r.p.m High starting torque when compared to squirrel cage induction motor.

Approximately 200 - 250% of its full-load torque.

A squirrel cage induction motor takes 600% to 700% of the full load current. But a slip ring induction motor takes a very low starting current approximately 250% to 350% of the full load current.

What happens if the motor is started as a normal induction motor?

If the slip motor is started with all the slip rings or the rotor terminals shorted, like a normal induction motor, then it suffers extremely high locked rotor current, ranging up to 1400%, accompanied with very low locked rotor torque as low as 60%. So, it is not advised to start a slip ring induction motor with its rotor terminals shorted.

In my next article, lets discuss on various starting methods & speed control of a slip ring induction motors.

Read more: http://www.brighthub.com/engineering/electrical/articles/43725.aspx#ixzz0sOWIe8nJ

HowDoesASlipRingMotorStart?

Introduction

In my last article, we have discussed everything on squirrel cage induction motors including its dis-advantages. To overcome these disadvantages, slip ring induction motors have been developed, with better starting characteristics and better current drawing characteristics. In this article, we will discuss on the starting method and characteristics of slip ring induction motors with its applications.

Starting Of Slip Ring Induction Motors

These motors are usually started with full line voltage applied across its terminals. As these motors have external resistance connected to its rotor circuit, the value of starting current is adjusted or kept minimum, by increasing the resistance of the rotor circuit. This external resistance can be assumed to be a form of rheostat, connected in star, thus kept at maximum when the motor starts and gradually cut-out as the motor gathers speed. It has been already discussed in my last article on Slip Ring Induction Motors..!!, that increasing the rotor resistance, not only reduces the rotor current, but the stator current too. This implies that the starting current of the motor is reduced when an external resistance is added in the rotor circuit. Thus because of this, the starting torque is increased due to the improvement in power factor.

The controlling rheostat may be of either stud or contactor type. It may be either hand operated or sometimes automatic. 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.

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

The slip ring induction motor has two distinctly separate parts, one is the stator and other is the rotor. The stator circuit 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.

Qualities Of Slip-Ring Motors: 1. suited for high inertia loads as it caters with excellent starting torque, 2. Low starting current when compared to squirrel cage induction motors, 3. Speed is varied with the help of external resistance, from 50% to 100% of the full speed, 4. Higher brush and slip ring maintenance required, 5. As the brush wears out, it may lead to intermittent contact, and thus heavy sparking. Thus we have seen in detail the starting arrangement of slip ring induction motors. In my next article, we will discuss on the speed control of slip ring induction motors. Read more: http://www.brighthub.com/engineering/electrical/articles/43726.aspx#ixzz0sOY1tIgY

Slip Ring Induction Motors BasicsHow Does A Slip Ring Motor Start?