Jim McKee (deceased)
Alabama Electric Motor Service 
Sheffield, Alabama 
Technical Education Committee Member 

The slip ring or wound rotor induction motor (WRIM) has been used in a variety of applications. Many of these applications use the WRIM’s high starting torque capabil­ity while limiting current to start and run very high inertia loads such as hammer mills, rolling mills, centri­fuges, and rotary kilns. Other applica­tions utilize the variable speed capability of the WRIM. Probably the most common use of WRIMs for variable speed is in crane and hoist service. Other variable speed uses for the WRIM include wiredraw ma­chines, fans, blowers, pumps and refrigeration compressors. 

Variety of solutions, options 
Many of these applications, if designed today, would utilize a standard induction motor and variable frequency drive (VFD), particularly those where speed control is the desired end result. When older WRIMs or their controllers fail, the best solution often is to replace both motor and control. There are situations, however, where the best solution may be to replace the old controller with a VFD and continue to use the WRIM. 

As always, the more knowledge we have about the equipment we work on the better equipped we are to make good choices about repairing replacing or upgrading this equipment for our customers. With this in mind, a review of how the WRIM and some of its control schemes work seems appropriate. David Gebhart, a former EASA engineer, wrote an excellent article on wound rotor induction motors titled “Secondary Resistor Selection for Wound Rotor Motors.” This article is in Section 3 of the EASA Technical Manual and begins with a description of how the wound rotor motor func­tions. Following is a summary of that very beneficial article. 

Speed/torque characteristics 
In contrast to a standard squirrel cage motor which has fixed speed versus torque characteristics once built, the WRIM can have an almost infinite number of speed/torque characteristics depending on the value of external resistance used in the rotor circuit. Figure 1, borrowed from the Technical Manual article, illustrates several curves for speed vs. torque for a given WRIM.

Curve G results from an open rotor circuit or infinitely high rotor resistance. Very little torque is developed in this incidence. Curve A occurs with the rings shorted or no external resis­tance in the circuit. By decreasing the rotor resistance from the condi­tion of Curve G, one can see that locked rotor torque increases up to Curve C. Further decreases in rotor resistance result in higher speed for a given load but a decrease in locked rotor torque. 

Torque produced by an induction motor is a result of the in-phase current flow in the rotor winding. Maximum in-phase current flow at locked rotor occurs when the resis­tance in the rotor circuit equals the reactance. This condition is repre­sented by Curve C. For values of resistance greater than reactance, rotor currents are almost directly proportional to torque. Curves G, F, E, and D are representative of these conditions. 

When resistance in the rotor circuit is less than the reactance, rotor current continues to increase but the torque decreases. This condition is represented by the family of curves between A and C. 

Since rotor current is reflected into the stator winding by transformer action, it follows that a reduction in rotor current also reduces starting current in the stator. The end result of being able to change the speed/torque curves is a motor which can develop 250% or greater locked rotor torque with relatively low stator currents. It also allows for variable speed by matching developed torque with load torque at any given speed. 

Methods for adjusting effective rotor resistance 
Several methods of adjusting the effective rotor resistance are used. The simplest way involves the use of several fixed resistors with a multi-position drum switch used to successively short out or bypass resistance in stages starting with maximum resistance and ending with the slip rings shorted in the final or run position. The drum controller usually provides five or more steps of resistance. Large motors often use  liquid rheostats where variable position electrodes are raised orlowered in a salt or caustic soda solution. Contactors are sometimes used instead of or in conjunction with drum controllers to short out stages of resistance. 

All of these methods of adjusting rotor resistance can present mainte­nance problems. Parts for older drum switches and controllers may be difficult to find. Heat dissipation in the resistor banks is a problem. Slip rings and brushes require frequent maintenance. 

While the WRIM offers an attrac­tive set of useful characteristics well suited to drive a variety of difficult loads, the motor and its control schemes are complicated and expensive. 

Because of the WRIM’s ability to develop high starting torque while limiting inrush and starting current, it is often used to start and run very high inertia loads at fixed speeds. WRIMs can be used to drive ma­chines such as slabbing mills or hammer mills subjected to very high but intermittent loading. 

In these applications the WRIM used in conjunction with a “slip regulator” is allowed to slow down and therefore release some of its kinetic energy to the driven load. Stator current is controlled by increasing rotor resistance as the motor slows down. These types of applications should not be considered for use with a VFD. 

Used for speed control 
WRIMs have also been used for speed control in a number of applica­tions. These applications may be good candidates for replacing the old speed control with a VFD. Before making the final decision to use a VFD, consider these points. WRIMs are expensive to repair and many of them are no longer manufactured. If the WRIM can be easily replaced with a standard motor, it should be. On the other hand, the WRIM may be in a special frame or be an integral part of a gear box and therefore replacement would be impractical. 

Let’s assume the decision has been made to control the WRIM with a VFD. What do you do with the rotor? There are two choices. Either short the slip rings or leave some fixed resistance in the rotor circuit. Both can be valid choices. All of our previous discussions regarding the speed vs. torque curves in Figure 1 are based on the fact that the rotor sees a line frequency (e.g., 50 Hz or 60 Hz) rotating field at locked rotor or 100% slip. When we start the motor with a VFD, the rotor at stand still is no longer at line frequency, but only a few hertz. This drastically reduces rotor reactance and therefore reduces the value of external resistance necessary to obtain maximum torque. In most cases no external resistance will be required. 

External resistance 
High inertia loads may need some external resistance. There is a tendency for these loads to cause the motor to be unstable at low speeds with no rotor resistance. Low inertia loads will normally start and run properly with the rings shorted. A safe way to determine what is required is to measure the last step of resistance used by the old controller. “Last step” means the value of resistance left in the rotor circuit just before the rings are shorted. Run the motor under the worst conditions with the slip ring leads shorted. If perfor­mance is good then the rings can be removed and the leads permanently shorted and insulated. If not, then permanently connect resistors with values as determined from measure­ment of the old controller. 

Effects of heating 
The motor may need to be derated slightly (up to 5% or 6%) due to additional heating caused by the VFD. Motor heating due to decreased ventilation should be considered if the WRIM will operate constantly at low speeds. If the application is a crane or hoist, limit the use of a VFD to hoist traverses and bridge trolley drives. Be sure that the VFD has the software and other features required for this type service. Unless you have specific knowledge and the expertise re­quired, do not use a VFD on a hoist lifting motor, or in any way modify the hoist control. 

A February 2004 CURRENTS article by Gary Braun addresses these issues for crane and hoist applica­tions. For starting high inertia loads be sure the drive is capable of delivering the required current, which may be 200 to 300% of nameplate. 

For applications where it is suitable the VFD and WRIM combi­nation will provide stepless speed control, improve efficiency, and reduce maintenance cost—all customer-pleasing enhancements. 

Categories: AC motors, Rotors, Wound rotors, Wound rotor motors, Drives/controls

Tags: Variable frequency drives Wound rotor induction motors

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