Resistance-start motor

Resistance-Start Motor. A resistance-start motor is a form of split-phase motor having a resistance connected in series with the auxiliary w inding. The auxiliary circuit is opened when the motor has attained a predetermined speed. 

Byatt. J.R., Selection of Primary Starters for Cage Motors and Secondary Starters for Slip-ring Motors, AOIP Electrical, UK. Kajiji, Y.H., Liquid Resistance Starting, AOYP Engineering Co. (P) Ltd, Mumbai, India. 

Tables 5.1-5.4 show the approximate resistance and reactance values in per-unit for two-pole and four-pole low voltage induction motors that are generally of the Design D type. Tables 5.5-5.8 show the approximate resistance and reactance values in per-unit for two-pole and four-pole high voltage induction motors that are of the reduced starting current type. In the absence of exact data from a manufacturer these data can be used for system studies such as starting motors, transient stability and fault current contribution. The data from a manufacturer should be used for calculations and system studies that are to be carried out during the detailed design phase of a project.

Secondary Control of Wound-Rotor Motors Wound-rotor motors may be effectively reduced-voltage-started or have their speed controlled by using external secondaiy resistance. The addition of resistance into the secondaiy circuit of a wound-rotor motor reduces the starting current and affects the speed under load conditions. 

When external secondaiy resistance is used for improved starting charac teristics, short-time-rated resistors are employed. As the motor accelerates, steps of resistance are cut out on a time or current basis to give the desired accelerating torque and current characteristics. 

The major differences between ac and dc starters are necessitated by the commutation limitation of dc motors, which is the ability of the individual commutator segments to interrupt their share of armature current as each segment moves away from the brushes. Normally 250 to 275 percent of rated current can be commutated safely. Since motor-starting current is limited only by armature resistance, line starting can be used only for veiy small [approximately 1492-W (2-hp)] dc motors. Otheiwise, the commutator would flash over and destroy the motor. External resistance to limit the current must be used in starting to prevent this. 

The higher the full load slip, the higher will be the rotor losses and rotor heat. This is clear from the circle diagram and also from equation (1.9). An attempt to limit the start-up current by increasing the slip and the rotor resistance in a squirrel cage motor may thus jeopardize the motor s performance. The selection of starling current and rotor resistance is thus a compromise to achieve optimum performance. 

The external resistance adds up to the total impedance of the motor windings and limits the starting current. It also improves the starting power factor. 

More importantly, such alloys also possess a very low temperature coefficient of electrical resistance (of the order of 220 idQ.IQ.rC, typical), which causes only a marginal change in its resistance value with variation in temperature. They can therefore ensure a near-consistent predefined performance of the motor for which the resistance grid is designed, even after frequent starts and stops. They are also capable of absorbing shocks and vibrations during stringent service conditions and are therefore suitable for heavy-duty drives, such as steel mill applications. 

If the insulation resistance is observed to be low the interior of the switchgear or the controlgear assembly should be dried out to attain the required value before energizing it. The procedure to dry the moisture is similar to that for motors and is discussed in Section 9.5.2. In this case during the heating-up period the insulation resistance may first drop and reach its minimum, stabilize at that level and then start to rise gradually. Continue the process until it reaches its required level as shown in Table 14.7. 

On the load side, the interrupter is connected through a cable to the equivalent circuit, with the required quantities of lumped resistance and reactance, to represent the motor to be tested, under a locked rotor condition. The circuit would also represent an interruption immediately after a start, to check for the most onerous operating condition for the interrupter to generate the highest surges as discussed earlier. 

Important features of a slip-ring motor Starting of slipring motors Hypothetical procedure to calculate the rotors resistance Speed control of slip-ring motors Moving electrode electrolyte starters and controllers... 

Torque is the turning effort developed by the motor or the resistance to turning exerted by the load. Usually torque is expressed in ft-lb however, the usual expression is as a percentage of the full load torque. Synchronous motors usually offer several types of torque. Starting or breakaway (called locked rotor) torque is developed at the instant of starting, see Figure 14-12. 

Start/delta, auto transformer, primary resistance Reduced voltage is applied to the motor ... 

Primary resistance A resistance is inserted into the supply to the motor. This reduces the voltage available at the motor terminals on starting, and this voltage increases gradually as the motor current falls when the motor speeds up. Once the motor has reacted at a predetermined speed the resistance banks are short-circuited. 

With this technique the motor has a wound rotor brought out to slip rings and an external resistance is connected into the rotor circuit. This resistance usually consists of a series of resistor banks, which are switched out progressively in a number of steps as the motor accelerates. The number and rating of each step is chosen so that starting current and motor torque are within requirements. 

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