IC8070 GE Rheostat

WHERE TO USE IC8070

• Speed control of dc motors. Use IC8070 rheostat to control field excitation.
• Power-factor control of synchronous motors. Use IC8070 rheostat to control field excitation.
• Voltage control of generators and exciters. Use IC8070 rheostat to control field excitation.
• Adjustment of voltage in control circuits. Use IC8070 rheostat to vary impressed voltage.
• Speed control of wound-rotor motors. Use IC8070 rheostat to vary resistance in motor secondary; not over 15 horsepower.

APPLICATIONS

A rheostat is a resistor provided with a ready means for varying its resistance. The usual application of rheostats is in the field circuits of motors or generators for the control of speed or voltage, or in control circuits. For most applications the size of the rheostat is determined by the characteristics of the load it must control. Other applications require a certain number of steps which determine the rheostat size.

GE rheostats are designed from no-taper to maximum-taper. The rheostat listing in this guide represents a portion of what is available from the GE Company. It has been found by experience that a 6-to-1 taper between the high-resistance end and the zero resistance end obtains satisfactory control of motors and generators as well as other applications for average installations. By a 6-to-1 taper it is meant that the last step of resistance has six times the resistance of the first step.

NO-TAPER RHEOSTATS

No-taper rheostats (i.e., resistance per step throughout the rheostat is uniform) have the same current rating throughout the rheostat. This is obtained by using similar resistor elements through the entire range of rheostat; hence, the current rating at one end of the rheostat is the same as the current rating at the opposite end.

TAPERED RHEOSTATS

On many applications the current-carrying capacity of the rheostat does not need to be uniform from one end to the other. For example, when controlling the voltage of a generator by field control, the field current at the lower desired voltage is less than the field current at the higher voltage. (On these rheostats the resistance elements are tapered; that is, the resistance per step and the current carrying capacity are not uniform, but are designed for a particular application.) When the rheostat is in the minimum resistance position, it must carry the maximum current of the circuit. However, as resistance is inserted in the circuit the succeeding resistor element does not have to carry as much current as the preceding
resistor element previously carried.

In a tapered rheostat, increasingly smaller diameter elements are used as resistance is turned in. The ohms per element increases as the current capacity decreases but the wattage value remains equal since P = I2R. Since any given area of a rheostat has the same wattage-dissipating capacity, greater utilization of the rheostat is obtained if tapered resistor elements are used.

CLASS RATING

It is GE practice to assign a class rating to all rheostats. The class rating consists of a group of digits identifying the ohmic value, maximum amperes, minimum amperes and circuit volts. A class rating of 250-2.1-.66-250 identifies a rheostat rated 250 ohms, 2.1 amp with resistance out, .66 amp with resistance in and for use in 250-volt circuit.