Specifications

107
Synchronous Speed The speed of the rotating magnetic field in a three-phase motor.
Synchronous speed is determined by the line frequency and the
number of motor poles.
Synchronous Speed (Ns) =
f = frequency
P = number of poles
120f
P
Thermal-Magnetic Used to describe a device that uses both heat and magnetism as
part of its operating principles. For example, a thermal-magnetic
circuit breaker can be tripped either by heat or magnetic force
resulting from an overcurrent.
Thermistor A device used to sense temperature that varies in resistance as
temperature changes.
Thyristor A family of multi-layer semiconductor switching devices that
includes silicon controlled rectifiers (SCRs), gate turnoff (GTO)
thyristors, triacs, and other similar devices. Thyristors are often
used in rectifier or power switching circuits.
Time-Current Curve A graph showing how long before a circuit breaker will trip at
each level of fault current.
Time Current Characteristics Curve
Siemens MG Frame Circuit Breaker
.1
3,000 1,000
100
10
1.0
.01
.1
1
10
100
1,000
10,000
I
i
= Instantaneous Pickup
-20% +20%
I
i
I
i
(Max.)
I
i
(Min.)
Times Continuous Current Rating (I
n
)
Time in seconds
Trip Unit 525
Timing Relay A control relay that incorporates a preset delay in contact
response. Some timing relays begin the time delay when the
relay is energized. Others begin the time delay when the relay is
de-energized.