29
Fig. 25 Motor temperature-rise ratio at Fig. 26 Motor temperature-rise ratio at
phase-failure phase-failure operation
When a strict phase-failure protection is necessary that includes also such a special case as over-current flows only to
l wire at the secondary side due to phase-failure at the primary side of the transformer, 3-element thermal overload
relay or 2E thermal overload relay should be selected.
3-3-3 Selection for Very Small Capacity Motor
The rated current of a very small capacity motor like the hydraulic pump motor is extremely small, so the heaters
of thermal overload relay corresponding to this rated current have a very high resistance value. Therefore, a voltage
drop caused by the thermal overload relay becomes large, so that full attention must be paid to insufficient starting
torque and voltage unbalance when using 2-element type thermal overload relay.
Where the load is light, unbalance of current due to voltage unbalance tends to become large. It is recommended
that 3-element type thermal overload relay be selected.
Where only a small-sized 2-element thermal overload relay can be installed due to limited space, its rated current
should be about twice the rated current of the motor exclusively for protection of restriction.
Reference 1. Insufficient starting torque
Example of RC0.1A ・・・・・・ Motor: 200V, 0.lA (rating)
Fig. 27 E (Heater voltage drop) = Is (Motor starting current) × R (Heater resistance value)
＝6 × 0.1 × 130 = 78 (V)
Vm (Motor input voltage) ≒200 － (√3 × 78) ≒ 65 (V)
input voltage ・・・・ 38%
starting torque ・・・ 15%
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