Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 24)

T The torque in the operating range is calculated as follows:
[ ]
Nm
n
I U
T
55 . 9 cos 3
⋅ ⋅ ⋅ ⋅ ⋅
=
η ϕ
U voltage across the motor [V]
I current [A]
cos φ power factor
η efficiency of the motor
n speed [min
-1
]
The rated operational currents, starting currents and the torque characteristic of cage induction
motors depend, among other things, on their design, especially the material and form of the
cage, as well as on the number of poles. The specifications provided by the motor manufacturer
apply in each individual case. Typical values for motors can be obtained from the RALVET
electronic documentation.
For switching asynchronous motors, under IEC 60947 the utilization categories AC-2 to AC-4
among others are defined to facilitate the user in the selection of suitable contactors
(
Tab. 1.1-1 ). These utilization categories make allowance for the loading of the switchgear by
the increased making currents when stationary motors are switched on and for the fact that the
effective switching voltage of a running motor is only around 17 % of the rated operational
voltage because the running motor develops a back-e.m.f. (counter voltage to supply voltage).
1.7.1.2.1 High efficiency motors
In the context of the efforts of saving energy and pollution control, the efficiency of electric
motors and drives has become an issue. This on the background of appr. 40% of the global
electricity being used for operating electric motors. The IEC standard 60034-30 (2008) defines
efficiency classes for general purpose induction motors of the power range of 0.75 …375 kW
and with 2, 4 or 6 poles (
Tab. 1.7-2 ). The term MEPS (Minimum Energy Performance Standard)
is being used in this context [25]. It is expected that efficiency class IE2 shall become the
minimum level for new motors in the area of the European Union, IE3 may be required in a
further step (minimum requirements, if any, are subject of national legislation).
IEC Class IEC Code EFF Code
1)
NEMA
Super Premium Efficiency IE4
Premium Efficiency IE3 NEMA Premium
High Efficiency IE2 EFF1 EPAct
Standard Efficiency IE1 EFF2
Below stand Efficiency ‘--- EFF3
1) CEMEP classification (CEMEP = European sector committee of Manufacturers of
Electrical Machines)
Tab. 1.7-2
Efficiency classes for general purpose induction motors according to
IEC 60034-30 (2008) in comparison to the EFF-codes of CEMEP and the codes used by NEMA. IE4 is
not yet defined and reserved for the future.
High efficiency motors that comply with the MEPS standard may have higher starting currents
and cause higher transient current peaks upon switching. The starting torque may be compara-
tively lower for the same starting current, while the breakdown torque may be higher.
When selecting switchgear for starting high efficiency motors, attention should be paid to the
selection of the proper release class of overload relays (start-up time could become longer at
higher starting current levels). In case of using (current-limiting) circuit breakers the choice of
LVSAM-WP001A-EN-P - April 2009 1-14