Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 79)

3.2 Direct starting of squirrel-cage induction motors
The direct starting (Direct On Line, DOL) is the simplest and most cost-efficient method of
starting a motor. This is assuming that the power supply can easily deliver the high starting
current and that the power transmission components and the working machine are suitable for
the high starting torques.
I> I> I>
Fig. 3.2-1
Example of a two-component starter for direct starting consisting of a motor protection circuit breaker and
a contactor
With direct starting, the poles of contactor and motor protective device are connected to the pole
conductors (
Fig. 3.2-1 ) and the operating current of the motor flows through them. The motor
protective device must therefore be adjusted to the rated operational current of the motor.
The contactor is selected according to the rated operational current I
e
and the respective
utilization category:
- AC-3 Squirrel-cage induction motors: Starting, switching off during running
- AC-4 Squirrel-cage induction motors: Starting, plugging, inching
Definition of utilization categories see Section 1.1 .
For AC-3 operation, allowance must always be made in practice for sporadic inching operations,
for example during commissioning, in case of faults or in service work. Contactors from
Rockwell Automation comply with these requirements and may be rated without risk according
to AC-3 values; for the large majority of devices, the rated operational currents for the utilization
categories AC-3 and AC-4 are the same.
A considerable proportion of AC-4 operations or exclusive AC-4 operation is in practice
relatively rare. In such cases, a high frequency of operation is often required at the same time
and a high electrical life span is expected. Thus the contactor must be selected according to
these two criteria. In most cases a larger contactor must be used than would correspond to the
maximum permissible AC-4 rated operational current. See also Sections
2.3.6.3 and 2.3.7 .
3.2.1 Starting time
The starting time is an important parameter in starter engineering, as the starting current can be
many times higher than the rated currents of motor and switchgear and correspondingly places
the latter under thermal loading. It depends on the torque of the motor and hence on the
selected starting method, as well as on the torque characteristic of the load. The difference
between the motor torque and load torque is the acceleration torque. In addition to the resistive
torque of the drive, the inertial mass to be accelerated has a key influence on the time taken for
motor starting.
The duration of so called no-load starting, i.e. starts without loading of the drive, typically lies,
depending on motor size, in the time range of under 0.1 to around 1 s, starting under load (but
without large flywheel masses) up to around 5 s. For centrifuges, ball mills, calenders, transport
LVSAM-WP001A-EN-P - April 2009 3-3