Duty cycle - Electric Motors:
Continuous steady-running loads over long periods are demonstrated
by fans and blowers. On the other hand, electric motors installed
in machines with flywheels may have wide variations in running loads.
Often, electric motors use flywheels to supply the energy to do the
work, and the electric motor does nothing but restore lost energy
to the flywheel. Therefore, choosing the proper electric motor also
depends on whether the load is steady, varies, follows a repetitive
cycle of variation, or has pulsating torque or shocks.
For example, electric motors that run continuously in fans and
blowers for hours or days may be selected on the basis of continuous
load. But electric motors located in devices like automatically
controlled compressors and pumps start a number of times per hour.
And electric motors in some machine tools start and stop many times
per minute.
Duty cycle is a fixed repetitive load pattern over a given period
of time which is expressed as the ratio of on-time to cycle period.
When operating cycle is such that electric motors operate at idle
or a reduced load for more than 25% of the time, duty cycle becomes
a factor in sizing electric motors. Also, energy required to start
electric motors (that is, accelerating the inertia of the electric
motor as well as the driven load) is much higher than for steady-state
operation, so frequent starting could overheat the electric motor.
For most electric motors (except squirrel-cage electric motors
during acceleration and plugging) current is almost directly proportional
to developed torque. At constant speed, torque is proportional to
horsepower. For accelerating loads and overloads on electric motors
that have considerable droop, equivalent horsepower is used as the
load factor. The next step in sizing the electric motor is to examine
the electric motor's performance curves to see if the electric motor
has enough starting torque to overcome machine static friction,
to accelerate the load to full running speed, and to handle maximum
overload. |
