Starting High Inertia Loads
There are a number of considerations when starting a high inerta load. It is important that the whole system is engineered correctly to prevent field problems and equipment failure.
A High inertia load, requires an extended starting time
to reach full speed. The start time is a function of the load inertia,
the load speed and the starting torque developed by the motor. If we reduce
the start voltage or current, the start time will extend.
The three key components to be considered are the motor, the starter and the supply. If any of these are not suitable for the high inertia load, the result can be disasterous.
Motor - Rotor
During start, there are high slip losses resulting in a
high power disipation in the rotor of the motor. This high power dissipation
causes a temperature rise in the rotor bars. The actual temperature rise
is dependant on the total power dissipated and the thermal mass of the
rotor bars. There is a limit to how much temperature rise the rotor can
tolerate. If the temperature is high enough, the bars can actually melt,
but every time the motor is started, there will be rotor bar heating and
this will result in expansion and contraction of the bars. The expansion
of the bars will put mechanical stresses on the bars and shorting rings
and over a period of time, the interface between the bars and shorting
rings can fail due to mechanical fatigue.
Motor - Start Characteristics
Under high start torque conditions, it is important to
ensure that the motor is "efficient" during start. That means
that the motor must produce a high torque for reduced current under high
slip conditions. If the motor produces a very low start torque, then it
is going to take much longer to start the load and this will impact on
the supply and on the starter. Correct attention must be paid to the starting
characteristics of the motor.
Motor - Stator
During start, there is a high start current flowing in the
stator. This will result in a high temperature rise in the staor windings
and if excessive will result in an insulation failure of the stator. Usually,
it is the rotor that is damaged by high inertia starts rather than the
stator. Stator failure usually results from continuous load conditions
whereas rotor failure is generally due to starting issues.
Starting a high inertia load requires a lot of energy to
be drawn from the supply. If you use any form of starter, (Full voltage
or reduced voltage) there will be a high demand on the electrical supply
during start. Reducing the voltage will reduce the instantaneous current
draw, but will extend the starting time. The heating effect in the supply
equipment will be the same but the voltage drop will be lower. If the
supply is not strong enough to start the load, there is a problem and
the supply will need to be strengthened.
The starter must be capable of withstanding the high overload
during start for the duration of the start. With a high inertia load,
to get the machine to full speed in a reasonable time, a high torque must
be presented to the load. Reducing the voltage too much will extend the
start time to an unacceptable period.
Engineering the start.
Commonly, start currents of 450% - 500% for 30 - 60 seconds are required to start high inertia loads assuming high start torque motors are employed.
These Calculations can be easily carried out using our Electrical Calculations software.
Step 1. Enter the load characteristics into the Edit Load Data page. The Load Torque is the work torque of the driven load. For a purely inertial load, this would be all zeros. Fan loads are usually a square law and for a freely ventillating fan would be approaching 100% at full speed.
Step 2. Enter the motor data from the motor data sheets into the Edit Motor Data page. Ensure that the effective inertia of the load is less than the "maximum Inertia" of the motor if this rating is given.
Step 3. Open the Acceleration curves and select the load and the motor and then select different starters and settings to see the effect of the reduced start current. Select the DOL start to get the maximum full voltage start time and compare this with the "maximum Locked Rotor Time" of the motor. If this is higher than the motor rating, try another motor.
Step 4. Specify a starter that can withstand the start current
and time for your application.
You can use this software to try a number of motors and starters to determine the combination that best suits your requirements.
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