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When the blades strike the plastic, the flywheel meets the resulting sudden torque peaks with the rotational energy it has stored; the motor does not feel these peaks directly and works against the average load. The price is paid at start-up: bringing a large flywheel up to rated speed takes a long time, during which the motor draws high current and heats up.
What the flywheel brings
The load in a granulator is not steady: when a thick-walled part enters the blades the torque rises abruptly, and drops once the part breaks. The flywheel dampens this fluctuation; as speed falls slightly, the stored energy is spent on crushing. The motor can therefore be selected for the average load plus a reasonable margin rather than the highest peak. On a machine without a flywheel or with a light rotor, a larger motor may be needed for the same crushing duty.
The difficulty at start-up
- Acceleration time depends on inertia; the heavier the flywheel, the longer the start.
- During a long start the rotor and winding heat up; repeated starts in quick succession are risky.
- If the overload protection is set for a short start, the motor may trip; setting it is the job of qualified electrical personnel.
- With reduced-voltage starting such as star-delta, starting torque falls and the start takes even longer.
The calculation behind this relationship is explained in the article on inertia and acceleration time.
Information needed for selection
- Total inertia, or weight and diameter, of the flywheel and rotor
- Pulley ratio and rotor speed
- The material being granulated: thin film, thick-walled injection-moulded part, pipe
- Hourly throughput and feeding method; overfeeding creates loads that even a flywheel cannot smooth out
- Number of starts per day
Application pages
Once the load and starting conditions are clear, you can look at the examples on the plastic granulator motors page and review heavy-duty cast iron models in the online motor list.