How can you tell whether a machine starts unloaded or loaded?

Whether a machine starts unloaded or loaded can be worked out by looking at whether material remains inside it when it stops, whether there is a device that removes the load before start-up, and how long the run-up takes. This information directly affects the motor's starting torque requirement and the choice of starting method.

Watch how it stops

The clearest clue is how the machine comes to a halt. Does it stop after the feed is cut and it has emptied, or is it left full after a power cut or an emergency stop? If a belt conveyor, for example, stops with material on it, the next start has to set the whole load in motion. Stones left in a crusher's hopper have the same effect.

Is there a device that removes the load?

• A valve on a compressor that releases pressure during start-up
• A damper on a fan that closes at start-up
• A discharge valve on a pump that is throttled at start-up
• A clutch or fluid coupling between machine and motor

If one of these is present and working properly, the machine in practice starts on light load. A faulty unloading valve, however, can overload a motor that was chosen for an unloaded start.

Do not confuse a heavy mass with a loaded start

A large fan impeller or flywheel takes a long time to come up to speed even with no material in it. Here the problem is not load torque but inertia. With a loaded start, by contrast, the machine resists from the very first moment and sometimes does not begin to turn at all. Having qualified electrical personnel measure the run-up time and the starting current helps distinguish between the two.

Effect on the selection

If a loaded start is identified, motor power and the starting arrangement are assessed accordingly; this assessment is based on nameplate data and a review by qualified personnel. The star-delta starting page explains the concept. For conveyor applications, you can look at belt conveyor motors and the three-phase models in the shop.