Page 8 - Industrial Technology EXTRA - 10th August 2020

current surges can cause a voltage disturbance on speeds, as well as positioning the crane itself. Both
the supply lines. To limit the starting current surge, wound rotor motors and DC motors are frequently
some large induction motors are started at a used to power these cranes and hoist heavy loads,
reduced voltage before receiving their full voltage which require a careful balance of force and control.
supply. Hoisting is pretty straightforward – the load
Star-delta is a commonly used starting method resists the motor, requiring positive torque against
in which the stator coils of the motor are initially gravity. But when the crane’s hook is loaded, the
connected in a star configuration and are switched load may overhaul the motor and requires a
to delta as the motor speed increases. This reduces dynamic braking resistor, such as those supplied by
the start current applied to the motor, reducing the Cressall, to ensure the load won’t run away and
risk of disturbances and interference on the crash.
electrical supply. A challenge to this solution can While VSDs have taken over a number of
arise when the motor coils in the configuration are traditional motor control techniques, many
briefly open-circuit, causing a voltage spike. To applications still require resistors to help keep them
eliminate this problem, a selection of small resistors turning effectively. Whether it’s for a wound rotor
are put into the circuit for a short period of time via motor for heavy industrial operations, or to limit
a third contactor. starting motor current, resistors still play a key role
For overhead cranes used in industrial facilities, in keeping motors in control.
resistors are used to control hoisting and lowering www.cressall.com

August 2020 • INDUSTRIAL TECHNOLOGY EXTRA! • p8