Until the motor gets up to speed, there is insufficient back EMF being generated to limit the current to its normal FLC. In the locked rotor condition, at the initial point where it starts to turn, the only limiting factor to current draw is the motors impedance (Z). This starting current can be very high, 6 – 8 times FLC.
Secondary resistance starting. On large machines this is often used with transformer starting. P 303
Motor Braking Mechanical Eddy-current Dynamic - (Regenerative) - (injection) Plug braking P 308
Mechanical brake for parking. Electric methods will not HOLD the motor and load still. Some form of mechanical brake is needed.
www.railway-technology Examples of eddy-current braking www.fitnessscape
www.-ph.postech.ac This principle is commonly used with lifts and hoists to regulate the rate of decent. Induction disc. Aluminium or stainless steel. Electro magnetics The retarding force is altered by the supply current. Eddy-current braking
www.nesea.,org www.uptenergy http//aevcenter www.forkliftsireland Uses for regenerative braking.
For a motor to accelerate a load from standstill the motor must be able to overcome the inertia, load resistance and losses within the system. Student exercise.. Make a list of ALL the losses in a system. A motor in a crane raises a load from the ground to the back of a truck. What energy is used and where is it used. L/O 3.4
Applied voltage as reference No load current The no-load current is the magnetising current and the current to overcome the no-load losses. In simple terms this current remains constant. The load current from the supply required to accommodate the load. This current is almost in phase with the applied voltage. Total load current. Phasor sum of no-load and on-load currents at the running pf.
When a load is placed on a motor eg; a conveyor is loaded, the motor slows, less back EMF is produced which allows more supply current to flow. BUT this is only a small part of the story. What happens in the magnetic circuits?
Prac work. <ul><li>Connect a star/delta starter </li></ul><ul><li>Observe injection brake </li></ul><ul><li>WRM with resistance in rotor circuit </li></ul><ul><li>Connect VF drive </li></ul><ul><li>Measure starting currents </li></ul>Demo pracs.