What is a locked rotor DC motor?
When an electric motor first starts running, the motor will draw approximately 5X the normal amp draw of the motor for approximately the first half second that the motor runs. The amps that an electric motor draws for about half a second that are about 5X the normal amp draw of the motor are called locked rotor amps.
What causes a locked rotor?
A compressor may draw locked current for several reasons. Simply confirming a high amp draw on the common winding is not enough. A single phase compressor has two sets of windings: Run & Start. Something as simple as a broken wire or weak capacitor can generate a lock rotor condition.
How do I know if my rotor is locked?
Out in the field, the rule of thumb for locked-rotor current is six times the full-load amperes marked on the motor’s nameplate. The result of multiplying the full-load amperes of a motor by six might be close to the actual locked-rotor current or the result might not be anywhere near the actual locked-rotor current.
What is locked rotor time?
The locked rotor time of the motor refers to the time it will take for the motor’s electrical components to reach a critical temperature when the motor is supplied with voltage to its stator (normally assumed to be full voltage) and with the rotor completely locked, i.e. unable to turn.
What happens when rotor locked?
Locked rotor amps is the current that an electric motor will draw when the rotor is locked (cannot move). This generally occurs at start up when the load is not moving. The locked rotor torque is the torque generated in this condition. If the locked rotor torque cannot move the load then you need to get a larger motor.
Is locked rotor current the same as inrush current?
Inrush current, also referred to as “locked rotor current,” is the excessive current flow experienced within a motor and its conductors during the first few moments following the energizing (switching on) of the motor.
What is locked rotor protection?
To protect a motor with a ”locked rotor” the maximum allowable time between a motor start attempt and the beginning of normal running operation can be protec- ted. When the permissable locked rotor time is less then the starting time the ”zero speed” input detects if the rotor is locked during the start attempt.
Is Locked rotor amps the same as full load amps?
The locked rotor current is the measured current with the rotor locked and with rated voltage and frequency applied to the motor. The amount of current a motor can be expected to draw under full load (torque) conditions is called Full Load Amps.
How do you calculate locked rotor amps?
To find LRA and starting kVA enter nominal operation voltage (such as 120 or 240), HP value, code letter from the nameplate label, and hit “Calculate” button. The general equation for single-phase devices is the following: LRA=1000*(kVA/HP)/Voltage. Note that this defines starting current at full nominal voltage.
What is locked rotor amps used for?
The very high current or amperage a motor draws when power is first applied, due to low counter emf.