locked rotor motor
- Thread starter rey-man
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luke warmwater
Senior Member
Re: locked rotor motor
If I may, "The rotor is locked".
If I may, "The rotor is locked".
luke warmwater
Senior Member
Re: locked rotor motor
In a motor, "locked-rotor" would indicate that the rotor has siezed. Are you looking for more info on this, like locked-rotor current??
In a motor, "locked-rotor" would indicate that the rotor has siezed. Are you looking for more info on this, like locked-rotor current??
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
Re: locked rotor motor
Starting at the instant you supply power to a motor, the current will reach its very highest current in a first few milliseconds. As the motor begins to move, however slightly, the current will start to drop. When the motor has reached its cruising speed, the current will drop to its minimum value. If at that time the motor is turning a mechanical load that is equal to the motor?s rating, then the current drawn will be the figure that is known as ?Full Load Amps,? or FLA. But the peak current that is reached in a few milliseconds would stay at that level forever (or until the motor burns up), if the motor does not move. That value, known as ?Locked Rotor Current,? provides useful information (to the design engineer) about the motor?s internal resistance and about how the motor is likely to behave during a short circuit fault somewhere in the power distribution system.
Starting at the instant you supply power to a motor, the current will reach its very highest current in a first few milliseconds. As the motor begins to move, however slightly, the current will start to drop. When the motor has reached its cruising speed, the current will drop to its minimum value. If at that time the motor is turning a mechanical load that is equal to the motor?s rating, then the current drawn will be the figure that is known as ?Full Load Amps,? or FLA. But the peak current that is reached in a few milliseconds would stay at that level forever (or until the motor burns up), if the motor does not move. That value, known as ?Locked Rotor Current,? provides useful information (to the design engineer) about the motor?s internal resistance and about how the motor is likely to behave during a short circuit fault somewhere in the power distribution system.
Re: locked rotor motor
It is of my experience that as rule of thumb for the locked rotor current of a across the line squirrel cage induction motor that is 7x the motors FLA rating. However, there is an event that preceeds that which causes a spike of current current that can reach 13 times the FLA rating of the motor and 17x or more with high/energy efficient squirrel cage induction motors. It is after that spike of current that counter EMF is able to reduce the current to the LRA value.
That high spike of current can creat difficulty in keeping a MCP closed when the motor is started. Hense, you have probably noted that that issue has been reviewed and revised over art430-52 of the NEC
It is of my experience that as rule of thumb for the locked rotor current of a across the line squirrel cage induction motor that is 7x the motors FLA rating. However, there is an event that preceeds that which causes a spike of current current that can reach 13 times the FLA rating of the motor and 17x or more with high/energy efficient squirrel cage induction motors. It is after that spike of current that counter EMF is able to reduce the current to the LRA value.
That high spike of current can creat difficulty in keeping a MCP closed when the motor is started. Hense, you have probably noted that that issue has been reviewed and revised over art430-52 of the NEC
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