What Is Normal No Load Idle Current?
What Is Normal No Load Idle Current?
(OP)
Hi All,
Someone told me today that as a general rule of thumb, motor idle current will increase as the motor nameplate speed decreases. In other words, a 3600 rpm (2 pole) motor might have 10-15% FLA at no load, an 1800 rpm (4 pole) motor might have 25-30% FLA at no load, and a 600 rpm (12 pole) motor might have 40-50% FLA at no load. Is this true?
The reason I ask is that I have a 4,500 HP Siemens-Allis 12-pole (600 RPM) squirrel-cage induction motor operating an automobile shredder. With the motor completely uncoupled from the shredder, the motor idles at about 250 amps, which is about 48% FLA. Does that sound normal to you guys? If not, what should the no load FLA be?
Thanks!
Someone told me today that as a general rule of thumb, motor idle current will increase as the motor nameplate speed decreases. In other words, a 3600 rpm (2 pole) motor might have 10-15% FLA at no load, an 1800 rpm (4 pole) motor might have 25-30% FLA at no load, and a 600 rpm (12 pole) motor might have 40-50% FLA at no load. Is this true?
The reason I ask is that I have a 4,500 HP Siemens-Allis 12-pole (600 RPM) squirrel-cage induction motor operating an automobile shredder. With the motor completely uncoupled from the shredder, the motor idles at about 250 amps, which is about 48% FLA. Does that sound normal to you guys? If not, what should the no load FLA be?
Thanks!





RE: What Is Normal No Load Idle Current?
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RE: What Is Normal No Load Idle Current?
If you have two motors both 100hp, both 480vac with one 2-poles and one with 12-poles:
They will both have roughly the same full-load current since that is related primarily to Hp and voltage (and lesser extent power factor and efficiency).
The low-speed motor will be a larger motor with a larger airgap and will therefore require higher magnetizing current. This means that for the low spee motor, the no-load current will be higher and the full load power factor will be lower (in general)
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RE: What Is Normal No Load Idle Current?
No load currents are an option the motor designers will handle to comply the torque, speed, efficiency, temperature rise, cost, etc. No standard regulate that parameter and could change from motor to motor and manufacturer.
RE: What Is Normal No Load Idle Current?
Again, many thanks!
RE: What Is Normal No Load Idle Current?
RE: What Is Normal No Load Idle Current?
RE: What Is Normal No Load Idle Current?
RE: What Is Normal No Load Idle Current?
RE: What Is Normal No Load Idle Current?
RE: What Is Normal No Load Idle Current?
"larger motor with a larger airgap"
Let me clarify. The airgap has a length (around the circumference), a height, and a depth (across the airgap). By larger I was referring to the length and the height (not depth). It should be obvious that length and/or height must increase when we increase the physical size of the motor.
The general form for inductance associated with an airgap is N^2 * depth / (A *mu0). For our cylindrical airgap, A is airgap length times core height.
As airgap length or height increases, A increases, magnetizing reactance decreases, magnetizing current increases.
The increase in length and height of airgap for slow speed motors is unavoidable and tends to cause increase in magnetizing current.
There are no inherent constraints associated with the speed of the motor that affect the depth of the airgap. However, for slow speed motors, manufacturers will tend to try to make the airgap smaller to compensate for the lower power factor caused by longer/taller airgap. There are limitations of how small a depth airgap you can make without unreasonable risk of pullover and rub, so this decrease in depth generally cannot compensate for increase in length and height.
Bottom line, the factor which causes decrease in power factor for slow speed motors is increase in length and height of the airgap.
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RE: What Is Normal No Load Idle Current?
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: What Is Normal No Load Idle Current?
Field B needs to stay constant. That means as A increases we have more flux. We need more exciting current (NI) to achieve that higher flux.
Mabye someone can provide a better explanation.
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Eng-tips forums: The best place on the web for engineering discussions.
RE: What Is Normal No Load Idle Current?
on an 8 pole this is only 15 degrees. The magnetic pull is around and not just radial. Thus making the magnetic pull arc shorter. Thanks for the clarity. Just being mechanical I was only concerned about the radial airgap.