Motor with no load
- Thread starter Karsten
- Start date
jraef
Member
Why does it matter? It means nothing and every motor will be different, it has to do with the internal design and construction.
The active power will be close to 0 kW.
Law of Conservation of Energy : if the motor does not do any work it will not consume energy from the electrical supply, it will only consume the small losses due to friction and heat
All the current you can measure in the cables is reactive intensity and the power factor will be very close to 0
Last edited:
widelto
Member
When I test a motor (full voltage) with no load and its current is higher than 25% of full load , my decision is not to buy it.
Bit_Bucket_07
Member
In my experience, something in the neighborhood of 30% of FLC is typical for an unloaded motor. Then again, as jraef said, it varies with motor design.
Low speed motors tend to draw more no-load current. Motors designed to provide high starting torque will also draw higher no-load current.
drbitboy
Lifetime Supporting Member
Alternate method to detect cavitation or empty inlet/sump, possibly preferable to adding a level switch?
The motor that generates more reactive power at no load usually has bigger and more robust windings, which is not a bad thing.
And if the plant has its reactive compensation system there should be no problem.
widelto
Member
Siemens used to have a motor plant here in Colombia , they closed it years ago.
All motors from that plant used to consumed almost 50% of full load current with no load. No body wanted to buy those motors.
All motors from that plant used to consumed almost 50% of full load current with no load. No body wanted to buy those motors.
That current does not count in the energy consumption.
Connect three capacitors in parallel with the motor and you will see that this current is greatly reduced.
widelto
Member
lfe:
Thanks for the info.
Thanks for the info.
BAJ
Member
If you are talking about a motor with permanent magnets on the rotor, like a typical servo motor, the current/power draw will be very low with no load. If it's an AC induction motor the magnetizing current can be a considerable value depending on the design/purpose, but most drives typically factor this in and report low power usage despite the higher magnetizing current.
Bit_Bucket_07
Member
BAJ
Member
kamenges
Member
Well, almost. While the the total kVA value of the magnetizing voltage and current is not converted to work (it is returned to the line or drive on the next half cycle) that current still needs to run through the motor supply conductors and the motor windings, where some of it is converted to heat. So, all other things being equal, a motor that draws lower no-load current will be more efficient, at least at partial load.
Keith
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