Cyndi Nyberg
Former EASA Technical Support Specialist
Typically, motor currents of interest are the no-load current, full-load current, service factor current, and starting (or inrush) current.
You will know the full load current from the nameplate, and you can calculate the starting current from the Code Letter on the nameplate.
Service factor current may or may not be on the nameplate.
The no-load current will not be shown on the nameplate. It is possible to obtain the value from the manufacturer, but not always. The no-load current is an important benchmark, and once it is established, you can estimate the load on the motor at any time.
After any repairs have been made, it is always a good idea to test a running motor. Not every shop has a dynamometer to load test motors, so normally the motor is tested without a load to check for proper speed, temperature and no-load current.
The measured current can be a good indication that the motor is wound properly. Too low a no-load current can indicate that the winding is too weak.
If the no-load current is too high, there may be a number of problems, including core damage, broken rotor bars, an incorrect connection, or a winding that is so strong that it is saturating the core iron.
Too low Or too high?
But when it comes to no-load amps, how low is too low, and how high is too high?
There are some general rules for what percentage of the full load amps the no-load amps should be, but every design can vary and there are always exceptions to the rules.
For a typical three-phase general-purpose motor, the current without a load will normally be between 25 and 40 percent of the full load current. But you may wonder where the current comes from.
The no load current consists of two components. First, the magnetizing component, or magnetizing current, is the current necessary to produce the rotating magnetic field. The bigger the air gap, the higher the magnetizing current. Also, as more flux is concentrated in the air gap, the higher the magnetizing current.
The second component of the no-load current comes from the losses produced in the rotating parts, mainly friction and windage losses.
Change of flux in air gap
The current at no load largely depends on the flux in the air gap. As the flux is increased, so is the no-load current as a percentage of the full load current.
The flux in a 2-pole motor is concentrated more in the back iron rather than in the air gap, so typically the no-load current is fairly low, maybe 1/3 of full load current, or less. As the number of poles is increased, the flux concentration in the air gap gets higher.
As a result, the no-load current of a slower speed motor is a higher percentage of the full load current.
An 8- or 10-pole motor can easily have the no-load current that is half of the rated current, or more.
For very slow speed motors, the no-load amps can be nearly the value of the full load current. There have even been a few cases where the no-load current is higher than the nameplate value, and drops down when the motor is loaded. That situation is very rare, but we have had confirmation from at least one manufacturer.
When you are test running a motor, keep in mind that there can be a wide variation of no-load amps from motor to motor.
But if you know the general range of values, then you can make an accurate assessment of what the level will be for a given motor.
Once the benchmark has been established and noted, the value can be used to estimate the load level on the motor when it is in service.
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