What is the no-load current of a motor, typically as a percentage of the motor’s rated current?

What was said above applies to general situations. When the motor power is low, the no-load current of the motor can sometimes exceed 50% of the motor’s rated current; whereas for motors with high power, the no-load current may be less than 20% of the rated current, and it is even lower for synchronous motors. At the same time, the more pole pairs a motor has, the greater its no-load current.

This applies to squirrel-cage motors; whereas for wound-rotor motors, the no-load current can reach 60-70% of the motor’s rated current. This is due to the large air gap in such motors, which requires a relatively high starting torque

Y-series motors are 30%~50%

Mnemonic: For the no-load current of an electric motor, estimate it at about 80% of its capacity； New large models are 40% off, old small models have higher kilowatt ratings. Note: The mnemonic is a formula for quickly calculating the specific value of the motor’s no-load current on-site; it is derived from numerous test data. It conforms to the principle that \"the no-load current of a motor is generally 1/3 of its rated current.\" At the same time, it is in line with practical experience: the principle that \"the no-load current of a motor should not exceed its rated power in kilowatts in order for it to be usable\" (applying to old-style, low-capacity motors after maintenance). The mnemonic \"Approximately 80% of the capacity is used to calculate it\" means that the no-load current of a typical motor is around 0.8 times the motor’s rated capacity in kilowatts. The no-load current of medium-sized, 4-or 6-pole motors is 0.8 times the motor’s capacity in kilowatts ； New series: high-capacity, 2-stage motors with a relatively low number of poles; the no-load current is calculated at 60% of the value for motors with a higher number of poles” ； For old, vintage series of motors with smaller capacity and a high number of poles – 8 poles or more – the no-load current is calculated based on the principle of \"few poles equal high kilowatt rating\"; in other words, the no-load current value is approximately equal to the motor’s capacity in kilowatts, though it is generally less than that value. The mnemonic formula can be used to calculate the no-load current of a motor. The value obtained using this formula differs slightly from the value stated in the motor’s manual as well as from the actual measured value, but it is more than sufficient for the daily work needs of electricians.

There’s nothing too specific; the best choice is a higher value, 70%

The no-load current of motors of grade 2, grade 4, and grade 6 must be different percentages of the rated current, right? Given that motor powers vary, the percentage of no-load current relative to the rated current should also differ, right?

The magnitude of no-load current is an important parameter reflecting the performance of a motor. The general rule is as follows: (1) For motors of the same base model, the greater the number of poles, the higher the relative value of the no-load current. (2) For the same pole number, the smaller the power, the larger the relative value of the no-load current. For example, in the motors of the Y series produced by a certain factory with base number 132, the ratio of no-load current to rated current is 31% for 2-pole motors, 45% for 4-pole motors, 56% for 6-pole motors, and 65% for 8-pole motors. For motors with a base number of 90, the ratio of no-load current to rated current is 48% for 2-pole motors, 60% for 4-pole motors, and 70% for 6-pole motors.

Modern variable-frequency motors have completely overturned the theory mentioned above, which states that the higher the number of poles, the greater the no-load current! Even the no-load current of 16-pole variable frequency motors is only 20% to 30% of the rated current; even for repaired motors, this value must not exceed 40%. These Y-series variable frequency motors are widely used in industries such as steel manufacturing, paper production, and textiles!

This post was last edited by zhaohh3211 on 2010-9-13 at 17:30. Motor technology is constantly advancing, and the no-load current of frequency conversion motors with a higher number of poles is significantly reduced.