Motor pulling too much current, how to limit this, what causes a motor to draw higher than normal.
High no-load current is the most common concern about high current with a three-phase motor.The broad issue of high no-load current isn't the only three-phase motor issue to which plants should pay attention.Let's look at the sources of these.
There is a situation in which a higher-than- expected no-load current can occur.There are some motors with no-load currents above or below the guidelines shown in Table 1 that are still satisfactory.
The actual test operating voltage should be considered when no-load current is high or low.The no-load current can be higher or lower than expected if the applied voltage is not within 10% of the motor's rated voltage.It is almost certain that a motor rated 200 volts on a 240-volt supply system will result in high no-load current.
Unusually high or low no-load current can be caused by a misconnection.The motor is intended to be connected to the parallel-delta.The motor could be mistakenly connected to the series-delta, which would result in a very low no-load current.The solution is to connect the motor.
Misconnection with a Delta instead of a wye is a possible issue.Some of the motors use the wye connection.The no-load current will probably far exceed the rated current if that is the case.
It is a good practice to check the no-load current and test the wye mode of the motor.If the current is very low, repeat the no-load test.If the no-load current appears normal in the wye connection, verify the correct connection for the application.There is an error in the winding of the connection.
A motor with a relatively high no-load current for an assumed power rating is a less common scenario.To determine the frame size from a NEMA dimensions chart, you need to measure the width from the bolt-hole center to the motor feet, as well as the height of the base and the shaft center.
Determine the number of poles.The number of poles is equal to 7,200 divided by the no-load rpm.If the no-load speed is 1,798rpm, the motor has 7 poles.
The frame size and number of poles can be used to estimate the motor power rating.The new estimated power rating can be used to evaluate the no-load current.
Incorrect winding data and stator core damage are the most common causes of high no-load current after a motor is rewound.
Increased no-load current can be caused by incorrect winding data that increases the magnetic flux levels.The effect on full-load current is not as significant, but higher starting current can trip protective devices.In extreme cases, this can cause contactors to blow open.Good practices, such as comparing the data to a database or using a computer program to calculate and evaluate winding magnetic flux densities, can be used by service centers to confirm that the winding data is correct.
A core test can be performed by service centers to evaluate the condition of a stator core.The results can be compared with industry standards.
There are a number of possible causes of high current with load.High current with load can be caused by an error in winding data.
It may be necessary to load-test the motor to confirm that it has acceptable current at full load if the inspection of the equipment does not indicate a mechanical cause for motor overcurrent.Caution should be used when evaluating actual motor current versus rated current.The 10% tolerance on rated full-load current is allowed by the motor and generator standard.If the rated current is 100 Amp, the actual current could be as low as 90 Amp or as high as 120 Amp and still be acceptable.
At full load, a winding with magnetic flux levels at least 10% greater than the original design densities will have higher-than-typical no-load current.A method of checking the performance of a magnetically strong motor is to measure the exact rpm at full load.The motor may be magnetically strong if it is significantly above rated speed.Basic calculations are required to determine whether the speed is high.
Tom Bishop has more than 30 years of hands-on and engineering experience at electrical machinery manufacturing and apparatus service firms.Bishop has presented many seminars on electric motor application, maintenance, and repair.He is also a member of the National Fire Protection Association electrical equipment maintenance committee.He can be reached at tbishop@easa.com.
Tom Bishop has more than 30 years of hands-on and engineering experience at electrical machinery manufacturing and apparatus service firms.Bishop has presented many seminars on electric motor application, maintenance, and repair.He is also a member of the National Fire Protection Association electrical equipment maintenance committee.He can be reached at tbishop@easa.com.
20% variation in motor slip rating is allowed by the standard.The slip is the difference between the two speeds.If a 4-pole, 1,800rpm motor has a full-load rating of 1,750rpm, 20% of the slip would be 10rpm.The actual speed could be between 1740 and 1,750rpm.A magnetically strong winding design can be seen if a speed greater than 1,760rpm is considered high.Check the line-to-line voltage before drawing a conclusion.It could be the cause of the high full-load speed if it is more than 10% above the motor rated voltage.
A pulsation in the output Torque of a motor can be caused by the open rotor bar passing under each phase of the motor winding.The net effect is a reduction in the steady-state output Torque; on average, the motor draws higher current that often pulsates.A single-phase open rotor test can be performed if an open motor is suspected.The test consists of applying about 1/6-1/4 of rated AC voltage, or variable voltage up to a current value of 75% to the two line leads of the motor while slowly turning it manually.The ammeter can be used to measure fluctuations in current.A broken bar will occur when the open bar passes under a pole if the current is greater than 3%.
Too much magnetic flux can cause high current with load.The square of the magnetic flux level is what determines the Torque capability of a motor.The Torque capability of the motor will decrease to about 81% if the winding flux is 10% lower than the design flux.If the motor was designed for 81hp, it would behave as if it were 100hp.It would be overload by 23% at full rated load because of its weak winding.The results include overcurrent and operation at a lower-than-rated speed.