Hi,

our workshop have receive one motor from our plant which specs are below:

Voltage: 3.3 kV
Power: 300 kW
No load current: 25A
Full load current: 68 A
Solo run (no load) current: 33.4A
Brand: ABB
RPM : 989 RPM


The problem is, the current during the solo run is 48% higher than permittable value which is 20% of the full load current (63A) which is 33.4A. We try to megger (using 5kV) the motor and the result is ok (more 100 MegaOhm).

As for that, we have open the motor, clean the stator (by air), balancing the rotor, change the bearing and do metal spray to the bearing housing and the rotor. then we re-test the motor but the result remains the same. We don't have any idea what cause the problem. As for extra information, the motor did not run (in storage) since 2003 until last week.
Anyone to help me....

This message has been edited. Last edited by: pisemude,

Pisemude:

High current under no-load (I am assuming that the motor is also not coupled) can be caused by a number of factors. It looks as if you have covered many of those.

There are several additional areas that should be evaluated, in order to determine the condition.

1) Was the motor rewound in the past? It is possible that a change to the windings may have occurred. In this case, the high current may the the result of a poor repair.
2) Also related to rewind: A damaged core will also cause a high no-load current. The reason is that the no-load current is the result of the circuit impedance, which include motor losses.

Most likely causes:

1) Check to ensure that you are evaluating the motor at the nameplate voltage. As you deviate from this voltage, either up or down, the no-load current will increase. The maximum deviation is +/- 10% for the safety of the motor, but changes to the values will start at about +/- 3% (average) of the nameplate. Voltage unbalance (from phase to phase) will also cause this result.

2) Rotor problems will also cause this as a direct result of a reduction in torque. Following is a short list of methods used to check the condition of the rotor:

Evaluating the condition of electric motor rotors is not a new concept. Electric motor repair shops have been using techniques such as growler testing, single-phasing, infrared, vibration and/or dye testing as methods used through the repair process. In the field, vibration, analog current tests and inductive testing has been used. Over the past twenty years, a variety of motor circuit analysis and current signature analysis technologies have entered the market providing even greater accuracy in the detection of rotor bars.

Growler testing is performed by applying power to a half-transformer with the rotor resting upon, or near, it. The induced current allows the repair person to check the rotor bars by using metal filings or magnetic paper. The term growler comes from its original purpose of detecting shorts in DC armatures. The power is induced into the armature and a hacksaw blade would be held just above the armature over each slot. If a short exists, the blade would begin to vibrate causing a ˜growling' sound. The accuracy of growler testing on AC rotors is improved by heating the rotor in an oven to approximately 200 degrees F prior to testing in order to cause expansion of any fractures.

Single phase testing involves the application of approximately 10% of the motor voltage across a single phase (ie: T1-T2), in an assembled motor with a good winding, then rotating the shaft slowly with a current probe attached. If the value stays within 3%, as the rotor is turned, then it is in good condition. This test is considered potentially dangerous to the technician.

Infrared testing is performed by winding a coil of wire through, or around, the rotor and applying a voltage and high current. This causes the rotor to heat and will identify smeared laminations and loose or broken rotor bars. Hot spots, identified with infrared, greater than 10 degrees C above the ambient rotor temperature identify faults.

Vibration analysis is performed with the motor assembled and under load, usually at least 50% of rated load. Signatures of twice line frequency with pole pass frequency and peaks of the number of rotor bars times the running frequency, will indicate rotor bar problems. This normally requires some degree of experience by the operator.

Analog current meters on switchgear/MCC's, or hand-held analog current meters, can be used by observing the meters for sharp pulsing of the current as the motor operates under load. This ˜ticking' motion will occur at pole pass frequency and is a strong identifier of multiple broken rotor bars.

Inductance testing can be used by viewing a continuous measurement of inductance or periodic testing of inductance through an arc or full rotation of the motor shaft, of an assembled motor. A repeating pattern indicates a good rotor and impact of the peaks or valleys of a few of the patterns indicate a broken rotor bar problem, impacts on the slopes of the patterns indicate casting voids (in aluminum cast rotors). There should be one pattern per pole of the machine being tested.

Current signature analysis is a classical method for analyzing rotor bars. Side bands of twice slip frequency approaching '35 dB down' indicate severe rotor bar conditions, that must be addressed.

(from ReliabilityWeb Motor Blogs by Howard W Penrose, Ph.D. - the 'Penrose Lecture Series')

Howard

Great info by Howard.

I would disagree on the comment that low voltage can cause high current during no-load run. Lowering voltage will always lower current during no-load run. So pisemude - check your voltage and make sure it's not higher than nameplate by more than 10%.

Also definitely worthwhile to check for balanced currents. If unbalanced might be related to power system voltages or motor.

Also is it possible that you have taken a wye motor and connected it in delta? That may increase the current substantially.

I do agree no-load current 33A~ 50% FLA sounds high for a 4-pole motor. But the criteria 20% FLA sounds a little stringent. Where did that come from? You listed "No load current: 25A"
Where did this info come from? It also exceeds your 20% criteria.

ElectricPete:

The current goes up at a higher rate at lower voltage, even unloaded. The effect has to do with the complete stator/rotor circuit.

At higher voltages, the increase in current is directly related to the excitation of the stator core, rotor core, and the complete circuit impedance. This is somewhat countered with the decrease in rotor slip frequency, which reduces the rotor current and resulting measured current at the connection box. At no load, the rotor is already close to synchronous speed (slip is generally within a few RPM of synchronous speed). This may cause a slightly exaggerated current at no load with high voltage.

At low voltage, the opposite effect occurs. At no load, the motor generates a lower magnetic field allowing the rotor to slip due to normal losses (which increase at the lower voltage, and play a greater part at no-load, the idle current is directly related to the losses). The increase in rotor frequency actually has a much greater impact on the increase in current.

This effect can be seen during the inrush phase of motor starting. After each phase completes a cycle [there is an effect called the 'half-cycle' peak, in which the current spikes as high as 21 times the nameplate current as the voltage increases (you are dealing with the actual DC resistance of the circuit), once it begins to decrease (the 'down-side' of the cycle - you are now dealing with fields and impedance), then the winding is inductive and we then deal with the standard 4-8 times, for standard, nameplate current inrush], the nameplate current can be about 4-8 times the full load current. This initial current is due to the locked rotor condition in the motor. Both the stator and rotor are seeing full line frequency (50 Hz, in this case), and the motor acts as a transformer, with the high rotor current reflecting back to the stator winding and measured at the connection box. As the rotor RPM increases, the rotor slip frequency decreases and the current drops.

Above 50% of load, the motor losses play a very small part. Below 50% of load (average), the losses play a very large part. However, a standard chart (NEMA) for voltage impact shows a much greater running current at lower than nameplate and a more gradual running current over nameplate voltage. This change remains regardless of actual load.

The high current due to broken/damaged rotor bars is the result of the same effect. With the loss of rotor bars, the rotor cannot generate enough torque and the slip increases, resulting in higher current (that pulsates at pole pass frequency - twice slip frequency)

I do have one other question, though. Where did the value of 25 Amps come from for idle current? Lower speed motors (6-pole in this case) tend to have higher idle currents than 4-pole machines. If you are just using a 'rule of thumb,' there may not be anything wrong, even at 300 kW. Similar 6-pole, 4160V, 60 Hz machines in the USA tend to have idle currents at about half, or just over half, of the full load current.

Howard

As voltage decreases, no-load current decreases.

From an equivalent circuit standpoint, at no-load we get close to s=0 so the rotor branch R2/S almost disappears (open circuit) and all that is left is the stator leakage reactance and magnetizing reactance....in other words a constant impedance (for all voltages below saturation). As we decrease voltage across a constant impedance, we decrease current.

Only if you assume that the voltage is at nameplate value. What is suggested is that the motor will run at synchronous speed at lower voltages.

The S~0 method of no-load test assumes that the motor is running at synchronous (or fractions of a Hz slip) at full voltage. This is meant as a method to provide for easier calculations to compare to blocked-rotor tests (performed at a calculated lower voltage) in order to estimate performance.

With a light rotor, such as with a small integral or fractional horsepower motor, this can be assumed, until the voltage drops enough. In a larger motor, the dynamics are a little different.

In either case, this subject may be moot if the stated no-load current is actually the correct idle current.

For initial decrease in voltage below nameplate by factor of 2 or more current will decrease. For even further decrease beyond that, I can see your point now that friction and windage can cause current to increase again.

I agree also this current reported in the original post may not be abnormal. Definitely the 20%FLA number mentioned is not realistic for 6 pole motor.

Dear Mr Howard,

The 25A current is from the name plate. Here, the frequency of the supply is 50Hz supply. Basically, we have settled the problem. This is due to unbalance supply from the main feeder. When we tested on our panel, we did not receive any high current. Therefore, I would like to both Mr Howard and electricpete for your valuable info and assistance.

pisemude - glad that your problem is resolved.

Howard/others - As a matter of curiosity, I wanted to investigate a little more how much voltage would have to decrease before we might see some increase in current above normal full-voltage no-load current.

For a typical motor, losses are < 5% of output power and friction/windage is 10% of losses. Therefore friction/windage is < 10% x 5% = 0.5% (.005) of output power.

So in theory you should be able to reduce voltage by a factor of 10, resulting in a torque reduction of 100, and still easily accelerate the motor. Motor should come up to a slip of 1/2 the nameplate slip. (0.5% friction torque / 1% rated torque = 50%.) Load component of the current would be presumably higher than frction load (0.5%) by the voltage ratio (10). i.e. load component of current is 10x0.5% = 5% of full load current. Considering that magnetizing current has decreased by a factor of 10 to at most 5% current, then the total current would be load component plus magnetizing component = 5% + 5% = 10% (ignored vector sum which would make the current even lower). This is still less than full-voltage no-load current.

My conclusion is that you would have to decrease voltage by more than a factor of 10 below nameplate before you would see increase in current above full-voltage no-load current. Does that sound correct?

(btw I'm not recommending anyone use 10% as voltage for starting a motor solo, I'm just asking a question).

Now, if we decrease voltage by let's say a factor of 30, then we are at the point that even if the motor fails to accelerate (not a good thing), the current could not be higher than locked rotor current over voltage ratio, ie ~ 500%/30 ~ 17% which would again be lower than full-voltage no-loade current.

So the window of voltage reduction which would increase no-load current, if it exists, would have to be somewhere in between 10x and 30x decrease below nameplate voltage. If voltage is outside this window on either side (but sill below nameplate voltage), then we would still have decreased current. Does that sound right?

This message has been edited. Last edited by: electricpete,