3-Phase Induction Motor Line Current Balance

If there is line voltage difference is of 2V then there will be phase to phase voltage difference of 4 to 5 volts in 2 phases and 8 to 9 volts in other phase to phase voltages so means there will be voltage difference of 2 % so 2 % voltage difference will leads to 4-5% difference in currents so there isn't much differences so carry one there will be not be such a huge difference

Motor will be kept running smoothly.

Current unbalance can be 6 -10 times the voltage unbalance ! There are many articles on the web about this, here is one example : Current unbalance

It is folly to look for energy savings here. Switching the motor off when not in use is a far better way of saving energy than looking at imbalance.

You misunderstand, the energy savings are aside from the imbalance question and are part of a much larger study of how to design high-efficiency hydraulic machines.

It looks as if the imbalance is reasonably normal, that even a small voltage imbalance can lead to the levels of current imbalance that I was seeing. All good then.

Well, that makes energy savings and imbalance two separate topics, thereby clearing the misunderstanding.

High-efficiency motors are the way to go. Hydraulics are an inherently "lossy" way to transfer power though very convenient for certain applications. One of the most efficient ways of transferring power is the derailleur-gear-shift-based bicycle chain, which is why hydraulics have made no headway in powering bicycles.

Is this the only unit to display these imbalances ? Is the RPM a close multiple of the Frequency ? If so have you investigated the possibility that the HPU is changing torque requirements each rotation ? If the motor is more balanced during no load or disconnected operation, you might have a clue where to look. Are there indications of vibration, during what operations, and etc. Do you have a fast pressure transducer to track and compare hydraulic pressures ? I am grasping at straws here, trying to think outside the box.

Thanks for all the helpful replies.

The machine in question is now being dismantled and packed into shipping containers so I can't do any more testing now but when it is reassembled on site I will see what effect rolling the phases has.

This is not the first time that I have seen current imbalance in these motors but this was the largest that I could recall, hence my original question.

As far as the hunt for energy savings goes, if it were just as simple as using high-efficiency motors then the hunt would have been over some time ago. However, things aint that simple. We build large applianceware machines that make the doors, cabinets etc for domestic appliances. These are typically multi-stage machines that bristle with servos, vfds, pneumatics and hydraulics. The hydraulics do all of the heavy-lifting jobs - pressing, notching, piercing, clinching etc and not surprisingly use the lion's share of the electrical power consumed. All loads vary in a complex, repeating pattern with machine activity.

The broad questions that we are looking into in the quest for greater energy efficiency are along the lines of : 1) whether it is better to use just one big HPU for the whole machine, or to use several smaller HPUs, 2) how to best use hydraulic accumulators to maximise efficiency, and 3) what is the most efficient way to electrically control the motor(s).

This is an ongoing project and it has already given us some valuable information. There's quite a bit more play in it yet however.

During reassembly/ installation at site please check the L-L inductance and resistance of the motor windings to identify the cause of current unbalance.

MANINDRA

More detail is nice. This does remind me of a warranty case where I was called in to support a sales engineer. We had supplied a motor but not the drive on an installation and the customer complained of noise and unbalanced currents in the motor. In this case the motor "drive" was a vfds by a competitor but was also new. When I heard the "motor noise" I suspected the drive and asked the customer to put the drive into bypass and run it straight off the power system. The noise and unbalance disappeared and I told them to contact the drive manufacturer. The problem turned out to be failed solid state device in the motor that was operating in parallel with several more good devices and allowed the drive to function, but did not allow it to do so correctly. The manufacturer of the drive remedied the situation quickly.

I am not suggesting that your problem is similar, but in the future it would be good to verify that you system is capable of supplying power in a balanced way, and then if you have unbalance in the motor, connect it directly to the power, bypassing the vfd. That way you can separate problems in the motor, connections, and vfd in to separate entities and correct problems rather than shipping them off to your customer, where the solution to any problem is much more costly than correcting it in a factory setting.

It would appear that none of the discussers has "hit the nail on the head" with respect to the problem. The problem we have here is of Positive and Negative Sequence. It is the Positive Sequence voltage (and power) that makes the motor rotate in the desired direction, . . . and it is the Negative sequence voltage (and power) that is trying to make the motor rotate in the opposite direction. You need to analyse the incoming voltage and determine how much negative sequence voltage you have. Any Negative sequence voltage will case the motor to be less efficient.

The useful power in the motor is (the Positive sequence power) - (Negative sequence power); while the power supplied to the motor is(Positive sequence power) + (Negative sequence power). One can see very readily that the efficiency of the motor will fall drastically with an increase in Negative sequence voltage and power. The standards do specify the limits of negative sequence for three-phase motor efficiency testing and operation.

If three-phase motors are to be installed on a system with excessive negative sequence, such installations may feature a "negative sequence regulator". Such a regulator will reduce the power consumption by increasing the efficiency of the motors.