Out of Balance Motor

Question: Out of Balance Motor
Being a tradesperson for many years I remember as a apprentice my foreman indicating 2 pole motors 3 phase will have sometimes a slight out of balance current characteristic compared to a 4 pole motor. At the time this was validated by a practical example,the motor resistence, voltages, were all equal and there was no shorts in the windings. 20 years on from this a apprentice has asked me why. Is this myth or can you provide some technical information for a explanation.



Answer:
The phase unbalance, in current, that you are seeing in 3600 RPM motors most likely has to do with the winding design. In high production, machine wound machines, the most common type of winding design is called 'concentric' winding and may sometimes be referred to as a 'basket' winding. The other type of winding is called 'lap' and appears almost as a vortex if you were looking through the motor. Lap windings tend to be very balanced, but time consuming to install. Concentric windings are used so that it is easy to pull them through the stator with little or no human intervention. There is a larger coil that goes across the diameter of the stator, then a slightly smaller coil, and so on. The first set of coils (one phase, usually) is positioned against the stator core, with the second set on top and the third set close to the rotor. The first set of coils is usually a little larger than the other two so that it can be positioned away from the core. The relationship between the position of the coils, the back iron of the stator core, air gap and the rotor, cause each phase to be slightly unbalanced electrically. The tolerance is set by NEMA as less than 5% impedance unbalance. As the coils have almost the same length of wire (ie: phase A might have 60 ft, phase B 55 ft, and so on), the phases will be balanced in resistance to within a few milli-Ohms. However, the position of each coil will impact the inductance of the coil, hence the impedance (consists of resistance, impedance, inductance, capacitance and frequency), which is the AC resistance of the coil. Because, simplistically, current is equal to voltage over impedance (not taking into account vectors), the current will have a resulting unbalance. This unbalance may appear to be greater in delta versus wye connected motors. This is also true in low voltage 4-pole motors, but not to the same extent because of the span (side to side distance) of the coils. To see the impact of the coils in the motor, you require the use of impedance-based measurement tools for de-energized testing.