What Voltage and Current Measurements Tell You about Your Induction Motor Part 1

Howard W Penrose, Ph.D., CMRP
President, SUCCESS by DESIGN®
Editor in Chief, IEEE Dielectrics and Electrical Insulation Society Web

Introduction

Barring some of the new technologies available to electricians or general maintenance these days, we still rely primarily on standard test equipment such as voltmeters and ammeters. The challenges we now face have more to do with a combination of experience and the new electrical environment. With the addition of even more power electronics and computerized/digital office equipment the harmonic content of what we are testing will cause problems with measurements, depending on the type of measurement instrument being used. Now, with the coming support and release of more and affordable plug-in electric vehicles (along with incentives) within the next 12 to 14 months, and companies rushing to be green by supplying charging ports for employees or customers, the heavier converters will add even more to the already dynamic electrical systems within facilities. New, green technologies that rely upon converters/inverters also result in some level of harmonic content that has not even been addressed in standards, to this date.

In our modern and changing electrical environment, how do we use our standard tools to test our electric motors accurately? Why is it that when I test with one instrument and then another I get different readings? Why do I get different readings when I am using the same instrument? How can I use this dynamic data to tell me what is going on in my machine?

Can I even rely upon my test instruments?

Now, to add to the complexity, electric motors that are designed for higher levels of production may have inherent current unbalances due to the winding style and connections. Almost all less than 600 Volt off the shelf induction electric motors use machine winding and insertion methods that result in a type of winding known as a ‘basket’ or ‘concentric’ winding. The inherent current unbalance levels in these machines are dependent upon a number of factors including whether they are connected delta or wye.

Can I rely upon my past experience with testing electric motors?

The answer to both questions is a qualified ‘maybe.’ Over the next several articles we are going to discuss the complexities of current measurements of induction electric motors and what those measurements can tell us. In this paper we are going to focus on the measurements in a sinusoidal, low harmonic content, electrical system. In future articles we will discuss the impact of harmonics and the differences between instrument types.

Standard Electrical System

As we move through this discussion, it is best to start with the basics of a 60 Hz (rules also apply for 50 Hz), low to no harmonic content system and then we will build from there. In this perfect electrical system, what are the issues that can affect the current readings at the motor, itself? There are three issues that have to be considered:

1. Power Supply: problems within the power supply ranging from impedance unbalance to unbalanced loads on the transformer causing voltage unbalance;

2. Motor Issues: problems within the electric motor such as winding shorts, rotor faults, damaged connections, over/underload conditions; and,

3. Combination: a combination of problems both in the power supply and electric motor.

With current measurements, alone, we cannot provide near enough information to be able to determine where a problem resides, or if a problem even exists. So, where do we start? By identifying ‘alarms’ that we need to consider and the proper selection of tools. For the purpose of this series of papers, we will consider the use of the voltmeter and ammeter for the basics and will call in the assistance of other standard tools for troubleshooting.

Voltage and Current Alarms

A question that appears over and over again in forums, emails, blogs, and classes is: “what is the limit on current unbalance?” The good news is that this limit is defined, kind of. At least it is defined for a sinusoidal power supply based upon voltage unbalance by the National Electrical Manufacturers Association, Inc. (NEMA) in the NEMA MG 1-2006 Standard for both low and high voltage induction motor applications.

From section 14.36: “When the line voltages applied to a polyphase induction motor are not equal, unbalanced currents in the stator windings will result. A small percentage voltage unbalance will result in a much larger percentage current unbalance. Consequently, the temperature rise of the motor operating at a particular load and percentage voltage unbalance will be greater than for the motor operating under the same conditions with balanced voltages.” [NEMA MG 1-2006]

Figure 1: Voltage Unbalance De-Rating
(Multiplier Vertical, Percent Unbalance Horizontal)

The reference to voltage unbalance identifies a voltage unbalance multiplier that is identical for all size induction machines, and reproduced here as Figure 1. The purpose of the derating factor is to take into account the additional heating that results from the current unbalance by multiplying the horsepower rating of the motor by the value based upon the current unbalance calculated as show in Equation 1.

Equation 1: Voltage Unbalance

V_ave= (V_1+V_2+V_3)/3

〖%V〗_unbal=V_max-V_ave /V_ave *100%

Where Vave is the average Voltage; Vn are the measured phase to phase Voltages;
%Vunbal is the percentage unbalance; and, Vmax is the largest difference from Vave

Current unbalance is calculated in the same manner as Equation 1 substituting A (current) for V.

From Section 14.36.1: “The effect of unbalanced voltages on polyphase induction motors is equivalent to the introduction of a ‘negative sequence voltage’ having a rotation opposite to that occurring with balanced voltages. This negative sequence voltage produces in the air gap a flux rotating against the rotation of the rotor, tending to produce high currents. A small negative sequence voltage may produce in the windings currents considerably in excess of those present under balanced voltage conditions.” [NEMA MG 1-2006]

The limits are defined in Section 14.36.5: “The locked rotor current will be unbalanced to the same degree that voltages are unbalanced, but the locked rotor kVA will increase only slightly. The currents at normal operating speed with unbalanced voltages will be greatly unbalanced in the order of approximately 6 to 10 times the voltage unbalance.” [NEMA MG 1-2006]

This means that in a motor drawing approximately 75 Amps and phase to phase voltage of 460V, 465V, and 483V, the voltage unbalance would be 2.9%. The phase current unbalance would range from 17.4% to 29%, or as low as 62 A on the low leg up to 88 A on the high leg in the 17.4% case (75 A +/- (75 * %unbal)) and as low as 58 A on the low leg up to 92 A on the high leg in the 29% case. If we do see a voltage unbalance and a current unbalance, based on this information, we have to determine if we have a supply issue or both a supply and motor issue.

The circuit impedance can also be an issue. A faulty power factor correction capacitor can cause current unbalances to appear as high as 50%, sometimes without a corresponding voltage unbalance. There are numerous cases where a technician has pulled a good motor due to a current unbalance from a system with a faulty power factor correction capacitor.

Part 1 Conclusion

Does this mean that even in a harmonic-free electrical system we are helpless? No, far from it. There are a number of tricks that can be used to help us determine what the issue is within a system. In Part 2, we will begin addressing how a system can be evaluated to determine if the unbalance is due to the supply, motor, or both.

Bibliography

H. W. Penrose, Ph.D., CMRP, Electrical Motor Diagnostics 2nd Edition: SUCCESS by DESIGN, Old Saybrook, USA, 2008.

NEMA MG 1-2006 Motors and Generators, National Electrical Manufacturers Association, Inc., New York, NY, USA, 2006.

Author’s Bio

Howard W Penrose, Ph.D., CMRP, is the President of SUCCESS by DESIGN® Reliability Services and Publishing as well as the Editor in Chief of the IEEE Dielectrics and Electrical Insulation Society Web. He has over 25 years in the electric motor and industrial energy and reliability industries from oil labs to motor repair journeyman to research and consulting on small to extremely large machines, motor management programs, diagnostics and hybrid vehicle machines. Dr. Penrose can be contacted via email at howard@motordoc.com or through his company website http://www.motordoc.com or IEEE site http://www.ieee.org/go/deis/.