Originally Published at Ask an Expert
Question: Turn calculation
How do you calculate the numbers of turn of
conductor per slot?
Answer:
The best approach would be to contact EASA (the Electrical Apparatus Service Association) through their website, www.easa.com. You can contact their technical support group, in the USA, at 314-993-2220 or easainfo@easa.com. I am attaching a copy of the catalog, which includes pricing. The books you would be interested in are: AC Motor Redesign EASA Motor Rewind Data AC Motor Verification & Redesign Program (software) The AC Motor Redesign book contains information on the 'Master Formula' for calculating motor design changes and how to determine turns. The EASA Motor Rewind Data book contains the rewind information specific to motor and model number. The AC Motor Verification and Redesign Program uses the master formula to calculate the impact of changes as well as what changes need to be made. This would include, if you have all the other rewind information, turn counts. The complex part of your question has to do with the impact of not knowing the number of turns. In modern low voltage (ie: <600 V) motors, there may be odd numbers of turns in each coil of your phase group. If you change this, there are a number of issues that may develop, including circulating currents and reduced/enhanced motor torque (by increasing or decreasing turns). If you wish to work from scratch (ie: there is no winding information), I suggest: "Electric Machines: Steady-State Theory and Dynamic Performance" by Mulukutla S. Sarma (ISBN 0534-93843-4) which may be purchased through Amazon.com (~$50 USD + shipping). Basically, a textbook on how to design motors. If there is anything else I can do to assist, please feel free to contact me.
Sincerely,
Dr. Howard W. Penrose, Ph.D.
General Manager
ALL-TEST Pro A Division of BJM Corp
123 Spencer Plains Rd
Old Saybrook, CT 06475
Ph: 860 399-5937
Fax: 860 399-7784
All Test Pro
Originally Published at Ask an Expert
Question: Polarization Index Test
I have a program to test the insulation of the AC and DC Motors, one this test is the Polarization Index ( PI ), but is a normal test to test the insulation in the Motors?
Answer:
Polarization Index is the relationship between the insulation resistance reading after one minute of applied DC voltage versus ten minutes. It is an accepted traditional test for evaluating the condition of the insulation resistance to ground. Prior to 1974, the absolute dangerous limit was a relationship of 1:1 (1.0 ratio) with 2:1 (ten minute to one minute reading ratio) being acceptable. This met the IEEE Standard 43 - 1974 requirements (IEEE Recommended Practice for Testing Insulation Resistance of Rotating Machinery). However, the release of the updated standard, IEEE Standard 43-2000 states that due to changes in the manufacturing and chemical makeup of insulation systems post-1974, some insulation systems polarize rapidly and would show a potential failure even on a new winding. Thus, IEEE Std 43 now states: "If the 1 minute insulation resistance is above 5,000 Meg-Ohms, the calculated PI may not be meaningful. In such cases, the PI may be disregarded as a measure of winding condition." IEEE 43-2000 Annex B, "Direct vs Alternating Voltage Testing," provides that an alternating voltage approach provides a more sensitive analysis of the winding.
Sincerely,
Dr. Howard W. Penrose, Ph.D. General Manager
ALL-TEST Pro, A Division of BJM Corp
123 Spencer Plains Rd
Old Saybrook, CT 06475
Ph: 860 399-5937
Fax: 860 399-7784
All Test Pro
I am finding very low RTG on large, approx 1000 HP, DC motors, some less than 10k ohms on armature and fields. I need information on the consequences of this and what safety and loss to production scenarios are possible.
Answer:
Although DC motors will operate under such low resistance to ground values it is generally considered a bad idea to risk equipment failure. Recently we evaluated a situation where low armature grounds <20Kohms was suspect in causing a drive failure ending in a flash fire inside the drive cabinet. In another situation a very large 3500hp DC motor used to press aluminum failed twice in one year from what was determined to be low armature grounds. What made this situation most interesting was that the troubleshooting identified a lower resistance(<10K) on the isolated power cables supplying the armature then what existed on the failed armature (100k). The fault had twice blown a hole in the armature windings. The US Navy used to operate their DC bus as low as 60K. After a DC Bus fire that rendered a submarine inoperable and resulted in personnel casualties they have increased their minimum to 200K. It would be our recommendation to contact the manufacturer of these motors and get some advise from them on how low they recommend you allow the resistance to get. I would like to see 1Meg and would be scheduling cleaning <200K.
Best Regards,
Noah P. Bethel Product Development PdMA Corporation
www.PdMA.com
Motor Fault Zones: Power Quality Case Study
an iPresentation Tutorial by Todd Gunderson & Noah Bethel, PdMA Corporation
Once again Noah and Todd have hit a home run with their latest Power Quality fault zone case study iPresentation Tutorial. Please join them in their 18 minute iPresentation exploring valuable techniques that you can use today to differentiate between Variable Frequency Drive faults or electric motor faults. Be sure an click the iPlayer download tab for a case study paper (pdf) also. Media Player required.
Can you please tell me the probable causes for 2 X line frequency in motors.
We had a problem of high vibration in one of our motors. Main contributor to vibration is 2 x line freq.. When we opened motor we found all things okay.
Can you tell me reasons for 2 x line frequency.
Answer:
The number one reason for any 2FL (twice line frequency) signature is electrical. Now, the type of electrical problem will be the question:
1) Stator eccentricity, shorted laminations, loose iron and a loose stator core will cause a high 2FL frequency, normally without sidebands.
This will also occur with unbalanced voltage incoming voltage.
2) Some 3600 RPM, and to a limited extent, some 1800 RPM, low voltage concentric-wound motors will show 2FL just because of the placement of the coils in relation to the stator core and rotor. This signature is more pronounced when the motor is connected for Delta.
3)Pole pass frequency (or twice slip frequency) sidebands around running speed and the 2FL peak indicate eccentric rotor conditions (usually dynamic eccentricity).
4) Broken rotor bars will show as multiples of running speed with pole pass frequency sidebands and may also show as rotor bar pass frequency
(RBPF) harmonics. 2FL sidebands around RBPF harmonics indicate looseness in the rotor bars.
5) Loose connections will appear as sidebands around a 2FL peak as 1/3 FL peaks with harmonic sidebands.
The good news is that motor current signature analysis is designed to quickly detect these issues using the motor current instead of mechanical vibration (the two technologies complement each other tremendously).
Please visit our site: www.alltestpro.com for more information on motor current signature analysis (MCSA).
Howard W. Penrose, Ph.D.
General Manager, ALL-TEST Pro
A Division of BJM Corp
123 Spencer Plains Rd
Old Saybrook, CT 06475
Ph: 860 399-5937
Fax: 860 399-3180
www.alltestpro.com
4/18/04
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