Surge Test Voltages and Standards
There are three main standards used for surge test voltages. Electrom surge testers meet the requirements of these standards:
Note that the standards are recommendations only. Any voltage can be agreed upon between the parties involved, and should be if there are any concerns.
For a summary with tables, graphs and recommendations, please scroll down to the Summary and Recommendations section.
ANSI/EASA Recommended Practice for Repair or Rotating Electrical Apparatus
For random wound and form wound coils and motors:
Test voltage, V = 2E+1000V, E = RMS rated Line to Line voltage of the motor.
This is the most commonly used test voltage formula for windings, stators, assembled motors and generators of all kinds. The standard does not distinguish between new and used machines, and is used for both random wound and form wound stators and assembled machines.
Note that for any device under test, the Insulation Resistance (megohms) should be higher than certain values listed in IEEE 43-2013 for the device to be over-voltage tested with a Surge test or Hipot test.
See details under Insulation Resistance.
IEEE Guide for Testing Turn Insulation of Form-Wound Stator Coils for Alternating-Current Electric Machines. This guide applies to:
- Individual stator coils after manufacture.
- Coils in completely wound stators of original manufacture.
- Coils and windings for rewinds of used machinery.
- Windings of machines in service to determine their suitability for further service (preventive maintenance testing).
The table below shows the IEEE Surge Test Voltage Formula for Form Coils and Form Wound Machines.
In the standard V₂ is called the momentary withstand capability across a coil. This is the voltage a new coil should be able to handle, and therefore what it should be surge tested to. Unless there are defects, new coils can handle much higher surge voltages.
The standard defines a withstand envelope (surge test voltage profile) that changes with the rise time Tr of the surge pulse. As the rise time increases, the peak surge test voltage should increase per the withstand envelope. V₁ should be multiplied by a higher factor “f” to get to the test voltage V₂. See graph below, the “Standard” profile. This is the voltage used for coils in machines where high-magnitude, fast-rising surges or switching transients occur frequently or continuously.
The “Alternate” profile in the graph can be used for coils not likely to see high-magnitude fast-rising surges, and the factor in the formula would typically be f = 2.
IEEE suggests a rise time Tr = 0.1 – 0.2µs and a factor f = 3.5 for turn to turn surge tests. The rise time is dependent on impedance properties of the load being tested. It is defined as the time it takes for the voltage to rise from 10% to 90% of the peak surge test voltage. Neither the users nor manufacturers of surge testers can control the rise time of a surge test pulse. Therefore, in practice, surge tests have been done with rise times longer than 0.2 µs in high impedance loads. This has been done with excellent results for a very long time using f = 3.5 as well as using the ANSI/EASA surge test formula.
Maintenance test and diagnostic tests of used machines:
IEEE 522 recommends testing at 75% of the formula when testing used machines.
Uncured and green” form coils:
The test voltage is reduced to 40% – 80% of the formula depending on the insulation materials and technology.
The standard says that coils may be tested at one or more of the following steps:
- Prior to insertion in the stator
- After coil installation, wedging and bracing, but before any connections are made. Each coil should be tested.
- After series connections are made, but before insulation them and before making connections between phase groups.
- After connecting into phase groups, but before connecting phase groups to each other. This can be done before or after insulating the series connections.
- After all connections are made and insulated. Maintenance tests are normally made in this condition.
IEC 60031-15 Edition 3.0, 2009
Impulse voltage withstand levels of form-wound stator coils for rotating AC machines
This standard applies to sample tests of form coils, typically of at least two form coils, “carried out on coils in new condition which adequately represent the configuration of the finished item to be used in the machine for the purpose of evaluating the manufacturing procedures and processes incorporated in the insulation system.” Note that these coils are not yet installed in a stator.
Front rise time T1 is defined as 1.67 times the interval between the instants when the impulse is 30 % and 90 % of the peak value.
For inverter fed machines (power systems using VFD, ASD etc.) the test voltage may be increased. This is to ensure that the motor insulation is good for the peak of VFD-associated switching transients. The standard states that the overshoot factor at the motor terminals can be as high as 1.7 for a 3-level inverter. For more information on this subject see Inverter (VFD/VSD/ASD) generated PD.
The standard states that “the test coils shall have completed their manufacturing process, including corona protection layer and stress grading if used, and shall be either embedded in earthed slots or fitted with the slot portion wrapped in earthed conducting tape or foil. The number of sample coils shall be at least two.”
The standard further says: “Routine tests may be performed on coils after insertion in the stator core but before processing and making the connections.”
In this condition, a reduced surge test voltage should be used, typically 40% to 80% of the surge test impulse voltage U′P. The test voltage depends on the insulation technology used, and should be agreed to between the manufacturer and user.
What Surge Test Voltages to Use – Summary & Recommendations
Following are graphs and tables showing the differences between the recommendations in the standards.
The two charts above show that the ANSI/EASA recommendation for used machines is similar to that of IEEE, except for machines operating at lower voltages. See red and light blue lines. For example, at 2,300V RMS L-L operating voltage, the ANSI/EASA test voltage is 5,600V, and IEEE is 4,930V, 670V or 12% less.
At 460V operating RMS voltage the ANSI/EASA surge test voltage is 1,920V, and the IEEE voltage is only 986V or almost 50% less. This is far too low for most applications. See table of test voltages below.
Electrom Instruments recommendations for surge tests of machines
For used machines:
Use the ANSI/EASA formula. The low surge test voltage for IEEE at lower operating voltages will result in fewer weaknesses being found, especially for inverter driven motors.
All the standards mention that the surge test formulas do not take inverter drive spikes into account, and that a higher surge test voltage may be considered for inverter applications.
Industrial users will sometimes de-rate surge test voltages down to the line to line RMS rated voltage for machines that are in operation, especially for those considered to be ‘dirty’. A test voltage lower than the peak operating voltage equal to RMS x 1.41 Volts, will not cover the whole operating voltage range of the motor. Although a de-rated surge test voltage can be prudent at times, it will not provide information about problems to come unless it is higher than the peak operating voltage.
For new machines:
Use the ANSI/EASA formula for machines with operating voltage of 1,000V or less.
Use either standard for higher voltages. The higher IEEE voltages for machines operating above 1,000V are still far below what the machine should be able to handle.
Keep in mind that Surge and Hipot tests should not be done if the insulation resistance megohm measurement does not support it. Click here for more information.
Surge tests of form coils
The options for testing form coils are many, and the standards address the subject differently.
IEC differentiates between sample coils that are surge tested before installation in the stator (bench test), and coils installed in the stator but not connected yet. The bench test of sample coils embedded in earthed slots or fitted with the slot portion wrapped in earthed conducting tape or foil, is at a much higher voltage than tests done on coils installed in a stator. IEC does not address testing of installed coils that are connected, or completed machines.
IEEE does not discuss bench testing of sample coils, but covers bench tests of all coils after manufacture, surge tests of coils at different stages of machine production, and tests of complete machines.
Sample testing and testing of all coils after manufacture should not be compared. They are two different types of tests. Once the coils are installed in the stator, the surge test voltages for the two standards are more similar. Installed in the stator, many users will also apply the ANSI/EASA formula which is in the same range as the IEEE and IEC formulas.
Electrom Instruments Recommendations for Surge Tests of Form Coils:
The user should agree with the manufacturer of the form coils what surge test voltages to use.
There are many different insulation materials and technologies, and each may require different test voltages at different stages of production of a machine.