When a switching device opens with the motor starting or with a locked rotor, current chopping and re-ignitions can produce overvoltages at the motor. IEC 62271-110 defines tests with 100 A and 300 A substitute circuits and sets no overvoltage limit: what matters is each application.
Short answer
IEC 62271-110 (edition 3.0, 2012) covers switching devices above 1,000 V that switch inductive loads, with or without short-circuit breaking capability. It includes the circuit breakers of IEC 62271-100 that switch high-voltage motors and shunt reactors, and the high-voltage contactors of IEC 62271-106 that switch motors.
For motors, the tests apply to three-pole switching devices with a rated voltage equal to or lower than 17.5 kV, which can switch three-phase squirrel-cage or slip-ring induction motors (clause 6.113.1). The voltage of the switching device may be higher than that of the motor if they are connected through a step-down transformer. The most usual arrangement is direct connection by cable.
The problem: breaking a starting current
The current of a starting motor, or one with a locked rotor, is high and highly inductive. The standard defines an inductive current as a power-frequency current with a power factor of 0.5 or lower. When the switching device opens it, these phenomena appear (clause 3):
- Current chopping: interruption at a point on the wave other than the natural current zero.
- Re-ignition: the current resumes between the contacts during an opening, with a zero-current interval shorter than a quarter cycle.
- Voltage escalation: an increase in the amplitude of the recovery voltage of the load circuit through energy accumulation in repeated re-ignitions.
The standard considers that switching a starting or locked motor is generally the most severe as regards overvoltages (clause 6.113.2). Opening a slip-ring motor during starting is usually less severe because of the effect of the starting resistor.
What is tested
The switching device is tested in the laboratory with a standardized circuit that simulates the locked motor (Figure 1 of the standard). Its parameters were chosen for a relatively severe case with respect to overvoltages and cover most applications. Tests can also be made in the field, with a real motor, and then they are valid only for switching devices working in similar circuits.
Because the iron core of the motor is nonlinear, its switching cannot be modeled exactly with linear components. The standard indicates that tests with linear components can be considered more conservative than switching real motors.
The supply and load circuits
Two supply circuits and two motor substitute circuits are used:
| Element | What it represents |
|---|---|
| Supply circuit A | A motor connected directly to a transformer |
| Supply circuit B | Cables in parallel on the supply side |
| Substitute circuit 1 | Current of 100 A, with a power factor lower than 0.2 lagging |
| Substitute circuit 2 | Current of 300 A, with a power factor lower than 0.2 lagging |
The substitute circuit is connected to the switching device with about 100 m of screened cable. Its natural frequency is 12.5 kHz with a tolerance of ±2.5 kHz and its amplitude factor is 1.7 ± 0.1.
The combinations form four tests: A with 100 A, A with 300 A, B with 100 A and B with 300 A. In each one, 20 operations are made with the closing and opening instants distributed at intervals of approximately 9 electrical degrees.
What the 100 A and 300 A cover
Clause 6.1 sets the scope of the type test with locked-rotor currents of 100 A and 300 A. It is considered valid for locked-rotor currents between 100 A and 300 A. It also covers up to the current associated with the short-circuit current of test duty T10 of IEC 62271-100:2008 (clause 6.106.1).
- Is the rated voltage of the switching device lower than or equal to 17.5 kV?
- No: The motor tests of clause 6.113 do not apply; consult the manufacturer
- Yes: Is the locked-rotor current within the range covered by the standard (100 A to 300 A and up to the current associated with test duty T10)?
- Yes: The 100 A and 300 A type test covers it
- No: Consult the manufacturer of the switching device for that current
What is measured and when it passes
During each test, the power-frequency voltage and current are recorded, along with the phase-to-earth voltage at the terminals of the motor or the substitute circuit, in all three phases. The report evaluates four values at the motor:
- The maximum overvoltage.
- The suppression peak overvoltage, which is the first peak of the transient voltage to earth on the load side.
- The peak phase-to-earth voltage on the load side.
- The maximum peak-to-peak excursion in a re-ignition or pre-ignition.
The test is considered passed if:
- The switching device behaves according to the criteria of IEC 62271-100, clause 6.102.8.
- The subsequent voltage tests meet IEC 62271-100, clause 6.2.11.
- The re-ignitions occur between the arcing contacts.
The standard sets no overvoltage limits, because they are relevant only to the specific application. Phase-to-phase overvoltages can be as significant as phase-to-earth ones. When they are hazardous for an application, the test may include overvoltage limiting devices, provided they are an intrinsic part of the equipment tested.
What this standard does not cover
- Switching of transformers on no load is not covered by IEC 62271-110. Its nonlinear behavior prevents it from being represented correctly with linear components in a laboratory.
- Short-line faults, out-of-phase making and breaking, and capacitive current do not apply to switching devices that switch reactors or motors.
Relation to the switch
IEC 62271-103 assigns a switch a motor breaking current, Imot. Unless otherwise stated, with the motor locked the breaking current is 8 times the rated motor current, and the standard refers to IEC 62271-110 for that duty.
What not to do
- Do not assume a starting current outside the range of the tests is covered without consulting the manufacturer.
- Do not compare only the breaking current of the switching device with that of the motor: ask for the overvoltage results for your application.
- Do not assume the maximum overvoltage of a laboratory test holds for another installation without considering the cable and the configuration.
- Do not forget phase-to-phase overvoltages.
Related topics
For the effect of starting on the network, see large motor starting and voltage dip. Motor protection against a locked rotor is explained in locked-rotor protection 51LR and 48 and the others in induction motor and its protections. The switching device is introduced in medium-voltage vacuum circuit breaker. To limit overvoltages, see the surge arrester.
The special-purpose switch for motors is explained in special-purpose switch, and the utilization categories of the contactor in contactor duty cycles and utilization categories.
The other parts of IEC 62271 are laid out in the standards for primary equipment testing.
When to call us
At Tiza Group we evaluate with you the switching of your medium-voltage motors: circuit breaker or contactor, cable, overvoltages and protections. See our primary engineering design service. Write to us and we will send you a quote.
Related guides
- Special-purpose switch
- MV contactor: duty cycles and utilization categories
- Large motor starting: voltage dip and study
- Locked rotor 51LR and prolonged start 48
- Medium-voltage vacuum circuit breaker: function and tests
References
- IEC 62271-110, High-voltage switchgear and controlgear - Part 110: Inductive load switching, 2012 edition: clauses 1.1, 3.101 to 3.111, 6.1 and 6.113 (Figure 1 and Table 1).
- IEC 62271-103, 2011 edition: clause 4.110.
Frequently asked questions
Why is switching a starting motor more severe than switching a running one?
IEC 62271-110 states that, as far as overvoltages are concerned, opening a motor during starting or with locked rotor is usually the most severe operation. The overvoltages when opening a running motor, which the substitute circuit does not represent, are generally considered less severe.
Which test currents cover the starting current of my motor?
The type test with locked-rotor currents of 100 A and 300 A is considered valid for currents between 100 A and 300 A, and up to the current associated with the short-circuit current of test duty T10 of IEC 62271-100. For starting currents outside that range, consult the manufacturer of the switching device.
Is there a maximum overvoltage limit in motor switching?
The standard gives no overvoltage limits, because they are relevant only to the specific application. It clarifies that phase-to-phase overvoltages can be as important as phase-to-earth ones. The test results serve as a basis for estimating the behavior of the switching device in other motor circuits.