Protection of electric motors from overload and short circuit

Protecting the electric motor from overload and short circuit currents (SC) is a prerequisite for the reliable operation of any asynchronous drive. Without properly selected thermal relays, circuit breakers and phase control devices, even a new motor burns out in minutes at the first failure in the network. In this article, Elektromotors engineers analyze the types of protection, formulas for calculating settings and real schemes that we implement at Tashkent enterprises.

What emergency modes threaten the engine

Before choosing protection, you need to understand what exactly we are protecting the winding, bearings and magnetic circuit from. Basic emergency modes of an asynchronous motor:

• Overcurrent - prolonged excess of the rated current by 15–50% due to jamming of the mechanism, wear of bearings or excess load torque.
• Short circuit (SC) - interturn, interphase or to the body. Currents reach 8–15 nominal values, winding destruction in a fraction of a second.
• Phase loss - the motor continues to rotate in two phases, the current in the remaining phases increases by 1.73 times, the winding overheats in 1–3 minutes.
• Phase imbalanceand voltage drop are typical for Tashkent industrial zones with outdated networks.
• Rotor jamming - the starting current (5–7 In) flows constantly, class B insulation burns out in 20–40 seconds.

Each mode has its own type of protective device. There is no universal protection - you need a complex.

Thermal relay RTL and PTT: overload protection

Thermal relay is the basis of protection against long-term overloads. Operating principle: the load current heats the bimetallic plate, which, when the set point is reached, opens the contact of the magnetic starter coil. The engine turns off.

The most common series in the post-Soviet space are RTL (Russia, KEAZ) and RTT (produced in Tashkent/Uzbekistan). Schneider LRD, Siemens 3RU, ABB TA are also used.

The thermal relay setting is selected according to the formula:

I_set = (1.1 - 1.15) × I_nom.dv

For example, for a 7.5 kW engine, 380 V, I_nom = 15 A, the relay setting is 16.5–17.3 A. We take the RTL-1022 relay with a range of 13–19 A and set the regulator to 17 A.

Comparison of thermal relays

Tripping class (10A, 10, 20, 30) shows the tripping time at a current of 7.2 × I_set. Class 10A - standard starting (up to 10 s), class 30 - heavy starting of compressors and crane motors. For crane motors class 20–30 is required, otherwise the relay will falsely operate on every lift.

VA-88 circuit breakers and fuses: short circuit protection

The thermal relay does not protect against short circuits - it is too slow. To disconnect short circuits, circuit breakers or fuses are used.

VA-88 assault rifle (KEAZ, IEK) is the most popular on the Tashkent market. Has thermal and electromagnetic releases. Thermal setting - 1.15 × I_nom, electromagnetic - 10–14 × I_nom (characteristic D for motors).

Example: 22 kW motor, I_nom = 42 A, starting current 7 × 42 = 294 A. We choose VA-88-32 with a rating of 50 A, characteristic D. The electromagnetic release will operate at ~500 A - the starting current will not cause a false shutdown, and the short-circuit current (thousands of amperes) will break the circuit in 10–20 ms.

Fuses PN-2, NPN, gG/aM are a simpler and cheaper solution, but require replacement after tripping. For the motor load, select the aM characteristic (short circuit only) together with a thermal relay - this is a classic protection scheme according to the PUE.

If you are planning to repair of industrial electric motors and at the same time modernize the control cabinet, check the selectivity: the upper circuit breaker must be 2 steps higher than the lower one, otherwise the entire section will be de-energized during a short circuit.

Protection against phase loss and voltage imbalance

Phase failure is the most common cause of engine combustion in Tashkent. Networks with overhead lines, oxidized terminals, burnt-out zero in a transformer substation - all this leads to operation on two phases.

Protection methods:

1. Differential thermal relays (RTL, LRD) - have an accelerated response mechanism when currents in phases are imbalanced. They work well, but do not catch a break at idle.
2. Phase control relay - RNPP-311, EKF RKF, ARIES FAZ-A - a separate module, measures the voltage of all three phases, turns off the starter coil in the event of a break, misalignment of more than 20%, or incorrect phase rotation. Cost from 200,000 to 600,000 soums, payback - one accident prevented.
3. Electronic starters and soft starterswith built-in protection - Schneider Altistart, Siemens Sirius, Hyundai Soft Starter. More expensive, but include a full package of protection.

We strongly recommend installing phase control relays on motors from 5.5 kW and above, especially for pumping stations and ventilation systems where phase failure goes unnoticed by the operator.

UZO or difavtomat: what to choose for the engine

The RCD (residual current device) and differential circuit breaker protect people from electric shock and equipment from leaks to the housing through damaged insulation. For industrial engines this is critical - breakdown of insulation on the housing leads to the appearance of a dangerous potential on the frame.

For motors with power over 5.5 kW, choose Type B RCD (electronic, reacts to direct leakage current, important when working through a frequency converter) with a setting of 100–300 mA. For drives up to 5.5 kW, type A is acceptable with a setting of 30 mA. Read more about leakage currents and insulation aging in the materials on electric motor diagnostics.

RTD and PTC temperature sensors: direct winding control

The thermal relay measures the current indirectly - it “does not see” the real temperature of the winding. If the motor is installed in a hot shop or is poorly cooled due to a clogged fan, the winding overheats at rated current. The relay will not work - the engine will burn out.

The solution is to embed sensors directly into the frontal parts of the winding at the stage of production or overhaul:

• PTC thermistors (KTU, KTY) are posistors with a sharp increase in resistance at a critical temperature (130, 150, 180 °C). Connects to protection relays UBZ-302, A-380, ARIES MKON. Cheap, reliable, a must for H class engines.
• RTD (Pt100) - platinum thermal resistances, give a linear dependence R(T). Used with multi-channel monitors in automated process control systems. They cost more, but allow you to see the temperature in real time and predict overheating.
• Thermocouples type K, J - for bearing shields, lubrication and wear control.

During major rewinding of the electric motor winding at our production in Tashkent, upon request, we install PTC sensors free of charge - this extends the service life of the machine by 2-3 times.

Typical integrated protection scheme

For an asynchronous motor 7.5–55 kW we recommend the following protection structure in the control cabinet:

1. Introductory machine VA-88 with characteristic D, setting 1.15 × I_nom.
2. Phase control relay RNPP-311 at the input.
3. Magnetic starter PML or KMI for motor current × 1.25.
4. Thermal relay RTL or LRD with differential protection against phase loss.
5. Type B 100 mA RCD for protection against leakage to the frame.
6. PTC relay (UBZ-302) with output to the starter coil.

This scheme protects against all described emergency modes and pays for itself in 1–2 combustions prevented.

Conclusion and assistance from specialists

Properly selected protection extends the engine service life by 3–5 times and pays for itself instantly. Do not skimp on thermal relays and phase control relays - the cost of rewinding the stator after a burned-out winding is tens of times higher than the cost of protective automation.

Do you need help with the selection of protection, cabinet repair or engine overhaul? Contact Elektromotors engineers in Tashkent - we visit the site, conduct an audit of the existing protection and offer the optimal solution to suit your budget. Go to contacts.