- Motor Thermal Overload Protection Definition: Thermal overload protection is a safety mechanism that prevents motors from overheating by detecting excessive current and stopping the motor.
- Causes of Overheating: Mechanical overload, locked rotor, low voltage, single phasing, and supply imbalance can cause a motor to overheat.
- Thermal Overload Relay: This relay uses a bimetallic strip that heats up and bends when current is too high, breaking the circuit to stop the motor.
- Thermal Limit Curve: This curve shows how long a motor can run at different overload levels without damage, helping to set protection limits.
- RTD Advanced Protection: Resistance Temperature Detectors (RTDs) provide accurate motor protection by monitoring temperature changes and triggering protective actions.
Motor thermal overload protection must allow normal starting current but disconnect an induction motor before sustained heating damages its insulation. The principle of three phase induction motor explains why current is high at start. A balanced three phase power supply applied to the stator windings creates a rotating magnetic field at synchronous speed. Conductive rotor bars shorted by end rings form the cage of a squirrel cage induction motor. The relative motion between that field and the rotor induces current and torque, which links motor current to heating in motor thermal overload protection.
The rotating magnetic flux induces circulating current in the rotor bars. At start, the stationary rotor has the greatest relative speed with respect to the stator field, so induced rotor current and stator current are high.
As the rotor accelerates, the relative speed and rate of flux cutting fall. If the rotor reached synchronous speed, induction and torque would fall to zero. A loaded rotor therefore runs slightly below the rotating magnetic field, leaving enough relative motion to produce torque.
Slip is the difference between synchronous speed and rotor speed, divided by synchronous speed. When load torque increases, the rotor slows relative to the rotating magnetic field. The resulting increase in slip raises induced current and torque until the motor reaches a new operating point. Excess load can push current and heating beyond the motor’s continuous rating.
Full-load slip for an induction motor depends on its design, rating and load. The following curve is an illustrative large boiler-fan start, not a universal motor characteristic.
The chart plots elapsed time on the vertical axis and stator current as a percentage of rated full-load current on the horizontal axis. In this example, standstill current is about 600% of rated current. Current falls as the rotor accelerates, reaching about 500% after 12 seconds at 80% of synchronous speed, then falling rapidly near normal running speed. Actual starting current and acceleration time must come from the motor and driven-load data.
Thermal overloading of an electrical motor occurs when heat is produced faster than the electric motor can dissipate it. Motor thermal overload protection must account for both current magnitude and duration.
Mechanical overload, a jammed load, prolonged starting and a locked rotor can all sustain high current. Low supply voltage can increase current or prevent acceleration when the mechanical load remains. Phase loss and voltage imbalance create unequal phase currents and extra rotor heating. Repeated loss and restoration of voltage can add successive high-current starts before the motor has cooled.
Overheating shortens insulation life and can damage windings. A complete motor thermal overload protection scheme should address these operating conditions:
- Mechanical overload.
- Stalling or locking of the motor shaft.
- Low supply voltage.
- Loss of one supply phase.
- Supply-voltage or current imbalance.
- Repeated loss and restoration of supply voltage.
A basic motor starter uses a contactor coil and a series protection circuit. The schematic below shows the control logic.
Pressing START energises the starter coil through the control transformer. Normally open main contacts 5 then close and apply supply voltage to the motor. Auxiliary contact 4 seals around the momentary START button so the coil remains energised after the button is released. Pressing STOP opens its normally closed contact and drops out the coil. Normally closed trip contacts 1, 2 and 3 are also in series with the coil. When any connected protection relays operate, the coil de-energises, contacts 4 and 5 open, and the motor stops.
The schematic uses an ANSI device 49 thermal element in the overload trip path.
The CTs reproduce the motor phase currents for the relay. In the illustrated electromechanical design, current heats a bimetallic element. Current above the relay setting for long enough bends the element, operates device 49 and opens normally closed contacts 1 and 2. The starter coil then drops out and stops the motor.
The motor’s manufacturer supplies the overload or damage limit used for motor thermal overload protection. A thermal limit curve states how long the motor can tolerate each current level from its known thermal state. The curve below is only an example.
The vertical axis is allowable time in seconds and the horizontal axis is current as a percentage of the stated rating. On this example curve, the limits are about 1,000 seconds at 200%, 100 seconds at 300% and 15 seconds at 600%. Do not apply those values to another motor without its manufacturer data. The upper region represents running overloads; the lower region approaches locked-rotor heating.
The selected relay’s time-current trip curve must remain below the motor thermal limit so it trips before the allowable damage time expires.
The same curve must remain above the verified starting-current trace so a normal start can finish. In the illustrated example, starting current is near 600% for 10 to 12 seconds. A relay that trips sooner at that current would interrupt a normal start. The acceptable coordination region therefore lies between the starting curve and the thermal limit curve. Select the relay trip class and settings from actual motor, starter and load data.
A bimetallic overload relay has an intentional inverse-time delay because its element must heat before it trips. In the example curve, a locked rotor may persist for 25 to 30 seconds before that relay operates. If this exceeds the motor’s locked-rotor withstand time, faster coordinated protection is required.
A high-pickup time-overcurrent element can provide separate stalled-rotor or severe-overcurrent protection. Its setting and delay must coordinate with normal starting current, the motor thermal limit and short-circuit protection. The associated over current relays should operate faster than the thermal element for the high-current region while leaving lower sustained overloads to the thermal model.
Motor thermal protection may therefore use a thermal model plus any phase-loss, imbalance, stall and fault elements required by the application.
Ambient temperature affects an uncompensated bimetallic relay’s heating and cooling. An ambient-compensated or electronic overload relay can reduce that error. Direct temperature sensing with an RTD adds winding or bearing temperature information that current alone cannot measure. Older analogue schemes can place the sensor in a Wheatstone bridge circuit.
In the illustrated bridge, the stator RTD forms one arm. As winding temperature rises, its electrical resistance changes and unbalances the bridge. Relay 49 operates at the set bridge current, opens the starter circuit and stops the motor. Modern numerical relays usually measure RTD resistance directly and can combine it with a thermal model.
