Motors - Thermal protection, Stalling, Negative phase sequence, Shaft currents

Thermal Protection in Motors

Thermal Protection is an important protection in motors. Motors can get heated due to overloading, high ambient temperature, variations in power quality, etc. Thermal overload can result in stator overheating, faulty operation and in some extreme cases even fire. Hence, all motors need to be fitted with protection against thermal overload.

Thermal overload protections can be classified into three types viz. Bimetallic, Magnetic and temperature sensing protection.

Bimetallic Protection
In bimetallic protections, a strip of two metals which are attached to one another is used. The motor current is made to pass through the strips. As current passes through the strips, the strips heat up and expand. Since, the strip is made up of two different metals and these metals have different rates of expansion, the strip bends in one direction. When the temperature of the strip reaches a particular value, it activates a mechanism which trips the motor.  This kind of protection is widely used and is simple in construction.  However, this method is not suitable in applications which require frequent starting and stopping of the motor.

The bimetallic protection gets reset faster than the motor cooling temperature and it may thus permit the motor to be started again when the motor has not sufficiently cooled from the thermal overload.

Magnetic Protection
This consists of a magnetic element whose field strength is a function of the motor current. When the motor current exceeds a preset value, the electromagnet inside the relay operates and trips the motor. The downside of this kind of protection is that it does not take into account ambient operating conditions such as temperature and ventilation which play an important role in the temperature rise of motors.

Temperature based thermal overload protection
This is method of protection that is relatively new. This method involves actually measuring the temperature of the motor and those of the winding hotspots using a temperature sensor such as the RTD (Resistance Temperature Detector). This method uses direct temperature sensing and is the most reliable and accurate, though, it is more expensive.

Stalling in Induction Motors, its effects and prevention

Stalling of the motor refers to a condition, where the motor is unable to rotate. This condition can be caused either due to any obstruction in the load or due to any problem with the motor such as bearing seizure, etc.

This condition is also known as locked rotor.

When the speed of the rotor decreases to a very low value or stops completely due to stalling, the slip of the induction motor increases. This causes higher voltage and consequently higher current to be induced in the rotor windings.

The stator currents also increase. The equivalent of the motor stalled condition is that of a transformer whose secondary is short circuited.

The high current drawn will cause damage to the windings and cause the rotor to heat up.

Stall protection devices work by monitoring the motor current and the speed. If the motor draws higher current at a preset low speed, the relay is activated.

Negative Phase sequence in Induction motors

     Negative phase sequence in induction motors is caused due to unbalanced voltages in the supply voltage applied on the stator terminals or unbalanced windings.
     Negative phase sequence components create a rotating magnetic field in the stator which moves in the opposite direction. This causes a decrease in the torque developed by the motor. The motor will thus have to draw a higher current for the same mechanical load.

     The rotating magnetic field which rotates in the opposite direction induces voltages in the rotor. These voltages have a frequency that is double the system frequency. Since the frequency of this rotor voltage is higher, it flows on the surface of the rotor due to the skin effect and causes surface heating which can lead to motor damage.

     Negative phase sequence relays can identify negative phase sequence condition and trip the machine. Negative phase sequence relays work by using a special filter which filters out the positive sequence and the zero sequence components. The filtered negative phase sequence voltage alone is measured. When the measured negative phase sequence voltage exceeds the set value, the relay trips the motor.

Shaft Currents in Motors and Generators

The magnetic field in a motor or a generator is ideally conducted along paths that are symmetrical. However, sometimes the magnetic fields within an electric machine such as a motor or a generator is asymmetrical. This asymmetry can be caused usually by variation in the quality of the steel used in the motor construction or in some cases to shaft eccentricity.

These asymmetrical magnetic fields which are varying over time can induce currents in the shaft of the motor or generator. These currents which are induced in the shaft tend to flow from one end of the shaft to the other through the bearings and then through the earth.

When these currents flow through the bearings, the tend to cause arcing and consequent pitting in the bearings. This can lead to failure of the bearings.

Shaft currents can be prevented by insulating one of the bearings. A Teflon layer is usually placed between the shell of the bearing and the bearing housing. This ensures that the shaft voltage induced does not have a return path. This prevents shaft currents from flowing.