Electrical Protection of 3 phase Motors: Types and Protection Schemes

To avoid unexpected breakdowns, costly repairs and subsequent losses due to motor downtime, it is important that the electric motor is fitted with some sort of protective device. Motor protection can be divided into the following 3 levels: (a) External protection against short circuit (b) External protection against overload (c) Built-in motor protection.

The basic electric motor protection scheme is shown below:

External protection against short circuit in the whole installation - External protection device is normally done using different types of fuses or short circuit relays. This kind of protection device is compulsory and legal and placed under safety regulations. 

External protection against overload of specific equipment - This type of protection is given to the electric motor to avoid overload and thereby prevent damage and breakdown of the motor. This type of protection reacts on current. Note that overload of  electric motors are a common cause of failure of  motors.

Built-in motor protection - This is usually done with thermal overload protection to avoid damage and breakdown of motor. The built-in protector always requires an external circuit breaker while some built-in motor protection types even require an overload relay. There are two types commonly used namely: (i) Thermostats (ii) Thermistors or Positive Temperature Coefficient Sensors (PTC).

Thermostats - Thermostats are in-built protection devices that are used in protecting an electric motor. With three-phase motors, thermostats are considered unstable protection against stall or other rapidly changing temperature conditions. In single phase motors thermostats do protect against locked-rotor conditions. Please note, Thermal switches cannot protect against locked- rotor conditions. The electric motor wiring diagram below illustrates internal motor protection by a thermal switch like the thermostat:

Thermistors Or Positive Temperature Coefficient Sensors (PTC)

Thermistors are built into the motor windings and protect the motor against locked rotor conditions, continuous overload and high ambient temperature. The symbol for a thermistor is shown below:

Thermal protection is then achieved by monitoring the temperature of the motor windings with PTC sensors. If the windings exceed the rated trip temperature, the sensor undergoes a rapid change in resistance relative to the change in temperature.

As a result of this change, the internal relays de-energize the control coil of the external line break contactor. As the motor cools and an acceptable motor winding temperature has been restored, the sensor resistance decreases to the reset level. At this point, the module resets itself automatically, unless it was set up for manual reset.

Note that the thermistor has to be connected to a control circuit, which can convert the resistance signal, which again has to disconnect the motor. They are commonly used in the internal windings of three-phase motors for thermal protection. The wiring diagram below illustrates the implementation of thermistor motor thermal protection in a three-phase motor:

PTC Sensors Lead identification

The colours on the PTC leads help determine what trip temperature the PTC sensor is made to handle. The table below shows lead colour identification for PTC sensors used in the internal windings of 3-phase electric motors for thermal protection:

Nominal Response temperature (degree C)	145	150	155	160	165	170
Coloured Leads	White	Black	Blue	Blue	Blue	White
	Black	Black	Black	Red		Green

PTC sensors come with trip temperatures ranging from 90°C to 180°C with an interval of 5 degrees

Internal Motor Protection Standards

The degree of protection that an internal protection device provides is classified in the IEC 60034-11 standard. In this standard, there is a thermal protection required for all electric motors. On an electric motor nameplate, there is a TP designation which stands for Thermal Protection. The IEC 60034-11 standard recommends that motors should have current as well as temperature-dependent protection.

Thermal Protection (TP) Designation for Electric Motors

TP is the abbreviation for thermal protection. Different types of thermal protection exist for electric motors and are identified by a TP-code (TPxxx) which indicates:

(i) The type of thermal overload for which the thermal protection is designed (1 digit)

(ii) The numbers of levels and type of action (2 digit)

(iii) The category of the built-in thermal protection (3 digit)

The table below illustrates different levels of thermal protection applied to motors:

Symbol	Technical Overload (1 digit)	Number of levels and function area (2 digits)	Category (3 digits)
TP 111	Only slow, constant overload	1 Level at cut off	1
TP 112			2
TP 121		2 Levels at emergency signal and cut off	1
TP 122			2
TP 211	Slow and fast, applies to constant overload and blocked condition	1 Level at cut off	1
TP 212			2
TP 221		2 Levels at emergency signal and cut off	1
TP 222			2
TP 311	Only fast, applies to blocked condition	1 Level at cut off	1
TP 312			2

Column 3 indicates the permissible temperature level when the motor is exposed to thermal overload. Category 2 allows higher temperatures than category 1 does.

The most common TP designation especially for pump motors are: TP 111 and TP 211. TP 111 is protection against slow overload while TP 211 is protection against rapid and slow overload conditions.

Common Fault Conditions Requiring Electric Motor Protection

Listed in the table below are the common faults that necessitates electric motor protection:

	Common Fault Conditions For Electric Motor Protection
1	Problems with the power supply quality: – Overvoltage – Undervoltage – Imbalanced voltages/currents – Frequency variation
2	Installation, Supply and Motor Failures
3	Slowly developing temperature rise: – Insufficient cooling – High ambient temperature – High altitude operation – High liquid temperature – Too high viscosity of the pumping liquid – Frequent starts – Too big load inertia (not common for pumps)
4	Quickly developing temperature rises: – Locked rotor – Phase breakage

Advanced External Motor Protection Systems

Due to the increasing use of electric motors in various aspects of industry, there has been significant advancement in external motor protection against damage of all sorts. More advanced external motor protection systems can protect against overvoltage, phase imbalance, too many starts/stops, vibrations, PT100 temperature monitoring of stator and bearings, insulation resistance and monitor ambient temperature.

These external motor protection relays are designed to protect three-phase motors against conditions, which can damage them in the short or the long run. In addition to motor protection, the external protection relay has features that can protect the motor in different situations:

(i)  Give an alarm before damage results from a process malfunction

(ii) Diagnose problems after a fault

(ii) Allow verification of correct relay operation during routine maintenance

(iii) Monitor bearings for temperature and vibration

Most advanced external motor protection systems can protect the electric motor against all or some of the following fault conditions when in operation:

(i)  Overload

(ii) Locked rotor

(iii) Stall / mechanical jam

(iv) Repeated starts

(v) Open phase

(vi) Ground fault

(vii) Overtemperature (using PT100 or thermistors signal from the motor)

(viii) Undercurrent

(ix) Overload warning