Smart Grid and Renewable Energy, 2012, 3, 62-66 Published Online February 2012 (
Ferrofluid Actuated Thermal Overload Relay
Badrinarayanan Rajagopalan, Mugundhan Hayagrivan, Mahesh Praveenkumar
Department of Electrical and Electronics Engineering, Panimalar Engineering College, Anna University, Chennai, India.
Received September 26th, 2011; revised October 26th, 2011; accepted November 4th, 2011
Thermal overload relays are economic electromechanical protection devices which offers reliable protection for electric
motors in the event of overload or phase failure. Presently there are two types of overload relays which depend on the
temperature characteristics of metals to provide protection by tr ipping the circuit. These relays lack accuracy as they do
not activate the trip circuit at any exact specified temperature. In this paper we introduce a new form of thermal over-
load relay actuated by ferrofluid. The ferrofluid has a very accurate transition temperature known as curie temperature.
It acts as a ferromagnetic material below the curie temperature and loses the property of ferromagnetism above this
temperature. By using this property of the fluid we propose an alternative method for more accurate operation under
overload condition. This relay finds application in the protection system of electrical machines. Thus, in this paper we
present a novel and simple technique for protection against thermal overloading using ferrofluid.
Keywords: Ferrofluid; Overload Relay; Thermal Protection; Motor Protection
1. Introduction
A relay is an electrically operated switch which is used to
control a circuit by a low-power signal with complete
electrical isolation between control and controlled cir-
cuits. Thermal overload relays are designed to cut the
power by opening the circuit when the motor draws too
much current from the supply for an extended time caus-
ing temperature rise. Presently there are two types of
thermal overload relays (Figure 1) in which one type has
a bi-metallic strip [1] which deflects as the temperature
rises until it causes the device to trip and open the circu it.
A second type of thermal overload relay uses a eutectic
alloy to retain a spring-loaded contact. When too much
current passes through the heating element for a long
time, the alloy melts and the spring releases the contact,
opening the circuit.
Ferrofluid are smart fluids which has the fluid proper-
ties of a liquid and the magnetic properties of a solid. We
use the thermal property of this fluid in the construction
of thermal overload relay. The temperature rise in the
Figure 1. Present thermal overload relays.
machine will cause the relay to activate the trip circuit
thereby opening the circuit by circuit br eaker.
The main motive of our technique is to isolate the
machine and to protect them from damage during the
event of thermal overload. Also this relay has a better
accuracy and shuts the power down at a specific tem-
2. Components
Some of the important components used in the construc-
tion of the relay are listed below.
2.1. Ferrofluid
Ferrofluid was originally develo ped by NASA in 1960’s.
They are highly viscous fluids made up of nanometer
sized pieces of magnetite (Fe3O4) suspended in a liquid.
These fluids are a special category of nano-materials
which exhibit simultaneously liquid and super-paramag-
netic properties. The possibility o f magnetic control over
their properties triggered many application orientated
researches. It behaves like a ferromagnetic material in the
presence of magnetic field but loses the property (be-
haves like paramagnetic material) in the ab sence of mag-
netic field. Also it chang es its density in prop ortion to the
strength of the magnetic field that is applied to it. How-
ever ferrofluid keeps its fluidity even if subjected to
strong magnetic fields [2]. The thermal stability of this
fluid depends on the type of surfactant used [3]. The
Copyright © 2012 SciRes. SGRE
Ferrofluid Actuated Thermal Overload Relay 63
property of ferrofluids losing their magnetic properties at
a specific temperature (Curie temperature) is used in the
construction of this relay. At present, ferrofluid technol-
ogy is well established and capable of solving variety of
technical pr oblems.
2.2. Magnetic Field Generator
The ferrofluid being a magnetically active fluid, it is ne-
cessary to have a magnetic field generator to produce the
required magnetic field. Relays are generally classi- fied
depending on the type of magnet being used to oper- ate
it. For this purpose either a permanent magnet or an elec-
tromagnet can be used. Relays using permanent mag-
nets are known as polarized relays and those using an
electromagnet are known as non polarized relays. The
property of the ferrofluid greatly depends on the direc-
tion of the magnetic flux hence it is necessary to choose
the perfect source of magnetic field based on the applica-
2.3. Heater Equipment
Electrical motors require a heating element to activate
the relay. In these machines the thermal overload relay is
used to sense the overload current. The temperature rise
of the heater is proportional to the amount of current
flowing through it. Hence the selection of heating element
is an important factor as it decides the current at which
the relay should activate the trip circuit. Selection is
based on the value of current flow during the full load
operation of the machine. The ambient temperature at the
motor and relay is the same in most applications. In
conditions where the temperature is variable, a sensor
based heater element is used.
3. Construction
It is very important to choose an adequate protective de-
vice for the safety of the machines during operation and
for their durability. The efficiency of protection methods
varies according to the application. The construction of
relay plays a major part in the protection of any electrical
machine. Motor terminal wire is connected to the heating
element which is wound on the upper part of the con-
tainer. The same heating element used in the current
techniques of overload can be used in this method of
thermal overload relay. A tube like cylindrical container
is used to hold the ferroflu id. Magnetic field generator is
placed concentrically around the container to hold the
ferrofluid in place. The power supply to the generator is
provided by the input terminal current of the motor.
Magnetic field generator is setup in such a way that it can
be moved along the vertical axis with the help of a step-
per motor. This arrangement is provided to ease the
process of resetting the relay once the problem is recti-
Ferrofluid is enclosed in the upper part of the container
so that it possesses a specific potential energy. Lower
portion of the container consists of a movable plate
connected to a spring mechanism. The spring arrange-
ment is made to provide the necessary restraining force
to keep the mova ble plate in position . In this relay a semi
automatic reset can be used where the relay is reset by a
controller based stepper motor. This method of reset is
used because the ferrofluid can be activated only by the
magnetic field generator which is energized by the sup-
ply current. A typical arrangement of the components for
the construction of th is relay is shown in Figure 2.
4. Operation
Thermal overload units are widely used on both the frac-
tional and integral horsepower manual starters for pro-
tection of motors from sustained electrical over-currents
[4]. This over-current could result from overloading of
the driven machin e or from excessively low line voltage.
An electric motor does not know en oug h to qu it when th e
load gets too much for it. It keeps going until it b urns out
[5]. If a motor is subjected over a period of time to inter-
nal or external heat levels that are high enough to destroy
the insulation on the motor win dings, it will fail and burn
out. A solution to this problem is to install some device
to protect a motor against expected overheating [6].
Hence, we use a thermal overload relay for this purpose
whose current sensing properties very similar to the heat-
ing curve of the motor. The operation of the protective
device is ideal if the motor is allowed to carry small,
short, and harmless overloads but is quickly disconnected
from the line when an overload has persisted for a long
time. The motor can do no more work than the overload
relay permits and provides protection against the over-
loads above full-load current when the motor is running.
Continuous over-current through the heater unit raises
Figure 2. Construction of ferrofluid relay.
Copyright © 2012 SciRes. SGRE
Ferrofluid Actuated Thermal Overload Relay
Copyright © 2012 SciRes. SGRE
the temperature of the heating element. Many types of
overload relay heater units are available so that the
proper one can be selected on the basis of the actual full-
load current rating of the motor.
It is necessary to achieve an inverse time relationship
with the relay operation so that normal motor starting
currents and momentary overloads will not cause th ermal
relays to trip. The operating time is approximately in-
versely proportional to the magnitude of the actuating
quantity. At values of current less than pickup, the relay
never operates. At higher values, the time of operation of
the relay decreases steadily with the increase of current.
The typical inverse time characteristic of the thermal
overload rel a y i s show n i n Fi gu re 3.
In this type of relay the inverse-time delay is achieved
by associating it with a heating element. The line current
is given to the relay through the relay input block. The
relay input block provides the supply to both the heating
element and the magnetic field generator. Generally the
heating element is connected in such a way that it is in
series with the motor starter. The magnetic field genera-
tor will get a continuous supply from the main supply.
Here a permanent magnet can also be used instead to
avoid continuous power supply. The magnetic field cre-
ated by the magnetic field generator holds the ferrofluid
in active region.
The base plate is now at its normal position where the
restraining force of the spring is greater than the weight
of the plate (Operating force < Restraining force). The
exact point at which the ferrofluid loses its magnetism is
decided by the selection of ferrofluid (based on the curie
temperature value), Magnetic field strength and the heat-
ing characteristics of the heating element. Considering a
constant magnetic field strength the control is purely
based on the heating element characteristic. When nor-
mal current flows through the motor, the temperature of
the heating element stays below the curie temperature of
the ferrofluid. As the amount of current increases and
flows for a given time period, the heating element reaches
the temperature value equal to the curie temperature. At
this temperature the ferrofluid loses its magnetism and
drops down to the base plate. The dynamics of fluid flow
in a tube are dependent on a dimensionless group called
the Reynolds Number [2].
UdInertial force
Reynolds NumberRevViscous force
 
Figure 3. Inverse time characteristics during overload.
Ferrofluid Actuated Thermal Overload Relay 65
The inertial force here is provided by the action of
gravity when the magnetic property is lost. The sudden
increase in weight causes the base plate to move down-
wards due to gravity. At this condition the weight of the
base plate is more than the restraining force of the spring
(Operating force > Restraining force). Hence, the base
plate pushes the movable contact downwards to short the
fixed contact following which the trip circuit is triggered.
The trip circuit eventually disconnects the motor from
the supply by opening the normally closed (NC) switches.
The operation of this relay can be clearly understood
from Figure 4 which shows the complete working. The
trip circuit is operated only during the fault condition by
closing the contacts. Once the fault is cleared, the supp ly
is given to the relay and the magnetic field generator is
used to bring back the ferrofluid to its original position.
This makes the movable contact to the non-operating
In this relay a semi automated reset mechanism is used
so that once the ferrofluid is lowered it can be moved to
its original position by the operation of stepper motor
operated magnetic field generator. Automatic reset over-
load relays are not normally recommended because of
the possible danger to personnel. The unexpected re-
starting of the machine may find the operator or electri-
cian in a hazardous situation as attempts are made to find
out why the machine has stopped.
5. Advantages
· Relays improve safety by providing complete electri-
cal isolation from high current and voltages during
fault in the system. It comes in all shapes and sizes
for different applications and they have various switch
contact configurations. As a result it can be used to
switch many contacts at once.
· Thermal overload relays provide flexible protection to
the motor from overheating and these relays have a
Figure 4. Operation of ferrofluid relay.
tendency to resist shock and vibration of normal ap-
plication. It also provide features like ambient tem-
perature compensation (Ambient temperature com-
pensation is an important factor as the temperature of
the overload enclosure is subjected to severe fluctua-
tion when the enclosure is expected to run either in
hotter or colder temperature surrounding).
· This relay can also be used in transformer protection
[7]. It is necessary to protect the transformer from
overheating. The overheating of transformer will da-
mage the transformer coil and generally the trans-
former temperature should be limited below 110˚C for
proper working. The relay can also be used in protect-
tion of generator protection. In certain cases the field
winding may get overheated due to high current pass-
ing through it. Hence it is very important to protect
the field winding of the generator from overheating.
· This relay has an improved accuracy as it trips the
circuit at a specific temperature. The other types of
overload relays are purely mechanical and dependent
on the temperature characteristics of the metal being
used. Here the ferrofluid relay operation has a very
sharp operating characteristic as it breaks the contact
when the temperature attains the curie temperature.
· It can withstand repeated trip and reset cycles without
need of replacement as the property of the ferrofluid
is completely reversible. Thus it is more advantage-
ous than using a fuse which need frequent replace-
ment after its operation.
· This ferrofluid relay also finds application in nuclear
reactor. It is of high importance to limit the nuclear
reactor to a specific temperature above which it may
become unstable. Hence in that case this relay can be
employed to slow down or shut down the operation
when the temperature crosses the safety limit.
6. Conclusions
In this paper a new type of thermal overload relay is re-
commended which provides an accurate tripping of the
circuit preventing the overheating of the motor. This relay
is based on the property of ferrofluid which behaves like
a ferromagnetic material in the presence of magnetic field
but loses the property (behaves like paramagnetic mate-
rial) in the absence of magnetic field. Another property
of the ferrofluid used in the relay operation is that the
fluid loses its magnetic property once the temperature
crosses the critical temperature known as curie tempera-
When the current flowing thro ugh the motor is nor mal,
the temperature of the heating element stays below the
curie temperature of the ferrofluid thereby keeping the
ferrofluid unaffected. As the amount of current flow in-
creases and prolongs for a given time period, the heating
element reaches the temperature value equal to the Curie
Copyright © 2012 SciRes. SGRE
Ferrofluid Actuated Thermal Overload Relay
temperature. At this temperature the ferrofluid loses its
magnetism and drops down. The ferrofluid eventually
falls on the base plate which is the operating force of the
relay. This force will be greater than the restraining force
of the spring which makes the circuit to trip.
Thus this new type of thermal overload relay makes
tripping of circuit during overheating of the motor more
accurate. It finds application in various industries and re-
search facilities where accuracy is an important factor.
Moreover in this paper the use of such smart fluid has
been exposed to find its application in the electrical do-
[1] “Motor Thermal Protection,” Danaher Motion, Document
Number: A-RT-000-18 Rev 3.
[2] T. A. Franklin, “Ferrofluid Flow Phenomena,” Massa-
chusetts Institute of Technology, Cambridge, 2003.
[3] C. Scherer and A. M. F. Neto, “Ferrofluids: Properties
and Applications,” Brazillian Journal of Physics, Vol. 35,
No. 3, 2005. doi:10.1590/S0103-97332005000400018
[4] S. L. Herman, “Electrical Motor Control,” 9th Edition,
Cengage Learning, Delmar, 2007.
[5] C. Bussmann, “Selecting Protective Devices Handbook,”
Motor Circuit Branch Circuit Protection, 2005, p. 132.
[6] Desensitizing Electric Motor Controls, Pacific Gas and
Electric Compa ny, August 2004.
[7] “Transformer Protection—The Importance of Effective
Transformer and Control Transformer Protection,” Little-
fuse POWR-GARD® Products, Littelfuse, 2010.
Copyright © 2012 SciRes. SGRE