What are the main functions of control
gear?
1. To provide a means of starting and
stopping the motor and, at the same time, of limiting the starting current if
required.
2. To give adequate protection to the
motor under all conditions.
3. To allow speed changing when
required.
4. To provide means of braking the motor
when required.
5. To reverse the direction of rotation
when required.
Protection of the motor must be automatic, but the other operations may
be arranged to be under the control of an operator, or may be partly or
fully-automatic.
What devices are required to give
adequate protection to the motor?
1. Under-voltage release to prevent
automatic restarting after a stoppage due to a drop in voltage or failure of
the supply, where unexpected restarting of the motor might cause injury to an
operator.
2. Overload relays for protection
against excessive current in the motor windings – e.g. in the event of overload
or failure of the motor.
3. Earth fault.
4. Single phase protection.
What provision must be made for
short-circuit conditions in motor circuits?
Since overload relays are not designed to operate and clear the circuit
in the event of a short-circuit. Circuit-breaker or fuse protection of sufficient
breaking capacity to deal with any possible short-circuit that may occur must
be provided.
What are the usual forms of overload
relay in motor-control gear?
In small contactor starters, generally thermal relays, either of the
‘solder pot’ or bimetal type. With large contactors or oil switches, magnetic
relays of the solenoid type with dashpots. Either type of overload relay may be
used within intermediate sizes.
How do thermal relays work?
The bimetallic thermal relay consists of a small bimetallic strip that
is heated by an element connected in series with the supply. When the current
rises above a preset value, the movement of the strip releases a catch which
opens the trip contacts.
In recent years more modern electronic relays are used which simulate
the thermal overload. Many of these relays also incorporate a memory, i.e.
simulates the temperature rise / cooling curve of the winding.
How does the magnetic overload relay
operate?
A solenoid connected in series with the supply contains a plunger whose
movement is damped by a dashpot. When the safe current is exceeded, the
solenoid pulls the plunger up – disconnecting the supply. The damping provided
by the dashpot prevents unwarranted tripping on short-time overloads.
How many overload relays are required in
the control gear?
On three-phase supplies where the neutral point of the system is
connected to earth, as is usually the case, three overload relays (one in each
line) are necessary for complete protection.
For 2-phase 3-wire and 4-wire supplies, two overload relays are
required, one in each phase line, none being connected in any neutral or earth
conductor.
With single-phase motors one overload relay in any conductor except an
earthed conductor or neutral.
What happens when one of the three lines
supplying a three-phase induction motor becomes open-circuited?
The motor, if already running, will continue to run as a single-phase
motor on the remaining single-phase supply. The condition is called
single-phasing. If the motor is loaded to more than about 30 per cent of full
load, the currents in the motor windings tend to become excessive and
overheating occurs.
With one line broken, the motor will not start up and, due to the heavy
standstill current, burn-out is likely unless the motor is quickly
disconnected.
What currents flow in a single-phasing
delta-connected motor?
Assuming that supply line L1 is open circuited as shown, typical line and phase currents, given as percentages of normal full-load three-phase current, at various loads will be:-
Thus, phase W connected across the two operative lines carries nearly
three times normal current under single-phasing conditions at full load, while
phases U and V, which are in series, carry more than full-load current.
What currents flow in a single-phasing
star-connected motor?
Assuming that line L1 is open-circuited as shown, the current flowing at full load in lines L2 and L3 and through the two phases in series will be of the order of 250 percent of normal full-load current, 155 per cent on 3/4-load and 98 per cent on 1/2-load.
Will
normal overload relays trip on single-phasing?
If correctly set, the normal overloads will trip when the motor is fully
loaded due to the rise in current passing through the closed supply lines. With
a delta-connected motor partially loaded, the rise in line current may not be
sufficient to operate the overload trip and one phase may became excessively
overheated.
What special protection can be provided
against single-phasing?
One method is to incorporate a combined overload and single-phase relay
in the control gear. A typical relay of this type includes three overload
relays with trip contacts so arranged that it will trip if the displacement of
one overload element differs from that of the others.
This type of relay will operate if single-phasing occurs at or near full
load with the same time delay as on overload, but at light loads, the time
delay for single-phase protection is longer. Another device is a phase-failure
relay in the control gear. Its principle is based on the fact that the currents
in the supply lines or the voltages between them at the motor terminals are
unbalanced when the motor is single-phasing.
What are the alternatives to the use of
overload releases?
Direct protection against overheating or burning-out of motor windings
may be built into the motor. Built-in protectors may take the form of
thermostats or thermistors embedded in the end windings of the stator while the
motor is under construction. These devices are sensitive to the winding
temperatures and are arranged in a suitable circuit so as to cause the motor to
be switched off if the windings heat up excessively.
How are built in thermal overload
protector arranged?
On smaller motors LV mush winding motors, these detectors are embedded
in the overhang of the winding. On the medium voltage motors these are placed
in between bottom and top coils in the slot portion of the core.
How do built-in thermal overload
protectors work?
Thermistors are very small semiconductor devices whose resistance
changes rapidly with temperature. Three thermistors are inserted in the
end-windings of the stator, one in each phase, and are connected in series. The
two thermistor terminals at the motor are connected to an electronic-amplifier-control
unit in the starter, through which the tripping circuit of the starter is
operated. The response of the thermistors to temperature change is extremely
rapid, allowing this type of protection to be effective under all motor
overload conditions.
Resistance temperature devices (RTD)
This is a resistance which increases linearly with temperature rise. The
most commonly used in motors is embedded in an epoxy glass type wedge which can
be inserted between the upper and lower coils. The resistance is measured with
an electronic amplifier control unit which is converted to temperature.
This unit has adjustable settings to allow for alarm and trip with
contacts which are then used in the motor starter circuit.
Thermocouples
A thermocouple is two dissimilar metals which are joined together and
with a change in temperature, creates a voltaic action. This gives out a
milli-volt signal which is then measured with an electronic amplifier control
unit converting the measurement to temperature.
When is direct-on-line starting used for
three-phase squirrel-cage motors?
It is usual for small LV machines; for larger motors it is often
necessary to use other methods of starting in order to ovoid excessive starting
currents. HV motors are usually DOL started. (since amps are low)
What are the connections for direct-on-line
starters?
The scheme of connections is merely three line leads in and three motor leads out. Direct-on-line contactor starters are designed round the basic circuit shown. An isolating switch may be incorporated in the starter. If reversing is required, two contactors one for each rotation, are required and are interlocked so that only one can close at a time.
A hand-operated oil switch with under-voltage trip coil may be used with
larger motors.
What methods are employed to reduce the
starting current of squirrel-cage motors?
Where the starting conditions are light, the starting current can be
lessened by some method of reducing the stator voltage when switching on. There
are four ways of starting on reduced voltage:
1. Primary-resistance starting –
introducing resistance between the supply and the stator windings.
2. Primary-reactor starting –
introducing a reactor in series with the stator windings, usually connected in
the star point.
3. Star-delta starting – connecting the
stator windings in star for starting and in delta for running.
4. Auto-transformer starting – supplying
the stator windings through tappings on an auto-transformer.
When
is primary-resistance starting employed?
Generally only for small motors on light-starting duty. The method is
easily adjustable to suit the load and gives a smooth breakaway against low
torque. If the resistance is adjustable, as in a faceplate starter, starting
can be very smooth and this is useful for motors that must be started without
any shock that might cause injury to the material being handled by the driven
machine.
When is the primary reactor method of
starting employed?
Mainly for high-tension motors on very light-starting load where a
fairly heavy starting current can be permitted e.g. boiler-feed pumps in a
large power station.
When is the star-delta starter used?
When the starting current has to be reduced and starting current and
torque values one-third of those obtained with direct-on-line starting are
suitable. It is necessary that the motor be designed to operate with the
primary winding connected in delta, but with six terminals brought out to allow
for connection in star during starting.
What
are the connections for a star-delta starter?
Motors arranged for star-delta starting have six terminals – the two
ends of each phase winding being brought out to terminals marked U1, V1, W1 and
U2, V2, W2. These terminals are connected to similarly-marked terminals in the
starter.
The basic circuit of a typical hand-operated air-break or oil-immersed
starter is shown in the diagram, the incoming supply being controlled by a line
contactor. With the change-over switch in the start position, the motor
windings are connected in star (U1, V1 and W1 together) and in the running
position in delta (U2 to W1, V2 to U1 and W2 to V1).
When is an auto-transformer starter used?
When more flexibility is required for starting a squirrel-cage motor
than is provided by the star-delta method, which is limited as far as starting
torque is concerned. Auto-transformer starting permits the stator to be wound
for running in star. The starting torque can be adjusted to suit the load by
changing the voltage tapping on the auto-transformer. Both starting torque and
current are reduced in the same proportion.
It is used for motors of medium and large size on light starting loads
(e.g. centrifugal pumps, fans, compressors and mills). Up to about 75kW the
simple auto-transformer starter is employed; above this, the Korndorfer
connection is recommended.
What does the simple auto-transformer
starter consist of?
The basic diagram is shown. The motor is started by connecting its
primary to tappings on the starting transformer; then after a time delay,
re-connecting direct to the supply. The winding on each limb of the
auto-transformer usually has three taps, 60, 75 and 85 per cent of line
voltage, but taps to give other percentages may be arranged as required. The
auto-transformer may be used in conjunction with a contactor panel, or
alternatively a hand-operated switch.
The accompanying illustration shows the wiring diagram of an
auto-transformer starter consisting of a line contactor interlocked with a
hand-operated change-over switch, three thermal or magnetic overload relays and
an auto-transformer.
What are the connections for the
Korndorfer system?
The simple auto-transformer starter has the disadvantage that at the
instant of transition from ‘start’ to ‘run’ the supply to the motor is
interrupted. This means that the insulation may be stressed by high transient
voltages.
The Korndorfer method keeps the motor connected to the supply
continuously by means of the connections shown in the diagram. On the first
step (a), switches 1 and 2 close and the motor accelerates at a reduced voltage
determined by the transformer tapping. On the second step (b), the star point
of the transformer (switch 2) is opened so that the motor continues to run with
part of the transformer winding in circuit. Next, this part is short-circuited
by the ‘run’ contactor or switch (switch 3 closes) and finally the ‘start’
contactor or switch (1) is opened, as shown at (c).
A fully automatic starter would comprise a triple-pole line contactor,
start contactor, running contactor, three single-pole overload relays,
auto-transformer with a set of links for tap-changing, a suitable timer, and
‘start’ and ‘stop’ pushbuttons.
Switching sequence for auto-transformer starting by the Korndorfer
method.
1. Motor at reduced voltage from
transformer.
2. Motor with part of transformer
winding in series.
3. Motor at full voltage.
What precautions should be observed when
applying reduced voltage starting to a load with rising characteristic such as
fans?
If the specified starting current is too low, the motor may start
correctly but not run fully up to speed. The result is that on changing over to
the running or full-voltage condition a very high current may be taken, thus
negating the low initial current. For this reason, even with fan drives, it is
not desirable to pin the starting current lower than about 200 percent of
full-load current.
What are the initial-starting line
current and motor torque when star-delta starting?
Both line current and torque are approximately one-third of the motor
standstill values on full volts.
Why are the above values of initial-starting
current and torque approximate?
Because the formulae given assume for simplicity that the
standstill/reactance of a motor is constant at all voltages -that the
short-circuit current varies in direct proportion to the applied voltage. Owing
to magnetic saturation, particularly of the slot lips, the standstill reactance
tends to be less on full volts than on reduced volts so the current and torque
values tend to be rather less than those obtained by the formulae given.
How do the various methods of starting on
reduced voltage compare as regards torque per ampere?
Star-delta and auto-transformer methods have the advantage over primary
resistance and primary reactor methods.
What mechanical methods of reducing
starting current can be adopted?
The starting duty can be reduced by fitting a centrifugal or other type
of clutch which only picks up the load when the motor is well up to speed.
What is sequence starting?
A system of starting by which several motors of similar rating are started
in sequence off one starter in conjunction with interlocked switching.
How are slip-ring motors started?
By first switching the supply on the stator winding with all the
external rotor resistance in circuit across the slip rings and then cutting out
the rotor resistance progressively as the motor speeds up until finally the
rotor winding is short-circuited.
What is the usual arrangement of
connections for a hand-operated slip-ring starter?
Small slip-ring starters usually consist of a contactor for the stator
circuit and a face plate-type starting resistance for the rotor circuit. The
basic essentials are shown, the three wires from the stator going to slip-ring
terminals R, S and T on the motor. An actual wiring diagram is also shown. The
starter must be fitted with interlocks to ensure that the resistance is all-in
when starting. With a contactor controlling the stator supply, interlocking is
simply effected, as shown, through electrical contacts on the arm of the rotor
starter, no current reaching the contactor coil ‘I’ unless the arm is in the
starting position.
What are the essentials of a
full-automatic stator-rotor starter?
An automatic starter would include a triple pole contactor to control
the stator circuit, together with rotor-resistance grids short-circuited by the
necessary number of accelerating contactors, the last of which must be
continuously rated to carry the full-load rotor current. Also required are the
necessary number of overload relays and timers controlling the duration of the
starting period. The number of timers and accelerating contactors correspond to
the number of steps of rotor resistance that are provided.
How is speed control of a slip-ring motor
effected?
By introducing resistance into the rotor circuit similar to a starting
resistance except that the heat losses in the resistance must be dissipated
continuously. Unless the duty is intermittent, all except small sizes require
some means of cooling the resistors.
Grid resistances with a motor-driven fan may be used in conjunction with
a drum controller. Alternative methods are oil-immersed resistances or a liquid
resistance cooled by circulating water through cooling tubes.
What is liquid resistance?
Insulated pots filled with a resistance solution of electrolyte, e.g.
caustic soda or washing soda. Plates connected to the slip rings dip into the
pots and are shorted out in the full-speed position.
Liquid starters and controllers are used for large motors.
What is the advantage of a liquid
resistance for starting purposes?
Resistance may be reduced continuously so that, with close control over
the current as indicated on an ammetre, a very smooth start can be obtained.
What is a slip resistance?
A fixed step of rotor resistance used to limit the current taken from
the supply at the instant when peak load is applied to the motor. It is often
desirable to do this on press drives, guillotines, etc. As the resistance value
is small, it is usual to have a conventional starter so arranged that the last
step of resistance is not cut out when the starting handle is right home. This
last step of resistance is continuously rated.
What is meant by motors in synchronous
tie?
When the two slip-ring motors are required to run at the same speed, it
is possible to do this by connecting their rotors together through the slip
rings in conjunction with a single slip resistance. The starter for such a
scheme includes a single rotor resistance, the last step of which is the continuously-rated
slip resistance, and two-stator contactors, one for each motor.