Topic 3 three phase motor protection devices

Topic 3 Three phase motor protection devices Motor protection Motor protection devices star point overloads Types of protection devices fuses and circuit breakers thermal overload magnetic overload under voltage and overvoltage relays electronic motor protection Isolation devices Timers Environmental protection Fault finding in three phase motor installations Motor circuit faults

• Motor protection

• Motor protection devices

– star point overloads

• Types of protection devices

– fuses and circuit breakers

– thermal overload

– magnetic overload

– under voltage and overvoltage relays

– electronic motor protection

• Isolation devices

• Timers

• Environmental protection

• Fault finding in three phase motor installations

• Motor circuit faults

Topic 3 Three phase motor protection devices

Purpose for motor protect

• To protect the motor from Electrical fault:

– Short circuit

– Insulation failure within a winding

• To protect against Mechanical fault:

– Overloading the motor

– Bearing failure

• To protect against environmental conditions

– Humidity

– Ambient temperature

Fuses and Circuit breakers

special requirements

times the normal running current of a

motor

blow almost instantly with 60A flowing through it

HRC Fuses

• These have silica sand to help interrupt the high fault current on short circuit

• Silver element with special design to

overcome short term overloads of motor starting

blow fuses

Circuit breakers

– Magnetic

– Thermal

circuit There are three versions, B, C and

D The D curve is used for motors

Circuit breaker tripping curve

Thermal overload

series with the motor

Bi metal strip

• Two dissimilar metals that have different

temperature co efficient of expansion

• When heated, the strip bends

• Can Protect against:

– Mechanical overload

– Low voltage

– Loss of phase

– Part winding short

Blue button on right is for manual

or auto reset

Single phase motor TOL

• These are a direct acting unit

• They turn off the motor current with their contacts, rather than the control circuit

• On Refrigerators, are normally self

resetting

Star point overloads

• These are placed inside some

hermetic compressors.

• Three direct acting thermal

overloads are connected to the

windings, at the star point, of a

three phase motor.

• They are all linked so as to

ensure all three phases are

turned off.

• Has caused some compressors

to be changed when they only

need wait for O/L to cool down.

Magnetic dashpots

• A large coil

connected in series

with the motor

• Oil filled cylinder,

with a disc,

connected to a

metal slug

• If excess current is drawn there is enough

magnetic field to lift the slug.

• The oil filled dashpot slows the slug down.

• If the overload lasts long enough it will raise the slug out of the oil and activate a switch.

• Should always be mounted vertically

• Viscosity and height of oil is critical

Phase failure

• As soon as one phase is lost it turns the

motor off

places where supply is not reliable

motor, all connected in series

• If the motor overheats, the resistance

increases

• This turns off an external relay

• The relay then turns off the control circuit

Thermistors

Motor must be below temperature

R1

efficiency of the cooling system

may clog air vents

Motor protection units

• Modern motor protection units can monitor motor condition and control the control circuit.

• Current transformers can convey information on current draw, replacing the TOL

• Phase fail and under voltage can be

incorporated.

• Thermistors can be incorporated.

• Short circuit protection can NOT be offered.

• Many soft starters and VSD units incorporate

these features.

Multiple starting

can be very high

• The stress that this can cause to the

insulation, due to heating of the winding, can be high

• Manufacturers will normally list a duty of starting depending on the motor type and starter fitted

Testing an electric motor

– Windings should be tested for continuity

• Insulation resistance test

– Each winding should be checked to frame at 500V with a reading of not less than 1MΩ

– If windings can be separated the windings should be tested at 1000V for 3 phase and 500V for single phase

6 terminal 3 phase motor

4 terminal single phase Split phase

or cap start motor

3 terminal single phase Split phase

or cap start motor

• X – Y, Y – Z, Z – X

continuity will all be

different The largest will

be the sum of the other

two

• X, Y or Z to earth @

500V not less than 1MΩ

X Z Y

E

4 terminal single phase cap start

cap run motor

3 terminal single phase permanent

cap motor

• X – Y, Y – Z, Z – X

continuity, two will be the

same and the other the

sum of the two The

largest will not be on the

common terminal

• X, Y or Z to earth @

500V not less than 1MΩ

X Z Y

E

Environmental protection

• IP ratings

– Protection against solid objects

– Protection against moisture

refrigerated rooms, where high moisture contents are found

Isolation devices

2.3.1 General

Electrical installations shall be provided with devices to prevent or remove hazards associated with the electrical installation and for

maintenance of electrically activated equipment.

NOTE: The measures specified in this Clause 2.3 are in addition to, and not alternatives, to the protective measures specified in Clause 2.4.

Electrical installations shall include all switching devices or other

means of disconnection necessary to enable operations, repairs and maintenance work to be carried out with safety.

2.3.2.2 Devices for isolation

2.3.2.2.1 General

Devices for isolation shall effectively isolate all active supply

conductors from the circuit A semiconductor (solid-state) device shall not be used for isolation purposes.

Devices for isolation shall—

(a) be capable of withstanding an impulse voltage likely to occur at the point of installation, or shall have an appropriate contact gap; and

(b) not be able to falsely indicate that the contacts are open; and

(c) clearly and reliably indicate the isolating position of the device; and NOTE: The symbols ‘O’ (OFF) and ‘I’ (ON) are deemed to satisfy this requirement.

(d) be designed and installed so as to prevent unintentional closure, such as might be caused by impact, vibration or the like; and

(e) be a device that disconnects all active conductors of the relevant supply Single-pole devices situated adjacent to one another may be used; and

(f) be readily available

Isolation devices

• Switchboard/Circuit

protection device

• Isolator

Timers

Timer contacts

• Normally open timed to close

• Normally closed timed to open

• Normally open timed to open

• Normally closed timed to close

Timed both ways

• N/O timed to open and close

• N/C timed to open and close