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

Topic 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 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 • Motor fuses and circuit breakers have special requirements • Motor starting current is between – 10 times the normal running current of a motor • A 10A fuse used on a 10A motor would 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 • Commonly known as motor start or slow blow fuses Circuit breakers • Two mechanisms – Magnetic – Thermal • Thermal reacts to overload • Magnetic operates very quickly on short circuit There are three versions, B, C and D The D curve is used for motors Circuit breaker tripping curve Thermal overload • Has or heating elements connected in series with the motor • These elements are wrapped around a bi metal strip • If the motor draws excess current the bi metal strip will move, activating a switch • The switch will turn off the control circuit 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 • Manual reset in most other applications terminal single phase cap start cap run motor • A – B, C –D will be the same values of continuity • A – E, C – E and A – C @ 500V not less than 1MΩ 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 Environmental protection • IP ratings – Protection against solid objects – Protection against moisture • Applicable mainly in cooling towers and 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 ... 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... – Loss of phase – Part winding short Phase failure • Phase failure relays • As soon as one phase is lost it turns the motor off • Does not wait for TOL to operate • Reduces damage to motor • Can... • In dirty environments when dust and dirt may clog air vents Motor protection units • Modern motor protection units can monitor motor condition and control the control circuit • Current transformers