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Section -9
Electric Motor , Circuits & Controllers
9-1
The method of starting motors shall restrict the current to limits laid down by the SEWA . All motors over 1 hp shall be provided with means of automatic disconnection from the supply in the event of excessCurrent flow or drop in voltage of over 15% the limits laid down are as set out below:
a) Motors with name plate rating of up to and including 11 kw (15 hp).
b) motors with name plate rating of above 11 kw (15 hp).
Motors that fall within category (a) may be connected for direct online starting with over current protection.
Motors that fall within category (b) shall not in any circumstances be connected for direct-on-line starting But shall be arranged for reduced voltage starting e.g open or closed transition star/delta starting . Auto Transmission starting , or other approved arrangement. All motors shall be rated at 415 volts 50 HZ three Phase or 240 volts 50 HZ single phase as required . Other voltage will not be accepted .
Table 9.1.A details the various insulation classes with their associated maximum operating temperature .The maximum class of insulation acceptable for use in UAE is “B” . Please note that insulation classes Y , A and E are not acceptable in any circumstances . However , when specifying the class of insulation To be used for electric motors or alternators , the actual site condition must be taken into consideration To determine if a higher insulation class is required . E.g. a class B motor will not operate satisfactorily If located in direct sunlight . In general , motors shall be of the drip proof type and be totally enclosed , fan cooled . where motors are required to operate in hazardous area or are required for special purpose
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The design of motor shall be suitable for this application .
The terminal box for all motors shall be weather proof . all motors shall of such construction as to make The temperature as uniform as possible in the different parts of the windings and core during operation , thus avoiding excessive heating at any point . all motors rated at 11 kw (15 hp ) and above shall be fitted With thermostatic control elements within the motor actuating directly the control circuit of the motor and Disconnecting it from the supply in the event of a temperature rise exceeding limits for its insulation class .
All motors shall be on their own separate circuit and be provided with control apparatus such as a motor Starter incorporating a suitable device affording protection against excess current in the motor or in the cable between the device and the motor . In addition , all motor starter for 3ph motors shall incorporate a phase failure device which will automatically disconnect the supply from the motor these two protection device shall be manually reset in all cases .
the different parts of each motor shall be capable of withstanding the highest mechanical and electrical stresses to which they may be subjected during their operation without any injury , failure or inferior or inferior performance .
9-2
Motor control panels shall be fitted with an ammeter or ammeter in each motor circuit.
9-3
The following tables details the type of insulation materials used , for each class for motor construction Together with its maximum winding operating temperature.
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9-4
Installation of 1-phase motors rated up to 3.7 kw (5 HP) and 3-phase motors up to 110 kw (150 HP) only shall Normally be permitted unless otherwise approved by SEWA . where a large number of motors above 150 HP are Proposed , the advice of SEWA shall be sought on availing a bulk supply .9-5
All electric motors shall be adequately protected against overload , short circuit , earth leakage and additionally , Against loss of one or more phases , voltage fluctuating , etc . as deemed essential to suit individual applications.9-6
Motor upto and including 11 kw (15 HP) may be connected for direct on-line starting with over current protection .9-7
All three phase motors over 3 HP and single phase motors above 1 HP shall be providing with current limiting Starting equipment to effectively keep the starting current within the following limits :Type of supply Rating of motor Max. Permissible starting current
Single phase 1 – 5 HP 5 times full load current
Three phase Less than 15 HP 5 times full load current
Three phase 15 HP to < 50 HP 2 times full load current
Three phase 50 HP and above 1.5 times full load current
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9-8
Starter shall be provided with overload relays of the thermal type with automatic compensations for Variation in ambient temperature between 2.80 C and 480 C.
The starting equipment to limit the current may consist of any of the following type or other approved By SEWA
a) star/delta
b) Primary – resistance C) Auto – transformer
9-9
All motors shall be provided with an isolator, for isolating the motor from the supply during periodsOf inspection or maintenance. Such means of isolation shall effectively interrupt the supply on all phases. The isolator may be integral with the control gear or separate, but shall be in close proximity to the motor. An emergency stop push button shall be incorporated in the control gear.
When motor starting gear is energized from an auxiliary circuit, the circuit shall also be isolated during maintenance.
All starters, isolators and push button shall be clearly marked in Arabic and English stating which machine they control and their function. To avoid confusion the words START and STOP instead of OPEN and CLOSED shall be used.
Motor and their control gear shall be located in well ventilated situations with adequate space for operation, inspection and maintenance.