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AMAZON multi-meters discounts AMAZON oscilloscope discounts Electric motor duty cyclesThe rated output of an AC induction motor given in manufacturer's catalogs is based on some assumptions about the proposed application and duty cycle of the motor. It’s common practice to base the motor rating on the continuous running duty cycle S1. When a motor is to be used for an application duty cycle other than the S1 continuous running duty, some precautions need to be taken in selecting a motor and the standard motors may be re-rated for the application. The duty cycles are normally calculated so that the average load over a period of time is lower than the continuous load rating S1. In the standards, several different duty cycles are defined. In IEC 34.1 and AS 1359.30, eight different duty types are defined by the symbols S1 to S8 as follows: S1: Continuous running duty • Operation at constant mechanical load for a period of sufficient duration for thermal equilibrium to be reached. • In the absence of any indication of the rated duty type of a motor, S1 continuous running duty should be assumed. • Designation example: S1; S2: Short-time duty • Operation at constant load, for a period of time which is less than that required to reach thermal equilibrium, followed by a rest and motor de-energized period of sufficient duration for the machine to re-establish temperatures to within 2°C of the ambient or the coolant temperature. • The values 10 min, 30 min, 60 min and 90 min are recommended periods for the rated duration of the duty cycle. • Designation example: S2 - 60 min S3: Intermittent periodic duty not affected by the starting process • A sequence of identical duty cycles, each comprising a period of operation at constant load and a period of rest when the motor is de-energized. • The period of the duty cycle is too short for thermal equilibrium to be reached. • Assumed that the starting current does not significantly affect the temperature rise. • The duration of one duty cycle is 10 min. • The following items should also be specified for this duty cycle - The cyclic duration factor, which represents the percentage duration of the loaded period as a percentage of the total cycle - Recommended values for cyclic duration factor are 15%, 25%, 40%, 60% • Designation example: S3 - 25% S4: Intermittent periodic duty affected by the starting process • A sequence of identical duty cycles, each comprising a period of significant starting current, a period of operation at constant load and a period of rest when the motor is de-energized. • The period of the duty cycle is too short for thermal equilibrium to be reached. • Assumed that the starting current is significant. • The motor is brought to rest by the load or by mechanical braking, where the motor is not thermally loaded. • The following items should also be specified for this duty cycle - The cyclic duration factor, which represents the percentage duration of the loaded period as a percentage of the total cycle - The number of load cycles per hour (c/h) - The inertia factor FI, which is the ratio of the total moment of inertia to the moment of inertia of the motor rotor - The moment of inertia of the motor rotor (JM) - The average moment of resistance TV, during the change of speed given with rated load torque • Designation example: S4 - 25% - 120 c/h - (FI = 2) - (JM = 0.1 kg-m^2 ) - (TV = 0.5TN) S5: Intermittent periodic duty affected by the starting process and also by electric braking • A sequence of identical duty cycles, each comprising a period of significant starting current, a period of operation at constant load, a period of rapid electric braking and a period of rest when the motor is de-energized. • The period of the duty cycle is too short for thermal equilibrium to be obtained. • The following items should also be specified for this duty cycle - The cyclic duration factor, which represents the duration of the loaded period as a percentage of the total cycle - The number of load cycles per hour (c/h) - The inertia factor FI, which is the ratio of the total moment of inertia to the moment of inertia of the motor rotor. - The moment of inertia of the motor rotor (JM) - The permissible average moment of resistance TV, during the change of speed given with rated load torque. • Designation example: S5 - 40% - 120 c/h - (FI = 3) - (JM = 1.3 kg-m^2 ) - (TV = 0.3TN) S6: Continuous operation, periodic duty with intermittent load • A sequence of identical duty cycles, where each cycle consists of a period at constant load and a period of operation at no-load (no-load current only), but with no period of de-energization. • The period of the duty cycle is too short for thermal equilibrium to be obtained. • Recommended values for the cyclic duration factor are 15%, 25%, 40% and 60%. • The duration of the duty cycle is 10 min. • Designation example: S6 - 40%. S7: Uninterrupted periodic duty, affected by the starting process and also by electric braking • A sequence of identical duty cycles, each comprising a period of starting current, a period of operation at constant load, a period of electric braking. • The braking method is too short for thermal equilibrium to be obtained. • The following items should also be specified for this duty cycle - The number of load cycles per hour (c/h) - The inertia factor FI, which is the ratio of the total moment of inertia to the moment of inertia of the motor rotor. - The moment of inertia of the motor rotor (JM) - The permissible average moment of resistance TV, during the change of speed given with rated load torque. • Designation example: S7 - 500 c/h - (FI = 2) - (JM = 0.08 kg-m^2 ) - (TV = 0.3TN) S8: Uninterrupted periodic duty with recurring speed and load changes • A sequence of identical duty cycles, each comprising a period of operation at constant load corresponding to a predetermined speed of rotation, followed by one or more periods of operation at other constant loads corresponding to different speeds of rotation. • The period of the duty cycle is too short for thermal equilibrium to be obtained. • This type of duty cycle is used for pole changing motors. • The following items should also be specified for this duty cycle - The number of load cycles per hour (c/h). - The inertia factor FI, which is the ratio of the total moment of inertia to the moment of inertia of the motor rotor. - The permissible average moment of resistance TV, during the change of speed given with rated load torque. - The cyclic duration factor for each speed of rotation. - The moment of inertia of the motor rotor (JM). - The combinations of the load and the speed of rotation are listed in the order in which they occur in use. • Designation examples: - S8 - 30 c/h - (FI = 30) - TV = 0.5TN - 24 kW - 740 rev/m - 30% - S8 - 30 c/h - (FI = 30) - TV = 0.5TN - 60 kW - 1460 rev/m - 30% - S8 - 30 c/h - (FI = 30) - TV = 0.5TN - 45 kW - 980 rev/m - 40% - (JM = 2.2 kg-m^2 ) Cooling and ventilation of electric motors (IC)All rotating electrical machines generate heat as a result of the electrical and mechanical losses inside the machine. Losses are high during starting or dynamic braking. Also, losses usually increase with increased loading. Cooling is necessary to continuously transfer the heat to a cooling medium, such as the air. The different methods of cooling rotating machines are classified in the standards IEC 34.6 and AS 1359.21. For AC induction motors, cooling air is usually circulated internally and externally by one or more fans mounted on the rotor shaft. To allow for operation of the machine in either direction of rotation, fans are usually of the bi-directional type and made of a strong plastic material, aluminum, or steel. In addition, the external frames of the motor are usually provided with cooling ribs to increase the surface area for heat radiation. The most common type of AC motor is the totally enclosed fan cooled (TEFC) motor, which is provided with an external forced cooling fan mounted on the non-drive end (NDE) of the shaft, with cooling ribs running axially along the outer surface of the motor frame. These are designed to keep the air flow close to the surface of the motor along its entire length, thus improving the cooling and self-cleaning of the ribs. An air-gap is usually left between the ribs and the fan cover for this purpose. Internally, on smaller TEFC motors, the rotor end-rings are usually constructed with ribs to provide additional agitation of the internal air for even distribution of temperature and to allow the radiation of heat from the end shields and frame. Special precautions need to be taken when standard TEFC induction motors are used with AC variable speed drives, powered by VVVF converters. For operation at speeds below the rated frequency of 50 Hz, the shaft mounted fan cooling efficiency is lost. For constant torque loads, it’s sometimes necessary to install a separately powered forced cooling fan (IC 43) to maintain adequate cooling at low speeds. On the other hand, for prolonged operation at high speeds above 50 Hz, the shaft mounted fan works well but may make excessive noise. Again, it may be advisable to fit a separately powered cooling fan. Larger rotating machines can have more elaborate cooling systems with heat exchangers. The system used to describe the method of cooling is currently being changed by IEC, but the designation system currently in use is as follows: • A prefix comprising the letters IC (index of cooling) • A letter designating the cooling medium, this is omitted if only air is used • Two numerals which represent 1. The cooling circuit layout 2. The way in which the power is supplied to the circulation of the cooling fluid, fan, no fan, separate forced ventilation, etc Code Description Drawing IC 01 - Open machine - Fan mounted on shaft - Often called 'drip-proof' motor IC 40 (New : IC 410) - Enclosed machine - Surface cooled by natural convection and radiation - No external fan IC 41 (New : IC 411) - Enclosed machine - Smooth or finned casing - External shaft-mounted fan - Often called TEFC motor IC 43 A (New : IC 416A) - Enclosed machine - Smooth or finned casing - External motorized Axial fan supplied with machine IC 43 R (New : IC 416R) - Enclosed machine - Smooth or finned casing - External motorized Radial fan supplied with machine IC 61 (New : IC 610) - Enclosed machine - Heat Exchanger fitted - Two separate air circuits - Shaft-mounted Fans - Often called Cac A motor ====17: Designation of the most common methods of cooling Degree of protection of motor enclosures (IP)The degree of protection (also called index of protection - IP) which is provided by the enclosure of the motor, is classified in the standards IEC 34.5 and AS 1359.20. The system used to describe the Index of Protection is as follows: • A prefix comprising the letters IP (index of protection). • Three numerals which represent. 1. The protection against contact and ingress of solid objects, such as dust. 2. The protection against ingress of liquids, such as water. 3. The mechanical protection and its resistance to impact. This third numeral is often not used in practice. First number: protection against solid objects Second number : protection against liquids Third number : mechanical protection Definition IP Tests: No protection No protection No protection Protected against solid objects of over 50 mm (e.g.: accidental hand contact) Protected against vertically dripping water (condensation) 150 60 0 Ø 50 mm Ø 12 mm Ø 2.5 mm Ø 1 mm Protected against solid objects of over 12 mm (e.g. finger) Protected against solid objects of over 2.5 mm (e.g. tools, wire) Protected against solid objects of over 1 mm (e.g. small tools, thin wire) Protected against dust (no deposits of harmful material) Protected against water dripping up to 15° from the vertical Protected against rain falling up to 60° from the vertical Protected against water splashes from all directions Protected against jets of water from all directions 15cm 20cm 40cm 150g 250g 500g Impact energy : 0.225 J Impact energy : 0.375 J Impact energy : 0.500 J Impact energy : 2 J 1m 0.15m ..m 6 77 8 9 6 Totally protected against dust. Does not involve rotating machines Protected against jets of water comparable to heavy seas Protected against the effects of immersion to depths of between 0.15 and 1 m Protected against the effects of prolonged immersion at depth .m 40cm 40cm 5000g 1500g Impact energy : 6 J Impact energy : 20 J ====18: Summary of the index of protection This system of degrees of protection does not relate to protection against corrosion. For example, a machine with an index of protection of IP557, is protected as follows: 5: Machine is protected against accidental personal contact of moving parts, such as the fan, and against the ingress of dust Test Result: No risk of direct contact with rotating parts (test finger) No risk that dust could enter the machine in harmful quantities 5: Machine is protected against jets of water from all directions from hoses 3 m away and with a flow rate less than 1 2.5 liters/sec at 0.3 bar Test Result: No damage from water projected onto the machine during operation 7: Machine is resistant to impacts of up to 6 joules. Test Result: Damage caused by impacts does not affect running of the machine. Construction and mounting of AC induction motorsModern squirrel cage AC induction motors are available in several standard types of construction and mounting arrangements. These are classified in accordance with the standards IEC 34.7 and AS 1359.2 2. Mounting position needs to be specified to ensure that drain plugs, bearings and other mechanical details are correctly located and dimensioned during assembly. The system used to describe the mounting arrangements is as follows: • A prefix comprising the letters IM (index of mounting) • Four numerals which represent... 1. Type of construction 2. Type of construction 3. Mounting Position 4. Mounting Position A summary of the mounting designations. A previous system of designation used letters B (horizontal mounting) and V (vertical mounting). This system has been superseded in both IEC 34.7 and AS 1359.2 2. The old designations are shown in the table in brackets. Foot Mounted Motors IM 1001 (IM B3)
IM 1071 (IM B8)
IM 1051 (IM B6)
IM 1011 (IM V5)
IM 1061 (IM B7)
IM 1031 (IM V6)
====19: Mounting designations for foot mounted motors Flange Mounted Motors IM 3001 (IM B5)
IM 2001 (IM B35)
IM 3011 (IM V1)
IM 2011 (IM V15)
IM 3031 (IM V3)
IM 2031 (IM V36)
====20: Mounting designations for flange mounted motors Face Mounted Motors IM 3601 (IM B14)
IM 2101 (IM B34)
IM 3611 (IM V18)
IM 2111 (IM V58)
IM 3631 (IM V19)
IM 2131 (IM V69)
====21: Mounting designations for face mounted motors Anti-condensation heatersWhen rotating electrical machines need to stand idle for long periods of time in severe climatic conditions, such as a high humidity environment, moisture can be drawn into the machine and absorbed into and onto the insulation of the stator and rotor windings. When a machine is de-energized after it has been running for a period of time, the internal temperature is high. As the machine cools, the low pressure inside the machine draws external moist air into the machine via the seals around the shaft. The moisture degrades the performance of the insulation materials by providing a partially conductive path between the windings and the frame of the machine. When the machine is energized, electrical breakdown of the insulation can occur. Standby motors or generators, which have not been used for some time, can fail to operate when they are needed. Under these conditions, where a motor is expected to stand idle for long periods in an environment of high humidity, it may be necessary to specify additional winding impregnation treatment and consideration should also be given to anti-condensation heaters. These are fitted inside the motor and their connections brought out to terminals. The heaters are energized from a 240 V supply when the motor is not in use to prevent condensation forming inside the windings. Anti-condensation heaters are normally in the form of a tape, which comprises a flat glass-fiber tape with a heating element woven into it. This tape is then inserted inside a glass fiber sleeve and wrapped around the stator winding overhang, braced and impregnated with the stator winding. One heater element is normally fitted to each end of the stator winding. A typical rating of a heater varies from 25 watts, for small motors, to 200 watts for large motors. Methods of starting AC induction motorsDirect-on-line (DOL) starting is the simplest and most economical method of starting an AC squirrel cage induction motor. A suitably rated contactor is used to connect the stator windings of the motor directly to the 3-phase power supply. While this method is simple and produces a reasonable level of starting torque, there are a number of disadvantages: • The starting current is very high, between 3 to 8 times the full load current. Depending on the size of the motor, this can result in voltage sags in the power system. • The full torque is applied instantly at starting and the mechanical shock can eventually damage the drive system, particularly with materials handling equipment, such as conveyors. • In spite of the high starting current, for some applications the starting torque may be relatively low, only 1.0 to 2.5 times full load torque. To overcome these problems, other methods of starting are often used. Some common examples are:
Most of the above motor starting techniques reduce the voltage at the motor stator terminals, which effectively reduces the starting current as well as the starting torque. From the equivalent circuits and formulae for AC induction motors, covered earlier in this section, the following conclusions can be drawn about reduced voltage starting: • Both the stator current and output torque during starting are proportional to the square of the voltage. During star-delta starting, the voltage is reduced to 0.58 of its rated value. The current and torque are reduced to 0.33 of prospective value. I_Start ∝ Voltage2 I_Start ∝ Voltage2 Motor selectionThe correct selection of an AC induction motor is based on a thorough understanding of the application for which the motor is to be used. This requires knowledge about the type and size of the mechanical load, its starting and acceleration requirements, running speed requirements, duty cycle, stopping requirements, and the environmental conditions. The following checklist and reference to the preceding sections provides a guide to the selection procedure. When selecting an electric motor, the following factors should be considered: • Type and torque requirements of the mechanical load • Method of starting • Acceleration time • Type of construction of AC induction motor:
• Environmental conditions
• Required degree of protection of the enclosure • Insulation class • Motor protection • Method of cooling • Mounting arrangement
• Cable connections • Direction of rotation • Duty cycle • Speed control (if required) In general, the selection of the motor is dictated by the type of load and the environment in which it will operate. The selection of a cage motor or slipring motor is closely related to the size of the machine, the acceleration time required (determined by load) and the method of starting (determined by the electrical supply limitations). From the point of view of price, reliability, and maintenance, the cage motor is usually the first choice. In general, slipring motors are required when: • The load has a high starting torque requirement, but the supply dictates a low starting current • The acceleration time is long due to high load inertia, such as a fan • Where duty dictates frequent starting, inching or plugging These are general comments because cage motors can be successfully used in all the above situations. Slipring motors are sometimes used for limited speed control. The slip can be controlled by controlling the external rotor resistance. As demonstrated earlier, the overall efficiency of this method is poor, so this method can only be used if the speed does not deviate too far from the rated speed. The slip power is dissipated as heat in the external rotor resistors. |
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