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You have to be current on electric motors
Short communications and letters
the IIR will be able to contribute towards efforts to protect the environment against possible damage caused by CFC emissions.
Les Commissions B1, B2, E1 et E2 qui sont comp~tentes darts les domaines de l'isolation, des machines frigorifiques, des pompes Zt chaleur et des syst~mes de conditionnement d'air ont constitub deux groupes de travail pour btudier les probl~mes et les possibilit~s de rbduire les bmissions d'hydrocarbures halogbnbs fluor~s. Ces groupes de travail presenteront leur avis et leurs recommandations d'ici trbs peu de temps. Ainsi, l'IIF contribuera aux efforts de protection de l'environnement contre les dbgZtts qui r6sulteraient du rejet de ces hydrocarbures halogbnbs.
You have to be current on electric motors It is with regret, because of my respect for the author and his contributions to refrigeration engineering, that I find it necessary to disagree with part of the editorial which appeared in the March 1987 issue of International Journal of Refrigeration. The title was 'Giving money to electric utilities'. I quote from the editorial: 'My concern is that so much electricity is wasted ... Even more importantly the electric motor may be the chief culprit. Have you ever checked its efficiency at its normal running load? At its rated maximum power it is probably very efficient but you are probably running it at only half-power most of the time; then what is it costing you?'. The editorial correctly pointed out that the efficiency of refrigeration installations can be improved by improving the system designs. The editorial implies that the use of electric motors which are larger than the minimum sizes required leads to a large waste of pwer and energy. Actually for small electric motors, a larger motor operating at half-load consumes about the same power as a smaller motor of equivalent design operating at full load (probably within 1-2 ~o) if each is used to drive the same piece of equipment. This letter presents comparative information on electrical motors which may be known to most of you. I am writing to be certain, however, that the editorial does not inadvertently lead some readers in the wrong direction.
without special procedures. Large refrigeration installations are usually designed for starting at reduced loads. Electric motor losses include I2R loses, eddy current and hysteresis losses, stray load losses, friction losses in the bearings, and windage or air circulation losses. Electrical engineers may know of additional minor losses, but the above include the largest losses. The I2R losses are very dependent upon the load, varying primarily with the square of the load. The effect of the change in electrical resistance with temperature, and thus, the effect of temperature on load is small. Most of the other losses are relatively constant at a fixed motor speed, although some losses are slightly dependent upon the load. As a result of the major effect of the I2R losses, the sum of the losses decreases significantly with reduced load, although losses due to rotation continue to no-load conditions. Comparisons of the input power required to deliver 3 horsepower (hp) to a piece of rotating machinery, for two classes of nominal 3 and 5 hp motors, are given below. A smooth torque-effort diagram, that is a rotating compressor or a reciprocating compressor with adequate
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Discussion
The power output of an electric motor is determined by the power required to operate a specific piece of equipment; the required power input into the equipment is determined by the operating characteristics of the equipment within the system in which the equipment is being used. The selection of an electric motor should be based on start-up conditions as well as on the range of normal operating conditions. Usually, electric motors are subjected to relatively high starting currents, since additional power is required to overcome the inertia of the motor and compressor. The equipment must be accelerated to operating rotational speeds. Small refrigeration systems are usually designed for starting 0140-7007/88/020106-03503.00 © 1988 Butterworth& Co (Publishers) Ltd and IIR 106 Int. J. Refrig. 1988 Vol 11 March
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5
Output (hp) Figure 1 Losses in small motors. 1, Nominal 3 hp; 2, nominal 5 hp. - - , Drip-proof; - - - , totally enclosed
Short communications and letters electrical and mechanical flywheel, is assumed to eliminate the effect of possible current pulsations. For convenience, published data sheets from the General Electric Company (USA) for Type KS (NEMA Design B) normal starting torque, continuous 40°C ambient, 60 Hz, 460 V motors were used. The conclusions apply to motors from other manufacturers, and motors operating at other voltages, frequencies and speeds. The published data sheets provide expected efficiences at full load, 3/4 load, and 1/2 load. Operating speeds at different loads are slightly different because induction motors have more slip with increased load. The motor losses were plotted as a function of motor load (Figure 1) to obtain the losses for the 5 hp motors delivering 3 hp. The input comparisons for 3 hp outputs are as follows: GE May 1983 data sheet for totally enclosed fan cooled motor nominal 3 hp, 1770 rev min- 1, losses = 0.39 hp, total = 3.39 hp nominal 5 hp, 1765 rev min- 1, losses = 0.41 hp, total = 3.41 hp GE July 1987 data sheet for drip-proof motor nominal 3 hp, 1765 rev min -1, losses=0.35 hp, total = 3.35 hp nominal 5 hp, 1755 rev min- 1, losses = 0.33 hp, total = 3.33 hp These comparisons indicate that there are essentially no differences in power for motors of similar design between nominal 5hp motors delivering 3hp and nominal 3 hp motors delivering 3 hp. The calculated power differences were less than 1 ~ and were also both positive and negative. The published data present expected efficiencies and the power differences may be slightly greater or less than those calculated and presented above. The important conclusion, however, is that moderately oversized motors do not consume more power. Furthermore, oversized motors do not overheat or burn out as easily as do tightly sized motors. Reciprocating eompresmrs
In the section above, brief reference was made to electrical and mechanical flywheels for electric motors driving reciprocating compressors. Large synchronous motors driving reciprocating compressors must have sufficient flywheel to prevent excessive current pulsations. Electric motor manufacturers request that the reciprocating compressor manufacturer provide an omega factor, which is the result of a simplified calculation of the integrated effect of the torque-effort diagram. The value of the omega factor enables the motor manufacturer to verify that he is providing adequate electrical flywheel or to request, if necessary, that additional mechanical flywheel be provided. To return to the I2R losses: alternating current electric motors driving reciprocating compressors do not rotate at uniform speed, although the average speed is the synchronous speed for synchronous motors and slightly less than the synchronous speed for induction motors. The motors accelerate positively and negatively as they rotate, depending upon the torque required at a given crank angle. The instantaneous current is not a smooth sine wave but has peaks and valleys depending upon the
phase angle between the current and voltage and on the torque-effort diagram. We will assume that the variation of electrical resistance with temperature is small. For simplicity, we will hypothesize a torque-effort diagram with an arithmetic average current of 10 A, which produces waves with minimum peaks of 5 A and maximum peaks of 15 A. The square root of the average of the 12 values is (25 + 125) 1/2 or 12.25 A. The motor will tend to overheat and will be able to deliver power only at an average current of 1(3/12.25 or at 82 ~o of the rated current, to avoid failure of the insulation and burn-out. This simplified calculation describes a problem which was encountered about 25 years ago. A nominal 15 000 hp synchronous motor was purchased to drive a multiservice compressor in a large cryogenic plant. During initial start-up of the plant, the compressors were operated at a maximum capacity of 80 ~o of their rated capacity because of the excessive temperature rise in the motor coils. Calibrated ammeters and voltmeters were used to measure the current and voltage in each phase, the power factor was measured, and the electrical energy input was calculated to be about 15000hp. Compressor power input was calculated to be about 12 000 hp, taking into account the volume of the clearance pockets which were used to reduce compressor capacity. We met with technical representatives from the motor and compressor manufacturers, and with electrical engineering consultants. We checked and rechecked our measurements and calculations. We could not explain the discrepancy until after we followed the recommendation of an 'old timer' and used an oscilloscope to observe and record the current and voltage. The sine wave for the current was not a uniform wave but had peaks and valleys at crank angles corresponding to the cylinders. One peak was about 200 ~o of the arithmetic average. Our electrical engineering consultant recommended modifying the 'amortisseur' bars to increase the electrical flywheel. This change enabled the motor to deliver 15000hp at a marginally acceptable temperature. We learned several things from this incident, some of which we should have known: i. most a.c. ammeters do not indicate amperes but instead indicate the RMS of the instantaneous current the square root of the integrated sum of the squares; 2. simplified methods for calculating the required electrical flywheel for reciprocating compressors can give erroneous results for compressors with unusual peaks in their torque-effort diagrams; unusual conditions are more likely to apply to multiservice compressors. For these compressors it is necessary to carry out the proper mathematical integration to determine the required flywheel; 3. good a.c. wattmeters multiply instantaneous current and voltage, and give correct power readings; and 4. some electric motor manufacturers underestimate the fan requirements for obtaining proper cooling. Additional comments on motors
Most process equipment vendors offer the least costly equipment which meet specifications. One consulting engineer has made the following recommendation to potential purchasers of cryogenic plants: 'Buy large electric motors about 2 0 ~ larger than those specified.
Rev. Int. Froid 1988 Vol 11 Mars
107
Short communications and letters The motors and switch gear may cost 5 % more, but you will avoid more costly motor problems due to burned out coils. In addition, you may save a small amount of power since the I2R losses will be lower'. I agree with the above recommendation. I have witnessed several crash repair programmes which were required to minimize down-time after motor failures. Replacing coils and realigning equipment, when coupled with lost production, can be very costly. In our costconscious atmosphere, we tend to make mistakes in minimizing first cost and in underestimating the true cost of equipment repair and down-time.
108
Int. J. Refrig. 1988 Vol 11 March
I would like to add a final comment to the effect that it is impossible to obtain uniform cooling air flow in a motor and that some spots are warmer than others. If you are lucky, your temperature sensors may measure the hottest spot, but the probability that this will occur is small. Furthermore, the insulation may have some weak spots. You can avoid costly repairs by selecting oversize motors which are less likely to fail.
J. M. Geist GeistTec, 2720 Highland Street, Allentown, PA 18104, USA
the IIR will be able to contribute towards efforts to protect the environment against possible damage caused by CFC emissions.
Les Commissions B1, B2, E1 et E2 qui sont comp~tentes darts les domaines de l'isolation, des machines frigorifiques, des pompes Zt chaleur et des syst~mes de conditionnement d'air ont constitub deux groupes de travail pour btudier les probl~mes et les possibilit~s de rbduire les bmissions d'hydrocarbures halogbnbs fluor~s. Ces groupes de travail presenteront leur avis et leurs recommandations d'ici trbs peu de temps. Ainsi, l'IIF contribuera aux efforts de protection de l'environnement contre les dbgZtts qui r6sulteraient du rejet de ces hydrocarbures halogbnbs.
You have to be current on electric motors It is with regret, because of my respect for the author and his contributions to refrigeration engineering, that I find it necessary to disagree with part of the editorial which appeared in the March 1987 issue of International Journal of Refrigeration. The title was 'Giving money to electric utilities'. I quote from the editorial: 'My concern is that so much electricity is wasted ... Even more importantly the electric motor may be the chief culprit. Have you ever checked its efficiency at its normal running load? At its rated maximum power it is probably very efficient but you are probably running it at only half-power most of the time; then what is it costing you?'. The editorial correctly pointed out that the efficiency of refrigeration installations can be improved by improving the system designs. The editorial implies that the use of electric motors which are larger than the minimum sizes required leads to a large waste of pwer and energy. Actually for small electric motors, a larger motor operating at half-load consumes about the same power as a smaller motor of equivalent design operating at full load (probably within 1-2 ~o) if each is used to drive the same piece of equipment. This letter presents comparative information on electrical motors which may be known to most of you. I am writing to be certain, however, that the editorial does not inadvertently lead some readers in the wrong direction.
without special procedures. Large refrigeration installations are usually designed for starting at reduced loads. Electric motor losses include I2R loses, eddy current and hysteresis losses, stray load losses, friction losses in the bearings, and windage or air circulation losses. Electrical engineers may know of additional minor losses, but the above include the largest losses. The I2R losses are very dependent upon the load, varying primarily with the square of the load. The effect of the change in electrical resistance with temperature, and thus, the effect of temperature on load is small. Most of the other losses are relatively constant at a fixed motor speed, although some losses are slightly dependent upon the load. As a result of the major effect of the I2R losses, the sum of the losses decreases significantly with reduced load, although losses due to rotation continue to no-load conditions. Comparisons of the input power required to deliver 3 horsepower (hp) to a piece of rotating machinery, for two classes of nominal 3 and 5 hp motors, are given below. A smooth torque-effort diagram, that is a rotating compressor or a reciprocating compressor with adequate
0.8
0.6
J
Discussion
The power output of an electric motor is determined by the power required to operate a specific piece of equipment; the required power input into the equipment is determined by the operating characteristics of the equipment within the system in which the equipment is being used. The selection of an electric motor should be based on start-up conditions as well as on the range of normal operating conditions. Usually, electric motors are subjected to relatively high starting currents, since additional power is required to overcome the inertia of the motor and compressor. The equipment must be accelerated to operating rotational speeds. Small refrigeration systems are usually designed for starting 0140-7007/88/020106-03503.00 © 1988 Butterworth& Co (Publishers) Ltd and IIR 106 Int. J. Refrig. 1988 Vol 11 March
~ 0.4
...;jJ
_q
-2J
0.2-
I I
I 2
I 3
I 4
5
Output (hp) Figure 1 Losses in small motors. 1, Nominal 3 hp; 2, nominal 5 hp. - - , Drip-proof; - - - , totally enclosed
Short communications and letters electrical and mechanical flywheel, is assumed to eliminate the effect of possible current pulsations. For convenience, published data sheets from the General Electric Company (USA) for Type KS (NEMA Design B) normal starting torque, continuous 40°C ambient, 60 Hz, 460 V motors were used. The conclusions apply to motors from other manufacturers, and motors operating at other voltages, frequencies and speeds. The published data sheets provide expected efficiences at full load, 3/4 load, and 1/2 load. Operating speeds at different loads are slightly different because induction motors have more slip with increased load. The motor losses were plotted as a function of motor load (Figure 1) to obtain the losses for the 5 hp motors delivering 3 hp. The input comparisons for 3 hp outputs are as follows: GE May 1983 data sheet for totally enclosed fan cooled motor nominal 3 hp, 1770 rev min- 1, losses = 0.39 hp, total = 3.39 hp nominal 5 hp, 1765 rev min- 1, losses = 0.41 hp, total = 3.41 hp GE July 1987 data sheet for drip-proof motor nominal 3 hp, 1765 rev min -1, losses=0.35 hp, total = 3.35 hp nominal 5 hp, 1755 rev min- 1, losses = 0.33 hp, total = 3.33 hp These comparisons indicate that there are essentially no differences in power for motors of similar design between nominal 5hp motors delivering 3hp and nominal 3 hp motors delivering 3 hp. The calculated power differences were less than 1 ~ and were also both positive and negative. The published data present expected efficiencies and the power differences may be slightly greater or less than those calculated and presented above. The important conclusion, however, is that moderately oversized motors do not consume more power. Furthermore, oversized motors do not overheat or burn out as easily as do tightly sized motors. Reciprocating eompresmrs
In the section above, brief reference was made to electrical and mechanical flywheels for electric motors driving reciprocating compressors. Large synchronous motors driving reciprocating compressors must have sufficient flywheel to prevent excessive current pulsations. Electric motor manufacturers request that the reciprocating compressor manufacturer provide an omega factor, which is the result of a simplified calculation of the integrated effect of the torque-effort diagram. The value of the omega factor enables the motor manufacturer to verify that he is providing adequate electrical flywheel or to request, if necessary, that additional mechanical flywheel be provided. To return to the I2R losses: alternating current electric motors driving reciprocating compressors do not rotate at uniform speed, although the average speed is the synchronous speed for synchronous motors and slightly less than the synchronous speed for induction motors. The motors accelerate positively and negatively as they rotate, depending upon the torque required at a given crank angle. The instantaneous current is not a smooth sine wave but has peaks and valleys depending upon the
phase angle between the current and voltage and on the torque-effort diagram. We will assume that the variation of electrical resistance with temperature is small. For simplicity, we will hypothesize a torque-effort diagram with an arithmetic average current of 10 A, which produces waves with minimum peaks of 5 A and maximum peaks of 15 A. The square root of the average of the 12 values is (25 + 125) 1/2 or 12.25 A. The motor will tend to overheat and will be able to deliver power only at an average current of 1(3/12.25 or at 82 ~o of the rated current, to avoid failure of the insulation and burn-out. This simplified calculation describes a problem which was encountered about 25 years ago. A nominal 15 000 hp synchronous motor was purchased to drive a multiservice compressor in a large cryogenic plant. During initial start-up of the plant, the compressors were operated at a maximum capacity of 80 ~o of their rated capacity because of the excessive temperature rise in the motor coils. Calibrated ammeters and voltmeters were used to measure the current and voltage in each phase, the power factor was measured, and the electrical energy input was calculated to be about 15000hp. Compressor power input was calculated to be about 12 000 hp, taking into account the volume of the clearance pockets which were used to reduce compressor capacity. We met with technical representatives from the motor and compressor manufacturers, and with electrical engineering consultants. We checked and rechecked our measurements and calculations. We could not explain the discrepancy until after we followed the recommendation of an 'old timer' and used an oscilloscope to observe and record the current and voltage. The sine wave for the current was not a uniform wave but had peaks and valleys at crank angles corresponding to the cylinders. One peak was about 200 ~o of the arithmetic average. Our electrical engineering consultant recommended modifying the 'amortisseur' bars to increase the electrical flywheel. This change enabled the motor to deliver 15000hp at a marginally acceptable temperature. We learned several things from this incident, some of which we should have known: i. most a.c. ammeters do not indicate amperes but instead indicate the RMS of the instantaneous current the square root of the integrated sum of the squares; 2. simplified methods for calculating the required electrical flywheel for reciprocating compressors can give erroneous results for compressors with unusual peaks in their torque-effort diagrams; unusual conditions are more likely to apply to multiservice compressors. For these compressors it is necessary to carry out the proper mathematical integration to determine the required flywheel; 3. good a.c. wattmeters multiply instantaneous current and voltage, and give correct power readings; and 4. some electric motor manufacturers underestimate the fan requirements for obtaining proper cooling. Additional comments on motors
Most process equipment vendors offer the least costly equipment which meet specifications. One consulting engineer has made the following recommendation to potential purchasers of cryogenic plants: 'Buy large electric motors about 2 0 ~ larger than those specified.
Rev. Int. Froid 1988 Vol 11 Mars
107
Short communications and letters The motors and switch gear may cost 5 % more, but you will avoid more costly motor problems due to burned out coils. In addition, you may save a small amount of power since the I2R losses will be lower'. I agree with the above recommendation. I have witnessed several crash repair programmes which were required to minimize down-time after motor failures. Replacing coils and realigning equipment, when coupled with lost production, can be very costly. In our costconscious atmosphere, we tend to make mistakes in minimizing first cost and in underestimating the true cost of equipment repair and down-time.
108
Int. J. Refrig. 1988 Vol 11 March
I would like to add a final comment to the effect that it is impossible to obtain uniform cooling air flow in a motor and that some spots are warmer than others. If you are lucky, your temperature sensors may measure the hottest spot, but the probability that this will occur is small. Furthermore, the insulation may have some weak spots. You can avoid costly repairs by selecting oversize motors which are less likely to fail.
J. M. Geist GeistTec, 2720 Highland Street, Allentown, PA 18104, USA