Motor speed modulation of air conditioning and heat pump systems

M o t o r speed modulation of air conditioning and heat pump systems R. Zigler

Modulation de la vitesse des moteurs des installations de conditionnement d'air et de pompe de chaleur Cet artic/e r@sume un s@minaire du m@me titre qui s'est tenu /ors de/a r@union de/'ASHRAE ~ Los Ange/es /e 6 f#vrier 1980. On examine/es possibi/it@s g#n#ra/es des vitesses variab/es pour le conditionnement d'air et /es pompes de chaleur et on donne que/ques renseignements particu/iers sur divers constituants principaux des syst#mes d'entraTnement @/ectrique. On pr#sente

This article is a s u m m a r y of a Seminar of the same title, at the Los Angeles A S H R A E Convention on February 6, 1980. The overall potential for variable speed air conditioning and heat pumps is discussed along w i t h some specific information on various major components of the electric drive systems. Results of a system simulation are first presented. This shows the output and energy consumption advantages of the variable speed system and develops some alternatives for size, fan speed, and efficiency. This seminar attracted an estimated 250 people. Four speakers, each experts in their own field, discussed the variable speed modulated heat pump and its major electrical components. System performance, compressor drive motors, the results from electronic generation of variable frequency and variable speed fan motors were all covered.

Output modulation conservation

There f o l l o w s the s u m m a r y of a paper w h i c h presented the general design characteristics and requirements of t w o speed single phase and three phase hermetic motors, that are presently being used. The third paper's discussion shows results found in the operation of an air conditioning system w i t h a specific variable frequency inverter d e v e l o p m e n t system. The final s u m m a r y considers the characteristic of various types of variable speed fan motors that are generally available to the air conditioner manufacturer.

of energy. By taking an analytical look at the total heat pump system, it provided a base and set the tone for the rest of this very interesting and timely session. The authors concluded that in a capacity modulated system, variable airflow is required for both comfort and maximum efficiency gains. Variable speed will allow tradeoffs between reduced equipment size and significantly improved seasonal performance.

and energy

The first presentation 1 showed the advantages of compressor output modulation in the conservation The author is with the Hermetic Motor Department, General Electric Co., Holland, Michigan, USA. This paper is a review of four papers presented at the ASHRAE semi annual and exposition meeting, Los Angeles, California, USA. Paper received 4 April 1980 Editorial footnote: no attempt has been made to convert units to Sl in this paper Note de 1'6diteur: dans cet article on n'a pas cherch6 convertir les valeurs en unit6s SI 01 40-7007/80/0401 96-09S02.00 © 1980 IPC Business Press Ltd and IIR

196

d'abord /es r@su/tats d'une simulation de syst@me. /Is montrent /es avantages pour/e rendement et /a consommation d'@nergie des syst@mes ~ vitesse variab/e et permettent de mettre au point des so/ufions de rechange pour/es dimensions et /a vitesse des venti/ateurs et /e rendement. On r#sume ensuite un rapport qui a present#/es caract6ristiques et /es spbcificat/ons g#n#ra/es de/a conception des moteurs herm@tiques monophas#s et triphas@s deux vitesses qu'on commence b uti/iser. L'examen du troisi#me rapport montre /es r#su/tats d'exp/oitation d'un syst@me de conditionnement d'air # convertisseur de fr6quenee variab/e sp@cifique. Le dernier r@sum# consid@re /es caracMristiques de divers types de moteurs de venti/ateur vitesse variab/e g#n#ra/ement disponib/es pour/e constructeur de conditionneurs d'air.

Miller stated that their performance data are from a computer simulation of the heat pump system. The simulation model provided for supplementary heat for cold ODT (outdoor temperature), cycling losses, control of house temperature and humidity, the compressor, and the heat transfer surfaces with variable loading. Speed range, air flow, and chassis size were studied to find their effect on the seasonal performance in both the heating and cooling modes. The scope of the study (Table 1) was limited to residential sized heat pumps in the Philadelphia weather profile. However, the relative results would apply over a wide range of locations.

International Journal of Refrigeration

T a b l e 1, L i m i t i n g p a r a m e t e r s imposed upon study Tab/eau 1. Param~tres /imitatifs imposds ~ /'~tude Split system air to air heat pumps

cycling. The effects of this degradation (Fig. 2) is described as part load efficiency which was shown to be as low as 76% of the steady state efficiency for the worst case. Single speed motors with various compressor displacements were assumed to have the same motor-compressor efficiencies as two speed or continuously variable speed systems at the same capacity and temperature conditions. Therefore, all variable capacity compressor types as well as single capacity units are represented as a function of displacement, chassis size and air flow.

Displacement rates from 0.25 to 2 times nominal Ambient temperatures from 0°F to 105°F Airflows from 0.75 to 1.25 times nominal Chassis sizes from 1 to 3 nominal tons Philadelphia weather A single speed unit has only one balance point heating and one balance point cooling where the output of the heat pump exactly matches the requirement of the house. A variable speed or other variable output unit can match the demand over a wide range of temperatures (Fig. 1). At all heating conditions, below the balance point temperature where the unit cannot supply the house needs, supplementary heat is added at a COP of 1.0. This reduces the overall efficiency of the heating system. During all operating conditions where the unit's steady state output exceeds the house requirements, the efficiency is degraded due to on-off

All the variable capacity systems in the same chassis size showed higher performance than the single speed system when integrated over the whole heating season to obtain the HSPF (heating seasonal performance factor) (Fig. 3). The same chassis size means the same heat transfer surfaces. A maximum HSPF of 2.2 for the single speed 3 ton chassis can be increased to about 2.7 (a 22.7% improvement) with a three level modulated system of 0.5, 0.75 and 1 per unit displacement with fan air flows of 0.75 and 1 per unit. In Fig. 3 the numerator of the fraction refers to the relative displacements 1.0 09J

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Fig. 1 Range of matching capabilities for single and variable speed units Fig. I Domaine d'app/ication des groupes b une vitesse et des groupes # vitesse variab/e

Fig. 2 /nfluence de/a d#gradation du rendement du fair du fonctionnernent par tout ou r/en o[~: facteur de charge =besoins en chauffage ( o u e n froid) d'une maison/puissance de chauffage par pornpe de cha/eur ( ou puissance frigorifique)

T a b l e 2. S e l e c t e d r e s u l t s o f e f f i c i e n c y g a i n f o r a 2.5 t o n c h a s s i s in P h i l a d e l p h i a Tab/eau 2. Rdsu/tats chois/s du gain de rendement pour un eh~ss/s de pu/ssance fr/gorffique de 7500 kca/ h -~ # Phi/ade/phie Displacement rate range

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Volume 3 Number 4 July 1980

197

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Fig. 3 Augmentation du facteur de performance saisonnier de chauffage des syst#mes ~ puissance variab/e par rapport # ce/ui des syst#mes bune vitesse pour des chassis de m#mes dimensions

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Fig. 4 Sup#rioritb du systbme b vitesse variab/e en re/at/on avec /e COP pour une sdrie de temp#ratures ambiantes

used and the denominator refers to the relative air flows used for the analysis. Similar analyses of the cooling season also showed significant improvements with the variable displacement systems• The opportunity exists to trade off first costs against operating costs in choosing the best combination of chassis size and displacements. A set of curves (Fig. 4) on a specific, continuously variable speed system in a compromise chassis size was shown to exceed single speed apparant COP in both heating and cooling by significant amounts over all outdoor temperatures except those within 1 or 2°F of the single speed balance point. The variable speed system had a two to one speed range (0.65 to 1.3 displacement), single speed, full output fans, and a nominal rating of 1.8 tons at the 1.0 displacement (speed) point. This compares to the 2 ton rating of the single speed unit. The shaded areas in Fig. 4

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Fig. 6 Coup/age et branchement des moteurs tr/phas#s, du type coup/e constant

show the improvements in the effective COP (includes cycling and supplementary resistance heat effects) of this variable speed system. Other selected results (Table 2) compared in 2.5 ton chassis were shown to give as much as 12.7% savings in energy used over the total heating and cooling season. Air flows of less than 1.0 werealso shown to be a requirement for good moisture removal on the low displacement rate designs in the cooling mode. The authors concluded that: reduced airflow is required for comfort control; maximum efficiency gains require variable airflow; displacement rate ranges should straddle the nominal rate; the same displacement rate range is good for both heating and cooling modes; and variable speed will allow tradeoffs between reduced equipment size and significantly improved seasonal performance•

Revue Internationale du Froid

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Fig, 9 T w o speed single phase motors w i t h shared main and phase w i n d i n g s

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Approximate torque ratio 4P/2P =0.74 Fig. 8 Three phase m o t o r c o n n e c t i o n s , variable t o r q u e w i t h tapped winding

Fig. 8 Coup~age et branchement des moteurs triphas#s, ~ coup/e variab/e, avec prises sur /e bob/nage

Windings and characteristics of t wo speed compressor motors Modulation of an air conditioner or heat pump can be accomplished with the use of a two speed motor. The various winding arrangements available for two speed operation of single and three phase motors will be reviewed 2. Three phase motors can be w o u n d so that the windings can be easily reconnected to give a two to one change in the number of poles and a two to one change in the speed. Single phase motors can also be w o u n d for reconnection into a two to one speed change. Mr. Gausman reviewed the three phase connections generally referred to as the constant horsepower, the constant torque, and the variable torque connections (Figs. 5-7). He stated that none of these seemed to accomplish the desired performance for the two speed hermetic. Even the so called 'constant torque' connection had a four pole torque capability of 1.25 times that of the two pole where the customer needed only a 0.8 torque ratio.

Volume 3 Num6ro 4 Juillet 1980

F/~7. 9 Moteurs monophas#s b deux v/tesses avec deux bobinages prmcipaux et deux bob/nages aux//ia/Tes

A variable torque connection with a tapped winding was selected for the compressor drive application (Fig. 8). This requires a third winding for each phase in addition to the standard series/parallel winding set, but with this arrangement, the torque ratio can be adjusted to the application by changing winding turns in the respective winding sections. Gausman showed performance curves for each of the connections. The high and low speed curves for the preferred 0.74 torque ratio connection in a specific motor were shown to give good efficiencies of 85% and 89% for the two operating modes. The two speed single phase motors present additional problems. When both the main and phase windings were shared in both speeds (Fig. 9), large harmonic dips shown below were found in the two pole speed torque curve (Fig. 10). This coupled with low efficiency made this connection unsatisfactory. Separate phase windings provide reasonable speed torque curves. However, the torque ratio was only 0.6, which is not adequate for the near constant torque hermetic application. Four pole mode efficiencies dropped to about 77% peak which is felt to be inadequate with today's needs for low energy consumption. As in the three phase case, a tapped single phase arrangement (Fig. 11 ) gives the best overall performance. A torque ratio of 0.75 was shown with four pole efficiency to peak at about 79% (Fig. 12) while the two pole, high speed efficiency peaked at about 86% (Fig. 13).

M o d u l a t i o n of heat p u m p s by f r e q u e n c y The results of a series of motor, compressor, and heat pump tests operating on a variable frequency power supply were described 3. This power supply is one developed to produce a modified sine wave of ac current referred to as ACS (alternating current synthesis).

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Fig. 11 Moteur ~ deux bobinages principaux et ~ un bobinage auxi/iaire

Motor tests with a dynamometer revealed good efficiency with the ACS compared to sine wave power when operating at 60 Hz. However, at 30 Hzthe ACS efficiency dropped at the heavier loads to a level of 20% below sine wave ,efficiency (Fig. 14). Liles commented later that this low efficiency may be due to the harmonic content of the wave form of the ACS system at 30 Hz. Setting the same RMS voltage to the motor as in the sine wave presents a lower fundamental frequency voltage. This results in

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lower torque and consequently lower efficiency at the heavier loads (Fig. 14). Calorimeter tests were run on the compressor over a wide range of suction and condensing temperatures at 30, 40, 50 and 75 Hz power. This data was used to prepare computer map tables of mass flow and kW input as a function of suction and discharge gas conditions. These tables were then used to predict the compressor performance in the computer simulation of the total heat pump system. The total heat pump unit was operated at the DOE rating points in an environmental test chamber. Curves showing the Btu/H output and COP resulting from these tests are shown in Fig. 15. The curves of COP show that below the 47°F (8°C) ODT rating point, the 30 Hz curve drops below the 40 Hz curve and at even lower temperature it falls below the 60 Hz curve. Liles said this is probably due to the increased sensitivity to motor inefficiencies as the suction density drops with outdoor temperature. Even with the low motor efficiency at 30 Hz, and including the ACS losses, the predicted energy

International Journal of Refrigeration

acity of 75 Hz (0 to 6%). Also shown were the negative effects of ACS losses (5 to 7%) and lower motor efficiency at 30 Hz (12 to 31%).

savings of the single phase modulated system was shown to be 5.0 to 13% depending on the DOE region (Table 3). These resultant savings are from the computer simulation which included the positive effects of: reduced compression ratios at lower condenser/evaporation duties (1 3 to 33%); modulation of fan/blower in step with compressor (0 to 5%); reduced cycling debits (1 to 7%); and reduced resistance heat requirements from the added cap-

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Liles stated that the tota} energy saving was be}ow the 20 to 30% they believed to be required for

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Fig. 15 COP par rapport ~ /a temperature ext&rieure (caisson)

T a b l e 3. Energy savings p r e d i c t e d by DOE f o r single phase A C S m o d u l a t i o n

Tableau 3. Economies d'#nergie prdvues par/e m/nist~re de/'dnergie pour/a modu/ation ~ courant a/ternatif de synth#se monophas# DOE region Line frequency

Annual energy consumption, kWh ACS modulated

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Volume 3 Number 4 July 1980 t J.,a. 3 / 4 - - e

201

Table 4. Energy savings predicted by DOE for ACS polyphase modulation Tab/eau 4. Economies d'~nergie pr~vues p a r / e minist~re d e / ' d n e r g i e p o u r / a m o d u / a t i o n ~ courant a/ternatif de synth~se polyphas~

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justification of frequency modulation. A polyphase motor does not exhibit the large loss in the 30 Hz efficiency that was shown for the single phase motor (Fig. 16). The comparison shows that at the 1.1 kW power point [approximately 60°F (1 7°C) ODT heating mode] the polyphase motor has an efficiency of near 78%, an improvement of about 23 points. According to Liles, when this polyphase motor was used, rather than the single phase motor, the energy savings in the six DOE regions were increased from the 5.0 to 13% range to a new saving of 12.5 to 23% (Table 4). When the secondary benefits from reduced suction gas preheat are included, they expect savings in the 20 to 30% range for most regions. This meets their expected requirement for economic justification of the frequency modulated system.

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Speed torque curves f o r a PSC multispeed motor

Fig. 18 Courbes coup/e-vitesse d'un moteur ~ condensateur permanent b p/usieurs v/teases

Revue Internationale du Froid

any point on this fan load line and any point on the efficiency load line of the next curve (Fig. 19, Table 5)

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Pole changing motors can be of the two winding main or one winding main type. The latter motor (Fig. 20) is limited to a two to one speed change as in a 2/4 or 4/8 pole combination while the two winding motor may be any combination such as a 4/6 or 4/8 pole motor. The efficiency curve (Fig. 21, Table 6) shows two distinct operating points since, the pole changing motors have inherently one operating speed for each given number of poles.

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Efficiency curve for a PSC multispeed motor

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Fig. 21 Efficiency curves for a two speed-one winding pole changing motor

Fig. 20 Schema des bobinages d'un moteur ~ deux vitesses, changement de po/ar/t6 et ~ un seu/ bobinage

F/g. 2 I Courbes de renc/ement d'un moteur ~ c/eux v/tesses, changement de po/arit6 et ~ un seu/ bobinage

low output cooling conditions. Variable air flow by variable fan or blower motor speed can also improve the overall efficiency or seasonal performance factor of the system. Timpone said the purpose of his paper4 was to look at the options available to create variable speed fan motors. He limited his discussion to those options that are economically feasible. He also limited his discussion to low horsepower type motors.

Variable~ freque~/

Volume 3 Num6ro 4 Juillet 1980

Variable voltage(SCR)

"6

The first motor he discussed (Fig. 17) was the PSC (permanent split capacitor) single phase motor. This motor consists of a main winding and an auxiliary or phase winding. Muitispeed operation is created by additional main winding sections added to reduce the strength of the motor. For each additional winding, there is one additional speed. The speed torque curves (Fig. 18) show a continuous fan load speed line through the three operating points on the motor speed torque curves, The motor design, can be altered to make it run at

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Efficiency curves for a PSC motor with variable input

Fig. 22 Courbes de rendement d'un moteur # condensateur permanent ~ a//mentat/on variable (tens~on ou fr@quence)

203

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65 30

65

Efficiency High-speed Low-speed Motor H P High-speed Low-speed

0.35 0.07

0.35

kWh used High-speed

80

803

Low-speed

31 3

-

Total

393

803

Energy cost at $0.05 (kWh) -~

$19.65

$40.17

Timpone pointed out that a combination of the multispeed tapped winding and pole changing windings could be designed to give additional operating speeds in one motor. For instance, two windings in each of two different pole structures could give four speeds in a PSC design. Pole changing three phase fan motors can also be designed in the same manner as those described in the paper by Gausman. PSC motors can also have their speed changed by variable input. A variable voltage supply or a variable frequency supply can be used. The variable voltage can be created by an SCR to give the approximate efficiencies shown in Fig. 22 and Table 7. The variable frequency motor efficiency shown,, assumed a standard high slip PSC motor design. An

2.04

optimum motor for this type application should be redesigned to make better use of the material. Later, in answer to a question, Timpone said that the efficiency was sacrificed by the high slip design used for the sample. For optimum performance, all variable input motors require redesign for the specific application. Even though low speed efficiencies are much lower than at high speed, the power input at the low speed is reduced significantly by the decreased fan load. This is reflected in the energy consumed when a comparison of a two speed and a single speed fan is made. He assumed it operated the same number of hours but only 10% of the time on high speed. If the two speed fan motor is now required to run continuously, as in a continuously modulated output heat pump, the energy consumed will of course go back up and the benefits must come from the overall system optimization. Timpone concluded that all aspects of the variable output system have to be considered in the motor design. Low speed and increased operating hours will have an effect on the bearing and insulation life. Motor efficiency always decreases as speed is reduced below the standard operating point. The power input may be less at the low speed but the total energy used relates to both efficiency and the number of operating hours (Table 8). Therefore, the total system operation must be evaluated to determine the relative energy costs of the various air moving systems in any output modulated air conditioner or heat pump.

References 1 Jaster, H., Miller, R. $. Steady state and seasonal efficiencies of heat pumps with continuously modulated compressors. Seminar at the ASHRAE semi annual and exposition meeting, Los Angeles, California, USA (1980) 2 Gausman, E. J. The characteristics of two speed windings for compressor motors. Ibid. 3 Liles, A. W. Variable frequency modulation versus line frequency operation of a conventional heat pump. Ibid. 4 Timpone, R. E. Ways to vary fan motor speed through design. Ibid. RSM and HJ are with the Thermal Systems Unit, General Electric Co., Schenectady, NY, USA. EJG is with A. O. Smith Corp., Tipp City, Ohio, USA. AWL is with Exxon Enterprises Inc., Florham Park, NJ, USA. RET is with Marathon Electric Manufacturing Co., Wausau, WI, USA

International Journal of Refrigeration