Prace Naukowe Instytutu Maszyn, Napędów i Pomiarów Elektrycznych
Nr 64
Politechniki Wrocławskiej
Nr 64
Studia i Materiały
Nr 30
2010
electrical machines, high efficiency motors,
permanent magnet, LSPMSM, power quality
Maciej GWOŹDZIEWICZ*, Ludwik ANTAL*
INVESTIGATION OF LINE START PERMANENT
MAGNET SYNCHRONOUS MOTOR AND
INDUCTION MOTOR PROPERTIES
High efficiency IM obtain higher efficiency than standard IM but simultaneously with lower
power factor. Moreover, they are material-consuming. The simplest way to increase both IM efficiency and power factor saving its simplicity, ruggedness and inherent self-starting property is replacement standard squirrel-cage rotor with squirrel-cage permanent magnet rotor and as it follows
change IM into LSPMSM. PMSM is both energy-saving and material-saving.
1. INTRODUCTION
Although many advantages of squirrel-cage induction motors (low cost, robust,
extreme reliability, low-maintenance, extreme life expectancy, quite good starting
properties), they have one essential drawback – they have relatively small and lagging
power factor so they need significant value of the reactive power to produce the magnetic field. This drawback does not take place in case of synchronous motors which
can operate even at leading power factor. They have also a little bit higher efficiency
and constant speed what can be useful in some applications [4]. Typical synchronous
motor has electromagnetic excitation which needs high maintenance and additional
DC supply. Moreover, synchronous motors have bad starting properties.
As we know magnetic field can be produced by electromagnet or by permanent
magnet. If we replace in synchronous motor electromagnetic excitation with permanent magnets we get rid of excitation winding, slip rings and the main drawbacks of
synchronous motor are eliminated. The whole or most of the magnetic flux is produced inside the machine and no or relatively small value of the reactive power is
absorbed by the working motor [2, 3].
_________
* Politechnika Wrocławska, Instytut Maszyn, Napędów i Pomiarów Elektrycznych, 50-372 Wroclaw,
ul. Smoluchowskiego 19, maciej.gwozdziewicz@pwr.wroc.pl, ludwik.antal@pwr.wroc.pl
14
Stators of induction motors and synchronous motors are practically the same so the
simplest way to obtain high both power factor and efficiency [advantages of a synchronous motor] and inherent self-starting property, simplicity and ruggedness [advantages of an induction motor] of a three-phase AC motor is installation of permanent magnets inside the induction motor rotor saving its squirrel-cage. This type of
motor is called Line Start Permanent Magnet Synchronous Motor. The typical solution
of this operation is shown in Fig. 1 [1].
(a)
(b)
Fig. 1. Cross-section of: IM (a), PMSM (b)
Figure 1 shows cross-sections of the Sh 90L-4 IM made by INDUKTA© [5] and
LSPMSM designed with the same stator as in the Sh 90L-4 IM [1]. As we can see the
only difference between these two motors- IM and PMSM, is the rotor, the stators,
sizes and external views are the same but they work and develop torque in completely
other principles.
2. RUNNING PROPERTIES
Let us compare running properties of IM and PMSM based on the 90L-4 IM type.
Comparison of their load characteristics is presented in Fig. 2. The obtained results of
the IM and PMSM rated performance are shown in table 1.
15
Fig. 2. IM and PMSM load
Table 1. Comparison of IM and PMSM
Parameter
Unit
IM
PMSM
Pn
kW
1,50
2,00
Un
V
400
400
In
A
3,61
3,54
ηn
%
79,8
87,2
cosϕn
-
0,755
0,935
nn
rpm
1419
1500
Tn
N·m
10,1
12,7
As we can see the PMSM has significant advantages in comparison with the IM:
greater power from the same volume of the machine,
− higher efficiency,
− higher power factor,
− constant speed.
Due to magnetic field produced by permanent magnets the PMSM absorbs significantly less reactive power [≈50% less] and for both motors reactive power absorption
depends on the load what is presented in Fig. 3.
16
Fig. 3. IM and PMSM reactive power absorption
The PMSM efficiency is higher because the rotor copper loss ΔPCu2 ≈ 0 due to no
slip like in case of an induction motor according to the formula (1):
2
ΔPCu 2 = I rotor
⋅ Rrotor = s ⋅ Pψ = 0 ⋅ Pψ = 0 ,
(1)
what is presented in Fig. 4.
Fig. 4. IM and PMSM power loss without stator copper loss
If we focus on the IM and PMSM currents in rated operation in Fig. 5 we can notice that the IM current is similar to the sinusoidal current. The PMSM current is significantly distorted and contains higher harmonics. The PMSM current higher harmonics cause absorption distortion power and increase of stray load loss.
17
(a)
(b)
Fig. 5. Rated currents of: IM (a), PMSM (b)
If we investigate temperature rises for the IM and PMSM in rated operation in Fig.
6 we can notice that for the IM rotor temperature rise is greater than stator temperature
rise. For the PMSM the situation is opposite. Moreover, for the PMSM temperature
rises are significantly limited so either insulation with lower class can be applied [and
simultaneously with lower price] or air flow can be reduced [and simultaneously
windage loss can be reduced].
Fig. 6. IM and PMSM temperature rises in rated operation
18
3. STARTING PROPERTIES
Let us compare running properties of IM and PMSM. If we focus on the IM and
PMSM torque and speed during starting for rated torque in Fig. 7 we can notice that
the PMSM starting torque is significantly reduced and oscillates because permanent
magnets during PMSM starting develop oscillating torque. It follows also that the
PMSM speed during starting tends both to increase and to decrease but the PMSM
resultant torque and acceleration are positive.
(a)
(b)
Fig. 7. Torque (a) and speed (b) for IM and PMSM during starting for rated load torque
If we investigate the IM and PMSM starting currents in Fig. 8 we can notice that
the PMSM starting current is higher than the IM starting current and the PMSM
starting time is longer than the IM starting time [almost 2 times longer] so for
PMSM there is greater hazard of overheating in case of large number of motor
startings per time.
19
(a)
(b)
Fig. 8. Starting currents for rated load torque of: IM (a), PMSM (b)
4. CONCLUSIONS
The main advantages of PMSM in comparison with IM are:
− greater power from the same volume of the machine,
− higher efficiency,
− higher power factor,
− constant speed,
− lower temperature rise.
The main advantages of PMSM in comparison with standard SM are:
− much lower maintenance,
− simpler construction,
− no excitation DC supply demand.
The main drawbacks of PMSM in comparison with IM are:
− lower starting torque,
− greater starting current,
20
− longer starting duration,
− only direct on line starting,
− absorption distortion power due to the significant current higher harmonics.
The main drawback of PMSM in comparison with standard SM is lack ability to
power factor regulation.
Line Start Permanent Magnet Synchronous Motors are very good alternative for
high efficiency induction motors because PMSM are both energy-saving and materialsaving.
Praca naukowa finansowana ze środków na naukę w latach 2010–2013 jako projekt badawczy
POIG.01.01.02-00-113/09.
REFERENCES
[1] ANTAL L., ZAWILAK T., Comparison of Induction Motor with Line Start Permanent Magnet Synchronous Motor- measurements results, Zeszyty Problemowe – Maszyny Elektryczne Nr 77/2007,
Wrocław 2007, pp. 277–282.
[2] FEI W., LUK P., MA J., SHEN J.X., YANG G., A High-Performance Line-Start Permanent Magnet
Synchronous Motor Amended From a Small Industrial Three-Phase Induction Motor, IEEE Transaction on Magnetics, Vol. 45, Iss. 10, Oct. 2009, pp. 4724–4727.
[3] HALKOSAARI T., Optimal U/f-control of high speed permanent magnet motors, IEEE Transaction
on Industry Electronics, Vol. 3, July 9–13, 2006, pp. 2303–2308.
[4] PARRISH J., MOLL S., SCHAEFER R.C., Synchronous versus induction motors: plant efficiency
benefits resulting from the use of synchronous motors, Industry Magazine, IEEE, Vol. 12, Iss. 2,
March–April 2006, pp. 61–70.
[5] www.cantonigroup.com (2010 r.).