Braking of induction motor has been the subject of many researches in order to improve its performances. Many braking induction motor's strategies have been developed. Conventional braking modes are electromechanical such as braking with servo motor and counter current braking; then electrical one as direct current braking (DC). These two techniques have major drawbacks which limit their use. The first one is characterized by high braking current which reaches 7 pu and for the second one, brake torque is important only low speeds. In this paper a new electrical braking mode based on
discrete variable frequency control (DFC) is suggested. Its is leading to significant reduction of braking current and improve the braking torque. The proposed strategy has been verified experimentally on a laboratory machine using induction motor feed by three phase inverter, AC thyristors monitored by a microcontroller PIC 16F867.
Index Terms- induction motor, braking, discrete frequency control.
1. INTRODUCTION
For repeated duty cycles and for safety reason, some times it is advised to brake the induction motor loaded [9-11]. So there are two braking types of the induction motor which are electrical and electromechanical braking techniques. In the last one will quote braking with servo motor which is characterized by a brake disc incorporated to the induction motor in order to maintain a fixed position at stand still or to prevent the rotation of the rotor. The other electromechanical braking with counter current
is obtained by inverting two phases of the stator winding, thus the inversion of the direction of the electromagnetic rotating field which permitting the deceleration of the machine. In this technique, an electric device is used to switch off the machine when speed reaches 0 rpm. The major drawback of this technique is the significant stator current that could reach seven times of the rated current.
For the electric technique, the main principle of DC braking is to switch off the motor from the network then apply a direct current in low voltage (20-24V) into stator winding. A resistive electromagnetic torque opposed to the motor one turn off quickly the induction motor. For this technique; more the current is higher more the braking is faster. In addition, in electrical braking mode, energy is transformed into heat in rotor bars due to losses [1],[2]. Consequently, the strong call of current and the braking heats limit the use of this braking technique. For either braking techniques, speed of the induction motor is reduced by the transformation of kinematic energy into heat [3]. To overcome these problems caused by electrical and electromechanical braking modes, and to improve the performances of the motor at braking such as reducing current and increasing braking torque, a new control strategy is proposed. This technique is based on hypersynchronous braking with DFC using AC thyristors. The new strategy allows a fast braking of the induction motor with a significant reducing current compared to electrical and electromechanical braking techniques. The main principle of the proposed strategy DFC followed
by DC is: the use firstly of DFC braking technique to slow down the machine until low speed is reached, then it will be followed by DC braking mode until the machine stops. The strategy of DFC braking is applied when the motor is operating under hyper synchronous speed. This paper deals with four sections. In the first one, the principle of braking with DC is described, that of braking with the DFC strategy has been carried out in the second section, the third one deal with simulation results and the last one deal with experimental validation of these techniques.
2. DC BRAKING TECHNIQUE
DC braking of induction motor is widely used in many industrials applications that require control position like robotic manipulation or actuation [4]. The main principle of this technique consists firstly to disconnect the machine from the net work. Then, to wait until the disappearance of the residual voltage in order to not deteriorate the stator winding. Finally to inject direct current into two terminals of the stator at low voltage [12]. This allows the slow down of the machine until its shutdown, but this deceleration is efficient only at low speed.
2010 7th International Multi-Conference on Systems, Signals and Devices
Figure 1 show the wave form of the residual stator voltage
when the induction motor is disconnected from the network.
Thus, for the DC braking mode the variation of electromagnetic
torque according to the speed is given by [13]:
(1)
Re sidual voltage (V) �o r-------------------------�
200
o
-200
o 1.0 1.2 14 1.6 1.8 Time (s)
M2 Where the scatter coefficient is a = 1- -- , the rotor time Figure 1. wave form of the residual voltage of the
LrLs machine
constant is Tr =..!:I. , w is the rotor angular velocity, Ls is stator Rr �
cyclic inductance, Lr is rotor cyclic inductance, rotor resistance is Rr and P is pairs of pole. According to this expression, electromagnetic torque is
maximum for OJ = OJm which is inversely proportional to the
rotor time-constant. Thus, the new value is [13]
(2)
As illustrated by Figure 2, it is clearly seen that DC braking
torque is efficient at low speeds. This can increases the time
deceleration and consequently the strong call of braking
current. This characteristic can damage the machine and
consequently limits the use of this technique.
3. D FC BRAKING TECHNIQUE
The theory of DFC technique is described in many literatures [6],[7]. In deed, this strategy is based on the use of AC thyristors like inverter [6],[8] in order to generate discrete frequencies which are sub-harmonic of the line one. The generated wave forms of new supply voltage and frequencies are obtained by inclusion or omission of half cycles of the supply frequency [5],[6]. Figure 3 shows some generated DFC voltage at some frequencies such as 12.5 Hz, 16.66 Hz and 25 Hz where the new rated speed is respectively 375 rpm, 500 rpm and 750 rpm. Considering the inversely proportional relationship between the power supply frequency and electromagnetic torque, the purpose of this technique is to improve braking electromagnetic torque with significant reducing current at high speed when comparing this technique with the DC and counter current braking modes. The implantation of this technique is possible while using the soft starter's fixed circuit. For the three-phase balanced power supply, considering the line frequency is fn = 50 Hz. The angular velocity of this supply
frequency is OJn . The expressions of the three-phase power
supply voltages are: ua' ub, Uc are [5],[6]:
ua = urn sin (wnt) . ( 27r )
ub=umsm OJnt- 3 (3)
' :>': --rTT----===========-� r.o
Figure 2. DC Braking torque of induction motor
Figure 3. Sub-multiples of line frequency: 12.5 Hz, 16.66
Hz, 25 Hz generated by DFC technique.
The generated pulsation Wo related to the generated
discrete frequencies and as well as the new supply voltages
systems is given by:
OJn=rOJo (4)
Where r is an integer. When taking phase A as the reference, we can deduce the expressions related to the principal generated wave forms of voltages [5],[6]:
(5)
2010 7th International Multi-Conference on Systems, Signals and Devices
Torque
Braking with Bruung wiJh DFC DC CUl1!!nt
fu-50Hz
Speed " .,
...----!' • • • • • -
Figure 4. Braking torque of induction motor with the
technique of DFC and DC current
Where induction motor is fed by this generated frequency, variation electromagnetic torque is given by figure 4. So this figure shows the variation of braking torque-speed characteristic as function of the generated frequencies. Firstly, operating point is Mo where the motor is fed by 50 Hz voltage frequency and the motor speed is nearly 157 rad/s. Then, applied frequency generated by DFC passes to 25 Hz. The operating point passes to M1. After what generated frequency passes successively to 16.66 Hz and 12.5 Hz. Operating point passes to M2 and M3 where motor speed becomes relatively low. Thus, the DC braking technique could be applied.
4. SIMULATION R ESULTS
Simulation results were carried out using the software PSIM. The induction motor is connected in delta. The parameters of the motor are given in the appendix. At the beginning the variation of speed, torque and braking current are simulated during DC braking. Then, braking with counter current technique has been simulated. Finally braking speed torque and current with the DFC technique is done. In deed, for the three types of braking; firstly induction
motor is disconnect from the network. Then waits until the cancelation of the residual voltage, as shown in Figure 1 for the 1.5 kW machine the residual voltage is canceled after 0.3 seconds. Finally, applies the braking technique. So, the main principle of the braking technique DFC followed by DC is detailed as follows:
A free stop of the machine is used until the residual
Figure 5. Braking of induction motor with DC under the nominal current, (a) braking speed, (b) braking torque, (c) stator current
4 .1. DC braking of induction motor
DC braking speed, torque and stator current of induction motor under the nominal current is given respectively in Figure 5.a, Figure 5.b and Figure 5.c. Thus, braking speed reached 375 rpm at 0.7 s and 0 rpm at 0.8 s. We note that time deceleration decreases if one induction motor is braking under more than the nominal current. As show in figure 5.b, braking torque with DC technique is average and reaches 0.4 pu. The wave form and the average of braking current are shown in figure 5.(c), where its average value reaches
Iaverage = 1 pu .
2010 7th International Multi·Conference on Systems, Signals and Devices
Figure 6. Braking of induction motor with counter current;
(a) braking speed, (b) braking torque, (c) stator current
4.2. Braking with the te chnique of counte r
cur r e nt
In the same way, the braking characteristics of induction motor with the electromechanical braking technique are given in figure 6. Where the wave forms of speed, torque and stator current are respectively given. Figure 6.a shows that with the counter current braking technique, the deceleration speed reaches 375 rpm at 0.6 s then 0 rpm after 0.7 s. As shown in figure 6.b, as soon as the braking technique with counter current is applied; immediately braking torque reaches 5 pu like peak value. The current variation is given in figure 6.c, where the peak current reaches 5.6 pu. With this strategy, the RMS current reaches 4 pu.
Figure 7. Braking of induction motor with the DFC technique under the frequency fo = 12.5 Hz followed by DC strategy; (a) braking speed, (b) braking torque, (c) stator current
4.3. Braking with the te chnique of D FC
followe d by DC
The main principle of this technique is to apply firstly the technique of DFC under the generated frequency 12.5 Hz, then to rock towards the technique of DC. So braking speed decreases firstly until reaches 375 rpm at 0.4 s, then it reaches 0 rpm after 0.5 s; consequently the shutdown of the machine. The braking torque is given in figure 7.b and it is significant during the application of the DFC technique. The braking current is given in figure 7.c; it is clearly
seen that with the technique of DFC the braking current is reduced. The RMS of braking current is evaluated as 1 pu. Comparison characteristics between the three types of braking techniques will be introduced later.
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Table 1. Simulation braking characteristics
Braking Time Current Average techniques deceleration(s) value(pu) torque(pu) DFC-DC 0.5 1 2
DC 0.8 1 0.4 Counter 0.7 4 0.9 current
Table 2. Experimental braking characteristics.
Braking Time Current techniques deceleration( s) value (pu) DFC 3 1 DC 32 I Counter 4 4 current
As referring to table 1, in order to reach 0 rpm, braking time by the technique of DFC is reduced compared to other techniques. Then braking current with the technique of DFC reaches respectively 100%, 25.5% of braking current with the technique of DC and counter current. In the same way braking torque with DFC reaches respectively 500% and 222% of that of the torque braking by DC and counter current. So, it is clearly seen that the technique of DFC improve the braking performance of the machine. The difference between experimental and simulation braking time due to that the simulation model of the induction motor is more simplify than the experimental one.
5. EXPERIMENTAL RESULTS
The experimental system consists of a three phase line voltage, induction motor, a mechanical load and an inverter. The induction motor's data and parameters are given in the appendix and are the same one of the simulation parameters. The experimental tests of the induction motor were done in
figure 8 and 9 where braking speed and current under the
respectively techniques DFC, DC and counter current are
given. Since the DFC technique is applied to allow
induction motor until the rated speed 375 rpm, we propose
in this section a comparison between the performances of
these techniques until this speed is reached. In deed time
deceleration and RMS current is summarized in table 2. As
shown in figure 8, it is clearly seen that speed braking with
DFC is the faster one than with DC and counter current
techniques; the same way for braking current given in figure
9. As referring to table 2, the techniques of DFC, DC and
counter current allow to reach the rated speed 375 rpm at
respectively 3 s, 32 s and 4 s. So, to slow down the motor
until this low speed; the time deceleration with DFC
technique is respectively 9% and 75% of this by DC and
counter current techniques. In the same way, braking current
with DFC is respectively 100% and 250% of braking current
with respectively DC and counter current technique. The
experimental results confirm the simulation one and show
that the DFC technique improves the braking performance
of the machine at high speed as comparing with DC and
techniques; (b )with DFC and counter current techniques
Stator current 10Aldiv
(a)
Time 500ms/div
Stator current lOi'./div
(b)
Time 500ms/div
Stator current lOAldiv
(c)
Time 500ms/div
Figure 9. Braking current: (a) with DFC technique;
(b)with counter current; (c)with DC under the nominal
current
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[4] M. A EI-Sharkawi, M. Akherraz, "Traking Control Technique for Induction Motors," IEEE Transactions
6. CONCLUSION on Energy Conversion, Vol. 4, No. 1, pp. 81-87, March.1989
Based on the fixed main circuit of classic soft starters, a new discrete variable frequency control technique is applied to improve electromagnetic braking torque with a significant reducing current. The strategy defines the triggering instants of soft starter to include partial half cycles of the new supply voltage system. Consequently the generated frequencies such as 12.5 Hz, 16.66 Hz, 25 Hz as well as generated voltage are obtained. Thus, the motor speed can be controlled to follow a desired value. Considering the inversely relation ship between frequency and electromagnetic torque of induction motor; reducing supply frequency can improve braking torque. Thus, the proposed strategy allows to improve the performances of the induction motor at high speed braking such as increasing braking torque with significant reducing current. A comparison study between the braking techniques with discrete variable frequency control, direct current and counter current show that the first one is the efficient braking technique of the induction motor. So, the proposed technique are applied to brake loaded motors at high speed and will be followed by direct current braking mode which develops a high braking torque near to low speed. Simulation results for the proposed strategy are presented. It has been also verified experimentally on a laboratory machine. Experimental tests confirm the simulation results.
A CKNOWLEDGEMENT
This work was supported by the Tunisian Ministry of High
rated speed Nn=1435 rpm, rated current In=3.2 A, stator
resistance Rs = 511, Ls = 485 mH, Rr = 4.5 11 ,
Lr = 485 mH, the magnetizing inductance Lm = 455 mH
7. RE FERENCES
[1] B. Karaliiinas, E. Matkevicius, "Investigation of Linear Induction Motor Braking Modes by Spectral Method," Electronics and Electrical Enginering, Vol. 75, No. 3, pp. 37-40, 2007
[2] Henrik Mosskull, "Robust Control of an Induction Motor Drive," TRITA-EE, Automatic Control School of Electrical Engineering, Stockholm, Sweden, pp. 1-263, 2006
[3] S. Cholula, A. Claudio, J. Ruiz, "Intelligent Control of the Regenerative Braking in an Induction Motor Drive," 2nd International Conference on Electrical and Electronics Engineering (ICEEE) and XI Conference on Electrical Engineering (CIE ), Mexico City, Mexico. September 7-9, 2005
[5] Zhao Kaiqi, Xu Dianguo, Wang Vi, "Discrete Variable Frequency Soft Starting on DSP-based Voltage Controller-Fed 1M Drive ," IECON '03. The 29th Annual Coriference of the IEEE Industrial Electronics Society, Roanoke, Virginia, USA , vol. I, pp. 758- 763, 2003.
[6] A.Ginort, R.Esteller, A.Maduro, R.Pinero, R.Moncada, " High starting torque for ac scr controller," IEEE Transactions on Energy Conversion, Vol. 14, No. 3, pp. 553-559, Sep.l999.
[7] Chi a-Chou Yeh, Nabeel A. O. Demerdash, " Fault Tolerant Operations in Adjustable-Speed Drives and Soft Starters for Induction Motors," IEEE 38 th Annual Power Electronics specialists coriference, Orlando FloriDa UAS, pp. 1942-1949, Jun. 2007.
[8] Hamdy. A. Ashour and Rania. A. Ibrahim, " Implementation and Analysis of Microcontroller based Soft Starters for three phase Induction Motors," EUROCON the international coriference on "Computer as a Tool", Warsa, pp. 2193-2198, Sep. 2007.
[9] Tan H. Fan, Philippe F. Winding, P.Lombard,
Sheppard J. Salon, Harun Ackgoz, "Dynamic Braking
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[10] G .EI-Saady, A .M .Sharaf,A .Mak ky, M.K.EI Sherbiny, G.Mohamed, "A novel technique for AC supply voltage and frequency modulation of induction motor drives," Industry Applications Conference, voLl, pp. 530-537, Oct.1995
[11] H.C. Specht, "Electric braking of induction motor," the 2nd meeting of the American institude of electrical ingeniering, Pittsburgh, pp. 627-640, ApriLl912
[12] Yunfei Wang , Geng Yang', Tao Hong, "Analysis and Implementation of AC Motor Braking Method without Energy Returning or Braking Unit," the Eighth International Conference on Electrical Machines and Systems, Vol.2, pp. 1447 -1451, Sept. 2005
[13] Jean Piere Caron, Jean Paul hautier, Modelisation et commande de la machine asynchrone, Editions Technip, Paris, 1995 ;
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