Speed Control of Dual Induction Motor Using Five Leg Inverter

Over the past decade, research efforts have been made to decrease the number of power electronic devices needed in multi-motor drive systems in order to condense the overall complication and cost of the drive. This paper proposes speed control for a dual three-phase induction motor system driven by five-leg voltage source inverter (FL-VSI) which is used in industrial manufacturing processes. Industrial applications frequently need a number of variable speed electric drives. This technique uses five-leg inverter instead of using conventional two three phase inverters. In the majority of cases, these multi-motor drive systems need independent control of individual motors. It is shown recently that it is possible to separately control two three-phase induction machines supplied through a five-leg voltage source inverter, with one inverter leg being common to both machines. The entire performance of the speed control for the five leg voltage source inverter fed dual-motor drive system is investigated using MATLAB/SIMULINK software.


Introduction
Dual-motor drive systems have received significant attention in recent years for the fact of reduced number of devices, size, and losses in the inverters. Various topologies like four leg inverter system, mono-inverter dual-parallel system and nine-switch inverter system have been researched extensively for dual motor drive system. The advantages of these systems are: 1. Reduced switching losses. 2. Reduced volume of the whole system 3. Less switching devices. 4

. Increased efficiency
Traditionally, 3 phase load control methods are of two types.
a. Use of individual inverters to control the loads separately. b. Use of single inverter to drive 2 loads. The disadvantage of the first method is that the number of switching devices increases, volume increases, more controllers requires, more losses and lesser efficiency.
The advantage of second method is that, both the loads can be connected in parallel, which reduces the number of switches, size, losses are reduced as compared to the first case, while the efficiency is improved. This paper proposes five legs voltage source inverter (FL-VSI) supplying the two induction motors is proposed. Five leg VSI can: 1) Reduce the number of power electronic devices.
2) Reduce the overall complexity and therefore the cost.
In the FL-VSI dual-motor drive system, as shown in Fig.  1, one phase of each motor is connected to the common leg of the FL-VSI. The other two inverter legs are connected to the remaining phases of each motor. The peak value of the common leg current can be higher than the other legs depending on the operating conditions. Many control methods have been proposed for FL-VSI dual-motor drive system for improving the voltage utilization factor and independent control. Some of them are Direct Torque Control (DTC) method, space vector modulation, hysteresis control method, double-zero sequence (DZS) method and two-arm modulation method. consists of a DC source, two induction motors, and the FL-VSI. Each leg of the inverter is composed of two IGBT switches with anti-parallel connected freewheeling diodes used to ensure a negative current path through the switches, and in total of 10 semiconductor switches and ten diodes. During operation of the inverter there are 32(2 5 ) possible switching configurations to prevent short-circuit on the dc-supply or open circuit on the load terminals. It should be ensured that the switches of same leg of the inverter should not be turned on simultaneously, as it would result in a short circuit across the DC voltage supply. DC supply is rated at 400V. In order to avoid undefined states in the FL-VSI and undefined AC output line voltages, the switches of any leg of the inverter cannot be switched off simultaneously as this will result in voltages that will depend upon the respective line current polarity. In Fig. 1, two machines share the inverter leg C. Inverter legs A and B are connected directly to phases A1 and B1, respectively, of the machine-1 (M1). Inverter legs D and E are connected to phases B2 and C2, respectively, of the machine-2 (M2). Switching functions Sfj are defined as Sfj = 1 when the upper switch is ON and Sfj = 0 when it is OFF.

Variation of speed by varying load torque
Variation of speed by varying load torque configuration is shown in Fig. 2. Phase delay of 36 0 is given for all gate pulses as ten switches are there in five leg inverter (360/10). Both motors of 10 HP, 1440 rpm, 50 Hz frequency ratings are selected. DC supply of 400 volts is applied as input to the inverter. Power = 7.5KW, N = 1440 RPM Power = Torque (T) * angular speed (ω) ω = (2*Pi*N) / 60 ω = 150.7996 radians/sec T = P/W T = 49.736 N-m T = 49.736 N-m, T/2 = 24.86 N-m, T/4 = 12.434 N-m, Torque at final it will be zero. At starting both the motors will run at synchronous speed i.e., at no load condition. After 5 seconds it will run at full load torque and speed will decrease automatically.

Fig. 2: Five leg inverter with varying torque
Torque is inversely proportional to speed. At 10 seconds motor will be at half load torque, speed will increase again. Again at 15 seconds motor will be at quarter load torque then speed will increase again. And finally at 20 seconds torque will be zero i.e., no load condition and speed will reach again to rated speed.
Parameters of both the motors shown in fig. 3 and pulse generation blocks are shown in fig. 4. Both motors are of same ratings such as 10 HP, 400V, 50 Hz, and 1440 rpm. And for pulse generation block frequency is 50Hz, so time is (1/50) 0.02 seconds. Pulse width is taken as 50%. Phase delay for each switch is 36 0 as shown in fig. 4.  Load torque is varied in steps. First 5 seconds it is zero torque, 5 to 10 seconds full load, 10 to 15 seconds half load, 15 to 20 seconds quarter load and finally 20 to 25 seconds again to no load. Full load torque step block parameters shown in Fig. 5 and half load torque step block parameters shown in Fig. 6.

Fig. 5:
Step block parameters with full load torque Step block parameters with half load torque Fig. 7 shows pulses of first five switches which are given to gates of each IGBT which will trigger the switches and Fig. 8 shows the pulses of pulse generator 6-10. The pulses are phase shifted by 36 0 each. Fig. 9 shows the line to line voltages of all five lines i.e., Vab, Vbc, Vcd, Vde, Vea. DC supply is given as 400V so the peak value of voltage is 400V.    Fig. 12 shows the speed and torque waveforms of both the motors. As the load torque decreases speed will increase, and vice versa.

Speed control of dual induction motor using control system
The speed control of induction motors plays very important role in many industrial applications. So controlling of speed of both the motors simultaneously using five leg inverter is achieved with the help of controllers. The pulses to the IGBT switches are varied using PWM method. The PI controller is used to vary the PWM. The circuit diagram of the complete drive system with controllers is given in Fig. 13. For speed control of both the motors actual speed is taken as feedback to compare with a reference speed. To convert the actual speed from radians to RPM it is multiplying with [60/(2*pi)]. The feedback speed which was taken from the motors is given to PI controllers to compare with the reference speed. Reference speed is given as 1200 RPM for motor 1 and 500 RPM for motor 2 as shown in Fig. 14. Actual speed which is taken from both the motors and reference speed which should be the output is given to the PI controllers.

Fig. 14: PI controller system
PI controller in the feedback control loop calculates the error by taking the difference between the outputs of the system. The KP and Ki values are tuned and found to be 20 and 10 respectively. Finding an error between actual speed and the reference speed is done. Saturation block is used to limit the input signal to the upper and lower limit saturation values. Error which is calculated in PI controller is given to the next controlling part which is named as frequency. The frequency which is in hertz is converted into radians by multiplying it with 2*Pi. Fig.  15 shows the pulse width modulation technique. All the three phases are phase shifted by 120 0 . All the sinusoidal waves are compared with the carrier wave. If sinusoidal waves are greater than carrier signal then the pulses will be generated or else no pulses are generated. The same procedure will continue for all three waveforms. In this way the pulses to fire IGBTs are generated and it will control the speeds which are required. NOT gate is used to ensure that no two switches of the same leg are triggered simultaneously.

Fig. 15: PWM method for generating pulses
Pulses generated using pulse width modulation (PWM) technique for both the motors is as shown in Fig. 16. Generated pulses are given to switches in sequence as given.  Fig. 18 shows the pulses which are generated by the PWM method and are given to all ten switches to get an appropriate speed which are required. Fig. 17 shows the pulses of switches S1, S6, S3, S8 and S5 and Fig.  18 shows the pulses of switches S7, S2, S9, S4 and S10 respectively.  The speed of both motors are shown in Fig. 21 and Fig.  22 respectively. The speed of motor 1 is set as 1200 RPM and motor 2 as 500 RPM.

Conclusion
This paper proposes a speed control method for dual motor system using FL-VSI. DC supply is given to the five leg inverter system. One of the legs is common for both the motors. For controlling speed of both the motors simultaneously speed of both the motors are taken as feedback. With the help of PI controllers the error signals between the reference speed and actual speed is calculated for both motors. PWM method is used for generating pulses. In these controllers three sinusoidal waves which 120 0 apart from each other are generated and are compared with carrier signal to generate the pulses which are used to fire the switches. These pulses will help to give the speeds which are required for proper operation. Reference speed for motor-1 is given for motor 1 as 1200 RPM and for motor 2 as 500 RPM. Finally the speed output is obtained and controlled. And this control system is checked for different speeds for both the motors like motor-1 speed as 1000 RPM and motor-2 speed as 800 RPM.