Induction Motor Starting with Fluid Coupling
Published on : Sunday 01-11-2020
Balakumaran G and Abhijit Mandal elaborate on the advantages of fluid couplings over electrical soft starters.
In modern industrial system, where high inertia loads such as conveyor belts, large fans, crushers, etc., are involved, the motor selection becomes a challenge for the design engineers in view of the starting requirements and design thermal limitation of the motors. Starting of such large loads on DOL can cause adverse effects to the system such as voltage drops caused by inrush current during starting for longer time thereby stressing the other loads connected to the system. Larger size motors are required in the view of starting requirements which is not economical.
Fluid coupling working principle
A fluid coupling works on Hydrodynamic principle. It has an Impeller and Runner inside with no physical connection between them. The impeller is connected to the shaft of driving equipment (Motor) and the runner is connected to the shaft of driven equipment (Load). Upon motor starting, the impeller pumps the fluid within the coupling to the runner, gradually increasing its torque. Once the torque of the runner reaches as that of load, it gets clutched with the load. The starting toque is gradually increased and power is transmitted smoothly accelerating the load thus resulting in soft start.
Types of fluid couplings
Fluid couplings are of two types:
1. Constant-fill couplings
Constant-fill couplings are sealed to the outside and pre-filled before commissioning of the equipment. Drive requirements determine the design and filling quantity. These couplings are mainly used for limiting starting torque to the load. Delay Chambers in constant fill fluid couplings are provided in some applications where the fluid in delay chamber is retained during startup and flows into the working chamber during the normal running condition.
2. Variable-fill (Scoop) couplings
Variable fill coupling are mainly used to regulate the speed of the driven equipment by changing the fill level (by scoop tubes, etc), during running condition. These couplings always have an external fluid circuit for changing the fill levels. Induction motor start without fluid coupling Refer to Fig 2 for Torque-Speed Characteristics of Motor & Load without fluid coupling. Full load torque of motor
T FL = P
ω
T FL = 60x P ……….(equ 1)
2πNr
The difference between the average motor torque (Tm) and average load torque (TL) is the acceleration torque (Ta), which is also the product of Moment of Inertia (J) and angular acceleration (α). The formula for calculating the starting time of the motor is derived as below:
Ta = Tm – TL……….. (equ 2)
Ta = J x α
Ta = GD 2 x ω
4 x ts
Ta = GD 2 x 2πN
4 x 60 x ts
ts = GD 2 x 2πN ………. (equ 3)
240 x Ta
Where,
P = Motor rated Power (KW)
T FL = Full load Motor Torque (Nm)
Tm = Average Motor Torque (Nm)
TL = Average Load Torque (Nm)
Ta = Acceleration Torque (Nm)
J= GD 2 /4=Total Moment of Inertia (kgm 2 ) at motor shaft.
α = Angular acceleration of motor (1/s 2 )
ω = Angular velocity of motor (1/s)
Nr = Rated speed of motor (rpm)
N = Change in speed (rpm)
ts = Starting time (sec)
Details of induction motor and load considered are:
Power (P) = 400 kW
Full load speed (N) = 1480 rpm
GD 2 of motor = 100 kgm 2
GD 2 of load at motor shaft = 700 kgm 2
Total GD 2 = GD 2 of Motor + GD 2 of Load
Total GD 2 = 700 + 100 = 800 kgm 2
Using equation 1, the full load torque of motor is obtained as
T FL = 60 x 400 x 1000 = 2582 Nm
2x3.14x1480
From the above fig 2, the average load torque and average motor torque worked out to be 80% and 150% of motor full load torque (T FL respectively. Using equation 2, the acceleration torque is computed as
Ta = 1.5 T FL – 0.8 T FL = 0.7 T FL = 0.7 x 2582 = 1807 Nm.
From Equation 3, the time for accelerating from 0 to rated speed (Nr) calculated as
ts = 800 x 2 x 3.14 x (1480-0) = 17.14 sec
240 x 1807
For consecutive 3 hot starts, motor should be designed for locked rotor withstand time of 17.14 x 3 = 51.4 sec, which is practically impossible in view of motor design restrictions. The starting current will be 6~7 times full load current throughout the starting period thereby increasing the temperature of motor during starting.
Induction motor start with fluid coupling
Fig 3 shows the Torque-Speed Characteristics of Motor, Fluid Coupling and Load. The motor is started lightly and accelerated to 85% of rated speed (Nr) till the clutching of fluid coupling with load.
GD² of Fluid coupling = 100 kgm² (considered equal to that of motor)
Total GD² till 85% Nr = GD² of Motor + GD² of Fluid Coupling
Total GD² till 85% Nr = 100 + 100 = 200 kgm²
From Fig 3, the average opposing torque of fluid coupling and average motor torque worked out to be 40% and 150% of motor full load torque (T FL ) respectively. Using equation 2, the acceleration torque till clutching of load is computed as:
Ta 1 = 1.5 T FL – 0.4 T FL = 1.1T FL = 1.1 x 2582 = 2840 Nm.
From Equation 3, the time for accelerating from 0 to 85% of rated speed (Nr) calculated as:
ts 1 = 200 x 2 x 3.14 x (0.85x1480-0) = 2.3 sec
240 x 2840
From Fig 3, the average load torque and average fluid coupling running torque worked out to be 80% and 130% of motor full load torque (T FL ) respectively. Using equation 2, the acceleration torque of load is computed as
Ta² = 1.3 T FL – 0.8 T FL = 0.5 T FL = 0.5 x 2582 = 1291 Nm.
Total GD² = GD² of Motor + GD² of Fluid Coupling + GD² of Load
Total GD² = 100 + 100 + 700 = 900 kgm²
From Equation 3, the time for accelerating from 85% of rated speed to rated speed (Nr) is calculated as:
ts 2 = 900 x 2 x 3.14 x (1480-0.85x1480)
240 x 1291
ts 2 = 900 x 2 x 3.14 x (1480 x 0.15) = 4.1 sec
240 x 1291
Total Starting time (ts) = Starting time (ts 1) + Starting time (ts 2 )
Total Starting time (ts) = 2.3 + 4.1 = 6.4 sec.
For consecutive 3 hot starts, motor should be designed for locked rotor withstand time of 6.4 x 3 = 19.2 sec, which is possible. Since the motor is started with light load till 85% of rated speed, the starting current will reduce considerably in a short time thereby limiting the temperature rise of motor during starting.
Advantages of fluid couplings
1. Fluid couplings offer soft start for loads with minimal stress on mechanical parts. For example, in case of the conveyor belt with DOL start, the maximum torque often exceeds 200 per cent of full load torque during starting providing shock to the belts thus surpassing its design limits. Whereas fluid couplings provide low starting torque and dampen the shock to the belts, increasing its life. The typical load acceleration time with fluid couplings varies between 20~60 sec based on the type of load (such as fans, conveyors, etc).
2. Motor starting time reduces with fluid couplings irrespective of the load as explained in the above sections.
3. Motor selection for operating duty rather than starting duty results in lower cost.
4. Fluid couplings are highly efficient (typically 96~99 per cent) due to low slip at rated operating duty.
5. By adjusting the fill level in fluid coupling, the smooth and controlled acceleration of the load can be achieved.
6. In multi drive system, loading between the drives shall be balanced by adjusting the fluid level in couplings.
7. Fluid couplings are provided with fusible plugs for overload protection. In the event of overheating, the plug melts and empties the fluid from the coupling effectively disconnecting power thus protecting the motor as well as load.
Fluid couplings in comparison with electrical soft starters
1. Fluid couplings are simple, robust and installed as part of drive system requiring less space in extreme working conditions. Soft starters are required to be installed in separate clean/air conditioned space.
2. Initial capital expenditure of fluid couplings is low compared to that of soft starters.
3. Starting current drawn by soft starters is high in the range of 350~500 per cent for most of the starting period till the load is accelerated. Whereas in case of fluid coupling, motor is accelerated to its rated speed in short time thereby reducing the starting current.
4. Fluid couplings are not impacted by any external failures as they are part of drive system whereas in case of soft starters, the failure of air conditioning system will have impact on their efficient operation.
5. Fluid couplings can be used in hazardous areas since there are no electrical components involved. Soft starters need to be kept away from hazardous areas due to chances of any sparks/leakages during switching operations.
6. No special training is required for installation, commissioning and operation of a fluid coupling. Soft starters require trained engineers/technicians for commissioning and operating them.
7. Fluid couplings can be easily replaced by available spares in the event of failures and the break down time of the system will be less. For soft starters, OEM support is required for assessment of failure/replacing any components within soft starters.
Significance of oil level in fluid coupling
1. Fluid coupling must be maintained with correct fill level, i.e., not too low nor too high. It has to be as per the actual power requirement of load and not the motor power.
2. Excess fill level above the load requirement will defeat the purpose of soft start of the load and have impact on motor performance. In some cases, fluid coupling may even get cracked due to the high pressure developed inside the coupling.
3. Lower fill level than the load requirement will lead to non-acceleration of load resulting in excessive heating of the fluid. Hence maintaining correct fill level is important for desired operation by fluid coupling.
Conclusion
Fluid couplings are being used in the process and power industries for many years. These are in use today, and will be used in the future too for their simple, robust, compact, minimal maintenance features. With proper selection and operation of fluid couplings for the said applications, it is still the most cost economical proposition.
Balakumaran G has done BE in Electrical and Electronics from BIT Mesra, Ranchi. He has an industry experience of over a decade and his expertise is in the field of Engineering for Power & EPC projects. He has been professionally associated with Reliance Group. Currently, he is working as Senior Manager – Electrical Design Engineering of Reliance.
Abhijit Mandal has masters in Power System from IIT, Roorkee. He has an industry experience of over 2 decades and his expertise is in the field of Engineering and Quality Assurance. He has been professionally associated with top conglomerates, like NTPC Ltd and Reliance Group. Currently, he is heading the Electrical Design Engineering & Quality Department of Reliance.
