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Home » Motor control » Choose motor starters (VFD or soft starter) base on your application
Choose motor starters (VFD or soft starter) base on your application
Soft starters are mainly a preference for easing the mechanical burden experienced by whatever the motor is driving.
The abrupt, direct online start for some mechanical systems creates avoidable wear and tear whereas the soft starter ramps up the speed easing the mechanical burden.
1. For small motors (below 5kW), powering pumps/fans & not requiring starting-current limitation a contactor + OLR is all that is needed. If however these need to be run most of the time or such motors are present in large numbers, there's a potential for energy saving so you're better off with VFDs.
1. Motors above 25kW or even smaller motors that are coupled to high-inertia loads do call for starting current limitation. There are penalties of upto 20% of the monthly power bill for just once exceeding the allowed maximum demand. The Max Demand is measured as the kWh consumed every half-hour multiplied by 2. This is a compelling reason enough even if the user is not keen on saving energy.
Another compelling reason is that high starting currents on large motors cause a dip in voltage for other machinery, this may cause trips and nuisance upsets in other machines. The cheapest method for limiting starting current is the star-delta starter. Next is the Soft-starter, followed by the variable frequency drive. Costs for the VFD are coming down steadily and increasingly their potential for energy saving is getting appreciated.
1. A soft starter is incapable of speed-control. It only restricts starting current by applying the voltage in a ramp. The consequence is that for much of the ramp, the torque is severely compromised, as torque is proportional to the square of voltage! For 20%, 50%, 80% voltage the corresponding torques available are just 4%, 25% & 64% of full load torque! But this is OK for fans and centrifugal pumps whose torque requirements are just 0.08%, 12.5% & 50% at these speeds, as these are proportional to the CUBE of their speed.
1. Now the variable frequency drive.... It can give full torque or even 200%(short-duration) at low-speeds even at or stand-still! It also starts motors in a ramp thus is also a soft starter! When used with pumps & fans, it can be configured to reduce the speed of fan/pump to match the load. It can also be configured in speed control or torque-control modes. Torque control is critical when the nature of loading fluctuates. For eg.. pumping fluids of varying viscosity or machines that sand plywood. Whenever thicker fluids/wood demands higher torque on the motor, the drive adapts by decreasing speed and increasing torque. When the torque demand is low, speed increases by itself. This is different from the more common speed control, where speed control (not torque) is the goal. Not all variable speed drives provide torque control. A class of drives which give full load torque at standstill (without pulsating) is called vector drive. This is commonly used for winding sheets of paper/steel/film where critical tension control is needed, along with speed + torque control in constant-power mode namely. Speed is inversely proportional to torque.
To learn VFDs, start with understanding the theory of Induction-motor's speed-torque curve. With this insight, you can configure any make of drive at first go without extra software.
The abrupt, direct online start for some mechanical systems creates avoidable wear and tear whereas the soft starter ramps up the speed easing the mechanical burden.
1. For small motors (below 5kW), powering pumps/fans & not requiring starting-current limitation a contactor + OLR is all that is needed. If however these need to be run most of the time or such motors are present in large numbers, there's a potential for energy saving so you're better off with VFDs.
1. Motors above 25kW or even smaller motors that are coupled to high-inertia loads do call for starting current limitation. There are penalties of upto 20% of the monthly power bill for just once exceeding the allowed maximum demand. The Max Demand is measured as the kWh consumed every half-hour multiplied by 2. This is a compelling reason enough even if the user is not keen on saving energy.
Another compelling reason is that high starting currents on large motors cause a dip in voltage for other machinery, this may cause trips and nuisance upsets in other machines. The cheapest method for limiting starting current is the star-delta starter. Next is the Soft-starter, followed by the variable frequency drive. Costs for the VFD are coming down steadily and increasingly their potential for energy saving is getting appreciated.
1. A soft starter is incapable of speed-control. It only restricts starting current by applying the voltage in a ramp. The consequence is that for much of the ramp, the torque is severely compromised, as torque is proportional to the square of voltage! For 20%, 50%, 80% voltage the corresponding torques available are just 4%, 25% & 64% of full load torque! But this is OK for fans and centrifugal pumps whose torque requirements are just 0.08%, 12.5% & 50% at these speeds, as these are proportional to the CUBE of their speed.
1. Now the variable frequency drive.... It can give full torque or even 200%(short-duration) at low-speeds even at or stand-still! It also starts motors in a ramp thus is also a soft starter! When used with pumps & fans, it can be configured to reduce the speed of fan/pump to match the load. It can also be configured in speed control or torque-control modes. Torque control is critical when the nature of loading fluctuates. For eg.. pumping fluids of varying viscosity or machines that sand plywood. Whenever thicker fluids/wood demands higher torque on the motor, the drive adapts by decreasing speed and increasing torque. When the torque demand is low, speed increases by itself. This is different from the more common speed control, where speed control (not torque) is the goal. Not all variable speed drives provide torque control. A class of drives which give full load torque at standstill (without pulsating) is called vector drive. This is commonly used for winding sheets of paper/steel/film where critical tension control is needed, along with speed + torque control in constant-power mode namely. Speed is inversely proportional to torque.
To learn VFDs, start with understanding the theory of Induction-motor's speed-torque curve. With this insight, you can configure any make of drive at first go without extra software.
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