SMC: Do You Need a Reduced Voltage Soft Starter?

| Tom Hopkins


Before we delve into the features and options of an SMC™ from Rockwell Automation®, let’s first look at your options.

Review of Motor Starter Options 

There are three devices to choose from when starting a motor. In order of features:

  • An across the line starter consisting of a contactor with an overload relay. This is the least expensive option, but it has some downsides, depending on the application. The motor will hit full speed very quickly. This can be jarring to equipment if the motor is large and is likely to wear mechanical components over time. A motor has an inrush of 6-10 times the full load amperage when started across the line. Depending on the size of the motor, the resulting voltage sag could cause issues for surrounding equipment or the utility. 
  • Soft motor controller or SMC (also called a reduced voltage soft starter). This device reduces the voltage to the motor and in so doing reduces the torque produced. Torque is reduced by the square of the input voltage, so a 50% reduction in voltage would equate to 25% of the torque. An SMC also allows the limitation of the current draw.
  • Drive. This device uses “pulse width modulation” to give more control over the process. A drive has many features in addition to a softer startup. If varying the speed of a motor is part of your process, you have to use a drive.

Depends on Your Application

As I said, if you need to vary the speed of the motor, a drive is the only option. But if once the motor is started it just runs full speed, you should consider an SMC. A soft starter will not jar the equipment at start-up or shut down. And if it is a larger motor, it will keep the inrush down, hence not tripping off other devices in the plant or incurring utility ire and demand charges. In the case of larger motors, an SMC is significantly less money, as well.

Motor Size

If you have a smaller motor of < 25 amps (15HP at 480V), Allen-Bradley® has what they call a starting torque controller (STC). It is a no-frills SMC with two adjustment dials. One dial for voltage reduction (10-80%) and another to adjust the application time (1/2 to 5 seconds). There are no communications for pulling status info. And it must be used in combination with a contactor. But the total cost of STC and contactor is only $200-300. 

For motors up to 50 HP, drive technology has come down in price so much that it may be just as cost-effective to buy a drive and have the extra features available.

Above 50 HP, if you do not need to vary the motor speed, an SMC has a HUGE price advantage. In the Allen-Bradley product line, you are talking 20% to 40% of the cost of a drive, depending on the HP. Depending on the series you choose, several helpful features are also available. 

Choices in SMC Platforms Over 25 Amps

SMC-3SMC-3: Uses simple dip switches to program it. Options include soft start and soft stop, current limit, and kick start (for those times when you need a nudge to overcome static friction). This device does not have any communications capability and is available up to 900 HP.  

SMC-Flex: This is a full-featured device with integral bypass and flexible communications with advanced performance, diagnostics, and protection. Available for motors up to 1000 HP.  

SMC-50SMC-50: This is the premier product with advanced monitoring and protection functions, superior communication capability, and energy saver mode.

The SMC-50 has jogging capability of plus or minus 15% speed for short periods of time. The “linear acceleration” mode feature is superior for startup in almost all applications, especially challenging ones like pumps. High-inertia loads such as band saws, centrifuges, mixers, flywheels, rock crushers, and hammer mills can also be improved using linear acceleration. Here is a link to a white paper about the technology.
Even though the SMC-50 is the premier product, this platform is slightly less money than the SMC-Flex, but only available for applications up to 450HP.

Horizon Solutions specialists are available to help you with your motor starting applications. Give us a call!

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