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Posted on Friday Jan 04, 2019 at 02:54PM in Electrical

Consider this; every motor needs a way to be started and stopped, and in some applications, reversed. The 3 most common will be discussed below, including the pros and cons of each method.


Direct On Line (DOL), also referred to as across the line starting, is the most common with the lowest initial cost, but with the most drawbacks. This is accomplished with a motor starter, which consists of a contactor and a motor overload. This method delivers high torque at startup, but puts the most stress on the system, mechanically and electrically. When using this type of starting, the starter immediately delivers full voltage to the motor, resulting in a temporary inrush current resulting in current draws up to 6-8 times the Full Load Amp (FLA) rating of the motor. An example of this is a 30HP motor with a nominal FLA of 40 Amps; starting DOL, the motor could see 320 Amps until the load gets up to speed. It stresses the motor windings, reducing motor life and can cause belts to squeal, reducing their life, and cause severe stress to couplings, gearboxes and whatever else is connected to the motor shaft, reducing their life as well. For reversing applications, the correct starter must be utilized. It consists of 2 contactors (to switch 2 of the 3 leads) to achieve reverse, and an overload to protect the motor. The overloads react relatively slowly versus the next 2 options. Note: The motor always goes to full speed.


 Soft starters are another method of starting a motor. It ramps up the voltage being delivered to the motor, resulting in a “soft” start.  It reduces the inrush current and can prevent belt slippage and is easier on couplings and the other components connected to the motor. There is a reduction in the startup torque with this method. It is easier on the motor windings because the inrush is typically half of the DOL method above. Most soft starters can be reversed with a switch closure to an input on the device. This is slightly higher in cost vs DOL. Again, the motor always goes to full speed but there is more control of the start and stop.


Variable Frequency Drives, also referred to as VFDs, can deliver full torque at start up and there is no inrush current with this method. Ramp up (acceleration) and ramp down (deceleration) times can be set for smooth starts and stops. Adjustable speed is another benefit of this type of motor control. It is by far the easiest on your motor and system, if the motor is suitable for use on a VFD. Minimum and maximum speeds need to be considered to ensure the correct motor is being used. This method comes at a higher cost than the previous 2 types, but in the right application, it can reduce energy consumption and there are incentives available from the utility companies to offset the initial cost, but the energy savings with also help pay for the VFD. To learn more about the incentives available, you can go to www.DSIREUSA.ORG . All that is required is the Zip Code of the customer location for a list of Policies and Incentives.


Feel free to contact your Purvis Industries sales representative to get connected with one of our Triad RAE for assistance in locating the best solution for you!





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