Learn how to evaluate your options for push button vs. PLC control for more effective motor starting, run time and stopping.
By Bill Bernhardt, senior commercial engineer, Rockwell Automation
When you look at how everything has changed over time, it’s amazing. Some things have changed for convenience, some out of necessity. In the case of industrial motor starting, it’s both.
Long ago, motors were started by a contactor closure on the line controlled by energizing or de-energizing a coil. This simple on/off push-button control worked great for small and large motors alike.
As time passed, other methods of motor control emerged, which included using variable-frequency drives (VFDs) and soft starters. Both added more control to the starting, run time and stopping of the motor.
Improved control reduces mechanical wear and tear on the system. And the energy savings when starting or reducing current during runtime is a bonus.
The most efficient way to run a starter with the least amount of heat loss is across the contacts of a contactor. When the contactor is closed, current passes through the now connected busbars. No electronic components generate heat.
Bypassing a soft starter while at speed is beneficial. Using a bypass contactor while at speed will allow the silicon-controlled rectifier (SCR) to be gated off, producing lower heat.
There are times when a purely solid-state device is desired, for example in a high-vibration or dirty environment. For those situations, solid-state devices are of use. For other applications, a bypass contactor is appropriate,
What about just starting a load with simple on/off control or in an emergency… say, an application needs to be energized quickly and with little concern about wear and tear — for example, control of town flooding? In those cases, contactors still are viable.
Closing a Contactor
As stated earlier, energizing a coil from a control power source connected to simple push buttons will perform the on/off control. A maintained button or two-position switch, used mainly in two-wire control, will hold the coil energized. The coil will be de-energized when the button is pushed again, and the voltage to the coil is removed. Of course, the push-button contacts have to be rated for the voltage and the current of the coil to which they are connected.
A momentary button (normally open) in combination with an auxiliary contact is used to energize the coil and make a hold-in contact to maintain the circuit. When another momentary button (normally closed) is pressed, this will open the hold in contact, removing power to the coil, and the motor will stop. This is three-wire control. Again, contacts are rated for the coil’s current.
These methods require some wiring of components. To reduce the use of push buttons and wiring, consider using a programmable logic controller (PLC) output to control a contactor coil. Control can be accomplished through the outputs and simple coding of the programmable logic controller (PLC) instead of using external push buttons.
Benefits of Contactor PLC
Using a contactor PLC input frees up space traditionally needed for push buttons, etc. In addition, a PLC that may already be in the system controlling other applications could be used, saving valuable panel space.
Using a PLC reduces the need to have push-button contacts rated for large inrush current for coils of larger contactors. Its control typically is in the mA range, while the hold-in current and associated inrush current would be seen by the control voltage source and would be much higher.
What about the control voltage itself? Does it have to match the PLC input voltage? The PLC input voltage typically does not match the coil voltage, say, 24VDC for the PLC input. Coil voltage could be 120VAC, and the coil could be controlled by the 24VDC PLC input.
This is just as an interposing relay would behave, but in this case it would be built into the coil. Control voltage would be different from the voltage being controlled.
Learn about Allen-Bradley motor control contactors from Rockwell Automation.
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