Safety is a major concern in a good many motor-driven applications. This is true in industrial applications where motion starts when power is applied and especially true when power is restored after an outage. In such cases, unwanted or unexpected motion is a risk to life or limb. The most simple solution to this problem is the simple, time-proven Run /Stop Relay Circuit.
With this control circuit, motion cannot commence (or restart) without the operator’s specific pushbutton command. While simple, there are numerous variations and enhancements as you will see, and the benefits are clearly obvious.
The limitation to the first circuit is that the load current flows through the control pushbuttons. The power source may be either AC or DC, and may be at any voltage from 6 to 240V. While this may be perfectly acceptable for low power applications, it is not recommended for high power.
Operation is simple. When the normally open (NO) Run pushbutton is pressed, K1 picks up, “shorts” or “seals” the Run contacts and powers the load—when the Run pushbutton is released, the K1 contact (also called “holding contact”) conducts instead of the pushbutton so that operation continues. Pressing the normally closed (NC) Stop pushbutton drops the relay thus removing the power from the load. The loss of power also drops K1 so it cannot automatically restart when power is resusmed.
The second circuit adds a relay contact to power the load so that the stop pushbutton does not conduct load current. The industrial relay suggested can handle 15A AC or DC (up to 30V).
Note that additional pushbuttons may be added as required—Run pushbuttons are connected in parallel, but Stop pushbuttons must be connected in series.
Relay — TE Connectivity (formerly Potter & Brumfield) K10 Series, DigiKey PB329-ND, $12 each
Note that the schematic is laid out with horizontal lines that resemble a ladder with the supply on the left and return on the right—this is referred to “ladder logic.” Ladder logic is also called “relay logic” and can perform relatively complex Boolean operations. Most PLCs (programmable logic controllers) can be programmed in “ladder logic” that is a high level program language and is displayed on the PC screen in a similar way.
This control has a number of enhancements over the initial circuit and is perhaps representative of a large number of systems actually in use. First is that the power relay is now a high current power relay called a “contactor.”
Contactor — TE Connectivity
Contactors are available with a current rating in the range of about 10A to several hundred amps, voltage rating of 240VAC to 600VAC, and 1 to 4 poles. Sometimes low current “auxiliary contacts” are added as an option to help perform relay logic functions and to eliminate additional relays.
The “jog” function is used to index machine position—it is generally “tapped” briefly so that the motor runs only for a fraction of a second until it is positioned correctly. The jog pushbutton is not sealed in so the motor stops when the pushbutton is released.
The transformer on the top of the schematic is called a “control transformer.” For industrial applications the output is generally 120VAC, but it is sometimes 24VAC or 240VAC. Its function is to transform the line voltage to the relay logic voltage and handle the contactor coil inrush current. Contactors with AC coils draw about 5 times rated current until the magnetic circuit closes—this high current can cause the voltage to sag if the control transformer is incorrectly sized.
Safety switch and overload relay contacts
On the right side of the K1 coil, are several contacts. These may perform several functions. The safety switch could be a door or safety guard that must be closed before the machine may be started. The thermal overload relay is a thermal device that monitors motor phase currents and opens a contact should the current exceed the threshold level. The motor thermal switch is a bi-metallic device that is embedded inside the motor stator windings. It provides motor over-temperature protection. There are also numerous other functions that can be added in series at this point.
Thermal Overload Relay
Grainger 3EA40, Telemechanique /Square D
Grainger 1D324, Square D, 30A, 3 Pole
The circuit breaker is an over-current device that opens upon excessive current. It also can perform the function of an input power disconnect switch. The circuit breaker trip current is a gross value that is generally much higher than the normal operating current. This is in contrast to the thermal overload relay that is closely matched to the nameplate motor current rating.
These pushbuttons are high reliability devices that cost in the order of $50 or greater. Generally the experimenter need not be concerned with this type of quality, but if you ever worked in an industrial environment, you could appreciate the value of such. It is not unusual to have an operator actually kicking a Jog pushbutton with his foot while positioning material in a machine with his hands.
Additional functions may be added or removed depending upon requirements. For this sort of equipment, look at Grainger. DigiKey is not generally into industrial controls.