What Is a Shock Resistant Relay?
A shock resistant relay is a safety device that prevents equipment from malfunctioning due to overload.
When a set current value is exceeded, a contact signal is output to stop the equipment. Shock Resistant Relays play an important role in preventing accidents involving property or personnel.
Compared to mechanical safety devices such as shock guards, shock relays have the advantage that they do not require extensive equipment modifications.
Uses of Shock Resistant Relays
Shock resistant relays are safety devices used in conveyors. The following are examples of shock resistant relay applications:
- Overload protection for winders
- Bite protection for chain conveyors and belt conveyors
- Overload protection in cranes
- Biting protection for crushers
Overcurrent protection devices include circuit breakers and thermal relays, which protect motors and upper circuits. These devices prevent the spread of accidents to higher-level circuits due to motor short circuits and burnout due to overloads. Shock resistant relays, on the other hand, are devices that prevent overloading of conveyors and other loads.
Reduction gear motors are used in chain conveyors and winders. Although the speed of the motor with a speed reducer is low, a strong torque is applied when the load shaft is overloaded. This strong torque may cause chain breakage or equipment failure.
If a foreign object is caught in the chain, it can be recovered in a short time, but if the chain breaks, it is difficult to recover immediately. In addition, if the broken chain recoils and strikes a human body, there is a risk of personal injury. To prevent this, shock resistant relays can immediately detect over-torque and stop the motor.
Principle of Shock Resistant Relays
A shock resistant relay consists of a current transformer, display/control unit, and output unit.
1. Current Transformer
The current transformer is a component that detects the current of the motor. It consists of two coils wound around an iron core, etc. There are integrated types and separate types. The integrated type is used for small capacity motors, while the separately mounted type is used for large capacity motors.
2. Display and Control Unit
The display/control section displays the current value and sets parameters. Rotary switches are often used to set parameters.
3. Output Part
The output part is a component that transmits digital signals of overcurrent to the outside. Relay contact outputs using electromagnetic coils are mainly used. This contact output stops a drive device such as an electromagnetic contactor and shuts off power to the motor.
How to Select a Shock Resistant Relay
Shock resistant relays are often specified in terms of recommended capacity by the manufacturer. If a current transformer is installed separately, match the specifications of the current transformer to the capacity of the motor used.
The type of output signal must also be selected. There are self-holding type and self-resetting type, which should be selected according to the control circuit. For the display, either a digital display or an analog display can be selected. In most cases, digital displays are easier to read, but analog displays are often more robust.
Some products may have functions other than overload. Products that detect and transmit missing phases or unbalance are also available, and products with the required functions are selected.
Other Information on Shock Resistant Relays
How to Use Shock Resistant Relays
Shock resistant relays are electronic control components that are generally sensitive to moisture and dust. Therefore, in most cases, they are stored inside control panels. The most common method of mounting a shock relay inside a control panel is to mount it on a circuit board using a DIN rail or the like.
Inside the control panel, they are connected to the electrical wiring and control circuits of the motor. Wrap the motor’s main circuit wiring around the current transformer and connect the output terminals to the control circuit. If a power supply for shock resistant relay is required, it is generally introduced from the control circuit.
Shock resistant relay settings must also be implemented. The minimum operating current value and operating time must be set, and these should be set by checking the load current and starting current during normal operation.
Depending on the specifications of the shock resistant relay, a secondary current of 0 to 5 A is generally used for the current transformer. In such cases, the operating current is often set around 0 to 6 A while watching the load current. The operating time is generally set between several seconds and several tens of seconds.