Regarding the protection of electric motors, what are the commonly used motor protection measures?

In some cases, frequent motor starts are required. For example, the lifting machinery working at the installation site of large equipment often requires the lifting machinery to slightly rise, descend or move back and forth, left and right to move the large equipment parts that are lifting, and the operation is often carried out in the way of "joging". This allows the components of the equipment to be precisely positioned and installed.

The corresponding electric motor on the lifting machinery will start and change direction frequently under heavy load.

In the control circuit that uses a thermal relay for motor overload protection, the high current of each motor start will cause the sensitive element (temperature sensing element) of the thermal relay to get heat. Due to the frequent start of the motor, under the repeated action of the starting current, the heat gradually accumulated by the sensitive elements of the thermal relay does not have time to dissipate, making the thermal relay particularly "sensitive". At this time, the overload protection function of the motor is in a state of imminent triggering, as long as the motor starts, the heat on the sensitive element of the thermal relay increases again, so that the thermal relay acts immediately, and the wrong "motor overload" protection action occurs.

If you want to start the motor smoothly, you have to wait for the temperature of the sensitive components in the thermal relay to cool down and return to a normal state.

In order to avoid the above-mentioned "motor overload" malfunction of the motor that starts frequently, the current relay can be used instead of the thermal relay to monitor and protect the current of the motor. The current relay has two adjustable links, one is to monitor the setting of the current, and the other is to set the delay time. Once the current relay detects that the current of the motor reaches or exceeds the monitoring current value set by the current relay, the current relay does not act immediately, but starts the timer until the delay time is reached, if the current is still above the monitoring current, the current relay will act to cut off the overload operation of the motor.

The current relay monitoring current value can be set to the rated current of the motor, and its delay time can be set slightly greater than the continuation time of the starting current of the motor to avoid malfunction caused by the starting current.

In motor overload protection circuits with frequent start-ups, current relays are more suitable than thermal relays.

The above analysis hopes to help you understand the principle of motor overload protection. If you have any questions, you can continue to ask them.

Mechanical protection measures:

Regular lubrication, regular minor repair, major repair, backrest wheel installation of protective net, vibration detection of large motor, temperature detection.

Electrical protection device:

Over-current protection (over-current protector), short-circuit protection (circuit breaker), anti-condensation device (space heater), insulation protection (leakage circuit breaker), under-voltage and over-voltage protection (circuit breaker), etc.

Do not touch water and keep ventilation.

There are many kinds, but the main ones are: overload protection and underload, overvoltage protection, and phase sequence protection, in addition to leakage protection, etc.

Short-circuit protection, over-current protection, over-current protection and under-voltage protection, etc.

1. Short-circuit protection.

When the insulation of the motor winding, the insulation of the wire is damaged, or when the electrical circuit fails, for example, the main contact of the forward contactor is not broken and the main contact of the reverse contactor is closed, a short circuit will occur. At this time, a large short-circuit current will be generated in the circuit, which will lead to excessive heat generation, which will damage the insulation of the motor, appliances and wires.

2. Overcurrent protection.

Overcurrents can be caused due to incorrect starting and excessive load torque, as well as frequent reverse braking. In order to limit the excessive starting or braking current of the motor, additional resistors are often inserted into the armature circuit of the DC motor or in the rotor circuit of the AC wound rotor motor. If this additional resistor is shorted during starting or braking, a large starting or braking current will be generated.

3. Thermal protection.

Thermal protection is also known as long-term overload protection. The so-called overload means that the current of the motor is greater than its rated current. There are many reasons for overload, such as excessive load, single-phase operation of three-phase motor, under-voltage operation, etc.

When the long-term overload, the motor heats up, so that the temperature exceeds the allowable value, the insulating material of the motor will become brittle, the life will be reduced, and the motor will be damaged when it is serious, so it must be protected.

4. Under-voltage and zero-voltage protection.

When the motor is running, if the power supply voltage disappears for some reason, in order to prevent the motor from starting itself when the power supply is restored, the protection is called zero voltage protection, and the zero voltage protection relay KHv is often used for zero voltage protection. When the motor is operating normally, the excessive reduction of the power supply voltage will cause some electrical appliances to be released, resulting in abnormal operation of the control line and possible accidents.

Service life. The life of the motor is related to various factors such as insulation deterioration, wear and tear of the sliding part, and dysfunction caused by bearing deterioration, etc., and most of them depend on the condition of the bearing. The life of the bearing is as follows: mechanism life and lubricating oil life.

The life of the bearing.

1. The life of the lubricating oil deteriorated due to heat.

2. Mechanical life caused by operating fatigue.

In most cases, the impact of heat on the life of the lubricating oil is greater than the impact of the load weight on the bearing on the mechanical life. Therefore, the life of the motor is estimated by the life of the lubricating oil, and the reason that has the greatest impact on the life of the lubricating oil is the temperature, which greatly affects the life time.