The practice has proved that the fuse is a simple and effective protection device against phase short circuit, single phase short circuit fault and overload of low voltage motor. However, if the type and parameter of the fuse are not selected properly or the use and maintenance are not favorable, the expected protection effect can not be achieved. The following points for attention in the selection and use of low-voltage motor fuse protection are presented for reference.

Key words: low voltage motor fuse maintenance

The practice has proved that the fuse is a simple and effective protection device against phase short circuit, single phase short circuit fault and overload of low voltage motor. However, if the type and parameter of the fuse are not selected properly or the use and maintenance are not favorable, the expected protection effect can not be achieved. The following points for attention in the selection and use of low-voltage motor fuse protection are presented for reference.

1. Selection problem

(1) If the expected short-circuit current is not too large (e.g., less than 4ka), the fuses of rm10, rl6 and rl7 series can be preferentially selected from the economic point of view. On the one hand, users can easily disassemble the melt themselves. On the other hand, they can be used for both short circuit protection and overload protection.

(2) If the short circuit current is expected to be large, fuse with high breaking capacity should be selected, such as rt12, rt14, rt15 series of fuses.

(3) The protection of motor fuses, generally do not require a large capacity and current limiting effect, but hope that the melting coefficient is small. Therefore, it is appropriate to choose zinc melt and lead tin alloy melt.

(4)rm1, rm2 and rc1 series fuses have been eliminated. Therefore, it is generally not allowed to use porcelain insert rc1a series fuses for motor protection. If the conditions to use it, also can barely use rated current for 15 a fuse, following as below 7  5 kw motor overload protection.

2 Parameter Selection

(1) The rated voltage of the fuse shall conform to the operating voltage of the motor. The working voltage of fuse is related to the length of fuse tube and the strength of insulation. Fuses should not be used in circuits higher than their rated voltage, nor should large fuses be fitted into small dissolving pipes.

(2) The rated current of the fuse should be greater than the maximum working current through the motor circuit for a long time. The current carrying part and the contact part of the shell will not be damaged by the working current. The rated current of the fuse shall not be less than that of the fuse.

(3) The ultimate break current of the fuse should be greater than the maximum short-circuit current flowing through. To ensure that when cutting off the fault current, do not burn the fuse.

(4) The rated current of the fuse shall be selected according to the following three conditions:

① Select according to normal working conditions:

The starting current of the motor can reach (4 ~ 8)ied, and the starting duration is about 5 ~ 10s. Under these conditions, fuses should be neither aged nor fusible.

The specific characteristics of the fuse should be according to the curve provided by the manufacturer. According to the test, the rated current of the fuse can meet the above requirements when it is about half of the maximum passing current.

The rated current of the fuse can be selected by pressing down

ie? rj≥iq/k

Eq. iq- motor starting current, generally (4 ~ 8)ied is 4 ~ 8 times the rated current of the motor.

k- proportionality coefficient. Generally, it ranges from 1.5 to 2.5. For infrequent starting motors, 2.5 shall be taken, and for frequent starting motors, 1.5 shall be taken. Winding type

The starting current of the motor is small, and the coefficient can be reduced to 1.25.

② Should be selected according to the time with the control appliance:

When the fuse is used together with the electromagnetic contactor, it should be ensured that the fuse first cuts off the short circuit or overload current, and the contactor is disconnected in the no-load behind. The known contactor action time is 0.04 ~ 0.06s. Therefore, the requirement for fuse fusing time of 0.02 ~ 0.03 s, the reliable coefficient of 

Kk = (0.04 to 0.06 s / 0.02 0.03) = 2

According to the fusing test, when the short circuit current reaches (20 ~ 25)ie? At rj, the fusing time can meet 0.02 ~ 0.03s. Therefore, press the formula below to select the melt current.

ie? rj≥idmax/20 ~ 25

Type of  idmax - through the maximum short-circuit current fuse

(3) According to the requirements of selectivity between superior and subordinate protection:

Be selective. The action time of the lower-level protection must be shorter than that of the upper-level protection, and the action value of the upper-level protection must be longer than that of the lower-level protection. The melt current should be selected according to the data on its protection characteristic curve and its actual error. If the matching margin of the circuit breaker time is considered as 10%, namely +5% ~ -5%, the following conditions must be met

T1 acuity 1.05 +. Zeta % / 0.95 - zeta % x t2

Where ζ%-- the error of fusing break time is determined by the product specification. If it cannot be determined, it is generally considered as 50%

t1-- corresponds to the fault current value, the fuse break time of the upper melt obtained from the characteristic curve, s

t2-- corresponds to the fault current value, and the fusing time of the melt protected by the motor is the next level obtained from the characteristic curve, s

In general, t1≥3t2 is considered.

If the protection characteristic curve cannot be found, it can also be selected by pressing the formula

ie? rj1≥kphie? rj2

Where iie? rj1, ie? rj2-- is the rated current of the superior and subordinate (i.e. motor protection) melt respectively

kph-- fit coefficient, generally 1.8 ~ 2.5. When the fuse has filler, it takes a smaller coefficient, while when there is no filler, it takes a larger coefficient. In general, the rated current of the fuse at the upper level is 2 ~ 3 levels higher than that at the lower level for the adjacent fuses of the same type and melt material.

3 Operation and Maintenance

(1) Before installation, check whether the rated voltage, rated current and ultimate breaking capacity of the fuse are consistent with the requirements. 

(2) During installation, the contact between the melt and the contactor and the contactor and the tool holder should be ensured to avoid high melt temperature