A method for motor thermal overload protection, comprising:
- acquiring a motor thermal limit curve, the motor thermal limit curve representing the relationship between the current in the stator coil of the motor and the maximum allowable safe time;
- selecting a first overload factor calculation point and a second overload factor calculation point on the motor thermal limit curve, and calculating a first overload factor of the first overload factor calculation point and a second overload factor of the second overload factor calculation point based on a preset heating time constant;
- calculating a modified overload factor based on the first overload factor and the second overload factor;
- selecting at least two time constant calculation points on the motor thermal limit curve, and calculating a modified heating time constant for each time constant calculation point based on the modified overload factor;
- obtaining a modified heating time function for each motor operating point based on the modified heating time constant for each time constant calculation point;
- calculating a stator heat level at each motor operating point based on the modified overload factor and the modified heating time function;
- comparing the stator heat level with a first predetermined threshold and a second predetermined threshold greater than the first predetermined threshold, and giving an alarm if the stator heat level is greater than the first predetermined threshold and less than the second predetermined threshold;
- stopping the motor if the stator heat level is greater than the second predetermined threshold;
- wherein calculating the first overload factor of the first overload factor calculation point and the second overload factor of the second overload factor calculation point based on the preset heating time constant comprises calculating the first overload factor and the second overload factor based on the following formula:
- k
- i
-
- (
- I
- eq
- I
- B
- )
- 2
- -
- (
- I
- eq
- I
- B
- )
- 2
- -
- 1
- e
- T
- i
- τ
- ′
-
- heat
- wherein,
- ki is the overload factor at the overload factor calculation point numbered i, wherein i is 1 or 2;
- Ti is the maximum allowable safe time at the overload factor calculation point numbered i;
- Ieq is the equivalent current of the motor;
- IB is the full-load current of the motor;
- τ′heat is the preset heating time constant;
- wherein calculating the modified overload factor based on the first overload factor and the second overload factor comprises calculating the modified overload factor based on the average value of the first overload factor and the second overload factor, the lower limit value of the overload factor and the upper limit value of the overload factor;
- and
- wherein the modified overload factor is calculated based on the following formula:
- {
- k
- =
- k
- *
- ,
- k
- 1
- +
- k
- 2
- 2
- ≤
- k
- *
- k
- =
- k
- 1
- +
- k
- 2
- 2
- ,
- k
- *
- <
- k
- 1
- +
- k
- 2
- 2
- ≤
- k
- **
- k
- =
- k
- **
- ,
- k
- 1
- +
- k
- 2
- 2
- >
- k
- **
- wherein k is the modified overload factor;
- k1 and k2 are the first overload factor at the first overload factor calculation point and the second overload factor at the second overload factor calculation point, respectively;
- k′ is the lower limit value of the overload factor;
- k″ is the upper limit value of the overload factor.