AC Motor control method

• US 4,499,414 A
• Filed: 11/02/1982
• Issued: 02/12/1985
• Est. Priority Date: 11/04/1981
• Status: Expired due to Fees

• Current Assignee: Fanuc Ltd (Fanuc Corporation)
• Original Assignee: Fanuc Ltd (Fanuc Corporation)
• Inventors: Fujioka, Yoshiki, Hirota, Mitsuhiko
• Primary Examiner(s): Smith, Jr., David
• Application Number:
  US06/438,692
• Time in Patent Office: 833 Days
• Field of Search: 318/803, 318/807-811
• US Class Current: 318/803
• CPC Class Codes: H02P 23/08 Controlling based on slip frequency, e.g. adding slip frequency and speed proportional frequency

Abstract

A method of controlling an AC motor which exhibits a constant torque characteristic at a rotational speed N below a base speed Nb and a constant output characteristic at a rotational speed N above the base speed Nb, comprising the steps of finding a base speed N_L after limiting the output power of the AC motor, N_L being derived from the base speed Nb and the ratio η between maximum output power Pmax₁ and maximum output power Pmax₂ before and after the output power of the AC is limited, respectively, rendering the slip frequency of the AC motor constant until the rotational speed N of the AC motor reaches the base speed N_L, varying the slip frequency in inverse proportion to the rotational speed N for N between N_L and Nb (N_L <N≦Nb), and varying the slip frequency in proportion to the rotational speed N for N above Nb (Nb<N), whereby the output power of the AC motor is rendered constant at a rotational speed N above N_L.

Claims

1. A method of controlling an induction motor which exhibits a constant torque characteristic at a rotational speed N below a first base speed N_b and a constant output characteristic at a rotational speed N above the first base speed N_b, comprising the steps of:

(a) finding a second base speed N_L after limiting the output power of the induction motor, N_L being derived from the base speed N_b and the ratio η

between maximum output power Pmax₁ and the maximum output power Pmax₂ before and after the output power of the induction motor is limited, respectively;

(b) rendering the slip frequency of the induction motor constant until the rotational speed N of the induction motor reaches the second base speed N_L ;

(c) varying the slip frequency in inverse proportion to the rotational speed N for N between N_L and N_b (N_L <

N≦

N_b), said step (c) comprising;

(ci) finding a slip frequency f_1s, at the first base speed N_b by multiplying said ratio η and

a constant slip frequency f_1s together, wherein the latter resides in a region where the slip frequency is constant and is stored in memory beforehand; and

(cii) varying the slip frequency in inverse proportional to the rotational speed N in such a manner that the slip frequency takes on the value f_1s, at the base speed N_b ; and

(d) varying the slip frequency in proportion to the rotational speed N for N above N_b (N_b <

N), said step (d) comprising;

(di) finding a slip s'"'"' at a rotational speed N above the first base speed N_b by multiplying said ratio η and

a constant slip s together, wherein the latter resides in a region where the slip is constant and the slip frequency is proportional to the induction motor rpm, the slip s being stored in memory beforehand; and

(dii) varying the slip frequency in proportion to the rotational speed N, with the slip s'"'"' serving as a proportional constant at a rotational speed N above N_L ;

whereby the output power of the induction motor is rendered constant at a rotational speed N above N_L.

2. A method of controlling an induction motor according to claim 1, in which a slip frequency f_s in the region N_L <

N≦

N_b is computed from

space="preserve" listing-type="equation">f.sub.s =a/N+b

where f_s =f_sl at N=N_L, and f_s =η

f_1s at N=N_b.

3. A method of controlling an induction motor according to claim 1, in which a correspondence table between the rotational speed N and a set value S_v corresponding thereto is stored in memory in advance, a particular set value corresponding to a particular rotational speed is read out of the memory, and clock pulses of a constant frequency F are frequency-divided by the read set value to generate pulses, the frequency of which is indicative of slip frequency.

4. A method of controlling an induction motor according to claim 3, in which S_v =A for N in the range 0<

N≦

N_L, S_v =B·

N for N in the range N_L <

N≦

N_b, and S_v =C/N for N in the range N_b <

N, where A, B and C are constants.

5. A method of controlling an induction motor having a base speed, said method comprising the steps of:

(a) maintaining a constant motor slip frequency from zero rotational speed to a lowered base speed corresponding to an increased output power range;

(b) varying the motor slip frequency inversely proportional to the rotational speed when the rotational speed is greater than the lowered base speed and less than or equal to the base speed; and

(c) varying the motor slip frequency in proportion to the rotational speed when the rotational speed is above the base speed.