Method of rotating a polyphase motor at less than rated speed
View Patent ↗A polyphase AC induction motor is connected to a power supply through a soft starter having three sets of inverse parallel connected silicon controlled rectifiers with each set corresponding to one particular phase. Low speed starting and operation of the motor can be accomplished through triggering circuits controlling the phases of the triggering pulses in relation to the phases of the supply. The low motor speeds are developed by a gating sequence that generates a low frequency waveform that is less than the main supply frequency to the motor. This low frequency waveform is current and voltage controlled by the gating sequence to permit the AC motor to smoothly operate at speeds less than 100% of rated while developing net positive torque at the low controlled operating frequency.
1. A method of adjustably controlling the rotational speed of a polyphase AC motor which is supplied by a polyphase source of AC electricity, comprising:
providing a plurality of input waveforms, having one for each phase of said polyphase source;
dividing each of said input waveforms into separate positive and negative pulsed waveforms according to a preselected output frequency and gating pattern;
selecting appropriate of said positive and negative pulsed waveforms which are aligned in time according to said preselected output frequency and gating pattern to form a combined AC output waveform having a preselected frequency; and
applying said combined AC output waveform to said polyphase AC motor which is adjustably rotated at less than the rated speed of said polyphase AC motor.
2. A method as described in claim 1 , wherein said polyphase AC motor is rotated in one of a forward and a reverse direction.
3. A method as described in claim 1 , wherein dividing each of said input waveforms into separate positive and negative pulsed waveforms according to a preselected output frequency and gating pattern sequence produces a rotating magnetic field in said polyphase AC motor that has a net positive average torque in the desired direction of rotation at the desired speed.
4. A method as described in claim 3 , wherein said preselected output frequency and gating pattern is selected based upon the desired rotational speed of the AC motor.
5. A method as described in claim 4 , wherein said preselected output frequency and gating pattern incorporates one of a 30 degree and a 15 degree conduction pattern for desired rotational speeds of the AC motor of less than 8⅓ percent.
6. A method as described in claim 5 , wherein said conduction pattern is 30 degrees and said AC motor is a three phase motor.
7. A method as described in claim 5 , wherein said conduction pattern is 15 degrees and said AC motor is a six phase motor.
8. A method as described in claim 4 , wherein said gating pattern produces an average net positive torque in the desired direction of rotation that has a reduced pulsating torque component than the maximum average net positive torque.
9. A method as described in claim 4 , wherein said preselected output frequency and gating pattern incorporates a one of a 60 degree and a 30 degree conduction pattern for desired rotational speeds of the AC motor of greater than 8⅓ percent.
10. A method as described in claim 9 , wherein said conduction pattern is 60 degrees and said AC motor is a three phase motor.
11. A method as described in claim 9 , wherein said conduction pattern is 30 degrees and said AC motor is a six phase motor.
12. A method as described in claim 4 , wherein said gating pattern produces the maximum average net positive torque in said polyphase AC motor.
13. A method as described in claim 1 , wherein said gating pattern may be adjusted when said AC motor is stopped.
14. A method as described in claim 1 , wherein said gating pattern may be adjusted during AC motor operation.
15. A method as described in claim 1 , wherein said input waveforms are variable for an associated preselected output frequency and gating pattern to produce said adjustable rotation.
16. A method as described in claim 1 , wherein said output waveforms are compared with a reference value and the preselected output frequency and gating pattern is automatically adjusted to cause the AC motor to correlate to the reference value and to minimize motor heating.
17. A method as described in claim 1 , wherein said preselected output frequency and gating patterns are preset in accordance with a fixed reference value.
18. A method as described in claim 1 , wherein said preselected output frequency and gating pattern is selected to minimize DC offsets in the motor phase currents.
19. A method as described in claim 1 , wherein dividing each of said input waveforms comprises the use of discrete SCR gate circuits for each of said input waveforms.
20. A method as described in claim 19 , wherein said preselected output frequency and gating patterns are utilized to control the operation of said discrete SCR gate circuits.
21. A method as described in claim 20 , wherein each output state of each SCR gate circuit is calculated as:
SCR
Output
State
=
11
-
(
INT
(
Current
Angle
*
12
)
Output
Period
)
wherein said Output Period is defined as:
Period
=
360
degrees
*
100
Speed
%
entered
by
user
.
22. A method as described in claim 21 , wherein said gating pattern is a 30 degree pattern.
23. A method as described in claim 22 , further comprising input waveforms A, B, C and said preselected frequency and gating pattern comprises:
Gated
Output State
SCRs
Firing angle
0
A+, B−, C+
120
1
A+, B−
150
2
A+, B−, C−
180
3
A+, C−
210
4
A+, B+, C−
240
5
B+, C−
270
6
A−, B+, C−
300
7
A−, B+
330
8
A−, B+, C+
0
9
A−, C+
30
10
A−, B−, C+
60
11
B−, C+
90.
24. A method as described in claim 20 , wherein each output state of each SCR gate circuit is calculated as:
SCR
Output
State
=
5
-
(
INT
(
Current
Angle
*
6
)
Output
Period
)
wherein said Output Period is defined as:
Period
=
360
degrees
*
100
Speed
%
entered
by
user
.
25. A method as described in claim 24 , wherein said gating pattern is a 60 degree pattern.
26. A method as described in claim 25 , further comprising input waveforms A, B, C and said preselected frequency and gating pattern comprises:
Firing
Output State
Gated SCRs
angle
0
A+, B−, C+
120
1
A+, B−, C−
180
2
A+, B+, C−
240
3
A−, B+, C−
300
4
A−, B+, C+
0
5
A−, B−, C+
60.