WO2007058782A2

WO2007058782A2 - Soft starter for single-phase motor and single-phase motor having such starter

Info

Publication number: WO2007058782A2
Application number: PCT/US2006/042808
Authority: WO
Inventors: Pingshan Cao; Jun You
Original assignee: Emerson Electric Co.
Priority date: 2005-11-11
Filing date: 2006-11-02
Publication date: 2007-05-24
Also published as: CN1964178A; CN100452634C; TW200822522A; WO2007058782A3; TWI393341B

Abstract

A soft starter for a single-phase motor is provided, wherein the single-phase motor includes a main winding, a start winding and a run capacitor, the main winding and the start winding are connected in series with each other and the run capacitor is connected across the main winding and the start winding connected in series, the soft starter comprises a reactor connected in series with the main winding, an AC contactor having a coil, the coil being connected in parallel with the start winding, when a rotation speed of the motor reaches a predetermined threshold, the AC contactor acts to bypass the reactor. The soft starter of the present invention is used to reduce the start current in the main winding, to increase the start torque and to shorten the start time during the motor's start. Moreover, the present invention also provides a single-phase motor having such soft starter.

Description

SOFT STARTER FOR SINGLE-PHASE MOTOR AND SINGLE-PHASE MOTOR HAVING SUCH STARTER

FIELD OF THE INVENTION

The present invention relates to a single-phase motor and in particular to a soft starter for a single-phase motor and a single-phase motor incorporating such soft starter.

BACKGROUND OF THE INVENTION

There are two ways to start a single-phase motor (hereinafter motor), one is direct start and the other is soft start. The direct start is a kind of full voltage start, and the motor in such mode will have a large start current, which will have a great impact on the power network. The soft start generally starts the motor by means of decreasing the start voltage, thereby reducing the start current.

Fig 1 is a diagram showing a high torque starter for a motor in the prior art. As shown in Fig 1 , the high torque starter includes a start capacitor 11 and a potential relay 12. The start capacitor 11 is connected in parallel with the motor's run capacitor 101 and is connected across a start winding 102 and a main winding 103 of the motor connected in series. The coil of the potential relay 12 is connected in parallel with the start winding 102 and the constant-close contact 1 of the potential relay 12 is connected in series with the start capacitor 11. When the main winding 103 is not energized, the constant-close contact 1 is closed. At the moment when the main winding 103 is energized, the rotation speed of motor is equal to zero and the induced voltage of the start winding is very low. Then the coil voltage of the potential relay 12 is also very low so that the relay 12 does not act and its constant-close contact 1 is still in closed state. In such case, since the start capacitor 11 is connected in parallel with the run capacitor 101, the start current in the start capacitor 11 is larger, which causes the start torque to increase and the start time to decrease. When the motor is i . started, with the increase of the rotation speed, the induced voltage of the start winding 102 also increases, whereas the start current gradually decreases. When the coil voltage of the potential relay 12 reaches the operating voltage, the constant-close contact is switched to open state disabling the start capacitor 11.

Such high torque starter could increase the start torque and reduce the start time, but the motor would still have a large start current during the start, which causes line voltage to drop too much.

In order to decrease the start current during the motor start operation so as to make the motor start smoothly and to reduce the impact on the power network, a soft starter can be used to control the motor. Currently a soft starter for motor mainly uses the thyristor to decrease voltage and to limit current.

Fig 2 is a diagram showing an example of the thyristor soft starter for a motor in the prior art. As shown in Fig 2, the thyristor soft starter comprises a start capacitor 21, a bidirectional thyristor 22 composed of two thyristors reversely connected in parallel, a control circuit 23 and three relays 24, 25, 26. The start capacitor 21 is connected in parallel with the run capacitor 201 of the motor. The bidirectional thyristor 22 is connected in series with the run winding 202 of the motor for decreasing voltage and limiting current. The control circuit 23 is connected in series with the main loop of the motor for controlling on and off of the relays 24, 25, 26 and the firing angle of the bidirectional thyristor 22. Relay 24 is connected in series with the main loop for controlling the operation of the main loop. Relay 25 is connected in series with the start capacitor 21 to control on and off of the start capacitor 21. Relay 26 is connected in parallel with the bidirectional thyristor 22 for bypassing the bidirectional thyristor 22 under the control of the control circuit 23. When the main loop is not energized, the relays 24, 25, 26 are opened. If the control circuit 23 detects that the main loop is energized, it signals to the bidirectional thyristor 22 so that the bidirectional thyristor 22 is enabled to be in the state of phase-shifting conduction. Then the control circuit 23 signals to the relays 25 and 24 in turn and the relays 25 and 24 are closed successively. After this, the motor begins to start. At this time, the voltage of the run winding 202 decreases as the bidirectional thyristor 22 is in the state of phase-shifting conduction, thereby the start current of the run winding 202 decreases. Moreover, the parallel connection of the start capacitor 21 and the run capacitor 201 may increase the start torque. With the increase of the rotation speed, the start current gradually decreases. When the motor is in normal operation state, the control circuit 23 signals to Relay 26 and Relay 26 is closed, so that the bidirectional thyristor 22 is bypassed. At the same time, the control circuit 23 controls Relay 25 to be in open state to disable the start capacitor 21. By then, the motor is soft started.

In this solution, the voltage of the run winding 202 can be adjusted through controlling the firing angle of the bidirectional thyristor 22, thereby obtaining different start currents.

However, such thyristor soft starter would result in long start time, severe harmonics in the start current deteriorating the motor's capability of low voltage start. Because the thyristor soft starter includes many components such as the control circuit for controlling the bidirectional thyristor, a start capacitor, three relays, the soft starter has less reliability. In addition, the cost of the soft starter is very high.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an improved soft starter for a single-phase motor so as to have smaller start current and shorter start time and to eliminate the harmonics in the start current, thereby improving the low voltage start capability of the motor.

Another object of the present invention is to provide a single-phase motor having such a soft starter, which has good low voltage start capability.

According to one aspect of the present invention, a soft starter for a single-phase motor is provided, the single-phase motor comprises at least a main winding, a start winding and a run capacitor, the main winding and the start winding are connected in series with each other and the run capacitor is connected across the main winding and the start winding connected in series, the soft starter comprises: a reactor connected in series with the main winding, an AC contactor having a coil, the coil being connected in parallel with the start winding, when a rotation speed of the motor reaches a predetermined threshold, the AC contactor acts to bypass the reactor.

Preferably, the AC contactor comprises a constant-open contact connected in parallel with the reactor.

Preferably, the soft starter further comprises a start capacitor connected in parallel with the run capacitor, wherein when a rotation speed of the motor reaches a predetermined threshold, the AC contactor acts to disable the start capacitor.

Preferably, the AC contactor comprises a constant-close contact connected in series with the start capacitor.

Preferably, the reactor comprises an upper core and a lower core forming a magnetic loop, a main coil and a short-circuit secondary coil wound around the upper core and wherein the main coil is connected in series with the main winding and the short-circuit secondary coil generates a magnetic field opposite to that of the main coil.

According to another aspect of the present invention, a single-phase motor having a soft starter is provided, which comprises: a main winding, a start winding and a run capacitor, the main winding and the start winding being connected in series with each other and the run capacitor being connected across the main winding and the start winding connected in series, a reactor connected in series with the main winding, an AC contactor having a coil, the coil being connected in parallel with the start winding, the AC contactor acting to bypass the reactor when a rotation speed of the motor reaches a predetermined threshold. BRIEF DESCRIPTION OF THE DRAWINGS

It is believed that the above-mentioned objects, features and advantages will become more apparent through the following description of the preferred embodiments of the present invention with reference to the drawings, in which:

Fig 1 is a diagram showing the high torque starter in the prior art.

Fig 2 is a diagram showing a thyristor soft starter for single-phase motor in the prior art.

Fig 3 is a diagram showing a soft starter for single-phase motor according to an embodiment of the present invention.

Fig 4 is a diagram showing an improved reactor.

DETAILED DESCRIPTION OF THE INVENTION

Both Fig 1 and Fig 2 show prior art starter for single-phase motors which have been described above and the corresponding description are omitted.

Fig 3 shows a soft starter for a single-phase motor according to an embodiment of the present invention. As shown in Fig 3, the single-phase motor at least includes a main winding 301, a start winding 302 and a run capacitor 303 wherein the main winding 301 is connected in series with the start winding 302 at connection point C and the run capacitor 303 is connected across the main winding 301 and the start winding 302 connected in series at connection point R and S. The soft starter of the present invention comprises a reactor 31 and an AC contactor 32, wherein the reactor

31 is connected in series with the main winding 301 and the coil of the AC contactor

32 is connected in parallel with the start winding 302, the constant-open contact B is connected in parallel with reactor 31.

Next, the operation process of this soft starter is described. When the main winding 301 is not energized, the constant-open contact B of the AC contactor 32 is opened so that the reactor 31 is connected in series with the main winding 301. When the main winding 301 is energized, the rotation speed of the motor is equal to zero and the induced voltage of the start winding 302 is very low. So the coil voltage of the AC contactor 32 is also low and consequently the AC contactor 32 does not act. The constant-open contact B of the AC contactor 32 remains open. In such a case, the reactor 31 decreases voltage and limits current, thereby reducing the start current in the main winding 301 of the motor. With the rotation speed of the motor increases gradually, the induced voltage of the start winding 302 increases and the start current decreases. When the rotation speed increases to a certain speed (i.e. a predetermined threshold), the motor enters normal operation state and the coil voltage of the AC contactor 32 connected in parallel with the start winding 302 also reaches the operating voltage. So the AC contactor 32 starts to work. The constant-open contact B is closed to bypass the reactor 31. In this way, the motor is soft started.

It can be seen from the above description that during the motor start, the start current in the main winding is reduced by the use of the soft starter of the present embodiment. Moreover, only a few components are used in the soft starter, so the soft starter has higher reliability and lower cost.

In another embodiment according to the present invention, besides the above-described reactor 31 and AC contactor 32, the soft starter also comprises a start capacitor 33, which is connected in parallel with the run capacitor 303 of the motor and connected in series with the constant-close contact A of the AC contactor 32.

In the embodiment, the operation process of this soft starter is same in general as that of the above-described soft starter. Here, only the function of start capacitor 33 is described. When the main winding 301 is not energized, the constant-close contact A of the AC contactor 32 is closed so that the start capacitor 33 is connected in parallel with the run capacitor 303. When the main winding 301 is energized, since the AC contactor 32 does not act, the constant-close contact A remains closed. In such a case, since the start capacitor 33 and the run capacitor 303 are in parallel connection, a large start current passes through the start capacitor 33 thereby increasing the start torque and decreasing the start time. When the rotation speed increases to a certain speed, the motor enters normal operation state and the coil voltage of the AC contactor 32 connected in parallel with the start winding 302 also reaches the operating voltage. So the AC contactor 32 starts to work. The constant-close contact A is opened to disconnect the parallel connection between the start capacitor 33 and the run capacitor 303.

It can be seen from the above description that during the motor starts, the start current is reduced while the start torque is increased and the start time is shortened by the use of the soft starter of the present embodiment.

In the above-mentioned embodiments, the reactor could be any kind of linear reactor in the prior art and the selection of the AC contactor depends on the operating rotation speed of the single-phase motor.

In the case that the prior art linear reactor is used, since the start current passes through the main coil of the reactor generating a over-saturated magnetic field in the core, thereby causing the core to operate in the non-linear zone, a non-sine deformation may occur in the start current resulting in severe harmonics in the start current.

In order to eliminate the harmonics in the start current, the reactor in the soft starter may be improved. Fig 4 shows an improved reactor 31. As shown in Fig 4, the reactor 31 includes an upper core 311 and a lower core 312 forming a magnetic loop, a main coil 313 wound around the upper core 311, which is connected in series with the main winding 301 for decreasing the voltage and limiting the current during the start of the motor. Moreover, the reactor 31 further includes a short-circuit secondary coil 314 wound around the upper core 311. During the motor starts, the start current passes through the main coil 313 and generates a over-saturated magnetic field between the upper and lower cores 311, 312. At the same time, the short-circuit secondary coil 314 generates a magnetic field opposite to that of the main coil 313 to weaken the magnetic field generated by the main coil 313, decrease the saturation degree of the core and eliminate the harmonics in the start current.

Further, the gap between the upper core 311 and the lower core 312 could be increased to consume the magnetic potential generated by the start current during the start, thereby eliminating the harmonics in the start current.

It can be seen from the above description that by means of the improved reactor, the harmonics in the start current could be eliminated; thereby the low voltage capability of the motor can be improved.

Moreover, the present invention also provides a single-phase motor equipped with such a soft starter. The motor comprises a main winding and a start winding connected in series with each other, a run capacitor connected across the main winding and the start winding connected in series. Also, the motor comprises a reactor connected in series with the main winding, an AC contactor having a coil connected in parallel with the start winding and a constant-open contact connected in parallel with the reactor. In addition, the motor further comprises a start capacitor which is connected in parallel with the run capacitor and connected in series with a constant-close contact of the AC contactor.

When the single-phase motor is started, firstly the main winding is energized. Because the rotation speed is equal to zero and both the induced voltage of the start winding and the coil voltage of the AC contactor are low, the AC contactor does not act and the constant-close contact remains closed and the constant-open contact remains open. In such case, the reactor connected in series with the main winding could reduce the start current in the main winding of the motor. Moreover, a large start current passes through the start capacitor, which increases the start torque and reduces the start time. With the gradual increase of the rotation speed, the induced voltage of the start winding increases and the start current decreases. When the rotation speed rises to a certain speed, the motor enters the normal operating state and the coil voltage of the AC contactor reaches the operating voltage. At this time, the AC contactor acts, its constant-open contact is closed to bypass the reactor and its constant-close contact is opened to disconnect the start capacitor. In this way, the motor is soft started and the motor begins to work normally.

The soft starter for single-phase motor and the single-phase motor integrated with the soft starter of the present invention are described above in detailed by way of exemplary embodiments, but these embodiments are not exhaustive. A person skilled in the art may make changes and/or modifications to the embodiments shown within the spirit and scope of the present invention. Therefore, the present invention is not limited to these embodiments and the scope is only limited by the attached claims.

Claims

We claim:

1. A soft starter for a single-phase motor that comprises at least a main winding, a start winding and a run capacitor, the main winding and the start winding being connected in series with each other and the run capacitor being connected across the main winding and the start winding connected in series, the soft starter comprising: a reactor connected in series with the main winding, an AC contactor having a coil, the coil being connected in parallel with the start winding, when a rotation speed of the motor reaches a predetermined threshold, the AC contactor acts to bypass the reactor.

2. The soft starter for a single-phase motor according to claim 1, wherein the AC contactor comprises a constant-open contact connected in parallel with the reactor.

3. The soft starter for a single-phase motor according to claim 1 further comprising a start capacitor connected in parallel with the run capacitor, wherein when a rotation speed of the motor reaches a predetermined threshold, the AC contactor acts to disable the start capacitor.

4. The soft starter for a single-phase motor according to claim 3, wherein the AC contactor comprises a constant-close contact connected in series with the start capacitor.

5. The soft starter for a single-phase motor according to claim 2 further comprising a start capacitor connected in parallel with the run capacitor, wherein when a rotation speed of the motor reaches a predetermined threshold, the AC contactor acts to disable the start capacitor.

6. The soft starter for a single-phase motor according to claim 5, wherein the AC contactor comprises a constant-close contact connected in series with the start capacitor.

7. The soft starter for a single-phase motor according to any of claims 1-6, wherein the reactor comprises: an upper core and a lower core forming a magnetic loop, a main coil wound around the upper core, which is connected in series with the main winding, wherein the gap between said upper core and said lower core is increased in said reactor.

8. The soft starter for a single-phase motor according to claim 7, wherein the reactor further comprises a short-circuit secondary coil wound around the upper core to generate a magnetic field opposite to that of the main coil.

9. The soft starter for a single-phase motor according to any of claims 1-6, wherein the reactor comprises an upper core and a lower core forming a magnetic loop, a main coil and a short-circuit secondary coil wound around the upper core and wherein the main coil is connected in series with the main winding and the short-circuit secondary coil generates a magnetic field opposite to that of the main coil.

10. A single-phase motor having a soft starter, comprising: a main winding, a start winding and a run capacitor, the main winding and the start winding being connected in series with each other and the run capacitor being connected across the main winding and the start winding connected in series, the motor further comprising: a reactor connected in series with the main winding, an AC contactor having a coil, the coil being connected in parallel with the start winding, wherein the AC contactor acting to bypass the reactor when a rotation speed of the motor reaches a predetermined threshold.

11. The single-phase motor according to claim 10, wherein the AC contactor comprises a constant-open contact connected in parallel with the reactor.

12. The single-phase motor according to claim 10 further comprising a start capacitor connected in parallel with the run capacitor, wherein when a rotation speed of the motor reaches a predetermined threshold, the AC contactor acts to disable the start capacitor.

13. The single-phase motor according to claim 12, wherein the AC contactor comprising a constant-close contact connected in series with the start capacitor.

14. The single-phase motor according to claim 11 further comprising a start capacitor connected in parallel with the run capacitor, wherein when a rotation speed of the motor reaches a predetermined threshold, the AC contactor acts to disable the start capacitor.

15. The single-phase motor according to claim 14, wherein the AC contactor comprising a constant-close contact connected in series with the start capacitor.

16. The single-phase motor according to any of claims 10-15, wherein the reactor comprising: an upper core and a lower core forming a magnetic loop, a main coil wound around the upper core, which is connected in series with the main winding, wherein the gap between said upper core and said lower core is increased in said reactor.

17. The single-phase motor according to claim 16, wherein the reactor further comprising a short-circuit secondary coil wound around the upper core to generate a magnetic field opposite to that of the main coil.

18. The single-phase motor according to any of claims 10-15, wherein the reactor comprising an upper core and a lower core forming a magnetic loop, a main coil and a short-circuit secondary coil wound around the upper core and wherein the main coil is connected in series with the main winding and the short-circuit secondary coil generates a magnetic field opposite to that of the main coil.