The structure of the AC servo motor stator is basically similar to the capacitor split-phase single-phase asynchronous motor. The stator is equipped with two windings that are 90° apart from each other. One is the excitation winding Rf, which is always connected to the AC voltage Uf; the other The control winding L is connected to the control signal voltage Uc. Therefore, the AC servo motor is also called two servo motors.
The rotor of the AC servo motor is usually made into a squirrel cage, but in order to make the servo motor have a wide speed range, linear mechanical characteristics, no rotation phenomenon and fast response performance, it should have a rotor resistance compared to the ordinary motor Two characteristics, large and small inertia.
There are two types of rotor structures that are currently used:
One is a squirrel-cage rotor made of high-resistivity conductive materials and made of high-resistivity guide bars. In order to reduce the rotor's rotational inertia, the rotor is made slender;
The other is a hollow cup-shaped rotor made of aluminum alloy. The cup wall is very thin, only 0.2-0.3mm. In order to reduce the magnetic resistance of the magnetic circuit, a fixed inner stator, a hollow cup, should be placed in the hollow cup-shaped rotor. The rotor's inertia is small, the response is fast, and the operation is smooth, so it is widely used.
When the AC servo motor has no control voltage, only the pulsating magnetic field generated by the excitation winding in the stator, the rotor is stationary. When there is a control voltage, a rotating magnetic field is generated in the stator, and the rotor rotates in the direction of the rotating magnetic field. Under a constant load, the speed of the motor changes with the control voltage. When the phase of the control voltage is opposite, the servo motor Will be reversed.
Although the working principle of the AC servo motor is similar to the split-phase single-phase asynchronous motor, the rotor resistance of the AC servo motor is much larger than that of the split-phase single-phase asynchronous motor, so there are three significant differences between the servo motor and the single-phase asynchronous motor. Features:
1. Large starting torque
Due to the large rotor resistance, there is a clear difference compared with the torque characteristic curve of ordinary asynchronous motors. It can make the critical slip rate S0>1, which not only makes the torque characteristics closer to linear, but also has a larger starting torque. Therefore, when the stator has a control voltage, the rotor rotates immediately, that is, it has the characteristics of fast start and high sensitivity.
2. Wide operating range
3. No rotation phenomenon
Normally running servo motor, as long as the control voltage is lost, the motor will stop running immediately.
When the servo motor loses the control voltage, it is in single-phase operation. Due to the large rotor resistance, the two rotating magnetic fields in the stator rotating in opposite directions and the two torque characteristics generated by the rotor and the combined torque characteristics of the AC servo motor The output power is generally 0.1-100W.
When the power frequency is 50Hz, the voltage is 36V, 110V, 220, 380V; when the power frequency is 400Hz, the voltage is 20V, 26V, 36V, 115V and so on.
The AC servo motor runs smoothly with low noise. However, the control characteristics are non-linear, and because of the large rotor resistance, large losses, and low efficiency, compared with the same capacity DC servo motor, it is large and heavy, so it is only suitable for small power control systems of 0.5-100W.
The main advantages of permanent magnet AC servo motors are:
1. No brush and commutator, so it is reliable and has low requirements for maintenance.
2. The heat dissipation of the stator winding is more convenient.
3. Small inertia, easy to improve the speed of the system.
4. Adapt to the high-speed and high-torque working state.
5. It has smaller volume and weight under the same power.
The rotor of the AC servo motor is usually made into a squirrel cage, but in order to make the servo motor have a wide speed range, linear mechanical characteristics, no rotation phenomenon and fast response performance, it should have a rotor resistance compared to the ordinary motor Two characteristics, large and small inertia.
There are two types of rotor structures that are currently used:
One is a squirrel-cage rotor made of high-resistivity conductive materials and made of high-resistivity guide bars. In order to reduce the rotor's rotational inertia, the rotor is made slender;
The other is a hollow cup-shaped rotor made of aluminum alloy. The cup wall is very thin, only 0.2-0.3mm. In order to reduce the magnetic resistance of the magnetic circuit, a fixed inner stator, a hollow cup, should be placed in the hollow cup-shaped rotor. The rotor's inertia is small, the response is fast, and the operation is smooth, so it is widely used.
When the AC servo motor has no control voltage, only the pulsating magnetic field generated by the excitation winding in the stator, the rotor is stationary. When there is a control voltage, a rotating magnetic field is generated in the stator, and the rotor rotates in the direction of the rotating magnetic field. Under a constant load, the speed of the motor changes with the control voltage. When the phase of the control voltage is opposite, the servo motor Will be reversed.
Although the working principle of the AC servo motor is similar to the split-phase single-phase asynchronous motor, the rotor resistance of the AC servo motor is much larger than that of the split-phase single-phase asynchronous motor, so there are three significant differences between the servo motor and the single-phase asynchronous motor. Features:
1. Large starting torque
Due to the large rotor resistance, there is a clear difference compared with the torque characteristic curve of ordinary asynchronous motors. It can make the critical slip rate S0>1, which not only makes the torque characteristics closer to linear, but also has a larger starting torque. Therefore, when the stator has a control voltage, the rotor rotates immediately, that is, it has the characteristics of fast start and high sensitivity.
2. Wide operating range
3. No rotation phenomenon
Normally running servo motor, as long as the control voltage is lost, the motor will stop running immediately.
When the servo motor loses the control voltage, it is in single-phase operation. Due to the large rotor resistance, the two rotating magnetic fields in the stator rotating in opposite directions and the two torque characteristics generated by the rotor and the combined torque characteristics of the AC servo motor The output power is generally 0.1-100W.
When the power frequency is 50Hz, the voltage is 36V, 110V, 220, 380V; when the power frequency is 400Hz, the voltage is 20V, 26V, 36V, 115V and so on.
The AC servo motor runs smoothly with low noise. However, the control characteristics are non-linear, and because of the large rotor resistance, large losses, and low efficiency, compared with the same capacity DC servo motor, it is large and heavy, so it is only suitable for small power control systems of 0.5-100W.
The main advantages of permanent magnet AC servo motors are:
1. No brush and commutator, so it is reliable and has low requirements for maintenance.
2. The heat dissipation of the stator winding is more convenient.
3. Small inertia, easy to improve the speed of the system.
4. Adapt to the high-speed and high-torque working state.
5. It has smaller volume and weight under the same power.
