Slow-speed AC motion
Original article date: October 1997
While AC synchronous motors have been on the market for some years, many engineers are not familiar with the potential advantages of the technology. Among the benefits are extremely rapid starting and stopping, as well as reversing, low speed and high torque.
These inductor motors offer a high torque to inertia ratio, so they can reach synchronous speed within 25ms, typically within 5deg. Stopping is typically within 5deg for a 60rpm motor operator on 50Hz mains.
Since the connection between the rotor and stator is magnetic, these motors can be stalled without damage. The magnetic field acts as a non-abrasive magnetic clutch, allowing the rotor to be stopped without overheating. The shaft speed of an AC synchronous motor is synchronous with the line voltage frequency. Speeds are relatively slow, because the internal rotor and stator construction is based on numerous salient teeth, forming multiple pseudo poles.
Synchronous inductor AC motors with permanent magnet rotors have a small amount of residual torque that holds the motor shaft in position when the motor is de-energised. If this torque is not sufficient, a DC voltage can be applied to one or both windings, with the AC voltage removed, this increasing the holding torque to a level required by the application.
Their design also makes AC synchronous inductor motors self-starting. However, because of their oscillatory starting characteristics, there is a limit to the inertial loads which these motors can start from rest. A load having a moment of inertia not exceeding the maximum rating for the motor can be rigidly coupled to the motor shaft. If the load exceeds the maximum, a resilient coupling that will flex sufficiently to allow a small amount of free shaft rotation will usually enable the motor to start the load. Inertia loads as high as 5-10 times the rated maximum can usually be started in this way. The backlash, or play, in chain and timing belt drives in many cases will provide adequate flexing to start high inertia loads.
AC synchronous induction motors can usually be operated directly from a two-phase or three-phase power supply. However, a phase-shifting network is needed when they are operated from a single-phase source. The phase-shifting network for a two-phase motor consists of a resistor and a capacitor, while a three-phase motor needs only a capacitor. Two or more motors can be operated form the same power source, if the total current requirement does not exceed the capacity of the supply.
In an easy-to-read format, Micromech has introduced a simple guide to slow-speed AC motion. It includes detailed wiring diagrams and motor ratings table.
Applications
Simple positioning requirements within 5deg of shaft rotation or where high torque and low speed are demanded. These motors find application in:
test machines
X-Y positioning
paper manufacturing
sign rotators
valve controls
packaging machinery
timing belt transmissions
The motor in action
The AC synchronous inductor motor consists of stationary multi-phase windings and a unidirectional magnetic field. Torque is produced when an AC voltage is applied across the stator windings. Both two-phase and three-phase stator winding designs operate in a similar manner.
With the two-phase example, the rotor and stator have numerous opposing teeth. The rotor has 50 teeth and the stator has 40 teeth on a 48 tooth pitch. The outer periphery of the rotor is a south pole.
The stator has eight poles with two-phase, four-pole windings. The diagram shows one of the rotor teeth aligned with the centre tooth of the stator (A). At stator (B), the centre tooth is a quarter of a rotor tooth pitch distance out of alignment with a rotor tooth. At stator A’, the centre tooth is a half of a rotor tooth pitch distance out of alignment with a rotor tooth. And at stator B’, the centre tooth is three-quarters of a rotor tooth pitch distance out of alignment with a rotor tooth.
With maximum AC current flowing in the field of stator A, the rotor alignment will be as shown in the diagram. One quarter cycle later, the current will be maximum to the field windings of stator B and a rotor tooth will align with the centre tooth of the stator. At each successive quarter cycle, the rotor will advance one quarter of a rotor tooth pitch and the rotor will advance one rotor tooth pitch for each current cycle.
October 1997