Instantaneous forward/reverse operation of three-phase AC induction motor

The difference between single-phase and three-phase AC induction motors is more than just the input power. There are a few things you need to know when using a three-phase AC induction motor in instantaneous forward/reverse operation.

What is Instantaneous Forward/Reverse Operation?

Momentary forward/reverse operation describes the motor rotating back and forth repeatedly between two positions. An example application could be a connector test fixture that inserts and retracts a connector to test its reliability. This can be done with an AC motor, brushless motor, servo motor, or stepper motor. They can all reverse direction. The deciding factor is how quickly and accurately you want the motor to stop when commanded to stop.

Image credit: Mathworks

What is the difference between single-phase and three-phase AC induction motors?

1. Winding

First, the winding design is different. The primary and secondary windings of a three-phase motor are more balanced than those of a single-phase motor. In this case, we are referring to the electrical specifications of the windings. See the table below for an example.

Power supply (AC) Engine primary winding secondary winding

U phase (ohm) V phase (ohm) W phase (ohm)

Single phase 200/220/2304IK25A-CW157.6157.1 Not applicable

Three-phase 200/220/2304IK25A-SW179.9179.9179.9

2. Performance

Different winding specifications affect the speed-torque characteristics of the motor. In the figure below, we compare the speed-torque curves of a single-phase motor and a three-phase motor.

By providing more available torque in the lower speed area, three-phase motors provide greater starting torque and are better suited for instantaneous forward/reverse operation. When a single-phase motor is stopped and reversed, the lower torque may cause the motor to accelerate to its rated speed more slowly. When a three-phase motor is stopped and reversed, the higher torque enables it to accelerate to its rated speed more quickly.

3. Starting torque

Just for reference, single-phase motors do not stop instantly. Unless they are used with some type of friction, electromagnetic, clutch-brake mechanism, or electronic brake pack, they will coast to a stop. The coasting distance, or overrun, depends on the friction and inertial loads, but can be as high as 30 revolutions (on the motor shaft; divisible by the gear ratio). This overrun does not apply to the "instantaneous" portion of instantaneous forward/reverse operation, because the motor now needs time to accelerate to its rated speed with each start. If you switch direction too quickly without waiting for the motor to slow to a stop, the motor may continue to spin in the same direction.

Once power is removed, the overspeed of single-phase and three-phase AC induction motors is the same, although the higher starting torque of three-phase motors makes them more suitable for instantaneous forward/reverse operation.

Tip #1: Stop a three-phase motor before reversing direction

There are two reasons why you should stop the motor before reversing its direction if you want it to last. The best approach is to let the motor come to a complete stop before switching directions. Otherwise, testing is recommended.

Gear damage

Power short circuit risk

Gear damage

In some cases, the gears in a geared motor can be damaged when reversing direction. If the motor's direction is switched too quickly, the load may still be rotating in the same direction, but the motor is actually trying to rotate in the opposite direction. Since the torque acts in both directions, the gears can be damaged. Minimizing the impact forces on the gears within the motor's gearbox can extend their life. The higher starting torque of a three-phase motor makes the problem worse.

Power short circuit risk

The wiring of the internal windings of single-phase motors and three-phase motors is different. Therefore, the external wiring method and switch type are different. In the figure below, we show the wiring diagram of single-phase motors and three-phase motors.

Wiring Differences:

Capacitors

change

The first thing you might notice is the capacitor shown in the single-phase motor wiring diagram. The capacitor turns the single-phase power into multi-phase power. Multi-phase power is necessary to create the rotating magnetic field inside the motor.

The second thing you may notice is the number of wires that need to be switched in order to reverse the rotation of a three-phase motor (MC). For single-phase motors, a SPDT (single-pole double-throw) switch can do the job. However, for three-phase motors, a "lossless" electromagnetic switch with an interlocking structure is required. This type of switch does not allow both contacts to be opened at the same time. If either of the two wires is energized at the same time, it may cause a short circuit in the power supply, and then the circuit breaker may stop the motor. There is no risk with single-phase motors because there is only one pole to switch.

Tip #2: Use an Inverter

Another way to control the direction of a three-phase motor is to use an inverter or VFD (Variable Frequency Drive). VFDs are designed to control the direction and speed of three-phase motors (and more), so instantaneous forward/reverse operation can be accomplished more easily. In addition to the popular World K Series of standard AC motors, Oriental Motor's new KIIS Series of high-torque three-phase AC motors are designed specifically for use with VFDs.

Provides speed-torque curves that describe the expected performance of the motor and VFD combination. More will be added as they become available.

Feel free to click on the curve data above to learn more about these motors.

Final Thoughts

Keep in mind that instantaneous forward/reverse operation is not limited to three-phase motors. Any motor can perform forward/reverse operation, the deciding factor is how "instantaneous" and "accurate" you want the operation to be. There is always a trade-off.

Different motors have different ways of performing forward/reverse operation. Therefore, products offer different overspeed and braking frequency specifications. For example, a single-phase reversible motor uses a friction brake to significantly reduce overrun for instantaneous forward/reverse operation. Although the overspeed of the motor shaft is reduced to 2 revolutions, the heat generated by the friction brake limits the duty cycle to 30 minutes at a time. This motor is perfect for machines that can accept a 2-turn overspeed stopping accuracy and can only operate for 30 minutes at a time. For applications that require instantaneous stopping but do not require specific stopping accuracy values, a brushless motor system with a dynamic braking system may be sufficient. Stepper motors or servo motors actually provide the best stopping accuracy, starting torque, and stopping accuracy for instantaneous forward/reverse operation, but controlling these motors requires more than an AC reversible motor.

Here are some comparisons between all motors that can do forward/reverse. Remember, these overtravel values ​​are on the motor. If you add a speed reducer, divide the overtravel by the gear ratio. These are just reference values.

Oriental Motor offers a full range of AC induction motors from 1 W (1/750 HP) to 2,237 W (3 HP). In addition to induction motors, there are reversible motors, electromagnetic brake motors, clutch/brake motors, and flush motors. Various types of gearboxes can be added for high torque applications. Product series groups vary by motor type and function. For example, the World K series is our standard AC motor series that includes several types of motors from induction to electromagnetic brake types. The KIIS series is the three-phase portion of the KII series, which retains the high torque characteristics of the KII series as well as the new three-phase speed control function.

With such a wide range of products, it is recommended to have a motor selection consultation with our technical support engineers to narrow down your product choices.

Here is a breakdown of our entire range of three-phase AC motors.

World K Series (1~150W): Single-phase & Three-phase; Standard

K2S series (30~200W): three-phase; optimized for VFD

Brother Mid G3 Series (1/2~3 HP): Three-phase; High power

On our website, we divide these into "constant speed" and "speed controlled" AC motors. "Constant speed" AC motors include both single-phase and three-phase types, while "three-phase AC motors for VFDs" focuses only on three-phase motors designed for speed control.