Reversible Motors: Understanding Their Functionality
When we talk about reversible motors, it’s essential to clarify what makes them unique. Permanent split capacitor type AC motors are generally reversible. However, induction motors aren’t as efficient when it comes to reversing direction because they require coming to a full stop before changing direction. Reversible motors, on the other hand, can reverse much faster—typically within just five revolutions compared to the 30-revolution overrun of induction motors. This makes reversible motors a better choice for applications requiring quick direction changes.
The reason for this difference lies in the additional friction brake components in reversible motors. These brakes apply pressure to the armature when commanded to stop, significantly reducing the motor’s overrun. The holding torque produced by the friction brake is approximately 10% of the motor's output torque, which can be increased through the gear ratio. However, this torque isn’t meant to hold loads vertically; rather, it’s designed to minimize the time the motor continues to spin after power is cut off.
In terms of design, reversible motors share many similarities with induction motors, except for the added friction brake system. This system consists of a spring that continuously presses the brake against the armature, allowing the motor to stop almost instantly. Additionally, reversible motors use balanced windings, meaning both the primary and secondary windings have the same resistance and inductance. This ensures consistent torque regardless of the motor’s direction of rotation, making it possible to change direction on the fly.
Due to the constant friction between the brake and the armature, reversible motors require a capacitor with a higher rating than those used in induction motors to provide sufficient starting torque. This increased torque demand leads to higher operating temperatures, necessitating a reduced duty cycle of 50% (maximum 30 minutes of continuous operation). Still, as long as the motor case temperature remains below 100°C, the motor should operate reliably over time.
Understanding the Theory of Operation
The operation of a reversible motor begins when power is supplied to the copper windings in the stator. This generates a rotating magnetic field around the rotor at the frequency of the AC supply. According to Fleming’s Left-Hand Rule, the moving magnetic field induces a current in the aluminum bars of the rotor, creating opposing magnetic fields per Lenz’s Law. These opposing fields interact with the stator’s magnetic field, causing the rotor to spin.
For those interested in learning more about the operational theory of AC motors, we encourage you to explore our white paper on AC Motor Fundamentals.
Single-Phase Reversible Motor Wiring
Single-phase reversible motors function similarly to single-phase induction motors. However, three-phase motors are commonly used with inverters or VFDs for continuous speed control, making three-phase reversible motors less common. Remember, the rotation direction is indicated when viewed from the output shaft side of the motor.
Capacitor Usage in Single-Phase Motors
A capacitor plays a crucial role in the startup of single-phase motors. Without it, you would need to manually rotate the shaft to initiate movement—a process similar to starting a vintage airplane’s propeller. Wiring the capacitor correctly is vital; incorrect wiring was a frequent issue during my time as a technical support engineer.
Here’s an example of how to wire a 4-terminal capacitor with a single-phase motor:
Even though the capacitor has multiple terminals, the internal wiring connects the two closest terminals, making it functionally equivalent to a traditional two-terminal capacitor.
Remember to always ground the motor using its dedicated protective earth grounding terminal (PE) to prevent electrical hazards.
Demonstration of Standard Wiring
To see how the standard wiring looks, check out this demonstration video.
Electromagnetic Brake Motors: A Step Further
Similar to reversible motors, electromagnetic brake motors incorporate a power-off-activated electromagnetic brake. While the base motor is the same as a reversible motor, offering a 50% duty cycle, electromagnetic brake motors provide shorter overrun and greater holding torque.
These motors are ideal for vertical applications, such as load elevators. The electromagnetic brake locks the motor shaft, ensuring safety in the event of power failure during operation. The brake reduces overrun from 30 revolutions to just 2-3 revolutions, enhancing safety and stability.
The electromagnetic brake operates by engaging when the magnet coil is de-energized. Energizing the coil creates an electromagnet that attracts the armature against the spring’s force, freeing the motor shaft for movement. When the coil is de-energized, the spring presses the armature onto the brake hub, locking the motor shaft in place.
Compared to induction and reversible motors, electromagnetic brake motors have a more complex wiring setup due to the additional components involved. A capacitor is also required for single-phase versions, while three-phase versions are available for variable speed applications, thanks to their continuous-duty induction motor base.
Here’s a wiring diagram for a bidirectional electromagnetic brake motor:
Following this diagram and using the specified switches ensures the brake engages when the motor stops and disengages when it runs. The SW1 switch manages both motor and brake power, while SW2 controls motor direction.
Watch this video for a demonstration of proper wiring, including circuit breakers, switches, and CR circuit modules for surge suppression.
Overrun and Duty Cycle Comparison
Let’s summarize the key differences among induction motors, reversible motors, and electromagnetic brake motors:
| Type of Motor | Overrun | Duty Cycle |
|--------------------------|---------------|--------------|
| Induction Motor | 30-40 revs | Continuous |
| Reversible Motors | 5-6 revs | 50% |
| Electromagnetic Brake | 2-3 revs | 50% |
The overrun refers to the motor shaft revolutions after power is cut. Methods to reduce overrun include adding a high-ratio gearhead, increasing friction, or decreasing load inertia.
The duty cycles mentioned are recommendations. Generally, as long as the motor case temperature stays below 100°C, the motor will perform well.
Stay tuned for our next post on the torque-speed characteristics of AC motors, and don’t forget to subscribe!
Learn More About KII & KIIS Series
Check out this brief video explaining the KII & KIIS Series AC induction motors, reversible motors, and electromagnetic brake motors, along with their intended applications.
And remember, reliable operation starts with understanding your motor’s capabilities!
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