Hysteresis Motor

The Hysteresis motor is a special kind of a motor. It uses magnetism to make it spin. Inside the motor there is the metal ring called the "rotor." This rotor is made from material that can hold the magnetism very well even after the original magnetizing force is removed. Applying electricity to the motor's coils generates a magnetic field. This magnetic field magnetizes the portion of the rotor. The magnetized part of the rotor try to line up with the magnetic field generated by the coils. As the magnetic field keeps shifting the magnetized portion of the rotor keeps chasing it causing the rotor to spin continuously.

The spinning rotor is connected to the shaft or the other mechanical parts to make them move or do the work. Hysteresis motors are simple, reliable, and efficient. They are often used in the small appliances, clocks, timers, and other devices that require the constant smooth rotation.

What is Hysteresis Motor?

A Hysteresis motor is a single-phase motor. Its rotor is a solid steel cylinder that can hold magnetism well. Electricity flowing through the motor's coils produces a magnetic field. This field magnetizes portions of the steel rotor. As the magnetic field changes direction, the magnetic parts of the rotor follow it, causing the rotor to spin continuously. The spinning rotor can power other rotating parts. Hysteresis motors are simple, reliable motors used in devices needing constant smooth rotation.

Hysteresis Motor Construction

A hysteresis motor has five main parts:

1. Stator
2. Single-phase winding
3. Rotor
4. Shaft
5. Shading coil

1. Stator : The stator produces the rotating magnetic field from the single phase electricity supply. It has the two windings first is a main winding and the second is auxiliary winding. In some designs the stator has shaded poles instead.

2. Rotor : The rotor is made of a special magnetic material that can hold magnetism very well even after the original magnetic force is gone. Chrome steel and cobalt alloys are common rotor materials. The rotor is a solid cylinder without windings or teeth. Its magnetic cylinder component is mounted on the shaft using a non-magnetic support, such as brass. The rotor has high resistance to reduce energy losses.

The rotating magnetic field from the stator causes the parts of the rotor to become the magnetized. As the field rotates the magnetized rotor portions chase it making the rotor spin quickly and continuously. The shading coil helps the motor start rotating.

Working Principle of Hysteresis Motor

Starting Behavior

• When the single phase AC power is supplied to the stator windings it creates a rotating magnetic field.
• This rotating magnetic field induces currents (eddy currents) into the solid steel rotor.
• The eddy currents create a force (torque) that starts spinning the rotor, like an induction motor.
• The rotor material has a strong magnetic memory, which improves the starting torque.

Running Behavior

• As the rotor speeds up close to the simultaneous speed the stator's rotating magnetic field locks onto it and pulls the rotor poles.
• Once at synchronous speed, there is no more relative motion between stator and rotor fields.
• The eddy currents stop, and only the "magnetic memory" torque remains to keep the rotor spinning in sync.
• At light loads, the rotor poles stay perfectly aligned with the rotating stator field.
• At higher loads, the rotor poles lag slightly behind the stator field but still chase and stay synchronized.
• If the load becomes too high, the lagging angle increases until the rotor falls out of synchronization.
• As long as loading is reasonable, the rotor acts like a permanent magnet locked to the rotating stator field.

So in starting it behaves like an induction motor, but at running speed it behaves like a synchronous permanent magnet motor using the rotor's "magnetic memory".

What is Hysteresis Power Loss Ph in the Hysteresis Motor?

Hysteresis power loss (Ph) in the rotor is the heat loss due to the constant reversing/flipping of the magnetic domains as the magnetic field changes direction. It depends on :

P_h = k_h f_r B_{max}

1. fr - The symbol "fr" means how often the magnetic field in the rotor changes direction.
2. Bmax - Bmax means the strongest magnetic field in the gap between parts.
3. kh - The kh means the hysteresis constant of the rotor material.

Higher reversal frequency (fr), higher max field (Bmax), and materials with higher hysteresis constant (kh) lead to greater hysteresis power loss and rotor heating.

What is the Equation of the Hysteresis Torque in Hysteresis Motor?

P_{mech} = P_h \left(\frac{1-s}{s}\right) P_h = k_h \cdot f \cdot B_{max}^2 \frac{T_h n_r}{5252} = k_h \cdot f \cdot B_{max}^2 \left(\frac{1-s}{s}\right) n_r = n_s(1-s) f_r = sf_s

T_h = \left(\frac{5252k_h f B_{max}^2}{n_s}\right) n_s = \frac{120 \cdot f_s}{P} T_h = \frac{5252k_h B_{max}^2}{120} \cdot \frac{P}{f}

In the above equation of the hysteresis torque makes it clear that the hysteresis torque is independent of the frequency and speed.

What is the Torque Speed Characteristic of the Hysteresis Motor?

A hysteresis motor produces the constant torque regardless of its speed. This constant torque allows the motor to accelerate and synchronize with any load up to its maximum torque capability. The normal operating range of our motor is shown into the dark vertical line on the torque's speed graph. At the start when the motor is stationary means zero speed it can provide its maximum starting torque. As the motor speeds up, the torque stays at its highest level until it reaches synchronous speed.

At First the synchronous speed the constant torque value represents the maximum load torque and the motor can handle while remaining synchronized. If the load is get increased beyond this maximum torque value the motor will fall out of the synchronization and the stall.

What is the Speed-Torque Characteristics?

The torque is nearly constant from the start to the full synchronous speed. At start the torque comes from eddy currents and hysteresis losses. At full synchronous speed, only the constant hysteresis torque remains. This constant hysteresis torque is the maximum load torque the motor can handle while staying synchronized. If the load exceeds the hysteresis torque value, the motor will stall/fall out of synchronization.

What is the Starting Torque of Hysteresis Motor?

The starting torque of a hysteresis motor is given by the formula :

T_S = K \cdot \phi_S \times \phi_r \times \sin\alpha

Where :

1. K is a proportional constant.
2. Φs is the stator magnetic flux at the synchronous speed.
3. Φr is the rotor magnetic flux.
4. α is the angle by which Φr lags behind Φs due to the hysteresis effect.

Types of Hysteresis Motors

1. Cylindrical Hysteresis Motors : These have a solid cylinder shaped rotor.
2. Disk Hysteresis Motors : The rotor in these is a circular disk or ring shape.
3. Circumferential Field Hysteresis Motors : The rotor is supported by the Non magnetic ring material that does not allow the any magnetic fields to pass through.
4. Axial Field Hysteresis Motors : In these motor the rotor is supported by the magnetic ring material that easily allows the magnetic fields to fully pass through it.

The differences in the shape of the rotor cylindrical or disk and the material used to support the rotor non magnetic or magnetic. But they all work on the same principle of the rotors " magnetic memory " interacting with the rotating stator magnetic field to produce the driving torque.

Advantages of Hysteresis Motor

1. No moving parts or windings in the rotor, so no mechanical vibrations occur. This makes our operation quiet and very smooth.
2. Well suited for accelerating loads with high inertia/momentum.
3. Can achieve multiple speed operations by using gears.

Disadvantages of Hysteresis Motor

1. Poor output power compared to an induction motor of the same size - only about 1/4th the output.
2. Low efficiency in converting the electrical power to the mechanical power.
3. Produces low torque/rotational force.
4. It Has the low power factor and poor utilization of the supplied electrical power.
5. Only available in very small sizes/capacities.

Applications of Hysteresis Motor

1. Sound equipment: Used in audio devices where smooth, vibration-free operation is needed.
2. Recording instruments: Found in high-quality sound and video recording equipment.
3. Record players: Provide the smooth, constant rotation required for playing records.
4. Timing devices: Used in timers, controls and instrumentation requiring precise timing
5. Electric clocks: The constant speed makes them suitable for clocks and watches.
6. Teleprinters: Allow vibration-free operation of teleprinter and similar communication machines.

In general, hysteresis motors are used in applications that require constant speed, smooth operation with no vibrations or noise. Their simple design with no rotating windings or commutators makes them well-suited for precision devices and equipment. However, their low power output limits them to small devices and instruments.

Conclusion

The hysteresis motor is a unique type of single-phase AC motor. It uses the magnetic memory of its solid steel rotor to create torque and rotation. It operates smoothly and quietly by avoiding any mechanical vibrations. While starting like an induction motor, it runs synchronized to the stator's rotating magnetic field using just the hysteresis torque effect. Though low in power and efficiency, hysteresis motors find applications in devices needing precise constant speed operation such as clocks, audio players, and timing instruments. Their simple construction with no rotor windings makes them suitable for small, vibration free precision equipment.