Effect of Changes in Supply Frequency on Torque and Speed

Hardly any important changes in frequency take place on a large distribution system except during a major disturbance. However, large frequency changes often take place on isolated, low-power systems in which electric energy is generated by means of diesel engines or gas turbines. Examples of such systems are: emergency supply in a hospital and the electrical system on a ship.

The major effect of change in supply frequency is on motor speed. If frequency drops by 10%, then motor speed also drops by 10%. Machine tools and other motor-driven equipment meant for '50 Hz causes problem when connected to 60-Hz supply. Everything runs (60 -- 50) * 100/50 = 20% faster than normal and this may not be acceptable in all applications. In that case, we have to use either gears, to reduce motor speed or an expensive 50-Hz source.

A 50-Hz motor operates well on a 60-Hz line provided its terminal voltage is raised to 60/50 = 6/5 (i.e. 120%) of the name-plate rating. In that case, the new breakdown torque becomes equal to the original breakdown torque and the starting torque is only slightly reduced. However, power factor, efficiency and temperature rise remain satisfactory.

Stepper motors

Stepping motors can be viewed as electric motors without commutators. Stepper motors consist of a permanent magnet rotating shaft, called the rotor, and electromagnets on the stationary portion that surrounds the motor, called the stator. All of the commutation must be handled externally by the motor controller. The motors and controllers are designed so that the motor may be held in any fixed position as well as being rotated one way or the other. To move the rotor the electric magnets on the motor are activated in the right order. Every change in this process moves the motor one step. The order in which those electromagnets are activated determines the rotation direction. The stepper’s resolution is based on the steps (typically 1.8 deg or 3.6 deg per step). In the stepper system, the driver advances one step, and the stepper motor follows. For example a 1.8 deg. stepper will turn a full circle in 200 steps. No matter how you gear it, a stepper motor still moves in discrete steps. Each step covers a specific range of “swing”. In a nutshell, a stepper (with or without gear-train) is a set of “preset” positions you can move to. Any position that’s not part of the “presets” is unattainable by that motor or motor-and-gear-train combination, and can only be reached as an approximation.

Stepping motors come in two varieties, permanent magnet and variable reluctance. Lacking a label on the motor, you can generally tell the two apart by feel when no power is applied. Permanent magnet motors tend to “cog” as you twist the rotor with your fingers, while variable reluctance motors almost spin freely. You can also distinguish between the two varieties with an ohmmeter. Variable reluctance motors usually have three (sometimes four) windings, with a common return, while permanent magnet motors usually have two independent windings, with or without center taps. Center-tapped windings are used in unipolar permanent magnet motors. For both permanent magnet and variable reluctance stepping motors, if just one winding of the motor is energised, the rotor will snap to a fixed angle and then hold that angle until the torque exceeds the holding torque of the motor, at which point, the rotor will turn, trying to hold at each successive equilibrium point.

For applications where precise measuring of a motors’ rotor position is critical, a stepper motor is usually the best choice. Stepper motors operate differently from other motors; stepper motors turn on a series of electrical pulses to the motor’s windings. Each pulse rotates the rotor by an exact degree. These pulses are called “steps”, hence the name “stepper motor”. Stepper motors are traditionally used in various motion control applications. Stepper motors are quite easy to wire and control. Stepper systems are economical. Stepper motors are video used in robotics control and in computer accessories (disk drives, printers, scanners etc.).

Stepper motors produce motion in discrete steps. Similar DC motors, steppers usually have permanent magnets on the rotor and coils on the stator with field movement provided by commutation from the power supply. Stepper motors have a specified number of steps per revolution (typically around 200 steps, or 1.8 degrees per step).

Stepper motors are usually controlled by digital signals from the controller to power drive, with one pulse corresponding to one step. Thus, the frequency of the digital signals controls the speed of the motor.

Stepper motors have limitations. They are available in limited power and their rotation speed is limited (usually maximum speed limit is about 2000 rpm). The energy of stepper motors is low and stepper motor systems have tendency to have resonances which needs to be avoided. Stepper motors have characteristic holding torque and pullout torque. Other torques can be difficult to achieve. Therefore, precise torque control is difficult with steppers. Because of open-loop nature of stepper motor controlling, they are not very good to be used with varying loads. It is possible for a stepper motor to loose steps if is loaded too much. Steppers are not recommended for high-speed or high-power applications, or for applications requiring precise torque control. The stepper motors typically have a rated voltage at what they can work without overheating. Operating the motor at this voltage limits the maximum speed and torque at high speed. The current limiting can be done by using power resistors. Stepping motors can be used in simple open-loop control systems; these are generally adequate for systems that operate at low accelerations with static loads, but closed loop control may be essential for high accelerations, particularly if they involve variable loads. If a stepper in an open-loop control system is overtorqued, all knowledge of rotor position is lost and the system must be reinitialized; servomotors are not subject to this problem.

Magnetism

The term magnetism is used to describe how materials respond on the microscopic level to an applied magnetic field; to categorize the magnetic phase of a material. For example, the most well known form of magnetism is ferromagnetism such that some ferromagnetic materials produce their own persistent magnetic field. However, all materials are influenced to greater or lesser degree by the presence of a magnetic field. Some are attracted to a magnetic field (paramagnetism); others are repulsed by a magnetic field (diamagnetism); others have a much more complex relationship with an applied magnetic field. Substances that are negligibly affected by magnetic fields are known as non-magnetic substances. They include copper, aluminium, water, gases, and plastic.

The magnetic state (or phase) of a material depends on temperature (and other variables such as pressure) so that a material may exhibit more than one form of magnetism depending on its temperature.

Sources of magnetism

There exists a close connection between angular momentum and magnetism, expressed on a macroscopic scale in the Einstein-de Haas effect “rotation by magnetization” and its inverse, the Barnett effect or “magnetization by rotation”.

At the atomic and sub-atomic scales, this connection is expressed by the ratio of magnetic moment to angular momentum, the gyromagnetic ratio.

Magnetism, at its root, arises from two sources:

Electric currents or more generally, moving electric charges create magnetic fields.

Many particles have nonzero “intrinsic” (or “spin”) magnetic moments. (Just as each particle, by its nature, has a certain mass and charge, each has a certain magnetic moment, possibly zero.)

In magnetic materials, sources of magnetization are the electrons orbital angular motion around the nucleus, and the electrons intrinsic magnetic moment. The other potential sources of magnetism are the nuclear magnetic moments of the nuclei in the material which are typically thousands of times smaller than the electrons magnetic moments, so they are negligible in the context of the magnetization of materials.

(Nuclear magnetic moments are important in other contexts, particularly in Nuclear Magnetic Resonance (NMR) and Magnetic Resonance Imaging (MRI).).

Ordinarily, the countless electrons in a material are arranged such that their magnetic moments (both orbital and intrinsic) cancel out. This is due, to some extent, to electrons combining into pairs with opposite intrinsic magnetic moments (as a result of the Pauli exclusion principle), or combining into “filled subshells” with zero net orbital motion; in both cases, the electron arrangement is so as to exactly cancel the magnetic moments from each electron. Moreover, even when the electron configuration is such that there are unpaired electrons and non-filled subshells, it is often the case that the various electrons in the solid will contribute magnetic moments that point in different, random directions, so that the material will not be magnetic.

However, sometimes (either spontaneously, or owing to an applied external magnetic field) each of the electron magnetic moments will be, on average, lined up. Then the material can produce a net total magnetic field, which can potentially be quite strong.

The magnetic behavior of a material depends on its structure (particularly its electron configuration, for the reasons mentioned above), and also on the temperature (at high temperatures, random thermal motion makes it more difficult for the electrons to maintain alignment).

Diamagnetism

Diamagnetism appears in all materials, and is the tendency of a material to oppose an applied magnetic field, and therefore, to be repelled by a magnetic field. However, in a material with paramagnetic properties (that is, with a tendency to enhance an external magnetic field), the paramagnetic behavior dominates. Thus, despite its universal occurrence, diamagnetic behavior is observed only in a purely diamagnetic material. In a diamagnetic material, there are no unpaired electrons, so the intrinsic electron magnetic moments cannot produce any bulk effect. In these cases, the magnetization arises from the electrons orbital motions, which can be understood classically as follows.

When a material is put in a magnetic field, the electrons circling the nucleus will experience, in addition to their Coulomb attraction to the nucleus, a Lorentz force from the magnetic field. Depending on which direction the electron is orbiting, this force may increase the centripetal force on the electrons, pulling them in towards the nucleus, or it may decrease the force, pulling them away from the nucleus. This effect systematically increases the orbital magnetic moments that were aligned opposite the field, and decreases the ones aligned parallel to the field (in accordance with Lenz's law). This results in a small bulk magnetic moment, with an opposite direction to the applied field.

Note that all materials undergo this orbital response. However, in paramagnetic and ferromagnetic substances, the diamagnetic effect is overwhelmed by the much stronger effects caused by the unpaired electrons.

Paramagnetism

In a paramagnetic material there are unpaired electrons, i.e. atomic or molecular orbitals with exactly one electron in them. While paired electrons are required by the Pauli exclusion principle to have their intrinsic (spin) magnetic moments pointing in opposite directions, causing their magnetic fields to cancel out, an unpaired electron is free to align its magnetic moment in any direction. When an external magnetic field is applied, these magnetic moments will tend to align themselves in the same direction as the applied field, thus reinforcing it.

Ferromagnetism

A ferromagnet, like a paramagnetic substance, has unpaired electrons. However, in addition to the electrons intrinsic magnetic moments tendency to be parallel to an applied field, there is also in these materials a tendency for these magnetic moments to orient parallel to each other to maintain a lowered energy state. Thus, even when the applied field is removed, the electrons in the material maintain a parallel orientation.

Every ferromagnetic substance has its own individual temperature, called the Curie temperature, or Curie point, above which it loses its ferromagnetic properties. This is because the thermal tendency to disorder overwhelms the energy-lowering due to ferromagnetic order.

Some well-known ferromagnetic materials that exhibit easily detectable magnetic properties (to form magnets) are nickel, iron, cobalt, gadolinium and their alloys.

Magnetic domains

The magnetic moment of atoms in a ferromagnetic material cause them to behave something like tiny permanent magnets. They stick together and align themselves into small regions of more or less uniform alignment called magnetic domains or Weiss domains. Magnetic domains can be observed with a magnetic force microscope to reveal magnetic domain boundaries that resemble white lines in the sketch. There are many scientific experiments that can physically show magnetic fields.

When a domain contains too many molecules, it becomes unstable and divides into two domains aligned in opposite directions so that they stick together more stably.

When exposed to a magnetic field, the domain boundaries move so that the domains aligned with the magnetic field grow and dominate the structure. When the magnetizing field is removed, the domains may not return to an unmagnetized state. This results in the ferromagnetic material’s being magnetized, forming a permanent magnet.