Premium Membership ♕

Save 50% on all EEP Academy courses with Enterprise Membership Plan and study specialized LV/MV/HV technical articles & guides.

Home / Technical Articles / The art of achieving synchronous motor transient stability (especially in weak grids)

Perfect, but with stability problems

Since the subject of this article is somewhat not-that-easy to understand, I suggest starting with the basics of the synchronous motor. In the synchronous motor, the basic magnetic field is obtained by direct current excitation rather than through the air gap from the armature, as is the case with induction motors.

The art of achieving synchronous motor transient stability
The art of achieving synchronous motor transient stability

Comparatively large air gaps are used, making practicable the manufacture, even in relatively low horse-power ratings of low-speed synchronous motors. In all low-speed ratings and in large high-speed ratings, synchronous motors are physically smaller and less costly to build than squirrel-cage induction motors of equivalent horsepower.

A synchronous motor can be applied to any load which can be successfully driven by a NEMA Design B squirrel-cage motor. However, there are certain types of loads for which the synchronous motor is especially well suited.

The correct application of synchronous motors results in substantial savings in several ways, like in the high efficiency, power factor (pf) correction, reduced maintenance and space savings.

A synchronous motor consists of a frame, a stator, a rotor, an exciter, and an amortisseur winding. These components can be seen in Figure 1 below.

Figure 1 – Synchronous motor construction and main components

Synchronous motor construction and main components
Figure 1 – Synchronous motor construction and main components

Apart from the synchronous motor advantages, it is prone to stability problems upon external disturbances more than the induction motor. The justification is complex, but the literature offers such an explanation extensively.

This article skims through motor starting methods, rotation directions, excitation modes, torque types, and a case study. The aim of this article is to describe the synchronous motor transient stability performance and formulate the relationship between the excitation mode and the synchronous motor stability performance.

Table of Contents:

  1. Synchronous Motor Method of Starting
    1. Motor Starting by Variable Frequency Drive
    2. Motor Starting with an External Motor
    3. Motor Starting by Using damper (Amortisseur) Winding
  2. Synchronous Motor Rotation Direction
  3. Synchronous Motor Excitation Modes
    1. Constant Excitation Mode and Varying Load Relationship
      1. Normal Excitation
      2. Underexcitation
      3. Overexcitation
    2. Varying Excitation Mode and Constant Load Relationship
      1. Excitation Decreased
      2. Excitation Increased
  4. Torque Types
    1. Starting Torque
    2. Running Torque
    3. Pull-in Torque
    4. Pull-out Torque
  5. Case Study of Synchronous Motor 13.2 kV, 12.8 MW at 0.9 pf

1. Synchronous Motor Method of Starting

The unexcited rotor is accelerated to the synchronous speed at which it is excited using DC source. At this moment, the rotor poles are locked into the stator poles, so they both run synchronously. The synchronous speed is given by the famous formula NS = 120 f / P.

However, the arrangement between the rotor and the stator is not absolutely rigid. If the load increases, the rotor tends to fall back in phase by a certain angle. The motor speed is constant, and the motor keeps running till the maximum load angle is reached.

Some of the commonly used methods for starting synchronous rotors are described in the following section.

Suggested Course – The Essentials of Motor Soft Starters, Motor Starting Methods and Applications

The Essentials of Motor Soft Starters, Motor Starting Methods and Applications

Go back to the Contents Table ↑

1.1 Motor Starting by Variable Frequency Drive

The rotor will be able to accelerate and lock in with the stator’s magnetic field without any difficulty if the stator’s rotating magnetic field in a synchronous motor rotates at a low enough speed. The supply frequency “f” is then gradually raised until it reaches its typical 50/60-Hz value, at which point the stator magnetic field can be accelerated to its rated operating speed.

It is now possible to continuously control the frequency of the supply connected to the synchronous motor all the way from a fraction of a hertz up to and even above the typical rated frequency thanks to the development of such modern solid-state variable frequency drive packages.

If a motor-control circuit contains such a variable-frequency drive unit, starting a synchronous motor with it simply adjusts the frequency to a very low value for starting, and then raises it to the desired operating frequency for normal running.

The internally generated voltage of a synchronous motor, which is typically referred to as the counter EMF, will be lower than usual when it is run at a speed lower than the rated speed. Because of this, it is necessary to decrease the terminal voltage applied to the motor in proportion to the frequency to maintain the stator current within the rated range.

It’s worth noting that any variable-frequency power supply’s voltage typically varies roughly linearly with the output frequency.

Further Study – Inside Variable Frequency Drive (VFD) Panel: Configuration, Schematics and Troubleshooting

Inside Variable Frequency Drive (VFD) Panel: Configuration, Schematics and Troubleshooting

Go back to the Contents Table ↑

1.2 Motor Starting With an External Motor

This technique involves mounting a pony motor as an external starting motor and using it to accelerate the synchronous machine toward its rated speed (though not quite to it, as the synchronization process may fail to indicate the point at which the main switch connecting the synchronous machine to the supply system closes).

The pony motor may then be removed from the machine’s shaft or the power supply to the pony motor may be cut off once the output of the synchronous machine has been synced or paralleled with its power supply system as a generator.

When the pony motor is turned off, the machine’s shaft slows down, the speed of the magnetic field in the rotor momentarily lags behind, but the synchronous machine keeps working as a motor. The synchronous motor can be loaded normally, just like any other motor, once it starts to function as a motor.

Premium Membership Required

This technical article/guide requires a Premium Membership. You can choose an annually based Plus, Pro, or Enterprise membership plan. Subscribe and enjoy studying specialized technical articles, online video courses, electrical engineering guides, and papers. With EEP’s premium membership, you get additional essence that enhances your knowledge and experience in low- medium- and high-voltage engineering fields.

Check out each plan’s benefits and choose the membership plan that works best for you or your organization.

Good To Know!Save 50% on all video courses by purchasing Enterprise plan.

Log In »Purchase »

Premium Membership

Get access to premium HV/MV/LV technical articles, electrical engineering guides, research studies and much more! It helps you to shape up your technical skills in your everyday life as an electrical engineer.
More Information

Salem Alshahrani

Electrical engineer (BEE & Meng). Specialized in substation design, especially in LV/MV switchgears and transformers. Passionate in power system planning, analysis, and stability studies.

Leave a Comment

Tell us what you're thinking. We care about your opinion! Please keep in mind that comments are moderated and rel="nofollow" is in use. So, please do not use a spammy keyword or a domain as your name, or it will be deleted. Let's have a professional and meaningful conversation instead. Thanks for dropping by!

three  ×  three  =  

Learn How to Design Power Systems

Learn to design LV/MV/HV power systems through professional video courses. Lifetime access. Enjoy learning!

Subscribe to Weekly Newsletter

Subscribe to our Weekly Digest newsletter and receive free updates on new technical articles, video courses and guides (PDF).
EEP Academy Courses - A hand crafted cutting-edge electrical engineering knowledge