This is the eighth and final post in my series on Motor Starting. What I’ll do here is summarise what we have been through and pull everything together.
Electric motors are one of the most common items of electrical equipment in service. Despite there usefulness of electric motors, there are issues in starting motors and various methods are used to overcome these. Of particular concern it the growth of magnetic fields and back emf during starting leading to large currents and torque during this period. These currents and torque can have negative effects on both the electrical system and mechanical load.
During the series, six starting methods have been discussed which try to address the problems of starting current and torque, each of which has it’s own advantages and disadvantages:
Direct On Line - is the simplest and most cost effective, with the motor simply connected to the supply. This method of starting suffers from high current/torque during starting. Due to it’s simplicity it is my preferred method of starting; however, there are many instances where it’s disadvantages make it impractical.
Star Delta - is a reduced voltage starting method whereby the stator windings are initially connected in a star configuration and then switched to delta when the motor has accelerated. In star, the voltage across each winding is reduced by a factor of √3 resulting in a lower starting current and a starting torque of approximately 33% the full voltage torque.
Auto-Transformer - is another reduced voltage method, whereby an auto-transformer provides the starting voltage. Using an auto-transformer allows the level of voltage (consequently current and torque) to one of a wide range of value. With tapped auto-transformers, it is also possible to vary the starting characteristics during the run-up period.
Primary Resistance - in this method one or more banks of resistors are inserted into the stator winding during starting. Voltage drop across the resistors result in a reduced voltage at the motor terminals and improved starting characteristics. As the motor current decreases (and voltage drop across the resistors), the voltage on the winding increases resulting in a fast increase in torque during starting.
Rotor Resistance - like primary resistance, starting is controlled by the introduction of resistance banks, however, in the rotor (not stator). During rotor resistance starting, the torque is approximately proportional to the motor current. Selection of the resistor banks can achieve a close match to the required mechanical characteristics during starting.
Electronic Soft Start - in which back to back thyristors are switched to ramp up the voltage during starting (or ramp down during stopping). Selection of different ramp characteristics, initial starting voltage and current limiting functions allow soft starters to match the requirements of the mechanical load, provide smooth acceleration and ensure that reasonable values of starting current are drawn from the supply.
The table below summarises the characteristics, advantages and disadvantages of various starting methods.
| Direct On Line |
Star Delta |
Auto Transformer |
Primary Resistance |
Rotor Resistance |
Electronic Soft Start |
|
|---|---|---|---|---|---|---|
| Cost | + | ++ | +++ | +++ | +++ | ++++ |
| Starting Current (xIn) | 4 to 8 | 1.3 to 2.6 | 1.7 to 4 | 4.5 | < 2.5 | 2 to 5 |
| Starting Torque | 100% | 33% | 40/65/80% | 50% | 10 to 70% | |
| Adjustability | - | + | ++ | ++ | ++ | +++ |
| Typical Load Inertia | Any | Low | Low | High | High | Any |
| Mechanical Impact | High | Moderate | Moderate | Moderate | Low | Low |
| Motor Type | Standard | 6-Terminal | Standard | Standard | Slip-ring | Standard |
| Resistor Bank | - | - | - | Yes | Yes | - |
While not part of the series the following posts are related to motor starting and give additional insight:
This is the end of the series and I hope everyone found some value in it; if not the full series at least parts of it. If you have any comments, opinions or suggestions, please feel free to add these. If you liked or found some value in the series, please also use the bookmarks to give a ‘like’ or ‘tweet’.
Steven