When a motor is running the back emf generated by the motor’s rotor negates part of the winding impedance to give the running current. The running current is dependant on the load being driven by the motor and is assumed to be the full load current, that is the motor is running at its full kWo output. However when a motor is first connected to the power supply only the winding resistance and reactance affect the current drawn this can be more than seven times the full load current. As with any electrical load that starting current can affect other electrical equipment on the same network as well as affecting the motors ability to start.
An example of the effect on other equipment on the same network is that of a car.
A car’s electrical system power source is it’s battery when the engine is not running, that battery has an internal resistance.
If a car engine stalls whilst the headlights are on and the starter motor is engaged to restart the engine the headlights noticeably dip in intensity, but the headlight switch has not been touched.
The amount of current drawn by the starter motor is so great that the battery voltage is dragged down by its internal resistance thus reducing the voltage at the headlights, and they subsequently have a noticeably lower light output.
This is explained using the diagram below.
Let the battery cells have a voltage V=12V
The battery internal resistance Rb=0.0125Ω
The starter motor resistanceRs=0.035Ω
Each headlamp at 60W has a nominal resistanceRh=2.4Ω
When the headlights only are on the current in the circuit is 9.897A which gives a battery volt drop of 0.124V which leaves the headlight voltage at 11.876V.
When the headlights are on and the starter motor is engaged the current in the circuit rises to 258.02A which gives a battery volt drop of 3.225V which leaves the starter motor and headlights a voltage of 8.775V.
Thus when the starter motor is engaged the voltage at the headlights drops by 26% which is why the lights dim.
Note for simplicity the wire resistances and switch contact resistances have been ignored in the above example; the figures have also been rounded to three decimal places again for simplicity.
The current drawn by a motor when it starts is dependant on the method employed to start the motor. The major starting methods are:-
| Direct on line where the full line voltage is applied in a single step to the motor in this article the starting current is taken as seven times the full load current. | |
| Star delta where the voltage in each winding is reduced by √3 in the star phase of starting, the current is reduced to √3 of the line current, in this article the starting current is taken as two and a half times the full load current. | |
| Auto transformer where the starting voltage is reduced according to the tap selected on the transformer. Typical tapings are 80%, 65% and 50% which equates to starting currents of 4.48, 2.95 and 1.75 times the full load current. In this article the 50% tap is assumed. | |
| Rotor resistance where the starting current can be reduced to the full load current. The choice of resistances will determine the starting current which for purposes of this article is taken as one and a half times the full load current. | |
| Soft start where an electronic unit controls the effective voltage to the starter. The maximum starting current is determined by a preset control on the unit. | |
| Variable speed start where an electronic unit controls the speed of motor. The control unit varies the speed by varying the frequency of the supply to the motor. At ‘switch on’ the output frequency is set via a preset control on the unit to limit the starting current. Once the motor is running the speed of the motor is controlled by external command. |
The multipliers for the various starting methods are worst case scenarios, motors with specific data may show lower multipliers, it is also possible that the specific data may show higher multipliers.
Another starting method is primary resistance, since this method is not in common use in England it is not considered in this article.
There are also variations on the major starting methods of which the most notable are closed transition star delta starting and closed transition auto transformer starting more commonly known as Korndorffer starting. Closed transition star delta starting is not in common use in England and is therefore not considered in this article, however closed transition auto transformer starting is more commonly used than standard auto transformer starting so closed transition auto transformer or Korndorffer starting is considered within this article, and open transition auto transformer starting is not considered.