When studying the passport data of any electric motor, in addition to the declared power, voltage, rated current load and the method of connecting the poles, the starting current is indicated. This parameter belongs to the most important indicators of the operation of any engine unit and therefore needs more detailed consideration.
Start-up features and current ratio
Starting currents are estimated according to those loads on the electric motor that their working windings experience at the moment the voltage is applied to them. Most often, they exceed the standardized indicator by 6-8 times. The real value that takes this difference into account is called the multiplicity and is entered as a correction factor (photo below).
When the coefficient introduced in this way is known, it will not be difficult to determine the magnitude of the starting current.
Additional Information: The two characteristics of the motor should not be confused: its rated and operating current values.
Their difference is manifested in the following:
- The rated current is the current flowing in the coils during long-term operation of the unit and is limited only by the stator temperature (the degree of its heating).
- The working indicator characterizes the real current at the present time.
- It is usually equal to or slightly below the nominal value.
The entered coefficient depends on the power of the electric motor and on the number of pole pairs in it. At a low declared power, it will be less. The same dependence is observed in the case of poles.
Features of the starting process
The magnitude of the starting currents depends on the design features of a particular electric motor and, in particular, on its inertial characteristics. So, with a small number of poles, the total resistance of the windings is small and, accordingly, they reach large values. For engine models with very high shaft speeds, a "hard" start is typical, since more effort is required to bring it to operating mode.
In technically literate language, this phenomenon is described as follows:
- in a static state, the slip coefficient of the motor shaft is equal to one;
- when it spins up during launch, this indicator tends to zero, but never reaches it (because of the always present losses).
As a result of transient processes, the EMF of self-induction increases sharply, and the impedance of the coils and the current component decrease.
Determining the exact value
You can get accurate data on the desired value as follows:
- On an oscilloscope that picks up a signal on the shunt during engine start.
- Measurement at reduced voltage on the windings (5-10 times lower than nominal).
- Taking readings with a clamp meter.
In the first case, it is possible to fix the voltage surge on the oscilloscope screen and compare it with the steady-state value. The second option is good because the motor rotor does not overheat during measurement and does not distort the reading process. Using a clamp meter is the easiest way, but it does not provide the required accuracy (photo below).