The current consumed by the electric motor at inclusion in the power supply network is called starting. As the size of starting current can exceed many times over nominal, it needs to be limited, having picked up automatic switches with the necessary current characteristic protecting the line of turning on of this electric motor or their group. For this purpose it is also necessary to calculate starting current.
It is required to you
- Technical documentation on the electric motor.
Instruction
1. Define electric motor type. It can be the electric motor of a direct current or three-phase alternating current. Calculate rated current of the electric motor of a direct current in amperes, using a formula: IH=1000PH / (ηHUH), and rated current of the electric motor of three-phase current on a formula: IH=1000PH / (UHcosφH √ηH), where: Rn — the rated power of the engine, kw; UH is the rated voltage of the engine, in; ηH — the nominal efficiency of the engine; cos fn — the nominal power factor of the engine. Take data on rated power, rated voltage, efficiency and power factor from technical documentation of the electric motor.
2. Calculate the size of starting current in amperes after calculation of its nominal size. For calculation use a formula: IP =IH*Kp where IH is the nominal size of current, and Rc - frequency rate of a direct current to its nominal size. Check technical documentation on the electric motor, in it the frequency rate of a direct current to its nominal size (Rc) has to be specified. Increase this number by the turned-out size of rated current and receive the size of starting current in amperes. Calculate it for each electric motor which is in a chain.
3. Pick up the automatic switch for protection of the line of inclusion depending on the turned-out size of starting current for all electric motors in a chain. For the choice it is necessary to know that automatic switches can be like shutdown In, With and. Switches with characteristic of shutdown of type B will be suitable for lighting networks of general purpose, with characteristic of shutdown of type C serve for disconnection of lighting chains and installations with moderate starting currents (engines and transformers). For chains with active and inductive loading and also for protection of electric motors with big starting currents are usually applied to use switches with characteristic like D. Having defined switch type, pick up necessary depending on the turned-out size of starting current.