60
4
chapter
AC motors starting and
protection systems
Presentation :
• AC motors starting and braking systems
• AC motors protection devices and failure analysis
• Protection devices selection guide
Summary
4. AC motors starting
and protection
systems
61
4.1
Asynchronous motor starting systems
62
4.2
Electrical braking of 3-phase asynchronous motors
69
4.3
Multifunction motor starter units
74
4.4
Motors protection
76
4.5
Motor losses and heating
77
4.6
Causes of faults and their effects
77
4.7
Protection functions
83
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4.1
Asynchronous motor starting systems
4. AC motors starting
and protection
systems
62
This section is devoted to starting and braking systems and the protection of
asynchronous motors of all types.
Motor protection is required to ensure the installations work properly and to protect
machines and equipment’s.
Technology, starting and speed control are mentioned briefly. Please refer to the
relevant sections with detailed descriptions in this guide.
Personal protection is not discussed in this section. For information on this, please
refer to specific works on the topic. Details of this important aspect can be found in
the Electrical installation guide published by Schneider Electric.
4.1
Asynchronous motor starting systems
b Introduction
When a motor is switched on, there is a high inrush current from the mains
which may, especially if the power line section is inadequate, cause a drop in
voltage likely to affect receptor operation. This drop may be severe enough
to be noticeable in lighting equipment. To overcome this, some sector rules
prohibit the use of motors with direct on-line starting systems beyond a given
power. See pages K34 and K39 of the Distribution BT 1999/2000 catalogue
and the tables of voltage drops permitted by standard NF C 15-100.
There are several starting systems which differ according to the motor
and load specifications.
The choice is governed by electrical, mechanical and economic factors.
The kind of load driven is also important in the cho