This note explains contactor selection for motors with long starting time. The note has been divided into three parts for easy understanding of the concepts involved. They are as follows,
1. Understanding Motor Inrush Current
2. Long Starting Time Applications
3. Contactor selection for motors with long starting time
Understanding Motor Inrush Current (Stator current)
A motor generally drives a load through some transmission system. During start, the motor draws a high starting current or inrush current. This current is about 6-8 times the motor rated current and can cause a significant voltage drop. This voltage fluctuation affects other devices connected to the same supply. Hence several other strategies are employed for starting motors to reduce its starting current; the most commonly employed being the Star–Delta starting. The starting value of the current is independent of the load attached; however it must be sufficient to overcome the inertia of the motor load system. However, inertia of the load impacts the starting time of the motor as explained in the next part. As the motor accelerates and nears its rated speed, the current gradually reduces and
settles down to a value equal to motor rated current or less depending on the actual load connected. The typical torque-speed characteristics of an induction motor are as given below,
Long Starting Time Applications
The total time from rest till the motor draws its rated current is called the starting time. The starting time of the motor is a function of the load inertia, load speed and the starting torque developed by the motor. A high inertia load requires an extended time to reach full speed and hence the motor also draws high starting current for a long time. The motor starting time is specified by the manufacturer in the motor data sheet. Since motor starting time is also a function of applied voltage it differs for different starting methods. For example starting time of the motor with Direct-Online starting would be different than with Star-Delta starting. The starting line current in Star Delta configuration is one third of the starting current of the same motor in DOL configuration. However applied voltage and therefore starting torque also reduces, leading to higher starting time
Long starting time applications are generally those applications in which the motor starting time is around 40 to120 secs.
Typical applications involving motors with a high starting time are,
• Induced Draft Fans (ID Fans)
• Forced Draft Fans (FD Fans)
ID and FD fans have a high inertia and hence motors required to drive them will have a long starting time. As a result the motor will draw high inrush current for an extended period of time.
The high inrush current drawn by the motor at start is carried by the contactors that are used for switching. Since, this current flows for an extended period of time, the contactor needs to be selected judiciously. Guidelines for selection of contactor rating is as follows:
Contactor Selection for motors with long starting time
Contactors are selected based on their overload current withstand capability. Overload withstand capability is defined in IEC 60947-4-1.
It means that a contactor with rated operational current equal to or less than 630A can withstand 8 times its rated Ac3 operational current for a period of 10 seconds. This rating is also called as the 10 sec rating of the contactors.
For Example:
Let Rated operational current (AC3 Utilization category) of contactor = 400A.
Then the maximum current it can carry for a period of 10 sec = 8 x I = 3200A e
Now let us look at an example, how to arrive at minimum AC3 Ratings of the Star, Main and Delta contactors
Motor specifications
Motor kW Rating: 160 kW
Motor Full Load Line Current: 304A
Motor Starting time in Star-Delta: 85 sec
Solution:
Delta contactor can be directly selected as per type 2 chart specified by the contactor manufacturer. This is because delta contactor is connected only when the motor has reached near its rated speed and motor current has reduced to its full load value