Contactor Selection for Motors with long starting time

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

Utilization categories

Contactors are most commonly used in applications concerning control of electric motors. Contactors are used to start, stop, reverse, jog and plug the motors depending upon the application requirement. Contactors along with thermal overload relays also provide protection to the motor against overloads.

The most basic data required for contactor selection is the motor HP rating and it’s rated current. However this data is alone not sufficient. The type of load, duty cycle of the load, switching frequency are some of the factors that influence contactor selection. The switching capability of contactors is majorly dependent on the type of application, and hence international standards (IEC 60947-4-1) specify utilization categories which cover a broad range of applications. These utilization categories and the data associated with them are used by manufacturers to establish contactor ratings.

The utilization categories as per IEC 60947-4-1 are as follow:

AC-1 : Non-inductive or slightly inductive loads, resistance furnaces

AC-2 : Slip-ring motors : starting, switching off

AC-3 : Squirrel-cage motors : starting, switching off motors during running 1)

AC-4 : Squirrel-cage motors : starting, plugging, inching

AC-5a : Switching of electric discharge lamp controls

AC-5b : Switching of incandescent lamps

AC-6a : Switching of transformers

AC-6b : Switching of capacitor banks

AC-7a : Slightly inductive loads in household appliances and similar applications

AC-7b : Motor loads for household applications

AC-8a : Hermetic refrigerant compressor motor to control with manual resetting of overload release

AC-8b : Hermetic refrigerant compressor motor to control with automatic resetting of overload release

AC-15 : Control of a.c electromagnetic lods

DC-1 : Non-inductive or slightly inductive loads, resistance furnace

DC-3 : Shunt-motors : Starting, Plugging, Inching

Dynamic braking of dc motors

DC-5 : Series-motors : Starting, Plugging, Inching

Dynamic braking of dc motors

DC-6 : Switching of incandescent lamps

1) AC-3 category may be used for occasional inching (jogging) or plugging for limited time periods such as machine set-up: during limited time periods, the number of such operations should not exceed five per minute or more than 10 in a ten minute period.

2) A hermetic refrigerant compressor motor is a combination consisting of a compressor and a motor, both of which are enclosed in the same housing, with no external shaft or shaft seals, the motor operating in the refrigerant. 

The utilization categories most commonly encountered in contactor applications are AC-3 & AC-4

Applications under utilization category AC-3 (Normal Switching) are: Compressors, Pumps, Fans, Conveyors, Mixers, Agitators, Air conditioners, Elevators etc

Applications under utilization category AC-4 (Plugging, inching) are: Printing presses, Wire drawing machines, Centrifuges etc

The making and breaking capacities of contactors are dependent on the utilization categories and the standard specifies that the contactors or starters shall be capable of making and breaking currents without failure under the conditions stated.

Over voltage causes and effects

Over voltages or surges in the power system are the abrupt rise in the voltage level in the system. There could be several reasons for over voltage. The normal operating voltage of the system do not stress the insulation severely. But the voltage stresses due to over voltages can be so high that they may become dangerous to both the cables and the connected equipment and may cause damage, unless some protective measure against over voltages are taken. Over voltages occurrence in the system can be categorized by reasons:

1. External over voltages:

These over voltage originate from the atmospheric disturbances, mainly due to lightning. These over voltages take the form of a unidirectional impulse whose maximum possible amplitude has no direct relation with the operating voltage of the system. They may be due to any one of the following causes.

a. Direct lightning stroke

b. Electromagnetically  induced voltages due to lightning discharge near the line

c. Voltage induced due to the changing atmospheric condition along the transmission line

d. Electrostatic ally induced over voltages due to presence of the charge clouds 

e. Due to friction of the charged particles like dust, snow in the atmosphere or due to change in the altitude of the line. 

2. Internal over voltages:

Caused due to changes in operating conditions of the network, further classifieds into two groups.

a. Switching or transient over voltages

    The over voltages are generally of oscillatory nature caused by transient phenomena which appears when the state of network is changed by switching operations or fault condition.

The frequency of oscillation is governed by the inherent inductance and capacitance of the system and may very from few hundres Hz to few kHz

b. Steady state or temporary over voltages

    These are over voltages developed due to the disconnection of loads at power frequency.