MOTOR STARTERS FOR INDUCTION MOTORS
There are various types of electrical motors which are used for domestic, commercial & industrial applications. These motors may be AC motors or DC motors.
There are ac & dc motors available but acInduction motor is widely used for different applications. Induction motors are available in single phase & three phase supply. These motors need protection from various faults so a special switching arrangement is required. Switching ON& OFF of induction motors are done with the help of a device called STARTER.
WHY STARTER IS REQUIRED FOR 3 PHASE MOTORS –
For single phase supply no starter is required while starter is required for 3 phase motors because 3 phase motors takes very high current when motor is switched on (due to induction effect). This high current may reach upto 5-7 times than that of full load current of the motor and this high current reduces the supply voltage to motor temporarily which causes voltage fluctuation in the line and fluctuation in voltage affects the operation of other equipment. So, it is important to reduce initial high current of the motor & that is done by starter. This high current can be reduced by either inserting resistance in series or by using special technique in the starter. Secondly, in order to protect motors from faults, fault sensing devices (relay, SPPR etc) can be put in starters.
In other words we can say that starter is used for three purposes – 1) to switch on and off the motor 2) to reduce starting current & 3) to protect motors from faults.
Motor Starters are of two types – 1) DOL (Direct ON Line) starter, 2) Y-Δ (Star – Delta) starter.
- DOL STARTER–
DOL starter serves to purpose only – 1) to switch on & off the motor & 2) to protect motors from faults. As DOL starter doesn’t reduce the starting high current, so it is not used for high capacity motors. DOL motors are used up to 5HP motors.
Construction – It consists of a backup MCB (or fuse), a contactor & a relay. All these items are installed in an enclosure. The MCB may be installed in or outside of enclosure. There are two types of wiring done for starter -1) Power wiring which should be able to carry full load of motor. 2) Control wiring which is used to connect START/STOP push buttons, NO/NC contacts of relays & contactor.
MCB – It serves two functions –a) To switch off/on starter when its operation required, b) It provides back up protection against short circuit & overloading.
Contactor – It’s a switching device which is switched on/off when start/stop button is pressed. It has NO/NC contacts that are used to know the status of motor. NC contact is used to indicate OFF status of motor & NO contact for ON status of motor.
Relay – The function of relay is to sense the current of the motor and when the current of motor crosses a set value, its NC contact gets open due to activation of bimetallic strips. When NC contact becomes NO, supply to the coil of contactor is disconnected & contactor gets switched off. Current setting for motor protection is done through the relay.
Working of DOL Starter –
- Switching ON motor – Initial conditions- MCB should be ON, contactor (K) is OFF & motor is OFF. In order to run motor, contactor must be ON but contactor coil is not getting supply because control circuit is open because of start button. When we press start button, current passes through the stop button, NC of over load relay (OLR) & contactor coil, this current activates contactor. Contactor gets ON & motor gets 3 phase supply & start running.At the same time, the NO contact of contactor becomes NC, when we release the STARTbutton, contactor coil gets continuous supply through NC contact of the contactor, stop button and relay NC contact.
- Normal switching off motor- Motor can be switched off by pressing STOP button which stops current flow to contactor coil & contactor gets switched off. At the same time NC of the contactor becomes NO which further ensures that supply doesn’t reach till contactor coil & contactor remains in off condition.
- Motor trips (switched off automatically) under fault condition – Motor trips under fault condition (overloading & short circuit) to protect motor from damage. This protection is provided by thermal magnetic relay. When fault occurs, relay’s bimetallic strip bends & changes NC (which is in the line of contactor coil) into NO, this disconnects supply to contactor coil & contactor is turned off.
Power & Control wiring of starter –
There are two types of wiring done in starters – one is power wiring which should be able to carry full load of motor & connects all equipment like back up MCB, Contactor, relay& finally motor (see the diagram above). The second is control wiring which connects NO/NC contacts of contactor, relay, start/ stop push buttons. These are thin wire & carry very small value of current.
NO/NC Contacts (NO= Normally Open, NC = Normally Closed) –
NO/NC contacts play an important play in control wiring. These contacts are part of relay, contactors etc which are electrically not connected but connected mechanically with them. These contacts are potential free contacts. They change their position with the operation of relay, contactors etc. i.e NO becomes NC & vice versa.
SELECTION OF STARTER –
Starters are available in various combinations of back up MCB/MCCB (or back up fuse), contactors, & relays. Capacity of these items depends on full load of motor.
Selection of Back up MCB/Fuse –
- Selection of back up fuse depends on immediate trip or time delay, For single phase & three induction motor –
- Instantaneous (or non-delay time) – 175% of full load current of motor,
- Time Delay – 300% of full load current of motor,
- Selection of back up Circuit Breaker depends on immediate trip or time delay, For single phase & three induction motor –
- Instantaneous (or non-delay time) – 250% of full load current of motor,
- Time Delay – 800% of full load current of motor,
Selection of Contactor – Contactor in starter should be able to carry full load current of motor at least 125% of full load current.
Selection of Thermal Overload Relay – Range of Thermal Overload relay varies between 70% – 110% of full load current of motor.
Min. setting of relay – 70% of full load current of motor,
Mix. Setting of relay – 110% of full load current of motor.
Protection of Motor against single-phasing–
Sometimes it happens that instead of 3 phases, a motor gets 2 phases only due to some problem in one phase (fuse blown, loose cable connection in terminal box, failure of closing of one phase of MCCB/MCB, contactor’s one phase not closing due to chattering/oxidation), in this case motor windings may damage due to following reasons –
- If motor is on – current in these two phases will increase to 2.4 times the full load current,
- Torque required to operate the will not be sufficient and become uneven which willreduce the motor speed, motor becomes noisy & will vibrate,
- If motor is in off mode – it will not start & motor will draw more current from two live phases which may burn motor winding.
So, by using following way, a motor can be protected against single phasing –
- By Electromagnetic overload relay – It works on the principal of electromagnetic effect, when single phasing takes place the current in other phases will increase. This increased current will actuate the relay which in turn will isolate the circuit.
- By thermistor – In order to detect high temperature in the winding (due to single phasing), thermistor are inserted in the winding and thermistors are connected with amplifiers which actuates the electromagnetic relay. When relay is operated, it isolates the supply.
- By Bimetallic strip– Increase in temperature in winding can be sensed by placing bimetallic strip. It bends on high temperature which actuates the relay & relay trips the motor.
- By SPPR (Single Phase Prevention Relay) – It’s an electronic relay. It’s a voltage operated relay. It is available for Single Phase or 3 phase supply. It has input & out- put terminals. Voltage supply is connected with input terminals which are sensed by the SPPR & its output (NO/NC) terminals are connected with the wiring of contactor coil. When single phasing takes place, the circuit of SPPR operates the relay inside it which changes NO/NC contacts & these contacts disconnect coil supply & contactor gets switched off.SPPR doesn’t only give single phasing protection but also revere phasing and unbalance supply (depends on the design of SPPR). It gives alert & alarm also.
Being an electronic relay, SPPR is more accurate & sensitive relay as compared to other method of protection from single phasing.
- STAR – DELTA STARTER–
Star-Delta starter serves to purpose – 1) to switch on/off the motor & 2) to protect motors from faults. It is used for high capacity of motors (more than 7.5Hp) as it reduce high inrush current (motor takes high inrush current when motor speed reaches to its full speed from zero speed. After reaching full speed, high in-rush current is reduced automatically to its normal rated current). A motor is designed to run in delta connection but we start the motor in star connection in order to reduce the high inrush current. Motor remains in star connection till motor speed reaches its 90% & after that motor connection changes to delta connection again.Let us understand how it happens –
A Star-Delta consists of 3nos -contractors, 1no- OLR (Over Load Relay), 1no – Timer & 1no – Back up MCB. There are two types of wiring – Power wiring & control wiring.
Power wiring is done as per diagram given above, this wiring diagram is used to carry motor current. While control wiring is used to run moor in star connection from zero speed to 90% speed of motor & then change to delta connection for normal operation. Control wiring diagram of a typical star delta connection is given below.
Working – Control wiring diagram is used to change motor connection from star connection to delta connection with help of timer. Sequence of operation is given below(before activation of timer) –
Press START BUTTON – current will flow through NC of OLR – through stop button – through start button – to timer & to NC of Timer (T1) – to NC (D1) of Delta Contactor (D) – to star contactor (S) which gets ON as other side of coil is connected with neutral.
When contactor (S) is ON – the NO (S2) of S becomes NC & current will flow throughS2 to coil of main contactor (M) which gets ON as other side of coil is connected with neutral. When M gets ON, two NO contacts (M1 & M2) of M becomes NC. These two M1 & M2 contacts ensure continuous supply of current through these tow contactors S&M. This way S&M contactors are switched ON which makes STAR winding connection of motor. Motor runs in star connection& motor’s speed increases.
Working after timer activation – when motor reaches its 90% speed (motor takes 5 seconds aprox), at this stage timer gets activated (timer gets activated as per set time delay) & NC contact (T1) of Timer (T) becomes NO (which disconnects the supply of S contactor, at the same time NO contact (S1) of S contactor becomes NC. Secondly when Timer gets activated, its NO contact (T2) becomes NC, now T2 & S1 ensure supply of current to the D coil & D contactor gets ON. At the same time NC contact of D contactor becomes NO which ensures no supply to the S contactor. Atthis stage contactors D&M makes Delta connection of motor.
Control wiring diagram after activation of timer is given below.