ELECTRIC MOTOR - Part 2

ELECTRIC MOTOR

PART 2
March 10, 2014
• Induction Motor – Basic Operation
• When 3-Φ supply is connected to the stator winding the currents which flow in the windings produce a multi-pole magnetic field.
• This field rotates at a speed called synchronous speed.
f = P x N/120
N – synchronous speed, rpm
P – no. of magnetic poles
f – supply frequency, Hz
• Example: What is the synchronous speed of a 6-pole motor supplied at 60Hz?
• Answer: 1200 rpm

The rotating magnetic field cuts through the rotor conductors and induces emf’s in them.
• Since the rotor conductors are connected together at the ends, the induced emf’s set up rotor currents.
• The rotor currents produce a magnetic field which interacts with the rotating magnetic field and torque is exerted on the rotor conductors.
• The direction of the torque on the rotor conductors causes the rotor to rotate in the same direction as the rotating magnetic field.

• Q: How is the rotor direction reversed?
• Simply swapping over any two-supply line connections at the stator terminal box. This reverses the direction of the rotating magnetic field.
• An induction motor cannot run normally at synchronous speed. This is because the rotor conductors would then be stationary with respect to the rotating magnetic field.
• No emf would be induced in the rotor and there would be no rotor current and no torque developed.
• Even when the motor is on no-load and rotor speed has to be slightly less than synchronous so that current can be induced in the rotor and torque produced.
• SLIP is the difference between the rotor speed NR and synchronous speed NS.
• Slip is usually expressed as the percentage of synchronous speed.
• s = (NS – NR)/Ns x 100%

• Example: If a 6-pole motor is supplied at 60Hz and runs with a slip of 5%, what is the actual rotor speed?
• At no-load the rotor speed is only slightly less than synchronous speed since only torque required is that needed to overcome the rotational losses on friction and windage.
• When load is applied to the motor shaft the rotor tends to slow down.
• This allows the constant speed rotating field to cut the rotor conductors at a faster rate which induces larger rotor currents.
• The result is an increased torque output at a slightly reduces speed.
• The curve shows the variation of torque with slip for a standard cage-type induction motor.
• Also shown is a typical load characteristic which indicates the torque necessary to drive load at different speeds.

• Control Equipment
• When the stator windings of an induction motor are connected directly to its 3- supply, a very large current (6-8 x full load current) flows initially.
• This surge current reduces as the motor accelerates up to its running speed.
• Induction motors can be Direct-on-Line (DOL) started in this way.
• The starting current will not cause damage to the motor unless the motor is repeatedly started and stopped in a short space of time. This is called fast cycling.
• When very large motors are started DOL they cause a disturbance of voltage (voltage dip) on the supply lines due to the large starting current surge.
• This voltage disturbance may result in the malfunction of other electrical equipment connected to the supply.

• To limit the starting current some large induction motors are started at reduced voltage and then have the full voltage supply voltage reconnected when they have run up to near rated speed.
• Most induction motors on ships are started DOL.
• Reduced voltage starting is used for large motors driving loads like cargo pumps and bow thrusters.

• Two methods of reduced voltage starting
• 1. Star-Delta (Y-) starting
• 2. Autotransformer starting

• Contactors perform the switching action in starters to connect and disconnect the power supply to the motor.
• The contactor is an electromagnetically operated 3-pole switch initiated from local and/or remote stop/start push buttons.

• If the current goes above the rated current for the motor the contactor will be tripped out automatically to disconnect the motor from the supply.
• Large motors use contactors of this type which are mounted on insulted boards inside switchgear compartments called starters.
• A compact, self-contained contactor for use with small and medium sized motors is shown below in exploded view.

• Direct-on-Line Starter
• The contactor coil is connected in series with a start button, stop button and overload trip contacts.
• This is called the control circuit and is energized from two lines of the 3- supply – usually via a step-down transformer.
• When the start button is pressed the control supply is connected to the contactor coil. The contactor closes and starts the motor.

• When the start button is released its contacts spring open. An auxiliary contact on the contactor keeps the contactor coil energized after the start button is released.
• Pressing the stop button breaks the control circuit to the contactor coil; the contactor trips and the motor stops.
• If the motor takes too much current because it is mechanically overloaded or stalled, the overload coils will either magnetically or thermally open the overload trip contacts which will stop the motor and prevent overheating.
• Note, the correct term is overcurrent rather than the commonly used overload.
• Star-Delta Starter
• If the motor is DOL started with stator winding Y connected, it will only take 1/3 of the starting current that it would take if the windings were  connected.
• The starting current of a motor which is designed to run  connected can be reduced in this way.

• Y- starters may be operated by a manual changeover switch or they can be automatically switched using contactors controlled by a timing relay.
• Automatic changeover using contactors:
• Y- starter sequence:
• Operator closes motor isolator IS then presses start button.
• Start button connects the supply to contactor coil S.

• Contactor contacts S close and auxiliary contacts S1 close.
• Supply is connected through S1 to contactor coil L. Contactor contacts L close, motor windings are Y connected to 3- supply, motor starts.
• Auxiliary contacts L1 close at the same time as contactor contacts L. The operator may now release the start button since supply to L is maintained through L1.

• After a time interval which allows the motor run up to a speed, auxiliary contacts L2 open and L3 close.
• Contactor coil S is de-energized and its contacts S open; so do the auxiliary contacts S1.
• Contactor coil D is energized and the motor is now delta connected to the 3- supply.
• In some cases a mechanical interlock is fitted between the contactor contacts S and D so that both cannot be closed at the same time.
• Q: Why is it essential that contactors S and D are not both closed at the same time?
• The starter diagram shows a full short circuit is applied across the supply.
• At start, when the supply has just been switched on and the motor has not yet started to rotate, there is no mechanical output from the motor.

• The only factors which determine the current taken by the motor are the supply voltage (V) and the impedance of the motor windings (Z).
• The starting current of a  connected motor can be reduced to 1/3 if the motor is Y connected for starting.
• (The torque is also reduced to 1/3 resulting in an increased run-up time for the motor and load.)
• When an induction motor is running on load it is converting electrical energy input to mechanical energy output. The input current is now determined by the load on the motor shaft.
• An induction motor will run at the same speed when it is star connected as it will when it is  connected.
• This means that the power output from the motor is the same when the motor is Y connected as when the motor is  connected, so the power inputs and line currents must be the same when running in either connection.
• If the motor is designed to run in  but is Y connected, and on full load, then each stator winding will be carrying an overload of 1.73 x Rated current.
• This will cause overheating and eventual burnout unless tripped by the overcurrent relay.
The motor copper losses are produced by the I2R effect so the motor will run 3 times hotter if left to run in the Y connection when designed for  running.
• This malfunction may occur if the control timing sequence is not completed or the Y contactor remains stuck in while a mechanical interlock prevents the  contactor from closing.
• For correct overcurrent protection, the overcurrent relays must be fitted in the phase connections and not in line connections.

• Autotransformer Starting
• Starting large motors with long run-up periods demands a very high current surge from the supply generator.
• This causes a severe voltage dip which affects every load on the system.
• Reduced voltage starting will limit the starting surge current.

• One way to reduce the initial voltage supplied to the motor is to step it down using a transformer.
• Then, when the motor has accelerated up to almost full speed, the reduced voltage is replaced by the full mains voltage.
• The transformer used in this starter is not the usual type with separate primary and secondary windings.
• It is an autotransformer which uses only one winding for both input and output.

• This arrangement is cheaper, smaller and lighter than an equivalent double-wound transformer.
• For induction motor starting, the autotransformer is a 3- unit, and, because of expense, this method is only used with large motor drives, e.g. electric cargo pumps.
• The supply voltage is connected across the complete winding and the motor is connected to the reduced voltage tapping.

• A number of tappings are usually available on the transformer winding, giving voltage outputs ranging from about 50% to 80% of the mains supply voltage, e.g. a 60% tap on an autotransformer supplied at 440V would provide an output of
• (60/100) X 440V = 264V
• The autotransformer with its range of tapping points gives a set range of starting voltages to limit the motor starting surge current to a reasonable value.

• Example: Estimate and compare the likely starting current surges for a motor that takes 200A on full load when started (a) DOL, (b) Star-Delta, (c) Autotransformer with a 50% tapping.
• (a) DOL: IS = about 5 x IFL
• (b) Y-: IS = 1/3 x IDOL
• (c) Autotransformer: IS = x 2 ( IDOL )
• x – tapping point
• The DOL starter is simple and cheap but causes a large starting surge. Y- starting reduces the surge but somewhat more complex, requiring 3 contactors and a timer.
• The autotransformer method can be arranged to match the motor surge current and run-up period to meet the supply limitations by a suitable choice of voltage tapping.
• This starter is considerably more expensive than the other 2 starter types.

• As with the Y- starter, the autotransformer may use what is called an open transition switching sequence or a closed transition switching sequence between the start and run conditions. In the former, the reduced voltage is supplied to the motor at start then disconnected and the full supply voltage rapidly reconnected to the motor.
• The circuit shows a manually operated open transition, autotransformer starter.

• The problem with open transition is that a very large surge current can flow after the transition from reduced to full voltage.
• Q: What causes the large current surge in open transition starters when going from the start to run condition?
• All motors generate a back emf against the supply voltage when they are running.
• When the supply is removed from a running induction motor the magnetic field does not immediately collapse.
• The motor begins to slow down but still generates an emf.
• When the supply is reconnected, the supply voltage and motor emf are not necessarily in phase (the condition is similar to synchronizing a generator onto the bus-bars).
• So each time the starter is operated a different surge current will be produced – sometimes very large, sometimes quite small.
• Large surges could cause appreciable voltage dip on the supply and so affect other equipment.
• Closed transition starters overcome this because the motor is never disconnected from the supply during the starting cycle.

References:
Practical Marine Electrical Knowledge- By Hall
Siskind Charles , Electrical Machine
Preventive Maintenance of Electrical Machine by Hubert, Charles (2nd Edition)
www.electricalmachine.com

Part 3 coming up next on line