1. Motors & Motor Starters
Prepared By:
Erik Redd
&
Jeremy Roberts

2. Motors AC-Motors Parts of an Electric Motor
A. Stator : Stationary Frame
B. Rotor : Revolving Part
The rotary motion in an ac-motor is caused by the fundamental law of magnetism.
This law states that like poles repel and unlike poles attract.

3. Diagram of an ac-motor This shows a three phase, two pole stator.
Where A, B, and C are the three phases

4. Diagram of the Three Phases
Fig Pg. 244
Poles 1 and 4 are at their greatest magnetic field at time equal to one, because phase A (red line) is connected to those poles, and the same for the other poles when their corresponding phases are at maximum current magnitude.

5. Synchronous Speed
Speed at which it takes the motor to go one cycle and one revolution.
S=[120*frequency}]
(# poles)
Example:
For a three-phase, 60 Hertz, 2 pole motor:
S=[120*60]/2=3600 revolutions per minute

6. Polyphase Squirrel-Cage Induction Motors
The most common three-phase motor
Does not have solid poles
Instead, it has laminations: numerous flat sheets held together in a package. They are insulated from each other (this reduces Eddy currents) making up the stator
The difference between induction and synchronous motors is that the rotor for an induction motor can travel at a different speed than the stator. This is called Slip.
slip= Syn. rpm – Motor rpm * Syn. rpm

7. Example.
A 2 pole, 60 Hz motor runs at a full-load speed of 1760 rpm.
What is the slip?

8. Ans. %slip= *100
3600
=51.1%

9. Single-Phase Motors Supplied by single source of ac voltage
Rotor must be spun by hand in either direction, does not have a starting mechanism
Has no starting torque
Three different types of single-phase motors: split-phase, capacitor start, permanent split-capacitor, and shaded-pole motors

10. Resistance Split-Phase Motors
Has a start winding and a main winding
Winding currents are out of phase by 30 degrees, this produces a flux field that starts the motor
Main winding current (IM) and start winding current (IS) lags supply voltage (VL)
Start (inrush) current is high
Needs centrifugal starting switch or relay to disconnect the start winding (protects it from over heating)
Efficiency is between 50-60%

11. Capacitor-Start Motors
Has the same winding and switch mechanism arrangement as split-phase but adds a short time-rated capacitor in series with the start winding
The time shift phase between the main and start winding is close to 90 degrees
IS leads VL
Efficiency is between 50-65%
Capacitor controls the inrush current

12. Permanent Split-Capacitor Motors
Winding arrangement is the same as the capacitor and split-phase motors
Capacitor can run continuously, rated in microfarads for high-voltage ratings
No centrifugal switch is needed
IM lags VL, while IS leads VL
Efficiency is between 50-70%

13. Shaded Pole Motors Simple construction, least expensive
Has a run winding only, shading coils are used instead of the start winding
Stator is made up of a salient pole, one large coil per pole, wound directly in a single large slot
A small shift in the rotor causes torque and starts the motor
Efficiency is between 20-40%

14. DC Motors Consists of an armature winding and a stator winding
Armature windings act as the rotor
Has three different classifications: constant torque, constant horsepower, or a combination of the two
Standard industrial dc motors are shunt wounded
Modifications of the dc motor are: shunt wound, stabilized shunt exciting fields, compound wound motors, and series wound motors

15. Armature Voltage Control
Is used for motor speeds below base speed
Output torque= T=k*ø*IA
k is machine constant
ø is the main pole flux
IA is the armature current

16. Shunt Field Control Is used for motor speeds above base speed
Horsepower, (HP)= Torque*rpm
5252
Where torque is in lb-ft

17. Speed Regulation Speed Regulation (IR)= no load rpm- full load rpm

18. Brushless DC Motors
Three phase ac power is converted into dc by the input side of the motor to charge up a bank of storage capacitors
These capacitors are called the Buss
The purpose of the buss is to store energy and supply dc power to transistors in the output side as the motor requires the power to start up

19. Brushless DC Motors
Figure 13-21, page 264 shows the input power section
It consists of three fuses, six diodes, a choke, and two capacitors
The fuses protect the diodes
The choke protects against line transients
The motor control may run at very low speeds at very high torques while drawing little current from the ac line

20. Brushless DC Motors
This picture is a representation of the encoders (rotor part of the motor) telling the corresponding transistors (stator) to turn on in order to get maximum torque from the motor

21. Picture of a Brushless Motor

22. Motor Control Starters
Motor will draw high inrush current while the starter will slow current down
Starter reduces the amount of torque needed to start the motor

23. Magnetic Motor Starter
Normally open contacts
Not always possible to control amount of work applied to the motor
Has overloads
Motor may be overloaded resulting in damage to the motor
Open due to excessive motor current, high temperature, or a combination of both

24. Full-Voltage Starter Contains one set of contacts
Motor is directly connected to the line voltage

25. Reversing Motor Starter
Contains two starters of equal size
Two starters connect to the motor
Interlocks are used to prevent both starters from closing their line contacts at the same time
Figure 14-4A

26. Reduced-voltage Motor Starter
Applies a percentage of the total voltage to start (50% - 80%)
After motor rotates, switching is provided to apply full voltage
Torque will be reduced when starting
Four types:
1) Autotransformer
2) Primary Resistance
3) Wye – Delta
4) Part Winding

27. Autotransformer Starter
Two contactors are used:
1) Start contactor
- Closes first and connects motor to the line
through an autotransformer
- Deenergizes
2) Run contactor
- Motor switches to this contacter which has
full voltage

28. Primary Resistor Starter
Two contactor
1) Line contactor
- First to energize connecting motor to the
line voltage through a resistor
- After preset time, contactor opens
2) Accelerating contactor
- Energizes
- Causes smooth acceleration to full voltage

29. Wye – Delta Starter Three contactors are used
1) Line contactor and start contactor
- Energizes first and connects motor in wye
putting about 58% of line voltage across
each motor phase
- Contacts open after preset time
2) Run contactor
- Energizes connecting motor in delta and
putting full voltage on the motor

30. Part Winding Starter
Starter supplies about 48% of normal starting torque
Not truly a reduced-voltage means
Two Types
1) Two-Step - one winding connected to
full voltage line and, after a preset time,
the other connects
2) Three-Step – one winding is connected in series
with a resistor to the voltage line; after interval, resistor
is shorted out and then second line is connected to
full voltage line

31. Solid-State Motor Starter
For lower starting torque and smooth acceleration
Used on conveyors, pumps, compressors, etc.

32. Standard Modes of Operation
Motor voltage gradually increases during acceleration
Creates a kick start pulse of 500% of full load amperage for high friction
Used when necessary to limit current
Used when motor requires a full voltage start