Motor Control (Reduced voltage starters)

Question 1

List the 5 reduced voltage starters.

Answer 1

1. Primary Resistance Starter
2. Primary Reactance Starter
3. AutoTX Starting (AKA: Compensator starter)
4. Wye-Delta Starter
5. Part Winding Starter (technically not a reduced voltage starter)

Question 2

What are reduced voltage starters commonly employed for?

Answer 2

3 Phase SCIMS in the 5 - 200 HP range. Large motors

Question 3

What are the advantages of a Primary Resistance Starter?

Answer 3

• Simple
• Reduces voltage and current
• Provides smooth acceleration
• OK for 3, 6, 9 & 12 lead motors

Question 4

What are the disadvantages of a Primary Resistance Starter?

Answer 4

• Power consumption
• Inefficient (worse than pri. reactance)
• Limited in capability

Question 5

What are the characteristics of a Primary Reactance motor?

Answer 5

Inductive characteristic:
limits current
keeps inrush low and line drops more tolerable

Question 6

Advantages & disadvantages to a primary reactance motor?

Answer 6

Advantages:

• similar to pri. resistance
• more efficient

Disadvantages:
-Size of inductors needed

Question 7

What are the 2 “Primary Impedance starters”?

Answer 7

1. Primary Resistance starter

2. Primary Reactance starter

Question 8

What are the advantages and disadvantages of an Auto TX starter?

Answer 8

Advantages:

• High torque to motor
• multiple taps permit on site selection
• Efficient

Disadvantages:
-More cost

Question 9

What are the usual tap options on an Auto TX starter?

Answer 9

50%, 65%, 80%

Question 10

Wye-Delta starter characteristics?

Answer 10

• Equally effective as an Auto TX
• Reduces inrush to 1/3
• Low starting current (33%)
• Low starting torque (33%)

Question 11

What are Wye-Delta starters suitable for?

Answer 11

Suitable for inadequate power supply situations or where line drop exists

Question 12

How many leads can you use for a Wye-Delta starter?

Answer 12

Must be 6 or 12 leads

Question 13

Wye-Delta starter disadvantages?

Answer 13

• Weak starting torque limits applications
  -Overloads must be set to I phase
  (still only 3 required)

Question 14

Part Winding starter characteristics?

Answer 14

• Must be a dual voltage motor
• limited torque (do not start under load)
• uses small size starters
• 6 wires to the motor
• inherently closed transition
• full voltage applied to half the windings

Question 15

list 3 reasons why across the line starting in the most common method for starting AC motors

Answer 15

1. High starting torque
2. Inexpensive
3. Simple

Question 16

Why might across the line starting not always be suitable?

Answer 16

• High starting current

- High starting torque damaging to some loads

Question 17

Is it permissible to short out the overload protection of 3 phase motors during starting? explain.

Answer 17

Yes, if the load is slow accelerating it may cause the overload device to open the circuit

Question 18

How can you reverse control for 3 phase AC motors?

Answer 18

Interchange any two line leads supplying the motor

Question 19

What is meant by the term open transition?

Answer 19

Motor momentarily disconnected from line leads during transition

Question 20

what is meant by the term closed transition?

Answer 20

Motor continuously connected to lines

Question 21

what advantage is offered by closed versus open transition starting methods?

Answer 21

Closed has less line disturbance