Three Phase Motors And Starters

Question 1

Describe a Single-Voltage Motor

Answer 1

1. Has three windings or 3 groups of windings
2. Each winding or group of windings receives their voltage from one of the phases
3. Contains 6 leads
   - T1 and T4
   - T2 and T5
   - T3 and T6
4. When viewed from the non drive end, Positive phase sequence (ABC) rotates counter clockwise. Negative phase sequence (CBA) rotates clockwise

Question 2

Describe a dual voltage motor

Answer 2

2. 6 windings or groups of windings, two windings or groups for each phase
3. Has 12 leads but only T1-T9 are brought out to the connection box, T10,11,12 remain in the stator

Question 3

What are the 2 types of motor starters

Answer 3

1. Across the line starters (full voltage at start)
2. Reduced voltage starters (reduced voltage at start, full voltage at run)

Question 4

Describe across the line starters

Answer 4

1. When a starter connects the motor leads directly across the supply usually by means of a switch

Question 5

Describe part winding starters and motors

Answer 5

1. Two or more sets of three phase windings
2. One winding is energized with full voltage at start
3. Once started, the starter energizes both windings in parallel to run the motor
4. Inrush current is reduced to about 60-65%
5. This type of starter is for loads with small to no load at start

Question 6

Describe a Reduced-Voltage starter

Answer 6

1. Reduces the inrush starting current and starting torque on a motor circuit

Question 7

Describe an auto transformer starter

Answer 7

1. Uses different taps on an autotransformer to reduce voltage to the motor
2. Starting current to the motor is proportional to starting voltage
3. The starting current to the auto transformer starter is proportional to the square of the starting voltage
4. The starting torque is proportional to the square of starting voltage

Question 8

Describe a wye-delta starter

Answer 8

1. Three windings or groups of windings that are connected in wye for starting but are reconnected in delta for running
2. By starting in wye the inrush current can be reduced to 33.3% along with starting torque

Question 9

Describe and electronic soft starter

Answer 9

1. Reduces starting current with electronic switches and programmable chips
2. Once the motor has accelerated, a contactor closes to bypass the switches and apply a full voltage to the motor
3. Smaller in size than conventional RVS

Question 10

What are the 2 acceleration modes of an electronic soft starter

Answer 10

1. Ramp starting (starting current increases at a constant rate)
2. Current limit starting (starter establishes a max current during starting period)

Question 11

What is the maximum horsepower available for a single phase motor

Answer 11

15hp max, not common for anything over 5hp

Question 12

What are some benefits of three phase motors over single phase motors

Answer 12

1. Cheaper
2. More readily available
3. more efficient
4. Smaller in size
5. Quieter

Question 13

What are the 2 types of phase converters

Answer 13

Rotary and static

Question 14

Describe a rotary phase converter

Answer 14

1. It is an induction motor without an output shaft
2. Uses high capacitance, electrolytic capacitors and contactors to supply a leading current
3. Cannot connect the motors on the load side until the phase converter generator accelerates to rated speed
4. Timing relays may be used to delay
5. Can handle multiple 3 phase motor loads

Question 15

Describe a static phase converter

Answer 15

1. Most successful when the mechanical load does not vary greatly
2. There is no motor generator so it is cheaper
3. Can only supply a single motor of a specific horsepower
4. Does not replicate a three phase source as well as a rotary