Chapter 19 Reduced Voltage Starting
The majority of industrial applications normally use ___ motors.
3Φ induction
What starting method is typically the cheapest?
Part-winding
What are the advantages of Autotransformer starting
Provides highest torque per ampere of line current, 3 different starting torques available through autotransformer taps, suitable for relatively long starting periods, motor current is greater than line current during starting.
_________ is used to reduce interference in the power source.
Reduced-voltage starting
__________ is used to reduce the large current drawn from the electric utility lines by an across-the-line start of a large motor.
Reduced-voltage starting
What are the disadvantages of Wye-Delta starting
Requires special motor, low starting torque, momentary inrush occurs during open transition when delta contactor is closed.
What device is typically used to control the applied voltage of a DC motor?
SCR
What are the types of part-winding construction and which one is most common?
Two - step and three - step and two-step is more common
Wye-delta starters have the disadvantage of being ___ transition.
open
The ___ starting method requires a nine-lead wye motor.
part-winding
Voltage and current readings are taken when troubleshooting ___ circuits.
power
Primary resistor starting is a reduced-voltage starting method that uses a resistor connected in each motor line (in one line for a 1Φ starter) to produce a ___.
voltage drop
When is Autotransformer starting preferred over primary resistor starting?
when the starting current drawn from the line must be held to a minimum value but the maximum starting torque per line ampere is required.
Wye-delta starting accomplishes reduced-voltage starting by first connecting the motor leads in a ___ configuration for starting.
wye
What are the steps to troubleshooting reduced-voltage starting circuits?
1. Visually inspect the motor starter. 2. Measure the incoming voltage coming into the power circuit. 3. Measure the voltage delivered to the motor from the reduced voltage power circuit during starting and running 4.Measure the motor current draw during starting and after the motor is running
A motor started in the wye configuration receives approximately______ of the normal voltage and develops approximately ______ of the normal torque.
58% , 33%
How many leads extend from a wye-delta motor?
6
Standard primary resistor starters provide two-point acceleration (one step of resistance) with approximately ___% of line voltage at the motor terminals at the instant of motor starting.
70
What two devices are used to for DC motor reduced-voltage starting?
A starting rheostat or solid-state circuit
Why is the current high when starting a three phase motor?
Because of the lack of CEMF
The current drawn by a motor when starting is excessive due to a lack of ___ at the instant of starting.
CEMF
___ readings can be taken at the incoming power leads or the motor leads.
Current
___ fuses are normally required when using part-winding starting.
Dual-element
What is the current required by a motor to produce full-load torque at the motor's rated speed.
Full-load current (FLC)
___ starting is the least expensive and most efficient means of starting a motor for applications involving small-horsepower motors.
Full-voltage
What is the current equation for finding both the starting and running current?
I = (E - Cemf)/R
What are the disadvantages of Autotransformer starting
It is the most expensive design in lower HP ratings, low power factor, large physical size
What are the advantages of Part-winding starting
Least expensive reduced voltage starter, most dual-voltage mtors can be started part-winding on lower voltage, small physical size
___ current and the resulting torque produced in the motor shaft (in addition to load requirements) determine whether the motor can be connected across the line or whether the current must be reduced through reduced-voltage starting.
Locked rotor
What is the steady-state current taken from the power line with the rotor locked (stopped) and with the voltage applied?
Locked rotor current (LRC)
What are the disadvantages of Primary Resistor Starting?
Low torque efficiency, resistors give off heat, starting time in excess of 5 sec, requires expensive resistors, difficult to change starting torque under varying conditions
What is the method of starting a motor by first applying power to part of the motor coil windings for starting and then applying power to the remaining coil windings for normal running?
Part-winding
What method of reduced-voltage starting is technically not a true reduced voltage starting method?
Part-winding
What are the advantages of Primary Resistor Starting?
Smooth acceleration, high power factor during start, less expensive than autotransformer starter in low HPs, available with as many as 5 accelerating points
What are the disadvantages of Part-winding starting
Unsuited for high-inertia, long-starting loads, requires special motor design for voltage higher than 230v, motor does not start when torque demanded by load exceeds that developed by motor when first half of motor is energized, first step of acceleration must not exceed 5 sec or motor overheats
The amount of reduced torque is adjustable when using the ___ starting method, which has taps.
autotransformer
The ___ circuit includes the motor starter (mechanical or solid-state), overload contacts, and timing circuit.
control
Part-winding starting is usually classified as reduced-voltage starting because of the resulting reduced ___.
current and torque
Why are limits on inrush current necessary?
for smooth, steady power regulation and for eliminating objectionable voltage disturbances such as annoying light flicker.
What are the pros of a DC motor over AC motor?
high torque and good speed control and the available source of power.
Reduced-voltage starting provides ___ current draw over a longer period of time than full-voltage starting.
incremental
An ___________ acts much like a short circuit in the secondary of a transformer when it is started.
induction motor
When any motor is started, high ___ current is produced.
inrush
In autotransformer starting, the motor terminal voltage does not depend on the ___ current.
load
The ___ starting method is used when it is necessary to restrict inrush current to predetermined increments.
primary resistor
The larger the horsepower (HP) size of a DC motor, the less the ___ and the higher the current.
resistance
The purpose of a starting ___ is to reduce the voltage (and thus current and torque) during starting.
rheostat
An induction motor acts much like a ___ in the secondary of a transformer when it is started.
short circuit
The motor ___ windings must be divided into two or more equal parts for a motor to be started using part-winding starting.
stator
What are the advantages of Wye-Delta starting
suitable for high-inertia, long acceleration loads, high torque efficiency. ideal for especially stringent inrush restrictions, ideal for frequent starts
Autotransformer starting uses a(n) ___ 3Φ autotransformer to provide reduced-voltage starting.
tapped
What is Increment current?
the maximum current permitted by the utility in any one step of an incremental start.
A ___ is provided in the control circuit of a primary resistor starting circuit to short the resistors after the motor accelerates to a specified point.
timer
Autotransformer starting may use its ___ advantage to provide more current on the load side of the transformer than on the line side.
turns ratio
When a wye-delta starter is energized, ___ contactors close.
two
The current drawn by a motor during starting is typically about how many times more then the current rating found on the motor nameplate?
two to six times
How does a wye-delta starter compare to a part-winding starter?
wye-delta starting gives a higher starting torque per line ampere than part-winding starting, with considerably less noise and vibration. However, wye-delta starters have the disadvantage of being open transition.
