Single-Phase Motors 030202e Flashcards
What is the horsepower range for fractional horsepower motors?
1/100 to 3/4 hp
What is the horsepower range for integral horsepower motors?
1hp to 10 hp. With 15 hp being special order.
What are two popular motor types?
Resistance-start split-phase motor (split-phase motor).
Two-value capacitor motor (cap-start/cap-run motor).
What is counter torque?
Amount of opposition the load presents to being moved.
Opposition to twisting/turning.
What is torque?
Twisting force
What is running torque?
Amount of twisting force needed to keep a load rotating.
What is starting current?
Also known as inrush current is the amount of amps the motor will draw while starting from no speed to full speed.
What is starting torque?
Amount of twisting force required to start a load to move.
What are the basic components of a split-phase motor?
Frame.
Stator.
Rotor.
Starting switch (cut-out switch)
What is the purpose of the frame?
Encloses the stator windings.
Supports the motor’s end bells.
Helps to maintain a air gap between the rotor and stator.
What is the stator?
Consists of a laminated steel core with slots to hold the insulated windings.
Has 2 sets of windings, one larger main(run) winding and a smaller start winding.
What is the difference between the windings in the stator of a split-phase motor?
Main winding is made of larger insulated copper conductors and mounted on the bottom of the stator.
Start windings are smaller conductors with fewer turns to have greater resistance and less reactance than the main winding.
In a 2 pole motor the start windings are placed 90 degrees from the centre of the main winding and 45 degrees away in a 4 pole motor.
What are the rotor-bar conductors made of?
Older ones were made of copper.
Modern ones are made of cast aluminum.
What is the purpose of a centrifugal type switch mounted inside the motor?
Serves to disconnect the start winding just before the rotor reaches full-load speed.
What are the components of a centrifugal switch?
A stationary component attached inside the end bell with normally closed contacts.
A Rotating component mounted on the rotor shaft known as an actuator.
At what point do the stationary contacts open?
Approximately 75% of the rated speed.
What is the most common cause for single-phase motor failure?
Faulty start switch.
What is another method for a start winding cut-out other than the centrifugal switch?
Current-sensitive relays.
Electronic switching devices like triacs.
How does a current-sensitive relay work for a start winding cut out?
Has a solenoid connected in series with the main winding with normally open contacts.
The high inrush current during motor start is enough to activate the solenoid coil and close the contacts to energize the start winding.
When current decreases to rated value the solenoid drops out and start winding is removed from the circuit.
How must current-sensitive relays be mounted?
Upright position so that the gravity-controlled contacts reopen successfully.
What is the advantage of current-sensitive relays over a centrifugal switch?
Can be remotely mounted as well as are not prone to issues due to dirt and debris
What is the advantage of electronic switching devices?
Does not have contacts that may wear out.
Can be located some distance from the motor.
What degree of phase displacement between the main and start winding creates the greatest torque?
90 degrees.
How much electrical phase displacement between the main and start windings is typical in a resistance-start split-phase motor?
Approximately 25 to 30 degrees.
What can happen if the start winding is kept in the circuit too long?
Small-diameter conductors will overheat and likely burn out.
What is cross-field effect?
Rotors magnetic field is physically oriented approximately 90 degrees behind the stator.
This causes the magnetic fields to build to maximum at different times.
The rotors magnetic field is attracted to the stators but since both fields are alternating they can never catch up and creates a continuous rotational torque.
Rotors magnetic field lags the stators magnetic field.
What is rotor-slip?
The difference between synchronous speed and rotor speed.
(The synchronous speed is always higher than the rotor speed.)
What is the most common size of resistance-start motor?
1/4 hp
How much is the starting current compared to the running current?
5 to 6 times the amount.
How much more starting torque is there compared to the running torque?
125% to 150%
How are the run windings connected in dual voltage split-phase motors for low-voltage and high-voltage connections?
Low-voltage - run windings and start windings must be parallel connected.
High-voltage - The run windings must be series connected and the start winding connected in parallel with only one run winding portion.
What are the basic components of a two-value capacitor motor also known as the cap-start/cap-run motor?
An electrolytic capacitor (start-cap).
An oil-filled continuous-duty capacitor (run-cap).
Cut-out switch (centrifugal or relay type).
Main winding.
Auxiliary Winding.
Why is the start winding called the auxiliary winding in cap-start/cap-run motors?
Continuously energized.
What are some characteristics of the electrolytic capacitor (start-cap)?
Rated for short duty time of only three seconds.
Readily accept AC without degradation as long as duty time is not exceeded.
Should not endure any more than 20 start ups per hour.
Common failure point in faulty cap-start/cap-run motors.
Microfarad ratings range between 100 to 1000 uF. Most common 150 to 350 uF.
What are some characteristics of the Oil-filled capacitor (Cap-run)?
Designed for continuous duty.
Oil inside acts as a coolant and outside container is made of tin.
Voltage ranges from 370 Vac to 416 Vac.
Can withstand voltage potentials higher than RMS values.
Common microfarad rating of 10 uF to 50 uF.
What is the advantage of the cap-start/cap-run motor of the split-phase?
Capacitance produces an increased phase displacement of 90 degrees between the run winding and start winding.
This produces greater starting torque and lower inrush current.
What is the starting torque percentage compared to the running torque in a cap-start/cap-run motor?
250% to 450%.
What are the general horsepower ratings of a cap-start/cap-run motor?
1/4 hp to 10 hp with 15 hp available as special order.
What part of a split-phase motor holds the rotor in place?
The end-bells.
