EC (Motors) Ch 13 Flashcards
(18 cards)
Why is it called a ‘squirrel cage’ induction motor?
The rotor resembles a spinning exercise wheel or ‘squirrel cage’ with conductive bars shorted at both ends with end rings.
What is the synchronous speed of a two-pole, three-phase, 60 Hz motor?
Ns = (120 × Frequency) / Number of Poles
Ns = (120 × 60) / 2 = 3600 RPM
What is the percent slip of a four-pole, three-phase motor with 1740 RPM full-load speed?
Slip (%) = ((Ns - Nr) / Ns) × 100 = ((1800 - 1740) / 1800) × 100 = 3.33%
Why is there no starting torque with one winding in a single-phase motor?
A single winding creates an alternating field, not a rotating one, leading to no starting torque.
What is the purpose of a centrifugal switch in many single-phase motors?
It disconnects the start winding or capacitor once the motor reaches ~70–80% of full speed.
How does a capacitor create a rotating field in a single-phase motor?
A capacitor creates a phase shift between the main and auxiliary windings, simulating a two-phase power supply.
What are the differences between capacitor start and permanent split-capacitor (PSC) motors?
Capacitor Start: Uses a large capacitor and centrifugal switch; only used during startup.
PSC Motor: Uses a smaller capacitor continuously; no switch needed, but less starting torque.
Why is the efficiency of a shaded-pole motor low?
Shaded-pole motors have high core losses and low starting torque due to inefficient magnetic fields.
How are DC motors generally classified?
By the way their field windings are connected:
* Series
* Shunt
* Compound
* Permanent Magnet
Why must a series motor be loaded at all times?
Without a load, the motor can overspeed dangerously due to rapid speed increases with light or no load.
What is the relationship between torque and flux, armature current, and a constant in a DC motor?
T = k × ϕ × Ia
Where:
* T = Torque
* k = Machine constant
* ϕ = Flux per pole
* Ia = Armature current
What is the formula for horsepower in relation to torque and RPM in a DC motor?
HP = (Torque × RPM) / 5252 (Torque in lb-ft)
How is speed regulation determined in a DC motor?
Speed Regulation (%) = ((No-load speed – Full-load speed) / Full-load speed) × 100
How is rectification done in a brushless DC motor?
Electronic controllers (inverters) convert three-phase AC to DC, then re-switch it into three-phase AC.
What is the difference in rotors between brushless DC and AC induction motors?
- Brushless DC Motor: Rotor has permanent magnets.
- AC Induction Motor: Rotor is a squirrel cage—no magnets; current is induced by stator.
What are Hall-Effect switches in a brushless DC motor?
Hall sensors detect rotor position, sending signals to the controller for proper phase switching.
What is speed regulation in a brushless DC motor?
It’s the difference between the set (desired) speed and the actual speed, usually measured in RPM.
