EI · 8 cards

Motors, Generators and Transformers flashcards

8 cards with the formulas, rules and traps from this lesson. Answer in your head, then flip to check.

Fact · 1/8
Motor, generator, transformer: what energy goes in and comes out of each?
Motor: electrical in, mechanical out. Generator: mechanical in, electrical out. Transformer: AC electrical in, AC electrical out at a different voltage, with no moving parts.
Term · 2/8
What does the commutator do in a brushed DC motor?
It is a split ring on the shaft fed by carbon brushes. It reverses the coil current every half turn so torque stays in one rotational direction and the motor keeps spinning.
Formula · 3/8
How does back emf set a DC motor's current, and what is stall current?
$I = \frac{V - E}{R}$ : supply minus back emf, over winding resistance. Back emf grows with speed. At stall E = 0, so $I = V / R$ , many times the running current.
Formula · 4/8
Motor efficiency and mechanical output power?
$η = \frac{P _{o u t}}{P _{in}} \times 100%$ , with $P_{in} = V I$ and $P_{o u t} = τ ω$ . Convert rpm with $ω = 2 πn /60$ rad/s. Efficiency over 100% means the fraction is upside down.
Formula · 5/8
Generator output frequency from poles and speed?
$f = \frac{p n}{120}$ : f in hertz, p the number of poles, n in rpm. Example: 6 poles at 1200 rpm give 60 Hz; a 2-pole machine needs 3600 rpm.
Term · 6/8
Slip rings versus split-ring commutator on a generator?
Slip rings are two continuous rings that pass the coil's alternating voltage straight out, giving AC (alternator). A split-ring commutator flips connections each half turn, giving pulsating DC.
Formula · 7/8
Ideal transformer voltage, current and power relationships?
$\frac{V _{s}}{V _{p}} = \frac{N _{s}}{N _{p}}$ , $\frac{I _{s}}{I _{p}} = \frac{N _{p}}{N _{s}}$ , and $V_{p} I_{p} = V_{s} I_{s}$ . The higher-voltage side carries the lower current.
Common trap · 8/8
Can a transformer work on DC or change frequency?
No to both. It needs changing flux, so steady DC gives no ongoing output (and can overheat the primary). Output frequency always equals input frequency; a rectifier is needed for AC to DC.

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