Skip to content

Quiz: Fan Motors and Enclosures

Test your understanding of induction and EC motors, frames, nameplate ratings, motor enclosures, motor accessories and motor bases with these review questions.

1. Which motor enclosure does FanPedia say should not be used in dirty or wet atmospheres?

  1. TEFC (totally enclosed fan cooled)
  2. ODP (open drip proof)
  3. TENV (totally enclosed non-ventilated)
  4. EXPL (explosion proof)
Show Answer

The correct answer is B. An ODP motor's internal fan pulls the surrounding air in and blows it across the windings, so anything the air carries reaches the windings. That gives good cooling at low cost in clean, dry locations, but FanPedia states that ODP motors should not be used in dirty or wet atmospheres. TEFC, TENV and EXPL motors all keep the windings sealed from the outside air.

Concept Tested: ODP Motor

See: ODP Motor

2. Twin City Fan's Premium Efficiency designation is comparable to which classes?

  1. NEMA High Efficiency and IEC IE2
  2. NEMA Standard Efficiency and IEC IE1
  3. NEMA Premium Efficiency and IEC IE3
  4. NEMA Super Premium and IEC IE4
Show Answer

The correct answer is C. FanPedia gives two TCF designations: Standard Efficiency, comparable to NEMA High Efficiency and IEC IE2 (A), and Premium Efficiency, comparable to NEMA Premium Efficiency and IEC IE3. IE4, super premium (D), is listed by FanPedia as a future designation that is not used. The "P" in the motor line "50 HP, 1800 RPM, IN, P, ..." is this Premium Efficiency field.

Concept Tested: Motor Efficiency Class

See: Motor Efficiency Class

3. Why does an induction motor's rotor always turn slightly slower than its synchronous speed?

  1. If the rotor caught up with the rotating field, no current would be induced in it and no torque would be produced
  2. The motor's bearings add friction that holds the rotor back by a fixed amount
  3. The permanent magnets in the rotor lag behind the electronic controller
  4. The nameplate deliberately understates the speed to allow for a safety margin
Show Answer

The correct answer is A. In an induction motor, rotor current is induced by the stator's rotating magnetic field. Induction needs relative motion, so if the rotor ever reached synchronous speed, no current would be induced and no torque produced. The small difference is called slip, which is why a 1800 RPM motor shows a somewhat lower full-load speed on its nameplate. Permanent magnets and a controller (C) describe an EC motor.

Concept Tested: Induction Motor

See: Induction Motor

4. Why can an EC fan's airflow be adjusted by changing a setting in its controller, while an induction motor fan on fixed power needs a sheave change or an external drive?

  1. EC motors have more poles, so their speed is easier to divide
  2. EC motors run on a higher supply frequency than induction motors
  3. EC motors have no bearings, so their speed is not limited
  4. In an EC motor, on-board electronics decide how fast the field turns, while an induction motor's speed is set by the supply frequency and pole count
Show Answer

The correct answer is D. An EC motor uses a rectifier and a controller to switch current through its windings, so the electronics, not the grid frequency, set the speed. That speed can be set from a controller or a 0-10 V signal. An induction motor's speed follows the supply frequency and number of poles, less slip, so changing it takes an external drive or a sheave change. Pole count (A), supply frequency (B) and bearings (C) are not the reason.

Concept Tested: EC Motor

See: EC Motor

5. What does a shaft grounding ring do on a motor run from a variable frequency drive (VFD)?

  1. It prevents the VFD from inducing any voltage on the motor shaft
  2. It gives VFD-induced shaft charges a low-resistance path to ground so they bypass the motor bearings
  3. It coats the bearing outer race with insulation to block current
  4. It grounds the conduit box so the supply wiring is safe to touch
Show Answer

The correct answer is B. Fast VFD switching can induce a voltage on the motor shaft that discharges through the bearing grease, pitting the races until they show fluting and fail early. A shaft grounding ring of conductive fibers gives the charge an easier path to ground. It does not stop the shaft voltage (A). Coating the outer race (C) describes insulated bearings, the related option that makes the bearing a harder path.

Concept Tested: Shaft Grounding Ring

See: Shaft Grounding Ring

6. What does "326T" mean at the end of the motor line "50 HP, 1800 RPM, IN, P, 3 Phase, 60 Frequency, 460 Volt, SD, TEFC, 326T"?

  1. A TEFC enclosure in a 326 frame
  2. A 326 frame with a C-face and a 3.26-inch shaft height
  3. A NEMA 326 frame of the standard T-frame foot-mount type, with an 8-inch shaft height
  4. A rating of 32.6 kW at the motor shaft
Show Answer

The correct answer is C. The frame field gives the NEMA frame size, and the letter T marks the standard post-1964 T-frame foot-mount motor. On NEMA foot-mounted frames from 143T up, the first two digits divided by 4 give the shaft height, so 32 / 4 = 8 inches. The T does not stand for TEFC (A); the enclosure has its own field. A C-face is marked with a C (B), and the frame number is not a power rating (D).

Concept Tested: NEMA Frame Size

See: NEMA Frame Size

7. A fan exhausts 250°F air and needs 9.0 BHP at that temperature. It has a 10 HP motor with a 1.15 service factor. At a cold start the fan is full of 70°F air. About how much power does it need at the cold start, and is the motor adequate?

  1. About 12.1 BHP, which is above the 11.5 HP service-factor limit, so the motor is too small
  2. About 9.0 BHP, because power does not change with air temperature
  3. About 6.7 BHP, well within the 10 HP nameplate rating
  4. About 10.4 BHP, within the 11.5 HP service-factor limit
Show Answer

The correct answer is A. Density varies inversely with absolute temperature, so the cold air is denser by (250 + 460) / (70 + 460) = 710 / 530 = 1.34. Fan power is proportional to density at the same speed and airflow, so the cold-start load is 9.0 × 1.34 = 12.1 BHP. The most the motor may carry is 10 × 1.15 = 11.5 HP. Option C inverts the density ratio. The motor must be sized for the worst condition, not the design temperature alone.

Concept Tested: Motor Service Factor

See: Motor Service Factor

8. A belt-driven Arrangement 1 centrifugal fan has a 326T motor. Which motor base fits?

  1. Bolt-on mount, which covers 48 to 286 frame
  2. Standard post mount adjustable base, which covers 48 to 215 frame
  3. Heavy-duty slide rails, which are for the 440, 500 and 5000 frame series
  4. NEMA type slide base, which covers 48 to 445 frame on Arrangements 1, 3, 9 and others
Show Answer

The correct answer is D. FanPedia lists the NEMA type slide base for motors from 48 to 445 frame on Arrangements 1, 3, 9, 9F, 9H and 9ST, plenum fans and pedestal plug fans. A 326 frame on Arrangement 1 is inside that range. The bolt-on mount (A) is for Arrangement 9 tubeaxial, vaneaxial and centaxial fans, and the post mount (B) stops at 215 frame. Heavy-duty slide rails (C) are for much larger frames.

Concept Tested: Motor Base

See: Motor Base

9. A European specification calls for "IP55 motors, general purpose, non-hazardous area." FanPedia's table maps IP55 to TEFC (general or severe duty) and also to the explosion proof enclosure. Which choice is best supported?

  1. A TEFC motor with general duty, because the area is not hazardous and an explosion proof motor would add cost without need
  2. An explosion proof motor, because it is the only enclosure that meets IP55
  3. An ODP motor, because a general purpose motor is always open drip proof
  4. A TENV motor, because IP55 requires a motor with no cooling fan
Show Answer

The correct answer is A. IP55 maps to both TEFC and explosion proof, so the IP code alone does not decide. The other conditions do: the area is non-hazardous and the duty is general purpose, so an explosion proof motor (B) adds cost the job does not need, and the motor line becomes GD, TEFC. ODP (C) maps to IP22, not IP55. TENV (D) maps to IP56, and IP55 does not require a fanless motor.

Concept Tested: TEFC Motor

See: TEFC Motor

10. A fan exhausts a solvent paint spray room, and the plant engineer has classified the area around the fan as a hazardous location for flammable vapors. Which plan addresses the whole hazard?

  1. An explosion proof motor alone, because it makes the whole fan safe for flammable air
  2. An explosion proof (DFG) motor for the customer's stated classification, plus spark resistant construction because the vapor passes through the fan
  3. A TEFC motor, because sealed windings are enough to stop ignition of flammable vapors
  4. Spark resistant construction alone, because the motor enclosure does not matter if the fan is spark resistant
Show Answer

The correct answer is B. The motor sits in the classified area, so it needs an explosion proof enclosure with DFG duty for the class, division and group the customer states. The motor enclosure only protects against ignition from inside the motor, though: a rubbing impeller or a dropped tool can still strike a spark in the housing, so the fan also needs spark resistant construction. Options A and D each cover only half of the hazard, and TEFC (C) is not built for hazardous locations.

Concept Tested: Explosion Proof Motor

See: Explosion Proof Motor