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Quiz: Motor Ratings, Speed Control and Positioners

Test your understanding of motor speed, electric power, efficiency, thermal ratings, intended duty, variable frequency drives, motor bases and positioners with these review questions.

1. Who determines the hazardous location classification (class, division and group) of an area where a fan motor will be installed?

  1. The fan supplier's application engineer
  2. The motor manufacturer
  3. The plant owner
  4. The installing contractor
Show Answer

The correct answer is C. The NEC classifies hazardous locations by class, division and group, and the plant owner, not the fan supplier, determines the classification of each area. The application engineer's job is to copy the owner's class, division and group onto the order so the hazardous location motor's nameplate matches them, never to choose those values (A). The motor manufacturer (B) builds to the classification it is given.

Concept Tested: Hazardous Location Motor

See: Hazardous Location Motor

2. Which motor positioner gives adjustment in both horizontal directions and vertically?

  1. Motor positioner
  2. Bi-directional positioner
  3. Vertical jack screws
  4. Tri-directional positioner
Show Answer

The correct answer is D. A tri-directional positioner combines horizontal positioners with vertical jack screws, so it adjusts the motor along and across the base and vertically. The basic motor positioner (A) adjusts horizontally in one direction, and the bi-directional positioner (B) in both horizontal directions. Vertical jack screws (C) adjust only vertically; they thread through vendor-tapped motor feet and are removed after shimming.

Concept Tested: Tri-Directional Positioner

See: Motor Positioners

3. How does a motor winding RTD differ from a thermostat or thermistor used for motor thermal protection?

  1. A winding RTD reports a continuous temperature, while a thermostat or thermistor gives a yes-or-no signal at a set temperature
  2. A winding RTD warms an idle motor to prevent condensation, while a thermostat protects against overload
  3. A winding RTD is mounted on the fan bearings, while a thermostat is in the windings
  4. A winding RTD is standard on every motor, while thermostats must be ordered as a modification
Show Answer

The correct answer is A. A thermostat opens at a set temperature and a thermistor changes resistance sharply at a set temperature, so both answer "too hot or not" and give a shutdown signal. A winding RTD's resistance changes in a known way with temperature, so it reports a continuous value that plants can trend, alarm and trip on. Warming an idle motor (B) is a space heater's job. Winding RTDs sit in the windings (C) and must be called out on the order (D).

Concept Tested: Motor Winding RTD

See: Motor Winding RTD

4. A fan ships with a 230/460 V dual-voltage motor and a disconnect switch supplied with the fan. Why must the order state a single site voltage?

  1. A dual-voltage motor can only run on the higher voltage once it leaves the factory
  2. Add-ons such as a disconnect switch or speed controller are built for one voltage
  3. The motor's synchronous speed depends on which voltage is chosen
  4. Dual-voltage motors are not allowed on fans with three-phase power
Show Answer

The correct answer is B. A dual-voltage motor can be wired for either voltage, but a disconnect switch or speed controller supplied with the fan is built for one. FanPedia says the customer needs to state the actual voltage whenever the motor has add-ons. The motor can still be wired for either voltage (A). Synchronous speed depends on frequency and poles, not voltage (C), and dual-voltage three-phase motors such as 230/460 are common (D).

Concept Tested: Motor Voltage and Phase

See: Motor Voltage and Phase

5. According to the chapter, what are the main reasons to order an inverter-duty motor for a fan on a VFD?

  1. Low-speed overheating, because the fan load stays constant as speed falls
  2. Higher service factor, because a VFD raises the usable margin above 1.15
  3. The drive's voltage spikes, which stress the insulation, and the shaft currents it induces through the bearings
  4. The need to change the motor's number of poles when the frequency changes
Show Answer

The correct answer is C. A VFD builds its output from rapid switched pulses that stress insulation with voltage spikes and can induce shaft currents through the bearings; NEMA MG-1 Part 31 governs these precautions. For a fan, load falls with the cube of speed while cooling falls roughly in proportion, so low-speed heating (A) is not the main concern. A common VFD service factor is 1.0, not higher (B), and a VFD does not change the poles (D).

Concept Tested: Inverter-Duty Motor

See: Inverter-Duty Motor

6. A belt-driven fan delivers 12,000 CFM at 2.50 in. wg and needs 10.0 bhp at 60 Hz. Its VFD is turned down to 40 Hz. Using the fan laws, about how much brake horsepower does it need at 40 Hz?

  1. 6.67 bhp
  2. 2.96 bhp
  3. 4.44 bhp
  4. 10.0 bhp
Show Answer

The correct answer is B. On a VFD the speed ratio equals the frequency ratio, 40 / 60 = 0.667. Brake horsepower varies with the cube of speed: 10.0 × 0.667³ = 10.0 × 0.296 = 2.96 bhp. Option A scales power linearly with speed, and option C uses the square, which is the rule for pressure. Airflow at 40 Hz would be 8,000 CFM, so a one-third cut in airflow cuts power by about 70 percent.

Concept Tested: Variable Frequency Drive

See: Variable Frequency Drive

7. What is the synchronous speed of a 6-pole induction motor on 50 Hz power?

  1. 1,000 RPM
  2. 1,200 RPM
  3. 1,500 RPM
  4. 750 RPM
Show Answer

The correct answer is A. Synchronous speed is 120 × f / p = 120 × 50 / 6 = 1,000 RPM. A 6-pole motor runs its field at 1,200 RPM (B) on 60 Hz power, so quoting the 60 Hz value on a 50 Hz job is the common error. A 4-pole motor gives 1,500 RPM (C) and an 8-pole motor gives 750 RPM (D) on 50 Hz. The actual running speed is slightly lower because of slip.

Concept Tested: Synchronous Motor Speed

See: Synchronous Motor Speed

8. A floor-mounted Arrangement 1 fan uses a 449T motor. Which motor base style fits?

  1. NEMA type slide base
  2. Standard post mount
  3. Pivot/bolted adjustable base
  4. Heavy-duty slide rails
Show Answer

The correct answer is D. The NEMA type slide base (A) covers 48 to 445 frame, and 449 is above that limit even though it is a 440-series frame. Heavy-duty slide rails are listed for the 440, 500 and 5000 frame series, so they are the base for a 449T. The standard post mount (B) stops at 215 frame, and the pivot/bolted adjustable base (C) stops at 445 and is used on tubular, plenum and plug fans rather than Arrangement 1.

Concept Tested: Slide Base

See: Slide Base

9. A refinery specification calls for "IEEE 841 motors on all rotating equipment." A quote offers a TEFC, SD (severe duty) motor. What is the best analysis?

  1. The quote meets the specification, because SD and IEEE 841 are two names for the same duty
  2. The quote should change to general duty, because a refinery motor room is clean and cool
  3. The quote should change to an explosion proof DFG motor, because every refinery motor is in a hazardous location
  4. The quote does not meet the specification; IEEE 841 is a stricter standard that must be supplied when specified, in a TEFC (IP56) enclosure
Show Answer

The correct answer is D. FanPedia's rule is that SD is used where a severe duty motor is needed but IEEE 841 is not specified. Here IEEE 841 is specified, so a plain SD motor does not meet it even though both are severe duty (A), and IEEE 841 requires a TEFC (IP56) enclosure. Nothing in the specification describes clean, cool air (B). A DFG motor (C) is chosen only when the owner has classified the area as hazardous.

Concept Tested: Motor Intended Duty

See: Motor Intended Duty

10. A fan needs 12 bhp. One proposal uses a 40 HP premium efficiency motor "for margin," and another uses a 15 HP standard efficiency motor. Why might the oversized premium motor waste more energy?

  1. Premium efficiency motors are less efficient than standard ones at full load
  2. A larger frame always has a lower efficiency class
  3. At 30 percent of rated load, the 40 HP motor runs below the 50 percent point where efficiency drops sharply, while the 15 HP motor runs near its efficient range
  4. Larger motors run at a higher synchronous speed, which raises the fan's power
Show Answer

The correct answer is C. FanPedia says most motors are designed to run at 50 to 100 percent of rated load, with peak efficiency near 75 percent, and that efficiency drops sharply below about 50 percent. The 40 HP motor would run at 12 / 40 = 30 percent load; the 15 HP motor at 80 percent. The chapter notes that an oversized premium motor at 30 percent load can waste more energy than a correctly sized standard motor. Options A, B and D are not true.

Concept Tested: Premium Efficiency Motor

See: Premium Efficiency Motor