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Hoja de ejercicios: control de motores

Twelve problems from the motor room: synchronous speed from the frequency and the poles, slip from a speed on load, poles from a speed — and ladder-logic questions where you decide whether a coil is energised from the state of its contacts, a seal-in, an overload or a timer. The key explains each one.

Motor control worksheet · sheet #1

NombreDateScore/ 12

Ns = 120 × f ÷ Pslip % = (Ns − N) ÷ Ns × 100P = 120 × f ÷ Nsseries contacts = AND · parallel = ORNO closes when actuated · NC opens

  1. What is the synchronous speed of a 2-pole motor on 60 Hz?

  2. An 8-pole, 60 Hz motor turns at 847 rpm on load (practice figure). What is its slip, in percent?

  3. A 50 Hz motor has a synchronous speed of 1,000 rpm. How many poles does it have?

  4. In a three-wire start/stop circuit the operator pressed Start and let go; the motor is running. Nothing else is touched. Is the starter coil energised now?

  5. What is the synchronous speed of a 6-pole motor on 60 Hz?

  6. A 2-pole, 50 Hz motor turns at 2,921 rpm on load (practice figure). What is its slip, in percent?

  7. A 50 Hz motor has a synchronous speed of 3,000 rpm. How many poles does it have?

  8. Two normally open pushbuttons, A and B, are in parallel feeding pilot light L. A is pressed and B is pressed. Is L on?

  9. What is the synchronous speed of a 4-pole motor on 60 Hz?

  10. A 2-pole, 50 Hz motor turns at 2,917 rpm on load (practice figure). What is its slip, in percent?

  11. A 60 Hz motor has a synchronous speed of 900 rpm. How many poles does it have?

  12. Two normally open pushbuttons, A and B, are in parallel feeding pilot light L. A is pressed and B is released. Is L on?

sparkytools.net/worksheets/motor-control — free practice sheet. Practice numbers only; nothing here is a code-book value.

Answer key · Motor control worksheet · sheet #1

Show the answers and the working
  1. 3,600 rpm Ns = 120 × f ÷ P = 120 × 60 ÷ 2 = 3,600 rpm
  2. 5.89% Ns = 120 × 60 ÷ 8 = 900; slip = (900 − 847) ÷ 900 × 100 = 5.89%
  3. 6 poles P = 120 × f ÷ Ns = 120 × 50 ÷ 1,000 = 6
  4. Energised The seal-in contact in parallel with Start closed when the coil pulled in and holds the rung → energised.
  5. 1,200 rpm Ns = 120 × f ÷ P = 120 × 60 ÷ 6 = 1,200 rpm
  6. 2.63% Ns = 120 × 50 ÷ 2 = 3,000; slip = (3,000 − 2,921) ÷ 3,000 × 100 = 2.63%
  7. 2 poles P = 120 × f ÷ Ns = 120 × 50 ÷ 3,000 = 2
  8. Encendido Parallel: either closed path lights it. A closed, B closed → L on.
  9. 1,800 rpm Ns = 120 × f ÷ P = 120 × 60 ÷ 4 = 1,800 rpm
  10. 2.77% Ns = 120 × 50 ÷ 2 = 3,000; slip = (3,000 − 2,917) ÷ 3,000 × 100 = 2.77%
  11. 8 poles P = 120 × f ÷ Ns = 120 × 60 ÷ 900 = 8
  12. Encendido Parallel: either closed path lights it. A closed, B open → L on.

How to use it

  1. Print the sheet (or save it as a PDF from the print dialog). The answer key prints on its own page, so you can hand the sheet out and keep the key.
  2. Work every problem on paper, with the formula strip at the top, before you look at anything.
  3. Mark it from the key. For each miss, read the working line and find the exact step you did differently.
  4. Press New sheet for another twelve; the sheet number is in the link, so the same sheet can be printed again.

Worked example

Slip from a speed on load

  1. A 4-pole motor on 60 Hz turns at 1,746 rpm on load (a practice figure). Find its slip.
  2. Synchronous speed first: Ns = 120 × 60 ÷ 4 = 1,800 rpm.
  3. The rotor lags by 1,800 − 1,746 = 54 rpm.
  4. Slip = 54 ÷ 1,800 × 100 = 3%.

An induction motor always turns a little below synchronous speed on load — the slip is what makes it produce torque.

Preguntas frecuentes

What does “at rest” mean for a contact?

Nothing is acting on it — the button is not pressed, the coil is not energised, the switch is not tripped. A normally open contact is open at rest and closes when actuated; a normally closed one is the other way round.

Can I print a different sheet?

Yes. Press New sheet and you get another twelve problems with their own answer key; the sheet number goes into the link, so you can print it again later or send it to someone and they get the same sheet. Sheet #1 is always the one this page opens on.

Does it cost anything or need an account?

No. Every worksheet is free, prints from your own browser (or saves as a PDF from the print dialog), and needs no sign-up.

Ver también

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