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Electrical Machines, Drives and Power Systems > Fundamentals of Mechanics and Heat > Flashcards

Fundamentals of Mechanics and Heat Flashcards

1
Q

What is the force of gravity?

A

F = 9.8m where m is the mass of the object

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2
Q

What is the equation for torque?

A

T = Fr

  • T = Torque
  • F = Force
  • r = Radius
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3
Q

What is the equation for mechanical work?

A

W = Fd

  • W = Work done
  • F = Force
  • d = Distance the force moves
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4
Q

What is the equation for power?

A

P = W/t

  • P = Power
  • W = Work done
  • t = Time taken to do the work
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5
Q

What is the power of a motor?

A

P = (nT) / 9.55

  • P = Mechanical Power
  • n = Speed of rotation [r/min]
  • T = Torque

9.55 is a constant to take care of units. To be exact, use (30/pi)

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6
Q

How can we measure the power output of a motor?

A

We can use a prony brake. It’s a stationary flat belt that presses against a pulley mounted on the motor shaft. The ends are pressed against two spring scales and the belt pressure is adjusted with a screw.

We can vary the power of the motor by changing the tension of the belt. The power developed by the motor is converted into heat in the motor.

Using this system, we can see the torque, as the torque is equal to the difference between the two forces on the spring scales, as the scales are attached to the belt.

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7
Q

What are the forms that energy can exist in?

A
  1. Mechanical Energy (Potential Energy and Kinetic Energy are examples)
  2. Thermal Energy (Heat and Friction are examples)
  3. Chemical Energy (Coal and Batteries are examples)
  4. Electrical Energy (Lighting and Generators are examples)
  5. Atomic Energy (Energy released when nuclei are modified)
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8
Q

What are the typical efficiency ranges of the following devices and why is the electrical motor the best one?

  • Steam Turbine
  • Internal Combustion Engine
  • Electrical Motor
A
  • Steam Turbine = 25-40%
  • Internal Combustion Engine = 15-30%
  • Electrical Motor = 75-98%

This is because the electrical motor converts electrical energy into mechanical energy instead of thermal energy into mechanical energy. The process of electrical into mechanical is much more efficient as there are less thermal losses

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9
Q

What is the Kinetic Energy of Linear Motion?

A

E = 0.5 * m * v^2

  • E = Kinetic Energy
  • m = Mass of the body
  • v = Speed of the body
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10
Q

What is the moment of inertia of a mass m revolving at a distance r around axis o?

A

J = m * r^2

  • J = Moment of inertia
  • m = Mass
  • r = Distance from o to the centre of mass
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11
Q

What is the moment of inertia of a solid disc of mass m and radius r centred on the point of rotation o?

A

J = (m * r^2) / 2

  • J = Moment of inertia
  • m = Mass of disc
  • r = Radius of disc
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12
Q

What is the moment of inertia of an annular ring of mass m with a rectangular cross-section, rotating about point o which is centred at the middle of the ring?

R1 is the distance from o to the exterior radius
R2 is the distance from o to the interior radius

A

J = (m/2) * ((R1)^2 + (R2)^2)

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13
Q

What is the moment of inertia of a straight bar of mass m pivoted on its centre?

A

J = (m * L^2) / 12

  • J = Moment of inertia
  • m = Mass of bar
  • L = Length of bar
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14
Q

What is the moment of inertia of a rectangular bar of mass m revolving around an axis o?

R1 is the distance from o to the closest edge of the bar
R2 is the distance from o to the furthest edge of the bar

A

J = (m/3) * ((R1)^2 + (R2)^2 + (R1 * R2))

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15
Q

How can we change the speed of a revolving body?

A

We need to subject it to a torque for a period of time

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16
Q

What is the equation for the change in speed of a revolving body when we apply a torque?

A

n = (9.55 * T * t) / J

  • n = Change in speed [r/min]
  • T = Torque
  • t = Time interval
  • J = Moment of inertia

The 9.55 is here to take care of units. To be more exact, use (30/pi)

17
Q

What happens when the torque developed by a motor acts in the same direction as the speed?

A

The motor delivers power to the load. For any other conditions, the motor receives power from the load

18
Q

What is the rate of change of velocity relative to the net torque?

A

n = (9.55 * (Tm-Tl) * t) / J

  • n = Change in speed [r/min]
  • Tm = Motor Torque
  • Tl = Load Torque
  • t = Time period that the net torque acts
  • J = Moment of inertia of all revolving parts
19
Q

What are some examples of a motion convertor?

A
  • Rope-Pulley System
  • Rack and Pin-Ion Mechanism
  • A Wheel on a Track
20
Q

What is the equivalent power equation for a car jack driven by a motor?

A

Power supplied to raise the load = Fv
Power input to the jack = (nT)/9.55

nT = 9.55Fv

  • n = Rotational Speed (r/min)
  • T = Torque
  • F = Force
  • v = Linear Speed

9.55 is a simplification of 30/pi

21
Q

What is the equation for specific heat capacity?

A

Q = mct

  • Q = Quantity of heat added to or removed from the body
  • m = Mass of the body
  • c = Specific Heat Capacity
  • t = Change in temperature
22
Q

What is the equation for the thermal power transmitted through a body via conduction?

A

P = (lambda * A * (t1-t2)) / d

  • lambda = Thermal Conductivity of the Body
  • A = Surface Area of the Body
  • (t1-t2) = Difference of Temperature Between Opposite Faces
  • d = Thickness of the body
23
Q

How can we benefit from convection within electrical motors?

A

It allows for efficient cooling of the motor due to the air flow. This is why we use a fan to force air flow and convection

24
Q

What is the equation for the heat loss by natural convection in air?

A

P = 3A * (t1-t2)^1.25

  • P = Heat Loss by Natural Convection
  • A = Surface of the body (m)
  • t1 = Surface Temperature of the Body (Celcius)
  • t2 = Ambient Temperature of the Surrounding Air (Celcius)
25
Q

What is the equation for the heat loss caused by forced convection?

A

P = 1280 * V * (t2-t1)

  • P = Heat Loss by Forced Convection
  • V = Volume of Cooling Air (m^3 / s)
  • t1 = Temperature of the cool air
  • t2 = Temperature of the warm air
26
Q

What is the equation for the heat losses by radiation?

A

P = kA ((T1)^4 - (T2)^4)

  • P = Heat Radiated
  • A = Surface Area of the Body
  • T1 = Absolute Temperature of the Body (K)
  • T2 = Absolute Temperature of the Surrounding Objects (K)
  • k = A constant which depends on the nature of the body surface

Electrical Machines, Drives and Power Systems (3 decks)

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