Motor Background Knowledge 2

See figure 12, the magnetization curve of the air gap

Φδ: Air gap flux
Sδ: Air gap area
Δ: Air gap length
Fδ: Air gap magnetomotive force(magnetic EMF)

We can see that when δ is longer, α is smaller, air gap flux Φδ is smaller. Motor speed will increase if the other parameters remain unchanged. On the contrary, when δ is shorter, α is larger, air gap flux Φδ is larger. Motor speed will decrease. We will see the same result as we see in 7.2. We usually pursue the maximum possible value of (Φδ*Fδ) in motor design.

Motor torque is proportional to D2*L. [D: diameter of the rotor L: length of the rotor]
Motor power is proportional to D2*L *n.

How to evaluate a motor? Typically industrial products can be evaluated with the following aspects. The most important characteristic of industrial products is low deviation.

The fundamental characteristics that customers require are assembling dimensions and outline dimensions. The dimensional deviation of a good product should meet product standard requirement. (GB standard, industrial standard or enterprise standard)

• Rated voltage: known parameter (unit: V)
• No load current: I0 (unit: A)
• No load speed: n0 (unit: rpm)
• Rated current: IL (unit: A)
• Rated torque: TL (Unit: g.cm)
• Rated speed: NL (Unit: rpm)
• Current in stall: Ist (unit: A)
• Torque in stall: Tst (unit: g.cm)
• Other parameters such as efficiency, power, electric potential constant, torque constant etc. can be calculated from the above data.

• Vibration: amplitude (unit: mm), vibration velocity (unit: mm/s), vibration acceleration (unit: mm/s2)
• Noise: sound pressure LP (unit: dB(A) and acoustical power LW (unit: dB(A). They are both relative values.
• EMC: This index is to evaluate the ability of the motor resisting the radio interference or the radio interference level that the motor generates.
• Environment test: This is to judge the load capability of the motor under high and low temperature. Alternating temperature test is the common test. Alternating temperature and humidity test is more severe test. Magnetic field of ferrite magnet decreases by 5-7% under -80 ℃. The motor electric performance is therefore deviated.Mechanical shock, external alternating magnetic field, aging (long time) storage will also weaken the magnetic field.
• Others: such as safety clearance, safety creepage distance, protection class, type of cooling etc.

• a. Try to get maximum effective conductors. In other words, make the direction of current the same in as many conductors under the same pole as possible. In some cases (such as 12-pole rotor), we will sacrifice the number of effective conductors to improve commutation. Such cases will not be discussed here.
• b. Minimize the electric potential of the commutating coil (the one that is shortcut). Typically the sides of that coil are placed at the edges of the magnetic poles or between the magnetic poles. So the carbon brushes are usually placed in the middle of the commutator bars that are connected to the coil.
• c. Electric angle between positive and negative brushes is 180°.

Conclusion: There is not a sole way to connect coil and commutator segments.

According to the characteristics of our motor models, we hereby describe in details by means of types of power supply and motor load.

The characteristic of such power supply is that they have large internalresistance, There is large voltage drop when the motor is applied with load.

The output power of the motor is limited by the capacity of the power supply.

When designing such motor, motor efficiency is not the only point to be considered.

How to get the largest output power from the power supply is the most important. That is To say, try to get maximum value of P1=V*I.

Practically, it is hard to achieve this target considering only the motor parameters. How to properly evaluate such motors is also a subject to us.

In such circuit, when the current changes, the output voltage changes. In actual application, AC input voltage is usually 120V-240V. Out of various reasons when the current increases, the resistor R1 or capacitor C will take more voltage drop. The output voltage is less. Motor speed is therefore lowered. Its operating point and characteristics will all deviate. The result will be different according to different application of the motors.

Take the well-known hair dryer for example, if the above change happens, air flow decreases. The temperature of the resistor R1 rises. The resistor gets more voltage drop. The output voltage is lower, making the situation a vicious circle. The motor will lose its function quickly.

This is the ideal power supply. The input voltage doesn’t change with the environment or motor load. The motor’s characteristic is decided by the motor parameter itself. The motor performance data we provide to our customers are tested under such power supply. In practical application, high capacity accumulator battery and AC/DC adaptor (variation of V less than 5%) are deemed as regulated power supply.

Startup of the motor with fan load is similar to the startup of the motor with no load. So there is no requirement to motor’s startup or stall torque. Sometimes we even need to restrict its torque from being too large. The most important feature of the motor with fan load is the stability and discreteness of its speed in mass production. The output power is in proportional with the motor speed. If the motor speed deviates a lot, the motor working characteristics will also deviate a lot. So the characteristic of the load at working point is the major point we look at.

Examples are cable retrieving devices for vacuum cleaners and tube retrieving fixtures for Irrigation machines.Similar to winching devices, the motor starts to work at its full load. The most important characteristic of such motor is its stall torque. The consistency of the stall torque is the key point during motor design and fabrication.Central door lock actuator also belongs to such winching load. Motors with such load usually work at short time working cycle.

Torque of such load is stable during work. The motor power increases linearly with the motor speed. It may reach its full load at startup. But in most cases it starts up with partial load. Usually it works under rated load for a very long time. We should consider various aspects including temperature rise in motor design. Reciprocating pump is the typical linear load.

There are still other loads like eccentric wheel, gearboxes that we are not going to discuss in this article.