1. Basic concepts

The motor uses the magnetic field as a medium to realize the conversion device between electrical energy and mechanical energy or electrical energy and electrical energy. Permanent magnet DC motors use permanent magnetic fields (provided by permanent magnets such as ferrite, neodymium iron boron, etc.) as a medium to convert electrical energy into mechanical energy. The operation of any motor must have two basic conditions: magnetic field and current.

2. Motor classification

There are many classification methods, and the traditional motor science is classified in the following way (pictured): The motors produced by our factory are strontium ferrite permanent magnet brushed DC motors.

3. Basic Theory

The motor or motor science is based on the following five laws. If you want to have a preliminary understanding of the basic principles of the motor, you must first understand the following five laws

(1) The law of electromagnetic induction (also known as Faraday's law)

The cutting of magnetic field lines by a moving conductor in a magnetic field produces an induced electromotive force (voltage). The direction of the electromotive force is determined by the rule of the right hand, and has E = B • L • VE: electromotive force (unit: volts) B: magnetic field magnetic induction (Tesla) = 104 Gauss) L: effective length of the conductor (unit: m) V: speed of movement of the conductor (unit: m / s) If the conductor is connected with a wire in the figure on the right, an induced current I will be generated (see figure)

(2) The law of electromagnetic force (Bisha's law)

The energized conductor will generate electromagnetic force in the magnetic field, the direction of which is determined by the left-hand rule, and has the following relationship (figure): F = B * I * LF: electromagnetic force (unit: Newton) I: current in the conductor (unit : Ampere) B: Magnetic induction intensity of magnetic field (unit: Tesla) L: Effective length of conductor (unit: m) The left-hand rule is also called the motor rule, and the right-hand rule is also called the generator rule.

(3) Kirchhoff's law (as shown on the left)

ΣI = 0: for any node of the circuit, the algebraic sum of currents flowing into (or flowing out of) this node is zero (inflow equals to outflow) ΣU = 0: for any closed loop, the algebraic sum of voltages of each section is zero.

(4) Law of conservation of energy

In a system with constant mass, energy is always conserved (form can be changed)

(5) Ampere's law of full current

Simply put, a magnetic field is generated around the energized conductor, and the direction is determined by the right-handed spiral rule, and there is (as shown): ∮H • dL = ∑I = IA + IB + IC +…

H: magnetic field strength (A / m), = B / µ

L: path (m)

I: current (A)

4. Basic principles

A simple armature (rotor) with two conductors (one-turn coil) for a two-pole commutator of a two-pole permanent magnet motor. According to the laws of electromagnetic force and the left-hand rule, the rotor rotates counterclockwise (CCW). Disadvantages: there is a dead point, which is the simplest Motor, but not practical (pictured)

5. Electric potential, torque and energy equation

(1) Electric potential (as shown on the right)

From V = E + 2 △ U + I • r, E = V-2 △ U-I • r, and there is: E = KE • Φ • n (armature back EMF) V: power supply voltage (volt) 2 △ U: Brush voltage drop (volts) I: Armature current (amperes) r: Rotor resistance (ohms) KE: Potential constant (dimensionless) = Z / 60 (for two-pole motors, the number of Z-conductors) Φ: magnetic flux ( Weber) = average magnetic density B • pole shoe (or pole) width • rotor effective length n: speed (1 / min)

(2) Torque

TE = KT • Φ • I (electromagnetic torque: Newton • meter) KT: torque coefficient (dimensionless) = Z / 2π Φ: magnetic flux (Weber) I: armature current (A)

(3) The relationship between power and torque:

P≈T • n ／ 97500 P: power (W) T: torque (g • cm g • cm) n: speed (1 / min) If the unit of torque T is “Newton • meter” (N • m) then P≈T • n ／ 9.55 (W)

(4) Energy equation (as shown on the left)

P1 ＝ 2 △ U • I ＋ I2 • r ＋ PE PE (electromagnetic power) = P2 (output power) + PFe (iron loss) + Pmec (mechanical loss) P2 ＝ P1-2－ △ U • I－I2 • r－ PFe－Pmec (W) efficiency η = ﹙P2 ÷ P1﹚ × 100% (PFe + Pmec), also known as no-load loss, recorded as P0 = PFe + Pmec, so PE = P2 + P0 and electromagnetic torque TE = T2 + T0

(5) Energy transfer diagram: (as shown on the right)

6. Working characteristics (as shown on the right)

Generally speaking, the operating characteristics of DC motors refer to the following four curves (straight lines), that is, n = f (T2) speed as a function of output torque; I = f (T2) current as a function of output torque; η ＝ f (T2) efficiency and function of output torque; P2 ＝ f (T2) function of output power and output torque;

(1) I = f (T2)

I = TE / (KT • Φ) = (T0 + T2) / (KT • Φ) = T0 / (KT • Φ) + T2 / (KT • Φ) = I0 + [1 / (KT • Φ) (constant) ] T2 This is a straight-line equation (see Figure 9). I0 is the no-load current. When the motor is blocked, n = 0 and E = 0. According to Figure 6, the blocked current Ist = (V-2 △ U) / r

(2) n = f (T2)

E ＝ V－2 △ U－I • r = KE • Φ • nn = (V-2 △ U－I • r) / (KE • Φ) = {V-2 △ U－ [I0 + T2 / (KT • Φ)] • r} / (KE • Φ) ＝ (V－2 △ U－I0 • r) / (KE • Φ) － [r / (KE • KT • Φ²)] • T2 ＝ n0－ [r / (KE • KT • Φ²)] • T2 This is also a straight line equation

(3) P2 = f (T2)

P2≈T2 • n / 9.55 = ﹛n0- [r / ﹙KE • KT • Φ²)] • T2﹜ • T2 / 9.55 = n0 • T2 / 9.55－ [r / (KE • KT • Φ²)] • (T2 ) ² / 9.55

P2 is a quadratic parabola (as shown below)

η = f (T2) = P2 / P1

η is a curve (the calculation is complicated, omitted! As shown in Figure 11)

7. Analysis of main parameters

(1) Change in coil turns and wire diameter (other parameters remain unchanged)

According to 5.1, when the number of coil turns increases, the potential constant KE increases, the rotation speed n decreases, and vice versa. Then the speed n rises and vice versa. In addition, the blocking current is inversely proportional to the resistance r; the change in the number of turns and the wire diameter are mutually restricted due to the influence of the slot full rate, and the relationship should be clarified when adjusting.

(2) Flux change (other parameters remain unchanged)

The use of high-density magnets and the lengthening of the core length will lead to an increase in the total magnetic flux Φ. According to 5.1 and 6.2, the rotation speed n decreases, and at the same time n is affected by the load (T2), the so-called characteristic "hardens" "On the contrary, it becomes soft.

(3) Air gap change

As shown in Figure 12, the magnetization curve of the air gap Φδ = ﹣μ0 • (Sδ / δ) • Fδ Φδ: air gap flux Sδ: air gap area δ: air gap length Fδ: air gap magnetic potential (magnetic pressure drop) Permeability angle: α = tg-1 [μ0 • (Sδ / δ)], it can be seen that the larger the air gap δ (longer), the smaller the α, the smaller the air gap magnetic flux Φδ, when other parameters remain unchanged As a result, the motor speed rises, and vice versa, resulting in the effect described in "7.2"; in fact, the general motor design should pursue the maximum value of (Φδ • Fδ).

(4) Effective volume D2 • L

The torque of a general motor is proportional to D2 • L [D: armature (rotor) outer diameter, L: armature (rotor) length] The power of the motor is proportional to D2 • L • n