Catalogue - Dagu - #3

WHICH MOTOR?

POWER SUPPLY

It is also important to consider the form factor "K_f" of the power supply unit used. If the current flowing through the motor has a form factor higher than one, the motor will overheat because dissipation increases. This proportionately shortens the life of the motor, because the brushes are exposed to excessive heat due to the greater current peak value. Subsequently, the use­ful power of the motor is de-rated by 1/Kf as shown in Figure 8. These factors considerably influence the speed of the motor; the speed increases with the increase in the form factor of the voltage. Therefore, when buying a motor, it is important to spe­cify the type of power supply used, as illustrated by the sample diagrams below:

a permanent magnet dc motor is preferable:

a) When the required speeds exceed the speeds linked the network frequency;

b) When a battery or accumulator power supply is required;

c) When a high drive torque is required (Fig. 6);

to

1/K,

d) W

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expensive electronics.

is required without resorting to

Fig.8

an asynchronous ac motor is preferable:

a) When fixed speeds determined by the frequency of power supply network can be used (e.g. 2800, 1400, 950 rpm);

b) When higher speed stability is required (Fig. 4);

c) When an inverter can be fitted for continuous speed change;

d) When the motor is exposed to heavy duty, thus reducing maintenance.

K,

PERMANENT MAGNET DC MOTOR

COMPARISON

C'a>Ca

ASYNCHRONOUS

Diagram С

Diagram D

Diagram В

Diagram A

"Duty" defines the load the motor is subjected to and may include (if applicable) the following phases: start-up, dynamic braking, idling and rest, as well as their duration and time sequence (EN 60034-1). The most common types of duty are defined below with codes from S1 to S9.

continuous running duty - S1

Operation at a constant load maintained for sufficient time to allow the machine to reach thermal equilibrium (Fig. 1).

short-time duty - S2

Operation at constant load for a given time less than that required to reach thermal equilibrium, followed by a time at rest and deenergized,of sufficient duration to re-establish machine temperatures within a 2°C of the coolant temperature (Fig. 2). Example: S2 60 min.

intermittent periodic duty - S3

A sequence of identical duty cycles, each including a time of operation at constant load and a time at rest and de-energized, see fig. 3.In this duty, the cycle is such that the starting current does not significantly affect the temperature rise. The appro­priate abbreviation is S3, followed by the cyclic duration factor (Fig. 3). Example: S3 25%

intermittent periodic duty - S1 and S3 A motor designed for S1 continuous duty, which during rated function dissipates a power equal to AP1, can also be used for S3 regular intermittent duty, dissipating AP3, on condition that the "N" run time and the "t" cycle comply with the ratio:

API N

= Load

= Electrical losses = Temperature = Time

= Operation time with

constant load _x -Maximum temperature

a b c d N

Fig.4

TORQUE CONSTANT K₍

Fig.5

Fig.6

If the following physical principles are not taken into consideration, a wrong choice of the motors and supply system could be made. Dagu S.r.l. is the ideal partner to produce the properly selected motor. It is possible to obtain a general guideline by evaluating the torque "C" and the torque constant "K_c". By making the two formulas equal for the useful power "P_u "of the motor:

Flg.1

Load

Electrical losses

Temperature

Time

Operation time with constant load _x -Maximum temperature

a

b

c

d N

P_V=E*I (2)

E = inducted electro-motive force

V = unloaded voltage

I = input current based on load

n₀ = revs at zero current

n = revs unloaded

n₀ s n

60

obtain (J =

(3)

we

2л

0

60ȣ

Fig.2

/с. =

(4)

where

2»7i»«₀ 2n • n

Load

Electrical losses

Temperature

Time

Operation time with

Formula (4) shows that by increasing the supply voltage and/or reducing the revs, the torque constant increases, therefore on parity of useful torque (3) supplied to the axis, the input current to the motor decreases. Formula (4) also shows that the torque constant does not depend on the size of the motor. With equal values for K_c, a bigger motor will absorb more current, and therefore supply more torque to the shaft. The graph in Figure 7 compares different sized motors.

The curves indicated in the graph illustrate the power calculated using the formula (1).

constant load 0_max -Maximum temperature R - Rest period t - Cycle duration Intermittence ratio:

-100%

ЛРЗ

ratio between S1

To use a motor design

and S3

S1 in

temperature curves.

Fig.3

DPGÖ

motori elettrici

DRGÖ

motori elettrici