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0.9 Degree Step Motors

Oct
29
by Jeff Kordik

Most popular hybrid step motors have a "step angle" of 1.8 degrees. The 1.8 refers to the angular distance between full steps and is a bit of an anachronism. Most applications now employ microstepping to electronically divide the step size to as low as 51200 steps/revolution. Nonetheless, another class of hybrid motors exists that provide a full step size of 0.9 degrees. In some applications, where cost constraints dictate the use of simple, low cost single chip drivers that are only capable to full or half step operation, the 0.9 degree motor can provide twice the position precision as the traditional 1.8. The very curious Stepper Guru has elected to evaluate a typical 0.9 degree motor and contrast it to a 1.8 degree model. We chose two Applied Motion Products NEMA 17 step motors with the same frame size, length and rated current.

On the left is the HT17-221, a 0.9° step motor. On the right is the equivalent 1.8° motor, the HT17-269. The table below compares the basic electrical and mechanical parameters. The rated current is the same, but the 0.9° has more resistance and inductance and less rotor inertia.

 

HT17-221

HT17-269

Unit
Step Angle

0.9

1.8

degrees
Connection

4 leads

parallel

 
Resistance

20

12

ohms
Inductance

23.0

13.5

mH
Holding Torque

0.22

0.24

N-m
Rated Current

0.50

0.50

A/phase
Inertia

38

54

g-cm2
Length

34.3

34.4

mm

Given the nearly identical physical size of the motors, one would expect a similar ability to dissipate any heat resulting from current passing through the winding resistance. At low speeds, when a driver is able to achieve full rated winding current, the copper losses of the 1.8° motor will be:

P_1.8

The 0.9° motor sports a pair of 20 ohm windings, so the copper loss will be higher:

P_0.9

We should expect the 0.9 to run hotter than the 1.8, at least at low speeds. The 0.9° motor also has more inductance than the 1.8, which might be expected to reduce dynamic torque at higher speeds, where the inductance slows the ability to switch current in and out of the motor windings. But remember, the electrical time constant of an LR circuit is proportional to the quotient of inductance and resistance:

t_equals_LR

For the 1.8° motor, L/R = 13.5/12 = 1.13. For the 0.9° motor, L/R = 23/20 = 1.15. Indeed, we found that the 0.9 had somewhat improved dynamic torque when operated by the same driver and power supply.

Finally, we couldn't resist dismantling the two motors to have a look at what's inside. Regular readings of Stepper Guru already know that the classic 1.8° motor contains a rotor with 50 teeth on each disk and a stator with a similar tooth pitch. The 0.9° uses similar construction, but with a 100 tooth rotor and equally fine pitched stator.

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