Permanent Magnet Stepper Motors

Permanent magnet stepper motors use a permanent magnet (PM) in the rotor and operate on the attraction or repulsion between the rotor PM and the stator electromagnets. Permanent magnet stepping motors are generally two-phase, with small torque and volume, and the step angle is generally 3.75 degrees, 7.5 degrees, 15 degrees or 18 degrees, etc. MOONS' PM stepper motors are widely applied in many innovative applications, including: computer equipment, photographic systems, optoelectronic devices, valve control, ATM equipment, CNC machine, automatic winding machines, electronic clocks and medical equipment.

What is a Permanent Magnet Stepper Motor?

The PM motor is an inner rotor type motor, and the outer surface of the cylindrical rotor is evenly distributed with N and S poles.

What is a Permanent Magnet Stepper Motor?

Structure

Operating Principles

Operating Principles In response to each individual control pulse and direction signal, the drive applies power to the motor windings to cause the rotor to take a step forward, a step in reverse, or hold in position. For example, in a 7.5 degree two phase step motor: When both phases are energized with DC current, the motor will stop rotating and hold in position. The maximum torque the motor can hold in place with rated DC current, is the rated holding torque. If the current in one phase is reversed, the motor will move 1 step (7.5 degrees) in a known direction. If the current in the other phase had been reversed, the motor would move 1 step (7.5 degrees) in the other direction. As current is reversed in each phase in sequence, the motor continues to step in the desired direction. These steps are very accurate. For a 7.5 degree step motor, there are exactly 48 steps in one revolution. Two phase stepping motors are furnished with two types of windings: bipolar or unipolar. In a bipolar motor there is one winding on each phase. The motor moves in steps as the current in each winding is reversed. This requires a drive with eight electronic switches. In a unipolar motor there are two windings on each phase. The two windings on each phase are connected in opposite directions. Phase current is reversed by turning on alternate windings on the same phase. This requires a drive with only four electronic switches. Bipolar operation typically provides 40% more holding torque than unipolar, because 100% of the winding is energized in the bipolar arrangement. Speed-Torque Characteristics The dynamic torque curve is an important aspect of stepping motor’s output performance. The followings are some keyword explanations.

A. Working frequency point express the stepping motors rotational speed versus the drive pulse rate.
n = q * Hz / (360 * D)
n: rev/sec
Hz: the frequency value or the driver pulse rate.
D:the subdividing value of motor driver
q: the step angle of stepping motor
B. Start/Stop region: the region in which a stepping motor can be directly started or stopped.
C. Slew Range: the motor cannot be started directly in this area. It must be started in the start/stop
region first and then accelerated to this area. In this area, the motor can not be directly stopped, either. Otherwise this will lead to losing-step. The motor must be decelerated back to the start/stop region before it can be stopped.
D. Maximum starting frequency point at this point, the stepping motor can reach its maximum starting speed under unloaded condition.
E. Maximum running frequency point at this point the stepping motor can reach its maximum running speed under an unloaded condition.
F. Pull-in Torque: the maximum dynamic torque value that a stepping motor can load directly at the particular operating frequency point.
G. Pull-out Torque: the maximum dynamic torque value that a stepping motor can load at the particular operating frequency point when the motor has been started. Because of the inertia of rotation the Pull-Out Torque is always larger than the Pull-In Torque.

What are the differences between PM and VR and Hybrid stepper motors?

Comparison of performance parameters of PM motor and VR, Hybrid stepping motor.

Permanent Magnet vs Variable Reluctance vs Hybrid Stepper Motors

Characteristic	Permanent Magnet stepper motors	Variable Reluctance stepper motors	Hybrid stepper motors
Cost	Cheapest	Moderate	Relatively expensive
Design	Moderate	Simple	Complex
Resolution	step angle: 3°~30°	step angle: 1.8°, 0.9° and smaller	step angle: 1.8°, 0.9° and smaller
Speed Torque curve	High Torque at low speed, more pronounced torque drop at high speed	Less pronounced torque drop at high speed Noisy	High Torque at low speed, more pronounced torque drop at high speed (Maximum torque at the same speed)
Noise	Quiet	Noisy	Quiet(quieter if using microstepping)
Heat	Low temperature rise	High temperature rise(Need heat sink)	Low temperature rise
Microstepping	Full, Half and Microstepping	Typically run in Full-Step only	Full, Half and Microstepping
Rotor material	Ferrite magnet or NdFeB	Silicon steel sheet or iron(Non-permanent magnet)	NdFeB

Technical about Rated Current

Rated current of PM stepper is limited by thermal characteristics.

PG15 Permanent Magnet Gearmotors

Miniature Permanent Magnet Stepper Motors with Integrated Gearboxes, Small Size , 15mm Diameter, 2 phase

Item	Phase	Length (mm)	Holding Torque (Nm)	Holding Torque (mNm)	Coil Type	Weight (kg)	Price (USD)	Compare
PG15L0050-15P020S0-02	2	22.5	0.05	49	Bi-Polar	0.011	$28.80

20mm Permanent Magnet Stepper Motors

Miniature Permanent Magnet DC Stepper Motor, Small Size, 20mm Diameter, 2 phase, 18°

Item	Phase	Length (mm)	Rated Current (A)	Holding Torque (Nm)	Holding Torque (mNm)	Coil Type	Weight (kg)	Price (USD)
20PM020L6-00702	2	19.3	0.43		12	Bi-Polar	0.027	$24.57
20PM020L6-02101	2	19.3	0.25		12	Bi-Polar	0.027	$24.57
20PM020S0-01002	2	14.3	0.33		8.5	Bi-Polar	0.021	$23.50
20PM020S0-02501	2	14.3	0.21		8.5	Bi-Polar	0.021	$23.50
PG22L45.2-20PM020L0-17	2	60	0.5	0.2	200	Bi-Polar	0.12	$45.00
PG22L71.7-20PM020L0-14	2	55	0.46	0.42	420	Bi-Polar	0.1	$55.00

25mm Permanent Magnet Stepper Motors

Miniature Permanent Magnet DC Stepper Motor, Small Size, 25mm Diameter, 2 phase

Item	Phase	Length (mm)	Rated Current (A)	Holding Torque (mNm)	Coil Type	Weight (kg)	Price (USD)
25PM024L6-00801	2	17	0.47	17	Bi-Polar	0.039	$23.99
25PM024L6-01401	2	17	0.35	18	Bi-Polar	0.039	$23.99
25PM024S0-00601	2	12.5	0.5	10	Bi-Polar	0.03	$22.95
25PM024S0-01602	2	12.5	0.32	11	Bi-Polar	0.03	$22.95
25PM048L8-00801	2	17	0.47	25	Bi-Polar	0.039	$23.99
25PM048L8-01401	2	17	0.35	27	Bi-Polar	0.039	$23.99
25PM048S8-00601	2	12.5	0.5	16	Bi-Polar	0.03	$22.95
25PM048S8-01601	2	12.5	0.32	16	Bi-Polar	0.03	$22.95

35mm Permanent Magnet Stepper Motors

Permanent Magnet Stepper Motor, Small Size, 35mm Diameter, 2 phase

Item	Phase	Length (mm)	Rated Current (A)	Holding Torque (mNm)	Coil Type	Weight (kg)	Price (USD)
35PM024L8-00502	2	21.3	0.63	42	Bi-Polar	0.09	$25.45
35PM024L8-01202	2	21.3	0.4	42	Bi-Polar	0.09	$25.45
35PM024L8-07501	2	21.3	0.16	36	Bi-Polar	0.09	$25.45
35PM024S8-00502	2	15.3	0.6	22	Bi-Polar	0.07	$24.43
35PM024S8-01201	2	15.3	0.38	22	Bi-Polar	0.07	$24.43
35PM024S8-08001	2	15.3	0.15	22	Bi-Polar	0.07	$24.43
35PM048L6-00501	2	21.3	0.63	62	Bi-Polar	0.09	$25.45
35PM048L6-01201	2	21.3	0.4	62	Bi-Polar	0.09	$25.45
35PM048L6-07501	2	21.3	0.16	62	Bi-Polar	0.09	$25.45
35PM048S0-00508	2	15.3	0.6	48	Bi-Polar	0.07	$24.43
35PM048S0-01203	2	15.3	0.38	46	Bi-Polar	0.07	$24.43
35PM048S8-08001	2	15.3	0.15	46	Bi-Polar	0.07	$24.43

42mm Permanent Magnet Stepper Motors

Permanent Magnet Stepper Motor, Small Size, 42mm Diameter, 2 phase

Item	Phase	Length (mm)	Rated Current (A)	Holding Torque (mNm)	Coil Type	Weight (kg)	Price (USD)
42PM048L8-00302	2	21.3	0.85	95	Bi-Polar	0.12	$26.98
42PM048L8-00703	2	21.3	0.55	95	Bi-Polar	0.12	$26.98
42PM048L8-06001	2	21.3	0.2	102	Bi-Polar	0.12	$26.98
42PM048S8-00501	2	15.3	0.65	74	Bi-Polar	0.09	$25.66
42PM048S8-01209	2	15.3	0.42	64	Bi-Polar	0.09	$25.66
42PM048S8-08001	2	15.3	0.15	64	Bi-Polar	0.09	$25.66
42PM096L8-00301	2	21.3	0.85	78	Bi-Polar	0.12	$26.98
42PM096L8-00701	2	21.3	0.55	78	Bi-Polar	0.12	$26.98
42PM096L8-06001	2	21.3	0.2	78	Bi-Polar	0.12	$26.98
42PM096S4-00501	2	15.3	0.65	54	Bi-Polar	0.09	$25.66
42PM096S4-01201	2	15.3	0.42	54	Bi-Polar	0.09	$25.66
42PM096S4-08001	2	15.3	0.15	54	Bi-Polar	0.09	$25.66