Design World – August 2019

104 August 2019 http://www.designworldonline.com DESIGN WORLD

Motion Control

by imagining a line parallel to the speed-

torque curve at roughly ½ to 2/3 the height

of the published curve. This imaginary line

represents an amount of torque that a step

motor can reliably produce with minimal

risk of stalling. See Figure 1 (typical speed-

torque curve of a step motor) for more on

this.

2

“The step motor is so hot;

there must be something

wrong with it.”

Step motors are designed to run hot. The

most common insulation class used in

step motors is Class B, which is rated for

operation up to 130° C. This means that

the surface temperature of a step motor

can reach 90°C or more before failing. This

temperature is much hotter than a person

could touch without burning the skin. For

this reason, mount step motors away from

areas with a high chance of human contact.

Step motors are designed to run at high

temperatures because of their use in open-

loop control systems. Because an open-loop

step motor operates without any current

feedback (or velocity or position feedback),

the current supplied by the drive is constant,

regardless of the torque demand.

To get the most torque from step motors,

manufacturers specify them with the Class

B insulation in mind; so, current ratings are

designed to maximize torque output without

overheating. The end result is that step

motors produce a lot of torque ... but they

also get quite hot in doing so.

3

“Can I use a 12-volt

power supply to power

my step motor and

drive?”

For any kind of electric motor, not just step

motors, the supply voltage is directly related

to motor speed. As higher voltages are

supplied to the system, the motor achieves

higher speeds. The rated supply voltage

specified for servo and dc motors correspond

to other rated specifications including

speed, torque, and power. If a step motor is

specified with a rated voltage, it is typically

no more than the step motor’s winding

resistance times the rated current. This is

useful for producing holding torque but of

very little use when the step motor moves.

Like all electric motors, when the shaft

starts moving, the step motor produces a

back EMF (BEMF) voltage that impedes

the current flowing into the windings. To

produce usable torque, the supply voltage

must be substantially higher than the

BEMF. Because no hard and fast rules exist

for how high to specify the supply voltage,

users should review the published speed-

torque curves for a given step motor, drive,

and power supply combination. The supply

DESIGN WORLD August 2019 105

Figure 1 — typical speed-torque curve of a step motor. In published data

from the manufacturer, only the solid line is shown, which indicates stall

torque versus speed. The user must estimate a usable torque range as

shown by the dashed line.

Figure 2 — two speed-torque curves of the same step motor and drive

combination. Only the power supply voltage is different. The dark green line

shows stall torque with a 48-V power supply. The light green line shows stall

torque with a 24-V supply. A 12-V supply would result in an even lower curve.

Motion Control 8-19_Vs3 NEW.LE.indd 104 8/5/19 1:52 PM