RMS vs Peak Current

Machine Design Article by Mindy Lin Cheng 8/22/2007:

It’s hard to stay up to date when you are floating in a sea of technical jargon. However, it’s imperative that engineers know the terminology associated with the areas in which they work. And that’s especially true if their assignments take them outside their engineering discipline. For example, a mechanical engineer specifying a stepmotor should understand the associated mechanical and electrical terms.

Stepmotors need the right current if they are to work correctly. Without it, the motors can overheat, miss steps, and even freeze in their tracks. Yet the one electrical specification that most confuses all engineers, from the recent graduate through seasoned veteran, is the rating for stepmotor current. No doubt this happens because stepmotor-current ratings come in many forms such as amps/phase, amps RMS, average current, and even amps peak current.

An understanding of how power is applied to a stepmotor to make it step gives insight into the different current ratings. Most stepmotors have two electrical windings or phases labeled A and B. The phases are placed at a magnetic angle of 90° apart. Current flows through the windings generating a magnetic field that forces the permanent magnet in the rotor to align with the field.

Power for the windings comes from the stepmotor driver. The most common drive technique uses a bipolar, current-controlled method. Bipolar means the driver periodically reverses the polarity of voltage applied to the winding. Current controlled, as its name implies, varies the amount of current the motor sees. When power is first applied, the permanent magnet in the rotor of the stepmotor aligns with the magnetic fields generated by the windings.

To step the motor, current in one phase is turned off. The change in direction of the magnetic field forces the rotor to align with the powered phase. With that, the motor has taken its first step. Power to the first winding is turned back on, but with opposite polarity. This reverses the magnetic field of that phase. The rotor takes its second step to align with the new polarities. The second phase is turned off for the third step, and then its polarity is reversed for the fourth step. Overall, the motor takes eight steps before the polarity sequence repeats.

An obvious problem with this method is that one phase is turned off every other step. The lack of phase current reduces motor torque. To compensate for this drop, the driver boosts current in the powered phase. The higher current flow in the powered phase keeps torque the same for all steps.

At first glance, it appears that current through the powered phase should double. Such is not the case, however. When both phases are powered, the rotor locks midway between them. That places the rotor at a 45° magnetic angle to each phase. The strength of the magnetic field felt by the rotor is the sin 45° for one phase and cos 45° for the other, or only 70.7% of each phase.

When one phase turns off, the rotor aligns with the powered phase. To keep torque the same, the single-powered phase must develop a magnetic field strength of 2 X 70.7%, or 141.4%. The current through the single-powered phase must be 1.414X higher than its value when both phases are powered.

Because current changes with each step of the motor, it’s not possible to specify a single value of current when the motor is running. Current ratings in stepmotors stem from the amount of power and, thus, heat that the motor winding can handle. Power, of course, is calculated using the square of the current multiplied by resistance, or P = I²R. Resistance in this case is the resistance of the motor windings. For each motor a value of I is chosen such that I²R does not exceed the standard power rating of the motor.

Because current is constantly changing in an operating motor, a statistical method is used to calculate the current’s effect on motor power. As power is a function of current squared, the method used is called the Root Mean Square, or RMS, method. Using this technique, the value of all currents are squared, the average of their squared values found, and then the square root is taken of that average value. The calculation shows that the RMS value of a stepmotor equals the amount of current when both phases have power. Thus, for stepmotors, average current, RMS current, and amps RMS are identical ratings.

Labels on motors and motor data sheets typically list an amps/ phase rating. amps/phase specifies how much average current each winding or phase can handle without burning out the motor. It should be obvious that this value is the same as the amps RMS rating.

Peak current or amps peak is the highest current that can flow through the motor. As previously shown, the peak current is 1.41 X amps RMS. Drivers and controller products cannot supply currents higher than their design permits. Therefore, they specify their current rating in terms of its peak value. Recent changes now have some companies adding both amps peak and amps RMS to their driver data sheets to make it easier for engineers to match what the motor can handle.

The key relationship to remember is that amps peak = 1.41 X amps/phase (or amps RMS). Regardless whether you remember the reason behind the 1.41 constant, understanding the relationship between amps peak and amps/phase is crucial because, for most manufacturers, stepmotors only list amps/ phase while drives only spec amps peak. Understanding that difference lets you talk the same language to drive and stepmotor makers alike.

Calculating the RMS valuePower dissipation in stepmotors is a function of the square of the phase current times the winding resistance, I2R. However, phase currents in a turning stepmotor are constantly changing. To determine power dissipation in the motor involves calculating the average power over a specific period of time. The current pattern repeats every eight steps, so averaging the power level of all eight steps should provide the average power. To find the average power, take the average of the square of the current — a process known as the root-mean-square or RMS value. To help with the calculation, assume the current for each phase is 1 A when both phases are powered, and 1.41 A when only one phase has power. Calculating the RMS value of current for the eight steps gives:   Notice that the RMS value is the same as when both phases are powered. Thus the amps/phase, amps RMS, and the average current are all the same value, and that the amps peak value is 1.41X higher.

motadistribution.com – Stepper Motors

EMP401/EMP402 is giving an incorrect e-stop error.

When e-stop is wrong, and you receive alarm.
Check connection: Pin 2 of CN1 of EMP.
And ensure Pin2 connect to 25, 25 to -DC24v, and 7 to +DC24v.

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What is the detent torque of a VEXTA stepper?

The detent torque value of a VEXTA stepper is about 3% of the Holding Torque value but Oriental Motor does not specify the value.  It is just a rule of thumb. 

Hyperterminal settings are all correct, but not seeing anything on the PC?

When all of your hyperterminal settings are correct, and you’re using the Oriental Motor communication cable, but you still can’t communicate through your PC’s COM port, you should check to ensure that your I/O has power. This is true for Alpha Step Plus Stepper Motor/Driver packages and the EMP_Series_programmable controller.

Do not use 5VDC and 24VDC at the same time on your power supply for your control signal pins.

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How fast can Oriental Motor’s AlphaStep accelerate?

Oriental Motor has tested the AlphaStep drives up to 2G’s (64.4 ft/s^2) of acceleration at a frequency of 150 Hz

Change the wires to reverse the direction of a 2-phase stepper motor.

Sometimes it’s necessary to mount a stepper motor in a different orientation, or the opposite direction is needed at the motor, but there is no access to the driver control – given the same pulse and direction signals.  Using 6 lead wire unipolar driver, how do I change the wires around to accomplish this?

A: The A-phase lead-wires are normally Black, Yellow, Green.  The B-phase lead wires are normally Red, White, Blue.  To make the motor go in the opposite direction, swap the Black and Red wires, and swap the Green and Blue wires.

How is CN3, the regeneration unit connector, to be used?

In occasional cases where the AS motor is driving very large inertial loads, a regeneration unit supplied by OM can be used. P/N: RGA50-A (100V), and RGA50-C (200V). CN3 on the driver is actually connected direclty to the rectified DC power from the driver, and is around 164VDC. The regeneration unit from OM connects to CN3 and, by using a current controlling circuit, absorbs any surges in current caused by back emf. Therefore, customers should not design their own regeneration unit for this purpose.

How do I apply power to the electromagnetic brake wires?

For the AS46 (size 17), apply 24VDC, 0.3Amin. to the red and black wires that come off directly from the magnetic brake unit. For the AS66, AS69, and AS98, you must use an extension cable, which is designed to have the power wires pigtail from the cable. Apply +24VDC to the orange/black wire (orange for flexible cable), and GND to the gray wire.

Can I use the END output signal as a trigger to turn on/off the magnetic brake via a relay?

Yes, but you need to have a programmed delay before the motion starts and after the motion ends. You can do this through the PLC’s timer function, or if using the SC8800, feed the END output signal from the AlphaStep driver into one of the SC8800’s inputs. From there, you can use conditional statements, e.g., If IN1=1, DELAY=0.2, MI, etc. Remember, the END signal comes on only when the deviation counter (the difference between the pulse counter and rotor position counter) is within +/- 1.8° of the commanded position. So pulses may have stopped, but the rotor is lagging due to inertia, velocity filter setting, etc. If you didn’t compensate for this slight delay, the brake would come on right after the pulse train stops, and that would lead to premature brake wear and possibly inaccurate positioning.

Are the commons on the AlphaStep inputs & outputs on the same node, i.e., is there a possibility of grounding loops occurring?

The common grounds for the AlphaStep inputs & outputs (Pins 2, 14, 16, 24) are all on the node, so that regardless of which voltage you use (5V or up to 30V), you have the same DC ground. The cathodes for the inputs & outputs (Pins 10, 12, 22, 32, 34 on the inputs and pins 24, 26, 30 on the outputs), however, are not on the same node since it relies on the grounding on the customer’s side. Our driver relies on photocoupled signals.