MechMate CNC Router Forum - Wiring the PK296A2A-SG7.2 motors & power supply voltage selection

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- - Wiring the PK296A2A-SG7.2 motors & power supply voltage selection (http://www.mechmate.com/forums/showthread.php?t=554)

Mike,
Didn't you also at one time advise on using a derating factor for the voltage from the max 32 X SQRT( Inductance) formula? I thought that was what Travis was doing with the .67 factor. Sorry Travis.

Yes, I usually derate my power supply voltage to about 75% of the maximum to keep the motors cooler; however, my line voltage tends to run high (about 122VAC instead of the 115VAC that the transformer lists in the data sheets).

I assumed that the 67% derating factor was for recomputing the Current or Amps requirement that many builders frequently use that is found on the Gecko website in the old step motors basics whitepaper. That 67% figure was intended to be used with the old style round motors. The newer square motors are more efficient and more likely to draw 100% of the current if they're pushed hard. In reality, I doubt that many CNC users would ever push all motors anywhere near 100% all of the time, but I would rather have excess current available than to have a motor skip steps because it had caused the power supply to enter a brown-out condition.

We have done current recordings with peak-hold meters over long periods of time across various cutting jobs and have concluded that the 67% is still valid for the square motors.

I have access to a set of new Pacific Scientific step motors that I think would work on my mechmate at a very low cost. I am trying to decide if they are worth messing with or if I need to purchase one of the popular motors already in use on mechmates.
The one problem I can see is that the motors have high inductance and are 4 wire motors. My question is, "What is the impact of the high inductance, if any, other than the obvious high power supply voltages required?".
With a inductance of 10.3 mh I calculate that the power supply voltage required is 102 volts. I am using Gecko 203v drives and would probably end up using a supply voltage of 70 volts so I would not exceed the gecko's max working voltage. Current would be set to 3.3 amps as recommended by the motor specs. It seems to me that this setup would give me the necessary torque for direct drive but I am not sure how much impact the reduced supply voltage will have on speeds. Any insight will be appreciated. Thanks.

Generally speaking,
Speed is proportional to Voltage,
Torque is proportional to Current.

You need voltage to make the current flow and thus voltage and current are very closely related to each other. This idea that speed comes from voltage and torque from current is misleading.

Voltage X Current = Power
Speed X Torque = Power

What we are looking for is Power, and we can increase that by increasing any of voltage, current, speed or torque.

Quote:

Originally Posted by Richards

Travis,

There is an mistake in the way that you figured the voltage.

The maximum voltage is calculated by the formula: 32 X SQRT( Inductance). So, 32 X SQRT( 6mH ) = 78.4 VDC.

The current (Amps) required is determined by adding together the current requirements of all motors. The PK296A2A-SG7.2 motor, when wired bipolar series, i.e. 6mH, can draw about 2.1A per motor, so 5 X 2.1A = 10.5A total for all five motors. The 67% multiplier was used with the old-style round stepper motors. With the new-style square motors, many of us don't de-rate the current requirement.

So, for those motors wired bipolar series, I would use a 70VDC 10A power supply (700VA). The Antek PS-8N70 would be a close match.

For better speed, I would wire the motors half-coil. When wired half-coil, each motor's inductance is 1.5mH, so the maximum voltage would be 32 X SQRT( 1.5mH ) = 39.2 V and the motors could pull up to 3A, so 5 X 3A = 15A. For that configuration, I would use a 35VDC 15A power supply (approx. 500VA). The Antek 5N35 would be a close match for that wiring configuration.

Hi, I have a 350 Watt 36V switchmode PSU and 4 x PK296A2A-SG7.2 motors. Unfortunately now I see that a switchmode PSU was not a great idea, but too late I guess. Using your calculation above, it looks like I am going to be underpowered if I wire up half-coil, is this correct assumption? 4 x 3A x 35V = 420VA. I have seen Gerald mentions that 300 VA is adequate, does he wire the motor differently or reduce the current with the Gecko resistor? Keeping what I've got (36V 350VA PSU, 4 x G203V, 4 x PK296A2A-SG7.2) what is going to be the best way forward for me, must I reduce the current on the Gecko? Thanks, Jonny
Thanks, Jonny

Jonny, you probably do not need to reduce the amperage. Mike uses very conservative numbers (for good reasons), and is assuming that all motors could drawn maximum amperage simultaneously. In practice, that doesn't happen, and Gerald's 300VA is based on practical experience. The issue with switched power supplies is that they are designed for fairly smooth loads, not spiky ones such as motor coils switching on and off. The best way to address this is to ensure you have sufficient capacitance to smooth out those load demands. Linear power supplies have those large capacitors built in as part of their design. So adding a large capacitor to the DC output of your power supply would go a long way towards keeping it happy.