2.5 amps per phase drive - 2.8 amps per phase bipolar motor ...

[kochevnik]

What happens when you use this combo ? Something's gotta give right ? Does the motor draw 2.8 amps and stress the controller, or does the controller only deliver 2.5 amps and stress the drive ? I've even seen 3.35 amps per phase motors (bipolar) sold with a 2.5 amps per phase drive on Ebay.

Anyone can help me figure this out ? Thanks.

[jeffs555]

When you set the current on a modern chopper type drive, that is the current it will try to supply, and will supply no more. The current rating on a motor is the maximum current that the motor can safely take. If you take a chopper drive and set it for 2.5 amps, it will try to supply 2.5 amps even if it is attached to a 1 amp motor or to a 5 amp motor. If you supply 2.5 amps to a 1 amp motor, it will probably burn up. If you supply 2.5 amps to a 5 amp motor, it will run but have half the static torque(torque is proportional to current).

Running with reduced current is not as bad as it seems. Reduced current will result in reduced low speed torque, but will have no effect on torque at higher speeds.

[snowshovelbmx]

ok I have a question about this as well. since the motor will only draw 2/3 the rated current when wired in parallel and only 1/3 in series than does this mean that a 6 amp/phase motor could be run at full potential with a 4 amp/phase drive parallel or a 2 amp drive in series? just been wondering about this any help is appreciated.
chris.

[jeffs555]

I think you are a little confused. If you connect two windings in parallel, and keep the voltage the same, the current draw will double. If you connect two windings in series, and keep the voltage the same, the current draw will be half. However, we are talking about chopper drives with a current setting, so the motors will draw whatever current is set on the drive.

With a chopper drive and an 8 wire motor, for the same current you will get the same torque whether you drive only one winding or drive two windings in parallel. The only advantage to parallel connection is that it usually allows you to safely run higher currents. Usually, with a parallel connection, you can safely run 1.4 times the single winding(ie unipolar) current. With 1.4 times the current, you would get 1.4 times the torque.

With a chopper drive and an 8 wire motor, for the same current you will get twice the torque with a series connection than you would with a single winding. However, with a series connection, you must reduce the current by at least 0.7 times or the windings will burn up. With series connection, and 0.7 times the unipolar current you will get 1.4 times the unipolar torque. The main disadvantage to series connection is that the inductance increases, so the maximum speed will be half that of the parallel or unipolar connections.

PS The 1/3x for series and 2/3x for parallel is just a general rule for sizing the power supply with a chopper drive, and is not the actual motor current.

[Mariss Freimanis]

OK. What happens depends on the drive. Say you have a 5A motor connected to a 2.5A drive and the drive is a chopper type.

The motor will stall way before its time at higher speeds because the drive will keep "chopping" way past the speed it normally would. The chopping waveform will interfere with motor phase-timing signals and force the motor into resonance and resulting stall.

Mariss

[snowshovelbmx]

ok thanks a lot I understand what you are saying, I will be going with drives that are rated higher than the motors anyway but was just curious about that as well. thanks guys!

[jeffs555]

Mariss,
Now I am a bit confused. My point about current setting not having an effect at higher speeds was that above some speed, the current will no longer reach the set point on the chopper drive. I wasn't thinking about resonance issues. I know that cutting the set current in half would keep the drive chopping at higher speeds, but so would doubling the supply voltage.
In this thread http://www.cnczone.com/forums/showthread.php?t=13678
you said that "Mid-band resonance cannot occur while the drive is in current mode."

Is the resonance you are talking about here something different from mid-band resonance?

Jeff