dc servo motor torque

[SJ781]

Is the torque constant on a dc sevo motor? Say it makes 1000rpm at 50v and 500oz/in torque at 10A. If you take and lower or raise the voltage to change the rpms but keep the current the same does the torque change?

[balsaman]

well,

Yes. A servo drive will adjust the voltage to the motor to maintain the rpm it is comanded to go. If you increase the load the voltage will increase to maintain the speed. So lets say you want to go 30 ipm, and your axis hits some heavy cutting, the voltage will increase to maintain 30 ipm. If we keep increasing the load, the voltage and (current) will increase until you reach the point where the motor can no longer maintain the speed, at which time the motor will be running at it's rated voltage and current (or more) and the drive will fault.

So, just because the motor is running at less than it's rated speed, it does not mean it's running at less than it's rated voltage and current. Those two are load dependant, not speed dependant.

Eric

[dpaulson]

I thought that most dc servo drivers are PWM type and to raise and lower the speed, the pulse width is adjusted. It would still output 50v regardless of what speed it is turning.

[vacpress]

i think that the drives are both PWM controlled, and "current compensated".


i believe pwm signals can have various frequencies, but still have the amperage altered to try and force a closed-loop feedback device to reach its goal.

someone who knows something comprehensive should comment on this topic.

[balsaman]

If the pulse width is adjusted, that is effectively adjusting the voltage

Eric

[vacpress]

balsaman - is the amperage controlled seperately from the voltage? or is it on some linear scale or similar preset curve?

[balsaman]

The amperage is not controlled. The motor just takes what it needs up to the limit of the drive. If you take a motor and load it, the motor draws more current. The only way to force a motor to draw more current is to load it.

Eric

[balsaman]

horse power is volts times amps. Lets say you have a load that required .1 hp to move. Lets say the motor is running at 100 volts. Now change the load so it needs .5 hp to move. If you are still supplying 100 volts then the motor will draw 5 times the current to move it.

Eric

[arvidb]

balsaman, I'm not sure I agree totally with what you are saying.

First, power (watts) is voltage (volts) times current (amps). To get horsepower (which is also a measure of power), divide by about 735 (or about 745 depending on if you are using SI horsepower or imperial horsepower - or better yet, stay with watts ).

Second, I believe that most servo drivers control current, and not voltage to the motor. It works something like this:

1) You tell the servo to go to a certain position (by sending it a number of step and dir pulses, for example). If the servo sees that it's at some different position than the commanded, it calculates what speed it should use to get to the right position "in reasonable time". This is called the "position loop". So, input to this loop is the commanded position and the actual position, and from this a speed is calculated.

2) Now the servo knows how fast it should turn the motor. It compares this speed reference with the actual speed, and if it sees that it is off, it calculates how much torque will be needed to reach the commanded speed. This is called the "speed loop". Here, input is commanded and actual speed, and output is a torque command.

3) Since motor torque is proportional to motor current, the servo drive applies the voltage needed to get the commanded current. It is the current that is controlled, but the only way to control current is by adjusting the voltage. This is usually done with PWM.

Since it is the current that is controlled, it's usually ok to use a somewhat higher input voltage to the driver than the motor's rated voltage. The drive should have a current limit, and if this is set correctly (by you) the current will not have any chance to become high enough to damage the motor, even if the PWM peak voltage is higher than the motor's rated voltage.

Hope this makes things clearer, instead of the opposite

Arvid

[ESjaavik]

Arvid: It's very well explained. Quite unexpected coming from a Swede. :-)

The question is a bit fuzzy, but assuming SJ781 wants to know if the *maximum available* torque is constant, then the answer is:
Yes, up to the design RPM, usually found on the label. From that point on to max RPM the HP is constant and as a result the torque falls off. It may be a point to note that if the motor is at standstill the HP is zero, no matter how high the torque is. And this is theory. Usually a motor doesn't follow the ideal curve.

[balsaman]

Horsepower is the same as watts which is power. You are correct that you need to multiply watts x 746 to get HP.

Current is controlled by controlling voltage, as you say. The drive only can control voltage tho and the current is the result, which is what I was trying to explain...perhaps badly.

Eric

[ESjaavik]

The instant voltage from a chopping servo drive is zero or full voltage. The integrated voltage is varying. What most chopper drives try to do is to keep a target current flowing into the motor. That's also one reason it does not need a regulated voltage power supply: it doesn't care about the voltage.
If the voltage drops, it will compensate by holding it longer in the on-state to get the target current.