# Thread: How to determine actual current draw for a servo

1. ## How to determine actual current draw for a servo

Hi
I just upgraded to a servo on the z axis. I`m considering changing the y and potentially the x. I have two power supplies now. One is 54 volts 15 amp DC and one 32 volt 10 amp. How do I determine the maximum amperage draw of a servo motor to help me decide which power supply to use for the various motors. One of the cnczone members mentioned that its best to get a amperage reading when the servo is in a stall condition...under a maximum load. How is this accomplished in a test condition. The motor I have now was purchased from Jeff at homecnc.He mentioned that the maximum current draw is potentially 20 amps. At that rate Id need a new power supply totaling 60 amps. It seems a actual current rating is needed to make a reasonable estimate of true current .
Regards Barry

2. YOu can:

1. Trust the info given that 20 amps is the stall current

or

2. Find a way to stall the motor and measure current draw with an ammeter while in that condition.

or

3. Buy or rent a recording ammeter that has the ability to do a "peak and hold". Some non-contacting ammeters have the ability to sample and hold peak inrush current. This would give you the spike value without locking a motor and zapping it which is hard to do and quite brutal.

NOTE: you don't want to start attaching ANY p/s to ANY motor based on current rating of the P/S. A motor designed for say 30 volts will draw WAY more current if you zap it with 54 volts and vice versa.

My suggestion: Trust Jeff at homecnc. Run what he says for rated voltage and current for a P/S.

3. When a motor is stalled, it looks like a simple resistor to the power supply. If you measure the motor resistance with an ohmeter, you will get an idea of what the stall current is ( I=V/r).

Just remenber that the motor cannot tolerate this level for more an a few seconds before burning up. It is usually sufficient to size the ps based on the motor's continuous rating, if you add a bit of headroom.

4. Do you have a servo drive that you can read current from directly? What brand servo do you have?

• Well....it's best to go by the label.....their is a nominal current rating and a demagnetization rating.

Use the nominal current rating.....

• What Viper said. The motor manufacturer can afford to take a few out to the test stand and smoke them. You don't have to supply the current that will smoke the motor, and it's ok to run out of current before then. Think about it, your motor, your amp, and your power supply all have max ratings. An amp with a reasonable design will have a setting for max current. If you set the amp below the smoke current of your motor and your power supply, the system will not produce smoke.

We bought an expensive motor for research and filled the lab with smoke. It was very exciting.

• Guys wait !!! motors are rated in watts or kilowatts (kW). This is similar to a power supply rating which is vA. To find the kW rating of the motor the formula is volts x amps, similarly for a power supply or transformer. The difference is the motor draws power and the power supply delivers it, stalling a motor is a good way to burn it out, so dont do it !! The power supply should always have a rating higher than the motor, a correctly motor rated trip or fuse between the amp and motor is the protection. i.e; the 54v 15amp motor is 810 watts (0.81 kW) the 32 volt 10 amp motor is 320 watts (0.32 kW). Also note, 0.75 kW= 1 hp.

• Originally Posted by philserveng
stalling a motor is a good way to burn it out, so dont do it !! .
As has been said here before, there is no direct connection between power supply and motor, there is a drive in between.
You can stall a motor, the secret is to limit the current that is equal to the stall torque. The maximum torque is the current limit before demagnetization occurs.
Al.

• Have you used one of the free motor sizing programs to get a ball park figure for the torque needed to run your machine?

Once you have a good idea of the average torque, use the torque constant of the motor to figure the current needed. Give yourself a 20-25&#37; head room and multiply by 2 (2.5 ties average current). An unregulated supply can deal very well with short term overloads, so as long as the average draw is less than the rated power the system will be fine.

You don't need a supply rated for 3x the single motor peak as you would not be maxing out all three motors for long periods of time as a norm.

Aaron