# How to calculate power using amps drawn?

Is there a formula that I can use to calculate the HP a motor is actually making if all I have is amp draw?
I am trying to size a gearbox, but the customer has no idea what torque is needed. All they know is the motor is drawing 5 amps when it spikes and 3.5 amps when it is running. I am thinking that their gearbox failures is due to shock loading when the amps peak as the gearbox changes direction.
The motor in question is a 3/4 hp 90 volt, 7.6 amp motor, 1750 rpm.
If there is a way to figure out what type of HP the gearbox is seeing when the amps spikes it will be a huge help.

Bystander
Homework Helper
Gold Member
You haven't included a time; the HP is, in principle, infinite.

Borek
Mentor
Perhaps I am missing something but trivial answer is that 90 V and 5 A is 450 W or 0.60 HP.

Tom.G
Gold Member
Sounds like there is an inertial load involved with near-instantaneous reversal. Infamous for wiping out undersized gear trains!

• Fisherman199 and Asymptotic
CWatters
Homework Helper
Gold Member
Perhaps I am missing something but trivial answer is that 90 V and 5 A is 450 W or 0.60 HP.
Which is also consistent with it being rated at 0.75HP max.

CWatters
Homework Helper
Gold Member
Power = torque * angular velocity

But if it's changing direction the angular velocity goes through zero. So at first glance the torque must be infinite. However this is a situation where normal calculation fails. I suspect the current might peak higher than 5A, how is it measured?

I think you need to look at the stall torque for the motor and add a considerable safety margin to get the gearbox torque.

It's possible a high inertial load might drive the motor backwards immediately after a change of commanded direction incereasing the torque beyond the stall torque specs for the motor.

Perhaps you need a speed controller or interlock that doesn't allow (such) rapid changes of direction.

• Asymptotic
Instantaneous reversals apply very large loads - depends on the inertias and motor torques. If you 'fix' it by increasing the gearbox, there is a good chance that the problem will break out in a new place (coupling, motor...). A VFD (or servo) is the way that this situation is typically addressed. Step 1 is to determine the required accel/decel rates - that will dictate everything else.

Depending on how often this 'reversal' occurs, IR heating in the motor may be significant - a blower may be required to compensate for the low speed / high current operation - the 'HP' rating of the motor assumes full speed cooling (I'm assuming that it's a fan-cooled motor).

• Asymptotic
jim hardy
Gold Member
Dearly Missed
The motor in question is a 3/4 hp 90 volt, 7.6 amp motor, 1750 rpm.

if it's a DC motor with permanent magnet field then just Watts = Volts X Amps is a reasonable approximation, subtract maybe 10% for inefficiency.

If it's an AC motor then no, you can't get within probably 50% - look up "Power Factor"

• Fisherman199, dlgoff and Bystander
I'm going to take a different route here and say lets not worry so much about the gear box and the motor. I don't presume to know your process, but what I would do to solve this problem is NOT instantaneously switch direction on this load if at all possible. Now, a 90V motor is a real odd-ball. As Jim suggested, is this a DC motor by chance? I would love to know more about why/how there is an AC motor in use if that is the case. So if you are interested in this path, riddle me this:

• Why are is the load switching direction so quickly?
• Is there a reason you are looking at a different gearbox than examining what I would consider a process flaw in any process I can think of?

• CWatters
Tom.G
Gold Member
Now, a 90V motor is a real odd-ball. As Jim suggested, is this a DC motor by chance?
90Vdc motors are fairly common in industrial machinery as variable speed drives. Their controllers even account for the resistance of the motor rotor for maintaining the set speed under a varying load... and it's all analog control in a roughly 8 x 6 x 4 inch (H,W,D) box with a speed knob on the front (for the smaller motors). If I recall correctly, 90Vdc motors are also used for some low performance position servos.

Cheers,
Tom

jim hardy
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