I have started reading a book that states that when applying the voltage drop formula to motor loads it is best to calc. vd for 1.5% due to the sudden inrush of current. For the first few seconds this sounded like a good idea..until I started thinking about whe n amps increase voltage decreases. I know there must be something Im missing (probably somthing I learned and have forgotten). Any help would be appreaciated.
voltage drop on motors
- Thread starter nathan
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Re: voltage drop on motors
Nathan, the tightest tolerance I have heard of is 3% voltage drop on sensitive equipment. I do not consider motor circuits sensitive.
You could obtain 1.5% on very short runs of 20 to 30 feet or less, but to go any further would require enormously oversized conductors.
Dereck
Nathan, the tightest tolerance I have heard of is 3% voltage drop on sensitive equipment. I do not consider motor circuits sensitive.
You could obtain 1.5% on very short runs of 20 to 30 feet or less, but to go any further would require enormously oversized conductors.
Dereck
ron
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Re: voltage drop on motors
When considering voltage drop on a motor, be sure to consider the PF. PF drops on startup, hence the reason for 695.7. If the VD is too high at the motor, they are afraid the controller relays will drop out.
When considering voltage drop on a motor, be sure to consider the PF. PF drops on startup, hence the reason for 695.7. If the VD is too high at the motor, they are afraid the controller relays will drop out.
jcormack
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- Pennsylvania
Re: voltage drop on motors
When sizing wires for a motor application it is sometimes advantageous to oversize the wires beyond the minimum requirements as spelled out in the NEC - this is an engineering, not safety, issue. Sometimes voltage drop on start is very critical on devices with large inertia or high torque requirements - remember that torque is related to the square of the voltage - so a drop to 75% of rated voltage at the motor terminals will result in a drop to 56.25% of available torque. So, yes wires to motors may be oversized to reduce voltage drop - depending on the application - like say a NEMA C 60hp motor with across-the-line-start (or even SCR) on a belt conveyor. Or even one with a fixed fill fluid coupling if torque is close. On a DC motor app the speed is directly related to voltage applied - so if you do not have tach-feedback the motor will slow down on heavy loads due to voltage drop - this may or may not be a problem depending on app.
When sizing wires for a motor application it is sometimes advantageous to oversize the wires beyond the minimum requirements as spelled out in the NEC - this is an engineering, not safety, issue. Sometimes voltage drop on start is very critical on devices with large inertia or high torque requirements - remember that torque is related to the square of the voltage - so a drop to 75% of rated voltage at the motor terminals will result in a drop to 56.25% of available torque. So, yes wires to motors may be oversized to reduce voltage drop - depending on the application - like say a NEMA C 60hp motor with across-the-line-start (or even SCR) on a belt conveyor. Or even one with a fixed fill fluid coupling if torque is close. On a DC motor app the speed is directly related to voltage applied - so if you do not have tach-feedback the motor will slow down on heavy loads due to voltage drop - this may or may not be a problem depending on app.
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