How do I measure how many amps my motor draws?
Make the measurement with a clamp meter capable of measuring DC current, taking a single conductor of the supply line inside the jaw. Record not the no-load figure but the steady current under real load and the start-up surge. The start-up current is typically 5-8 times the rated current and lasts 0.5-3 seconds.
The steps to follow for a correct measurement:
- Use a DC clamp meter. Clamps that measure only AC will not show DC motor current; make sure the instrument measures DC (Hall effect).
- Take a single conductor. Take only the positive or only the negative conductor inside the jaw; if you take both together the fields cancel each other and you read zero.
- Run it with the real load. Current measured with no load misleads. Move the door, the shutter or the load under normal working conditions.
- Record three values: the no-load current, the steady current under load and the start-up surge.
- For the start-up surge use the instrument's peak/inrush hold function; the normal averaging mode misses the surge.
- Try the worst case: cold weather, tight mechanics, the heaviest load.
The steady value you obtain is the basis of the sizing; the start-up surge, on the other hand, must fit within the driver's peak window. The KS250 carries its 45 A peak value for 5 seconds, the PT500 its 40 A maximum value for 15 seconds. If the measured steady current is approaching 15 A, the PT500's 30 A continuous rating is more suitable.
The full topic: DC Motor Driver Selection Guide