How does a digital multimeter measure which hole of a power socket is the live wire?
If it is DC, it is very convenient. Just plug the two test leads directly into both ends of the power supply. If the displayed voltage is positive, the red test lead is the positive pole. If the displayed voltage is negative, the black test lead is the positive pole.
Because alternating current has no positive and negative poles, this method is not applicable to alternating current.
So, how do we use a multimeter to distinguish the live and neutral wires of AC power?
The first method: Now adjust the multimeter to the AC range (if it is divided, adjust it to a range greater than 220VAC), and connect the black test lead directly to the ground (you can connect it to the hole in the middle of the three-phase socket, if not, you can directly connect it to the ground or wall), the red test leads are connected to the neutral wire and live wire socket holes respectively. The larger number is the live wire.
The second method: Now adjust the multimeter to the AC setting, leave the black test lead directly idle, and connect the red test lead to the neutral wire and the live wire socket hole respectively. The larger number is the live wire. The voltage measured by this method is not the actual AC voltage, but it can distinguish the neutral line and the live line.
Of course, if you can't find the contact point of the neutral wire or live wire, you don't need to peel off the insulation layer of the wire. One test lead is grounded, and the other test lead is directly close to the insulation layer of the wire. The one with a larger reading is the live wire, and the smaller reading is is the zero line.
How to use a multimeter to determine motor speed
Ordinary three-phase squirrel cage motors have two speeds: one is the synchronous speed N1. The so-called synchronous speed is the speed of the motor's rotating magnetic field. This speed has a strict relationship with the number of magnetic pole pairs of the motor, that is, n1 - synchronous speed, rotation speed. /point;
J--frequency of current, cycles/second;
P--The number of magnetic pole pairs of the motor.
The second is the asynchronous speed, which is the speed of the motor shaft. When an electric motor drives production machinery, the speed of the motor shaft is
The rotational speed of the rotating magnetic field is slightly lower, but the difference is almost the same. For example, for a pair of magnetic pole motors, the speed of the motor wheel is 2950 rpm, for a two-pole motor, it is 1430 rpm, and for a three-pole motor, it is 920 rpm. Based on the above principle, a multimeter can be used to determine the number of magnetic poles of the motor, and then determine the speed of the motor;
