Q&A on basic knowledge of single-phase motors, stepper motors, etc.

Question: How much motor can a 0.75 kilowatt frequency converter carry?

Answer: When the working current does not exceed the rated current of the inverter, it can drive a 1.1KW motor

Question: I would like to ask if I will get an electric shock if I wear insulated shoes and hold an electric wire 380v or 220v with two hands.

Answer: Yes

Question: Is the air switch or the leakage protection switch installed first in the wiring process?

Answer: The air switch is also called an air circuit breaker. As long as the line current does not exceed the maximum current allowed by the switch, the circuit will not be disconnected even if leakage occurs; and the leakage protection switch has the function of an air switch, and one more is important The function is to cut off the circuit in time when the equipment (home) line leaks. If the leakage switch is in the front and the air switch is in the back, once the equipment has a leakage phenomenon, a skipping trip will occur. This will affect the abnormal shutdown of other normal electrical equipment, so normally the air switch should be in front, and the leakage switch should be behind.

Question: How to distinguish between PNP type and NPN proximity switch?

Answer: PNP is high-level output, that is, 24V; NPN is low-level output, that is, 0V. When wiring, the PLC common terminal of PNP sensor should be connected to 0V; the PLC common terminal of NPN sensor should be connected to 24V.

Question: What is the standard for the wire diameter of the ground wire of the three-phase asynchronous motor?

Answer: The grounding wire should be no less than 1.5 times the diameter of the phase wire, and the grounding resistance should be no less than 4 ohms

Question: Why do single-phase motors have two capacitors? What role does each play?

Answer: Since single-phase electricity is different from three-phase electricity, a single-phase motor cannot form a rotating magnetic field when it starts. It uses the characteristic of the current in the capacitor to generate a rotating magnetic field to start the motor. So the starting capacitor is essential in single-phase motors. But the running capacitor is not necessary.

The running capacitor plays a role of compensation and can increase the output power of the motor. Therefore, when the load power is large, a running capacitor needs to be added, and when the power is small, the running capacitor can be omitted and only a starting capacitor is reserved.

Q: What is the use of two capacitors for single-phase motors? When to use a capacitor?

Answer: The most commonly used classification of single-phase asynchronous motors is based on the starting method. Different types of single-phase motors have different methods of generating starting torque. The common ones are as follows: ① single-phase capacitor split-phase motors; ② single-phase impedance split-phase motors; ③ single-phase capacitor-operated motors; ④Single-phase capacitor start and running motor; ⑤Single-phase shaded pole motor.

Single-phase split-phase motors need two capacitors to start and run; one capacitor is a starting capacitor and the other is a running capacitor; a single-phase split-phase motor has a running winding and a starting winding embedded in the stator, and the difference between the two windings on the stator is 90º Electric angle, and all connected to the same single-phase power supply; because the single-phase power supply cannot produce a phase difference, it must use the 90° lead characteristic of the capacitor to give the motor a starting torque and generate a rotating magnetic field; when the rotor speed reaches the motor after starting At 70% to 80% of the rated speed, a centrifugal switch installed inside the motor will automatically separate the starting capacitor from the starting winding, and the running winding and the running capacitor will continue to maintain normal operation. This kind of split-phase motor can change the direction of rotation by changing the two wire ends of any one of the two sets of coils.

Question: How to determine the number of pulses required for a stepper motor to rotate for one revolution?

Answer: 1. Motor step angle: generally 0.9 or 1.8; if the step angle is 0.9 degrees, the motor rotates 400 pulses per revolution (360/0.9)

2. Subdivision: Adjust according to the instructions on the drive: if the subdivision number is 2, that is, every two pulse motors rotate one step, that is, one pulse motor rotates (0.9/2) 0.45 degrees, and the motor needs to rotate (360/0.9 )*2=800;

Question: After the servo is turned on, the panel displays 00 and there is no signal input. After measuring the encoder voltage, the encoder voltage is 24V regardless of whether it is working or not. Is the encoder broken?

Answer: Is 24V the encoder power supply voltage or the signal output voltage? According to the high and low level category of the encoder output signal, measure whether the voltage value of the output signal line of the encoder is 24V to determine whether the encoder is good or bad.

Q: What role does the series voltage divider play in the actual circuit?

Answer: In the circuit, the voltage is divided in series and the current is divided in parallel; according to Kirchhoff's law, the current of the series branch is equal, and because the terminal voltage is constant, the terminal voltage of different loads connected in series is different

Question: Teacher, how do the voltage and current values of the transformers pass into the PLC? Are there any related tutorials?

Answer: PLC can only receive standard signals, such as 0-10V, 0-20ma, ±10V and other signals! So there is no tutorial alone

Question: Is there no positive or negative connection between p1 and p2 in the Siemens LOGO power supply to 24v?

Answer: DC 24V is divided into positive and negative!

Question: How to determine the number of pole pairs of the motor through the motor nameplate?

Answer: It can be determined according to the model on the motor nameplate; such as model Y355L2-4; the last number 4 represents the number of magnetic pole pairs, which is a 4-pole motor with 2 pairs of magnetic poles

Question: I would like to ask whether the three-phase 220v power supply can be powered by a switching power supply 220v?

Answer: No!

Q: Are there any tips for looking at circuit diagrams?

Answer: Look at the main circuit first, then the control circuit, and finally the auxiliary circuit. Through the control circuit and the correlation between the main circuit and the auxiliary circuit, we can understand the logic relationship of the entire control.

Q: What is the rated current of a 315KVA dry-type transformer?

Answer: According to the three-phase power calculation formula: p= √31V Cos

P: Power unit watt (kw)

√3: 1.732

I: current (primary or secondary) unit: ampere

V: Voltage (primary or secondary) unit: volt

COS: power factor

High voltage (10KV) rated current:

I=P/1.732/U=315/1.732/10≈18(A)

Low voltage (0.4KV) side rated current:

I=P/1.732/U=315/1.732/0.4≈455(A)

Question: What does the Q letter in the electrical diagram mean?

Answer: Q represents the switch of power circuit such as circuit breaker, isolating switch QF

Question: How to understand the rising edge and falling edge instructions?

Answer: The rising edge (positive transition contact command) allows energy to flow for one scan period after each off-to-on transition. That is, the first scan period when the signal changes from 0→1 is called the rising edge

The falling edge (negative transition contact command) allows energy to flow for one scan period after each on-to-off transition. That is, the first scan cycle with 1→0 is called the falling edge

Question: A transformer is needed to amplify the protection current. How should the motor integrated protector and current transformer be wired?

Answer: You can put appropriate current transformers on the three main lines of the load. The K1 of the secondary lines of the three transformers respectively pass through the three transformer holes H1, H2, H3 of the protector and then return to the K2 of the transformer.

Question: What are the commonly used low-voltage electrical appliances?

Answer: We briefly list some commonly used low-voltage electrical appliances: circuit breaker series, various switches, contactors, relay series; various switches, buttons; indicator lights, inverters, PLCs, etc.

Question: What is electricity, current, voltage, resistance, circuit, electric power?

Answer: Electricity---The quantity of electric charge carried by an object is called electric quantity, and the unit is coulomb.

Electric current---The electric current is formed by the directional movement of electric charge in the conductor. The unit is called ampere, or ampere for short. The electricity passing through the cross-sectional area of the conductor in 1 second is 1 coulomb, and its current intensity is 1 ampere.

Voltage---The potential difference between two points in the circuit is called voltage, and the unit is volts.

Resistance---The hindering effect of current in a conductor is called resistance. The unit is ohm.

Circuit---The path through which current passes is called a circuit. The circuit consists of four basic parts: voltage, electrical appliances, switches and connecting wires. The power supply is a device that generates electrical energy; electrical appliances include light bulbs, electric fans, etc.; connecting wires are used to connect power supplies, electrical appliances and switches, and must be connected to a circulation path.

Electric power---indicating how much work is done per unit time, referred to as power, and the unit is watts. One kilowatt of electricity consumed in one hour is called "1 kilowatt-hour".

Question: What does the PLC special register sm0.0 mean and how to use it?

Answer: plc is always connected to 1 in the running mode; when it is about to be connected as soon as the power is turned on, the normally open contact of this register command can be used

Question: Delta connection, why the contactor coil will burn when the motor drives the load for about 2 minutes after starting

Answer: The delta connection is only used for motors above 4KW. This failure first depends on whether the contactor rated current selection is reasonable? Is the wiring secure?

Q: What is a safe voltage

Answer: When the human body is in contact with electricity, the voltage that will not cause any damage to all tissues of the human body is a safe voltage. The industry stipulates that the safety voltage is not higher than 36V.

Q: What is Sanden?

Answer: Three powers refer to planned, economical, and safe use of electricity.

Q: Will there be any harms caused by unbalanced three-phase electricity?

Answer: Due to the unbalanced three-phase voltage, a reverse-sequence rotating magnetic field will be generated on the stator of the induction motor. At this time, the induction motor is running under the action of two forward and reverse magnetic fields. Because the forward-sequence rotating magnetic field is much larger than the reversed magnetic The direction of rotation of the rotor rotates in the positive sequence direction, but the reverse sequence resistance of the rotor is small, so the reverse sequence current is relatively large. Due to the presence of the reverse sequence current and the magnetic field, a large braking force is generated, which will reduce the power.

Q: Why does the AC contactor overheat during frequent operation?

Answer: When the AC contactor is started, due to the large gap between the iron core and the armature and the small reactance, the exciting current that can pass through the coil is very large, often more than ten times the working current. If frequently started, the exciting coil will pass a large The starting current will cause the coil to overheat, which will burn out in severe cases.

Q: What is the point of zero connection?

Answer: The following requirements should be met when connecting to zero:

(1) In the same system, it is strictly forbidden to ground a part of electrical equipment and ground another part of electrical equipment to zero, which is extremely unsafe.

(2) Switches and fuses are not allowed to be installed on the zero line of the three-phase four-wire system.

(3) Pay attention to the laying quality of the neutral wire. In order to prevent the occurrence of a neutral wire disconnection fault, sufficient repeated grounding devices should be installed. The grounding resistance of repeated grounding should not be greater than 10 ohms.

(4) All electrical equipment connected to the neutral line should be connected in parallel, and then the neutral line or branch line should be connected.

(5) The cross-sectional area of the zero line should not be less than 1/2 of the cross-sectional area of the phase line.

Q: What are the requirements for the protection connection?

Answer: The neutral point of the transformer or generator grounding is called the zero point, and the wire connected to the zero point is called the neutral line. Protecting the neutral wire is to connect the non-charged metal shell of the electrical equipment with the neutral wire under normal conditions. In the power supply system where the neutral point below 1000V is directly grounded, the protective grounding is generally not used, but the protective zeroing is used.