Motor taking high current ?
No need to worry. In this article, We are going to know why do motors take high current without load. We will also discuss its preventive steps. First, We do need to learn about the rated current, No load current and high current.
This post covers :
Rated current
Selection of Motors depend upon power, torque and RPM requirement. Further, Motors are designed for particular voltage range. According to power rating, Motor takes current when rated voltage get applied across it.
“Rated current” is the safe current limit, Up to which motor works precisely without excessive noise, temperature and vibration.
Normal No load current
At no load, an Induction motor draws a fraction of its rated full-load current. This current can be as high as approximately 50% of the rated current in some motors, while a rough estimate for many motors is around 20–40%, depending on the motor’s size and design.
The no-load current serves two main purposes:
- Magnetization: It establishes the magnetic field required for the motor to operate.
- Supplying no-load losses: It provides the active power needed to compensate for core losses, friction, windage, and stator copper losses.
High current in motor
Current more than rated current is considered as “high current“. High current produces an excessive stress on winding, Motors noise, vibration and temperature goes out of the range.
High current; by deteriorating the insulation of motor winding; reduces the useful life of a motor. In worst circumstances, High current can damage the motor permanently.
Reasons of motor drawing high current without load
There are multiple reasons of motor drawing high current without load. Such as Damaged bearing, Wrong power supply and wrong connection.
Reasons of motor drawing high current without load are discussed in a table below :
| Reasons of motor drawing high current | Definition | Causes | Cure | Prevention |
| Damaged Bearings or bushes | Rotor rotates freely in between stator with the help of bearings or bushes. Damaged bearing or pushes produces an excessive load on the motor. | Material insertion in bearing or bushes. Exceed running life of bearing. Wrong selection of motor. | Replacement of bearings and bushes | Motors history card for Periodic bearing or Bushing change scheduling |
| Single phasing | Single phasing is the phenomenon in which one of the phase get disconnected and only 2 phases remain connected across the motor. Such condition motor takes high current. | Loose connection. One connection is skipped. Contactor burnt Thermal overload relay or MPCB’s contact burnt or contact get stuck. | Tight all the connection. Cross verify the connections at least twice. If faulty, Change contactor or Thermal overload relay or MPCB | Work should be done by skilled workforce. Every connection must be verified at-least twice . Right selection of MPCB Contactor and thermal overload relay. Scheduling of replacement of thermal overload relay, MPCB and contactor. |
| Wrong power supply | Supply voltage out of the range connected across the motor is known as wrong power supply | Unskilled workforce. Loose connections. Not reading the name plate. | Disconnect the power supply to motor. Connect after confirming the power supply | Voltage monitoring device. |
| Wrong or loose connections | Wrong winding connection. Wrong connected star or delta. | Unskilled workforce. Loose connections. Not reading the name plate. | Before connection always read the name plate. Verify the connections at least twice. | Skilled workforce at verification at least twice. |
| Air Gap or magnetic circuit problem | Increased Gap in between stator stamping and rotor stamping | Damaged bearing or bushes which results Rotor stamping and Stator stamping get rubbed against each other. | – | Monitor the motor parameters on regular basis |
| Shorted turn in stator winding | Winding coils shorted to each other. | Insulation breakdown, Overheating, Mechanical damage | Rewinding | Monitor the motor parameters on regular basis such as its insulation, Noise and vibration, Ampere and voltage parameters. |
| DOL Starter | DOL- Direct online starter, Motor get directly connected to power supply without any change in winding or power supply or frequency. In DOL starter motor consumes high current at starting. | Since full voltage get available across the motor. Motor will take high starting current due to high starting torque. | DOL starter running motors take high current at starting. It should be replaced with other alternatives such Star delta starters and Variable frequency drives. | DOL starter can be used up to 5.5 KW. Above that rating other starting means can be used such as Star delta starters and Variable frequency drives. |
Some of the reasons of high current represented through an images below :
Preventive steps for motor drawing high current
Next, Some of the preventive steps are discussed below to prevent the high current.
Thermal overload Relay
Thermal overload relay provides protection from overload conditions. Power supply directly passes through these Thermal overload relay. These relays do not directly interrupt the power supply rather these relay produces the trip signal.
Motor protection circuit breaker
Motor protection circuit breaker also provides the protection from overload. Unlike thermal overload relay, There MPCB interrupts the line current directly. Through auxiliary contacts trip signal can also be generated.
Voltage monitoring device
Voltage monitoring devices ensures that only rated power supply must be available across the motor. The power supply connected to the motor, Parallelly connected to these Voltage monitoring devices. Voltage out of the range; is triggered by these monitoring device. Which further interrupts the starters of the motor.
Starting method other than DOL starters
DOL stands for direct online starters. In this starter, Power get directly available across the motor without any change in voltage, frequency and winding configuration. Full voltage get available across the motor. Full voltage at starting produces high torque across the motor. High torque means high current at starting.
To limit that much high current, Alternate starters are used. Such as Auto transformer starters, Star delta starters and VFD(Variable frequency drive).
Regular interval monitoring
Regular basis monitoring helps in maintaining the motors parameters. Parameters such as “voltage “,”Ampere”, “Noise “and “vibration”. Parameters variation can be used to detect the alteration in a process.
Such as When bearing over the time when it get degreased or due to material ingress, Rotor’s noise and vibration get increased. That can be taken as preventive indication to replace the bearings.
How to troubleshoot the problem step by step
The motor in a doubt of high current. First, We need to verify the some of the parameters of the motor without energizing it.
- Check the name plate of motor. Its rated power, Voltage, frequency, Current, and connection type.
- Check overall condition of motor. Whether load directly coupled or not.
- If load is directly coupled, disconnect the output load. Than Observe the bearing noise and vibration by rotating the rotor of motor if possible. If doubt about the motor’s bearings or something is obstructing the rotor to rotate than rectify it first.
- If load is not directly coupled. Observe the bearing noise and vibration by rotating the rotor of motor. If doubt about the motor bearings or something is obstructing the rotor to rotate than rectify it first. If it is bearing, Change the bearing.
- After that check the resistance of motor’s winding. (Make sure motor’s power line is disconnected) if 6 terminals available across the terminal box, Check resistance of all 3 coils. Resistance value must be equal in all 3 coils. if resistance value varies in large than there are chances winding is faulty.
- Check the voltage available and working of starter without connecting to motor.
- Connect the motor according to its rated name plate such as connection type, Voltage, frequency. Starter selection must be accordingly.
Frequently Asked Questions (FAQs) related to motor taking high current
1. Why does a motor draw high current without load?
An induction motor may draw high current without a mechanical load due to excessive supply voltage, voltage imbalance, incorrect star-delta connections, winding faults, or internal mechanical friction. First, compare the three phase currents and supply voltages with the motor nameplate specifications.
2. What is the normal no-load current of a three-phase induction motor?
The no-load current varies with motor size, design, pole count, and rated voltage. For many three-phase induction motors, it may be approximately 20–40% of the rated full-load current, but this is only a rough guide. Always refer to manufacturer data or compare measurements with a known healthy motor of the same type.
3. Can a faulty motor winding cause high current without load?
Yes. Inter-turn short circuits, winding insulation damage, or incorrect winding connections can cause abnormal current and overheating. However, a normal winding resistance reading does not always rule out an inter-turn fault. Further testing may be required.
4. Can voltage imbalance cause high no-load current in an induction motor?
Yes. Unequal voltages across the three phases can produce unbalanced currents and additional heating. Measure all three line-to-line voltages and phase currents under the same operating conditions. Even a relatively small voltage imbalance can cause a significantly larger current imbalance.
5. Why does a motor draw high current when its coupling is disconnected?
If the motor still draws excessive current after being safely disconnected from its driven equipment, the cause may be electrical, such as a supply or winding problem, or mechanical, such as bearing friction. Compare the readings with the motor’s normal no-load current and investigate any abnormal noise, vibration, or heating.
6. How can I check whether a motor has a winding fault?
Start by isolating the motor from the supply and following appropriate electrical safety procedures. Inspect the terminals and connections, measure winding resistance between phases, and perform insulation-resistance testing using a suitable insulation tester. These checks can identify some faults, but an inter-turn short may require specialized testing. Never perform resistance or insulation tests on an energized motor.