DC Motor Protection

September 23, 2011  |  Articles

The purpose of D.C. Motor protection is to extend a motor’s lifespan by protecting it from conditions that can damage that the motor’s windings, both electrically and mechanically. Motor winding damage can result from any of the following conditions1:

  • Mechanical damage
  • Excessive moisture
  • High dielectric stress
  • High temperature

While each of the above conditions can lead to winding damage, the apparent failure is “thermal degradation of the insulation or burnouts. Insulation life is reduced by about half for each 10oC increase in winding temperature.” 2 To avoid thermal degradation of the insulation, there are a number of methods, devices and circuits used to monitor potential motor hazards and fault conditions and deeneergize the motor when these conditions are met.

Potential Motor Hazards and Fault Conditions

A review of motor hazards and common fault conditions is useful in understanding the different approaches taken to protect motors. These fault conditions are divided into the following categories3:

  • Motor-induced faults
  • Load-induced faults
  • Environment-induced faults
  • Power source-induced faults
  • Application-induced faults

Motor-induced faults4 are directly related to the motor and its associated wiring. Common motor-induced faults include burnt out insulation, bad bearings, loss of field, and other mechanical failures. Wiring problems, chafed or exposed wiring, cabling faults or abrased insulation can cause “short circuits between power phases or between a power phase and earth ground in the motor winding or its connections.”5 (Note: Even though wiring or cabling faults are related to power-source induced faults, they are categorized as a motor-induced fault.)

Load-induced faults6 are “the prolonged overloading as a result of the application of excessive mechanical load”7 Jamming (locked rotor) is a common load-induced fault that causes an apparent overload or high inertia (Wk2d). In pump applications, for instance, oil that is cold or highly viscous may cause a fault; oil heaters are possible solution to correct this fault condition.

Environment-induced faults8 include high ambient temperature, cold/damp environment, high contaminant level, blocked ventilation, among others. These conditions can increase the temperature of the windings by collecting moisture, degrading by corrosion or insulating the windings from contaminants. Loss of ventilation especially at low speeds also increases winding temperature.

Power source-induced faults9 typically will cause high motor currents that can thermally degrade the motor windings from I2R heating. These fault conditions are numerous and include, overvoltage, undervoltage, phase reversal, open phase failures10, unbalances, ground-faults, power transients, harmonics, and loss of field.

Application induced-faults11 are caused by operating conditions that typically cause overcurrent or overload conditions. These conditions include high duty cycle, jogging, rapid plugging (or plug reversing), overspeeding12, and synchronization problems.

Motor Protection Methods

Motor protection methods include devices and circuits that are used within the motor or used with the motor’s control circuitry to monitor fault conditions. They include:

  • Thermal overload relays
  • Transient voltage protectors
  • Ground fault relays
  • Distance relays
  • Fuses, contactors and circuit breakers
  • Undervoltage protection
  • Locked rotor protection
  • Loss of field relays
  • Reversed current protection
  • Isolation transformers
  • Harmonic filters
  • Power conditioners

Thermal overload relays13 protect motors from overload conditions. There are two main types: inherent and external. Inherent thermal overloads14 are bi-metal devices embedded in the motor’s windings. They are essentially thermostats with two dissimilar metals bound together that will bend to open (in some cases, close) a trip switch15 at a temperature setpoint, which is proportional to motor current in an overloaded condition. The switch is connected to the motor’s control circuitry to alarm and/or deenergize the motor. External thermal overload protection 16 use heaters that are connected in series with the motor’s windings and mounted on the motor contactor or circuit breaker. There are two types of heaters: solder pot and bimetal strip. Solder pot overloads will melt when the heat generated by the motor current in an overload condition occurs; this action opens the motor control circuit and trips the motor off the power line. Bi-metal strip17 thermal overloads operate similar to inherent overload protection. While thermal overload protection are most commonly used, electronic and magnetic overload protection are also used for overload protection.18 Electronic overloads are current sensors. They sense actual motor current and when the motor current reaches a predetermined level, a relay will trip and open the motor control circuit. Magnetic overloads use electromagnetism to sense an overload. When an overload condition is sensed, a relay coil will pull in (close) and trip the motor off the power line.

Fuses and circuit protectors are not overload protectors; rather, they are overcurrent protectors designed to “protect the motor from a direct ground or short circuit condition” 19 in the motor or its associated wiring and cabling. Short circuit protection is incorporated into a motor contactor with “high breaking capacity fuses” or a circuit breaker with “instantaneous attracted armature type relays.” Ground fault relays or interrupters are another type of overcurrent protection. They monitor “unintentional current paths between a current-carrying conductor and a grounded surface”20. For motors, ground fault current paths are typically found through dust, water, or worn insulation. Ground faults pose worker safety hazards. 21 Reverse current relays are a protective feature used in motor-generator applications where a standby battery is being charged by the generator. The reverse current relay prevents the battery from discharging and motorizing the generator. 22

For D.C. motors, the loss of field can potentially cause a dangerous, overspeed condition, 23 Hence, loss of field relays are used to monitor the motor’s field. They are connected in parallel with the field and monitor the D.C. motor’s field current. In the event that the field current decreases below a certain limit24, the loss of field relay will drop out and deenergize the motor’s armature.

When a motor fails to start or accelerate after it’s been energized, it is exhibiting a locked-rotor condition. In this condition, the “motor is subject to extreme heating, much more so than in an overload condition since the heat has very little time to be dissipated in the conductors and the other parts of the motor.” 25 Locked-rotor conditions can be protected by an overcurrent relay set for permissible I2t times and currents. But for large D.C. motors, another solution is to build a zero-speed switch into the motor. 26 If the motor does not accelerate to open the zero speed switch, the motor’s power supply is deenergized. However, there’s a disadvantage to the zero-speed switch. In situations where the motor starts but locks up at less than full speed, the zero-speed switch can close and deenergize the motor’s power supply. Locked-rotor protection can also be accomplished by a distance relay27.

Power source-induced faults include undervoltage, overvoltage, open phasing, phase rotation and phase imbalances. (Note: Generally speaking, phase imbalances, phase rotation faults and open phasing are associated with A.C. motors and will not be covered in this article. But it should be noted that if a D.C. motor is powered by a D.C. converter, this controller protects the motor from these conditions.) 28) Undervoltage faults can cause either high motor currents or a failure to start. Hence, most undervoltage protection is part of the motor starter. However, for prolonged undervoltage conditions, an inverse time undervoltage relay can be used to proect from this condition. 29

Rather than using discrete components to protect a D.C. motor from overvoltages or surges, D.C. drives, isolation transformers30 and power conditioning equipment31 are typically used to provide this type of protection. However, MOVs32, arrestors33, harmonic filters34 and power factor correction capacitors35 can also provide overvoltage protection.

Interlocks: Indirect Motor Protection

Interlocking is used to “prevent [motor] contactors from being energized simultaneously or closing together and causing a short circuit.” 36 In this respect, interlocking is an indirect type of motor protection and generally is used with motor starters for reversing and/or auxiliary control. There are three types of interlocks:

  • Mechanical
  • Electrical
  • Auxiliary Contact

Mechanical interlocks will physically prevent two motor contactors (Forward and Reverse) from closing simultaneously. “This interlock locks out one contactor at the beginning of the stroke of either contactor”. On the other hand, electrical interlocks use a pushbutton control or auxiliary contact to electrically isolate one contactor while energizing the other contactor. 37 Auxiliary contact interlocking is a wiring modification of pushbutton interlocking. There are two types of auxiliary contacts: Normally Closed (NC) and Normally Open (NO). For interlocking protection in a reversing circuit, a normally closed (NC) auxiliary contact is wired in series with the opposing motor contactor coil. Thus, when a motor is running in the forward direction, the forward contact coil is energized through the NC auxiliary contact. When the reverse direction is selected, the NC contact will open and deenergize the forward contact coil while the reverse coil will energize through its NC auxiliary contact.

Environmental Protection

Environmental contamination can adversely affect normal motor operation. Dust, air particulates, explosive vapors, water, humidity, high ambient temperatures can all shorten the lifespan of a motor. To protect a motor from these environmental conditions, the National Electrical Manufacturers Association (NEMA) and the International Electrotechnical Commission (IEC) have classified motor enclosures based upon the level of protection they provide. 38 The two major classification of motor enclosures are open and totally enclosed. Open motors are further classified as dripproof, splashproof, weather protected, semi-guarded and guarded. Totally enclosed motors are classified as totally enclosed non-ventilated, fan-cooled, explosionproof, dust ignition proof, air-to-water cooled and air-to-air cooled.

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